Facade Research Document

FACADE RESEARCH
NICHOLAS SOCRATES
Facade Research Document

Lloyds of London, Richard Rogers
& Partners, 1986
adaptive functions of facades adaptive functions of facades
Thermal comfort
Acoustic comfort
Visual comfort
Comfort
Thermal comfort:
convection
radiation
Comfort
ComfortThermal comfort:
convection
radiation
Comfort
Thermal comfort:
convection
radiation
Comfort
Comfort depending to
requirements
Percentage Predicted
Dissatisfied (PPD): 5-10%
Visual comfort
Lightness
Glare
Atmosphere
Acoustic comfort
adaptive functions of facades
Ventilation
adaptive functions of facades
Heating
cooling
Ventilation
Light
Sun protection
Natural ventilation:
Gap ventilation
Window
Chimney
Ventilation
2
AR3B430 Facade Technology
Adaptive Facades
Understanding of the Facade as a skin/ organ of the
building
Mick Davies / Polyvalente Wall, 1984
Lloyds of London, Richard Rogers & Partners,
1986

Heating
ventilation
Gap ventilation
Window
Gap ventilation
Window
Chimney
mechanical ventilation
ventilation
Gap ventilation
Window, Chimney
Mechanical ventilation
ventilation
Gap ventilation
Window, Chimney
Mechanical ventilation
Radiator Radiator
Convector heating
Radiator
Convector heating
Under floor convector heating
Radiator
Convector heating
Under floor convector
heating, Façade heating
Cooling
Night time cooling Night time cooling
Chilled ceiling (concrete floor
activation)
Night time cooling
Chilled ceiling
Cooling ceiling
Night time cooling
Chilled ceiling
Cooling ceiling
Cooling wings
Sunscreen
Shading
Sunscreen
Brises soleil
Sunscreen
Brises soleil
Fixed lovers
Sunscreen
Brises soleil
Fixed lovers
Green
Sunscreen
Brises soleil
Fixed lovers
Green
Roller blinds
Sunscreen
Brises soleil
Fixed lovers, Green
Roller blinds
Venetian blinds
Sunscreen
Brises soleil
Fixed lovers
Green
Roller blinds
Venetian blinds
Horizontal sliding blind
AlternatingFacade
Twin Face Façade Development
SecondSkinFacade
DoubleFacade
BoxedWindowFacade
Twin face facade: extra glass layer for wind and noise
protection and integration of installation
CorridorFacade
ChimneyBoxed
WindowFacade
ComponentFacade
Short Cuts for Air:
Box-window
Closing the opening for Special climate reasons:
Boxed window
„intelligent“ use: depending of the period of the year the
inner / outer Window has to be removed / opened
Closing the
opening for Special
climate reasons:
Boxed window
„intelligent“ use:
depending of the
period of the year
the
inner / outer
Window has to be
removed
Box-window:
Frankfurt Westhafen
Controls unclear Second
Skin Facade
Second Skin Facade:
XX Hamburg
Second Skin Facade:
Tore Agbar - Barcelona
Problem sound transport
Corridor façade
Corridor façade: Stadttor
Düsseldorf
Düsseldorf
Düsseldorf
Corridor façade: University of
Frankfurt
height leads to more energy
Chimney-box-window
3

Chimney-box-window:
ARAG Düsseldorf
Chimney-box-window:
ARAG Düsseldorf
Alternating Facade Alternating Facade:
Debitel Stuttgart
Alternating Facade:
Debitel Stuttgart
Alternating Facade:
Debitel Stuttgart
Alternating Facade:
Surface partly glazed, Window elements in the effectiv
areas of the façade: Concept Schüco
Alternating Facade:
Debitel Stuttgart
Alternating Facade:
Concept Schüco
Twin Face Facades
Main problems with twin face facades:
Acoustics
Acoustics, Heat Twin face facade in Madrid: temperature
in the
facade at sunny day in winter
Acoustics, Heat,
Condensation
Copyright: W. Heussler / Schüco Copyright: W. Heussler / Schüco
Acoustics, Heat,
Condensation
So twin face facades
are mainly used at
skyscrapers,
windy areas and
noisy places.
and the user has
to live with the
building.
P+C Kaufhaus / Köln
Renzo Piano
Building Workshop
Twin Face Facades:
„Component Facade“: Integration of Mechanical Components
Decentralized mechanical services:
Heating
Ventilation
Air
Light
Component Facade
Component Facade:
Posttower Bonn, Decentralized Mechanical
Services in the Floor Slab
Component Facade
“Component façade”
Facade mock-up / Detmold
Partly twin face facade and decentralised mechanical
services
Component Facade
Component Facade, Capricon Düsseldorf:
TE-Motion Facade from
Wicona
Decentralized
mechanical services in
the facade panels
Decentralized mechanical services in the facade panels
Component Facade
4

Component Facade
Integrated design
Mick Davies / Polyvalente Wall
for Lloyds of London
Smartbox / CEPEZED, Leven, ECN, TNO,
v d Vlugt etc.
TE-Motion Facade from Wicona
Decentralized mechanical services in the
facade panels
Mick Davies / Polyvalente Wall for Lloyds of London
Understanding of the Facade as skin / organ of the
building
E² facade by Schüco /
Uni Stuttgart (Prof. Behling) Component Facade
Active Facade
Daniel Westenberger / Technische Universität München
Sunscrean Daylight controle Open view Electric light Daylight controle Active Facade
logical, maybe even mandatory. Particularly if one considers the in-depth planning require-
ments and high level of prefabrication needed for facades and building services elements, as
well as the dramatic increase in complexity. The projects Posttower Bonn / Helmuth Jahn with
Transsolar and Capricon in Düsseldorf / Gaterman + Schossig are examples of the developmen-
tal steps described – both integrating building services modules into the facade. At the same
time, several suppliers have worked on incorporating these technical possibilities into their
products – such as t-motion by Wicona, smart box by CEPEZET as well as E² by schüco – however,
these products have not yet been implemented on a large scale (1/6/7/8).
previously seen (3/4).
Today’s developments are – besides various design trends – primarily driven by material-related
or technological innovations. This is evident in fully glazed constructions, a trend that started at
the end of the last century, whereby the technological finesse is part of the esthetic fascination.
Examples are the works by Peter Rice and Ian Ritchie, Mick Eekhout and Rob Nijsse (4/5). The
conception of glazed double facades must be considered as part of this development.
In addition to a continuing dissolution of the envelope, this concept also tried to fulfill the de-
mand to optimize the building climatically, since the many winter gardens proved as climati-
cally and energetically problematic and a longer usable period was required. A generation of
double facades evolved that, with its four main variants second-skin façade, corridor façade, box
window facade and shaft-box façade, was no longer merely a building envelope, but rather an
integral part of the building’s climate design by integrating climatic concepts (3/4/6/7).
Based on this fact, the next step – the integration of building services into the facade – seems
The façade technology of the 20th century can be characterized by the dissolution of the mas-
sive wall into a separation of structure and façade. With regards to esthetics and construction,
the development of façade technology today, after 60 years of curtain wall systems, 30 years of
element façade systems, 15 years of double skin façades and 10 years of experience with the
integration of environmental services, it seems that the peak of optimization of the existing
façade systems has been reached. By continuing the path of adding extra layers for each addi-
tional technical function will limit further developments to small technological steps.
1. Background
From a historic point of view, facades have evolved from two structural directions: light con-
structions based on the requirements of nomadic people, that allow for simple and quick assem-
bly, as well as massive constructions derived from the permanent need for protection at a given
location and the locally available materials (1). Following the fulfillment of these fundamental re-
quirements, the facade’s esthetic design developed with the goal to emphasize certain building
parts, functions or entire buildings. One example in this context is the step from Romanesque
to Gothic. The conception of light in the interior space served as a motor for the development
of ever bolder constructions with maximum window dimensions and minimal gravity-loaded
vault structures. In contrast, driven by the necessity of functional buildings to be efficiently
constructed, industrialization brought about the separation of the master-builder into the dis-
ciplines architecture and engineering – with the result of an alienation of these disciplines and
the creation of individual esthetic forms of expression (2). One step in the direction of esthetic
qualities still favored today is the dissolution of the wall, associated with modernity, into load-
bearing components – the building skeleton – as well as enveloping components – the facade.
The goal to seemingly dissolve the structure into individual parts created an esthetic form not
Figure 1 – Facade Principles : second-skin façade, box window façade, corridor
façade and shaft-box façade (1)
Figure 2 – Facade principles with building services integration: ventilation,
humidification, light (1).
5

is more efficient to combine the outer leaf with the inner leaf and insulation in the factory to one
prefabricated element. Due to their layered structure, these elements are called sandwich ele-
ments, but there is no mechanical cooperation between the inner and outer leaf. The inner and
outer leaf are quite thick, so a tight connection between these two will create internal forces due
to differences in temperature. A strong connection, to activate the mechanical sandwich action,
is not necessary either, because the inner leaf can easily resist the wind forces.
These sandwich elements could serve, like single interior leafs, as a load bearing element. How-
ever, this is not frequently done. The risk of damage to the exterior during the earlier rough
building phase is considered a disadvantage. Also the dimensional tolerances in this phase are
quite large. These tolerances would become visible in the irregularity of the width of the joints
in the rain screen.
From a distance and with a rough texture, these variations could be made less obvious.
In recent projects, however, load bearing prefabricated sandwich elements are used, especially
in high rise projects (fig. 1.3.5.). The logistic advantages of a single hoist will make more strict
dimensional tolerances economical.
However, the sandwich elements are generally non load bearing as a parapet element or as a
full storey high element.
The cross section of the sandwich elements is largely determined by considerations related to
their heavy weight and the fixing procedure.
Parapet element (fig. 1.3.2.)
The centre of gravity lies between the inner and outer leaf, so a crane may lower the element
vertically and directly onto the floor. There is no need to pull the element inwards.
Please note: With just a separate inner leaf of concrete, a façade element supported by consoles
on the columns is a more attractive solution. These consoles, however, penetrate the inner leaf.
This is no disadvantage because these openings can be closed from the outside. For a sandwich
element this is not a good solution because closing the airtight joints around the console is only
possible from the inside.
If the inner leaf is put on top of the floor, it is not logical to make the inner leaf underneath the
floor also of concrete. This would complicate the element and make positioning more difficult.
As a consequence, in that position light panels will be used.
The wind load from the bottom on the windows will be resisted by the concrete parapet panels.
At the top of the windows the wind load must be distributed to the underside of the floor. Then
it is logical to consider the inner leaf above the windows as an extension of the window frame.
The inner and the outer leaf may be produced separately and connected in a later production
stage. Then both the outer surface of the outer leaf and the interior side of the inner leaf may
be put at the bottom side of the formwork (the procedure is explained later on). Then on both
sides fine detailing is possible. With this production method it is also possible to form an open,
ventilated cavity.
Storey high element (fig. 1.3.3.)
Also with these elements, the centre of gravity makes
it easy to lower the element onto the floor. The el-
ement must be pulled horizontally between the
floors. Then a wide gap is needed to accommodate
dimensional tolerances between the floor levels.
The connection to the floor is made with angled pro-
files or steel strips.
A simpler fixing is found in adding a continuous
ridge on the inside of the top. The element may now
be hung instead of stand, making it at once stable
after positioning. The horizontal joint between the
elements is easily accessible at floor level. If the ridge
is unacceptable in the interior, a lowered floor edge
is used. This is simple in a prefabricated structure
with separate beams and floor elements (fig. 1.3.4.).
In the building of Civil Engineering the collected water is guid-
ed to the outside and downwards over a recessed surface cov-
ered with strips of black tiles. The tiles don’t absorb the water
and the different colour doesn’t show the staining.
Some surface treatments, like sandblasting, make the surface
more sensitive to staining.
1.3. Types of façade elements
An exterior leaf of concrete, like other materials, could be
hung on an already positioned inner leaf. Because of the heavy
weight, however, a crane must be used. Also the fixing and
adjustment is more elaborate than with lighter panels. Strong
and expensive anchors are needed (fig. 1.3.1.). Until 1980, con-
crete outer leafs were fixed directly to the floors with consoles
extending from the concrete elements. Requirements for
thermal insulation and the prevention of cold bridges did not
play a role. As an inner leaf, light panels or brickwork was used.
This type of fixing is no longer accepted.
To avoid additional adjustment and double use of the crane it
Dust will be collected on horizontal surfaces and this material will be washed away by periodical rain. In an early
stage of a period with little rain, the limited amount of water will remove some dust, but lower on the façade this
water will be absorbed by the porous concrete. This leaves the dust on the façade in specific areas.
On the other hand, during heavy rain a lot of dust is washed away, but this water flows rapidly downwards. If all the
dust is removed, the rest of the water will partially clean the façade.
With this mechanism an uneven staining appears, very soon giving the façade a rundown appearance. Because
the eastern façades in The Netherlands are exposed to irregular rainfall, these façades generally show more stain-
ing than the others. Especially south and west façades receive much more water due to the prevailing winds.
By adequate detailing, the staining can be slowed down and also be made more even. In the literature on concrete
façades, a lot of attention is paid to this aspect.
Some of the measures are:
− drip rails to stop the water, hanging on underside surfaces, from flowing backwards.
− external rims keeping the water from the surface as much as possible.
− horizontal surfaces, sloping inwards and collecting the water sideways to concentrated spots.
− hidden gutters below the windows and draining these gutters to a pipe system.
In fig 1.2.4 several solutions are given.
Dust will be collected on horizontal surfaces and this material will be washed away by periodical rain. In an early
stage of a period with little rain, the limited amount of water will remove some dust, but lower on the façade this
water will be absorbed by the porous concrete. This leaves the dust on the façade in specific areas.
On the other hand, during heavy rain a lot of dust is washed away, but this water flows rapidly downwards. If all the
dust is removed, the rest of the water will partially clean the façade.
With this mechanism an uneven staining appears, very soon giving the façade a rundown appearance. Because
the eastern façades in The Netherlands are exposed to irregular rainfall, these façades generally show more stain-
ing than the others. Especially south and west façades receive much more water due to the prevailing winds.
By adequate detailing, the staining can be slowed down and also be made more even. In the literature on concrete
façades, a lot of attention is paid to this aspect.
Some of the measures are:
− drip rails to stop the water, hanging on underside surfaces, from flowing backwards.
− external rims keeping the water from the surface as much as possible.
− horizontal surfaces, sloping inwards and collecting the water sideways to concentrated spots.
− hidden gutters below the windows and draining these gutters to a pipe system.
In fig 1.2.4 several solutions are given.
An important aspect of concrete façades is staining (fig. 1.2.3.)
Fig. 1.2.2 Some examples of surface treatment
Fig. 1.2.3 Uneven stainingFig. 1.2.4 Precautions to prevent water from running over
the façade surface.
Fig. 1.3.1 Adjustable anchors for
façade panels Fig. 1.3.2. Parapet element
6

medium rise buildings.
A better rain proofing may be wanted in sandwich panels without a cavity. These are normally
insulated by polystyrene foam, a material with a hard surface. In the joints, the sides of the in-
sulating layer do not catch water, and this may easily flow along to the interior leaf and airtight
joint. To improve the effectiveness of the rain proofing, one or more slotted grooves may be
added in the vertical joints of the rain screen and also an overlapping form in the horizontal
joint.
To improve the rain proofing even further, a flexible strip may be put into the vertical joint (fig.
1.5.3.). The openings in the rain screen are enough to equalize the pressure between the exterior
and the cavity. In this way a very effective and reliable joint is achieved, fit for high rise buildings.
The formwork, however, is complicated by these refinements and during transportation the fin-
er detailed edges are vulnerable. A straight edge, closed with a mastic joint is an alternative, but
this solution has some disadvantages. The life of the material itself is limited and the adhesion to
the concrete depends to a great extent on the condition of the surfaces. The mastic material may
also spread some of its components over the surface of the concrete near the joint. This staining
is not visible in itself, but dirt sticks to it and causes dark stripes beside the joint.
Sooner or later the mastic joint between the exterior panels will show some defects. This does
not have serious consequences if enough outside air can flow behind this joint to equalize pres-
sure differences over these joints.
Trying to completely close the vertical and horizontal rain proof joints with mastic may easily
lead to leaks into the interior. The airtight joints on the interior side will have some small air leaks
and these reduce the pressure in the cavity if the rainproof joints are closed to a high degree.
This air leakage to the interior increases the pressure difference over the rain screen, creating a
large influx of water. This will finally find its way to a small opening on the interior side, were it
will be pushed to the inside. The origin of the water is then impossible to detect. A mastic joint
may function well, but then the pressure equalisation to the outside must be guaranteed by
adequate local openings.
Sometimes the vertical rain proof joint is closed with elastic strips (fig. 1.5.4.). These profiles have
a limited range of compression in which they have sufficient contact pressure to the sides of the
opening. If this pressure is too low, the strips may fall out of the joint. This can happen in cold
periods when the concrete panels shrink and the elasticity of the material is low. Repeated ther-
mal movements can also gradually push the profile to the outside. The strip cannot be used in a
position accessible for vandalism.
1.6. Fixing
An exterior leaf of concrete may show a considerable variation in temperature. However, the
maximum temperature, caused by the sun, will not be as high as that reached by thin light
weight panels. The thermal mass is much higher, absorbing about 500 Wh for every 10 oC rise
in temperature. By the slower rise in temperature the sun will have lost it maximum strength
before a stationary balance is achieved.
The orientation is also less influential. For light weight panels the SW-façade is generally heated
up the most, caused by the high air temperature in the afternoon and the more perpendicular
angle of the sunrays to the surface. For a massive concrete panel, the loss of the intensity of the
sun soon restricts the warming up.
With a variation of the temperature from -20 oC to 55 oC, the change in length of a 7.2 m1 wide
façade panel is:
ΔL=ΔT*α*L=75*12*10-6*7200=6.5 mm
The interior leaf has a fixed temperature and does not follow these movements. Then, a rigid
connection between exterior and interior leaf is unacceptable, for this would cause high internal
stresses. A concrete connection would also form a serious cold bridge.
To prevent this, an exterior leaf of a sandwich element is fixed with stainless steel anchors. These
must be strong enough to resist the gravity load and the wind load, but they must be flexible
enough to limit the tresses caused by the forced movement.
By positioning the fixed and rigid supports in the middle of the panel, the movements at the
joint are limited. Around the perimeter flexible, hinged or sliding connections resist the wind
load only. In fig. 1.6.1 an example is given of the positioning of the anchors and their purpose.
In the middle, two anchors resist wind forces and the vertical loads and one anchor resists wind
loads and fixes the panel in a horizontal position.
To restrict the movements of the exterior panel with relation to the interior leaf, a solution may
be to combine for example two outer leafs of 3.6 m1 wide with an interior leaf of 7.2 m1 wide.
A smaller panel also has the advantage of less deformation caused by warping or bending in
the curing stage. During warming by the sun, some bending will occur, because the warming
1.4. Manufacturing
For sandwich elements two production methods are used.
− The element is produced in one cycle of operation. These elements have an exterior surface,
shaped by the bottom of the formwork and the interior surface of the interior leaf is flat and just
levelled.
− The element is produced in two phases. First, the interior leaf, with the interior side at the bot-
tom of the formwork, is cast. On the topside (cavity side) anchors are cast in. After hardening, the
insulation material is applied around these anchors and fixed to the inner leaf. Next, the outer
leaf is cast, with the outer surface at the bottom of the formwork. The inner leaf is lowered with
the anchors in the not yet hardened concrete. By keeping a distance between the outer leaf and
the insulation, a cavity may be formed.
The second way of manufacturing is of course more expensive, but the freedom of form and the
high quality on the inside may be a good ground to do so. However, a ventilated cavity is seldom
required for a concrete exterior leaf. Concrete is permeable to vapour, it can absorb a lot of water
and it is resistant to frost. For very dense exterior surface materials, however, like enamelled tiles,
the ventilated cavity may provide extra safety.
The choice of treatment of the exterior surface, besides the aesthetic demands, is also influ-
enced by the shape of the element.
To wash away the cement, to show the pebbles better, the curing of the cement is delayed by a
chemical put on the surface of the formwork. For vertical planes in the formwork, for example
around the windows, this chemical may flow downwards and spread over the bottom. This may
cause irregular effects in appearance. Also, a curved surface may become irregular. For these
forms, methods like blasting with grit may be a better choice.
Sometimes a surface with a rough appearance must be given flat edges to accentuate the joints.
In this way the attention is drawn to these stripes and the real joint with its varying width is less
obvious. These strips must be covered with grit blasting, but the cover must be very strong to
withstand the blasting. Then washing the surface may be favoured.
Concrete exterior leafs are produced in a factory, because there controlled working conditions
exist. To compact the concrete, vibrating tables are used, and these can only be placed in a fac-
tory.
Also for quality control, a factory is a better location than the building site. It can guarantee an
even surface quality and colour. This is important because the slightest difference will be visible
and it will ruin the image of a whole façade.
Recently, however, the production process was changed.
The development of high strength concrete resulted in thin self compacting mixtures. This is
also very favourable for the production of façades. In the factory, the noise of the vibrating ta-
bles is avoided and the formwork is much less stressed. Occasionally, concrete façades are cast
in situ. On the building site a high quality is easier to achieve, without air bubbles, uneven mixed
pebbles and discolouration.
1.5. Joint constructions
The joints between concrete exterior panels must be quite wide, for the dimensional tolerances
are large in comparison to products made of other materials. On average, the joints are 2 cm
wide.
Concrete will shrink during hardening and sometimes the elements show some warping. Then
the width of the joints will be variable and also in the depth differences appear between the
adjacent panels. The positioning of the elements is also less precise, caused by their large weight
and more difficult handling.
Due to their width, the joints have a major influence on the appearance of the façade. Therefore
it is important to use visual tricks to make them as even as possible. The detailing should be
aimed at hiding the differences.
One possibility is to chamfer the edges of the panels or to make details, suggesting a wider
joint (fig. 1.5.1.). In this way the variations in width are relatively less important. With chamfered
edges also the risk of damage at hoisting the element from the formwork or during handling is
reduced.
At the corners of a building, dimensional tolerance in two directions must be dealt with. Here
a grave risk exists of an unequal and tapered joint width. By designing overlapping joints the
tolerances may be masked.
The location of the joint must be chosen in such a way that fragile edges are avoided (fig. 1.5.2.).
Due to the thickness of the exterior panel, the joint is quite deep. Then, an open joint is a good
solution, for the edges will efficiently catch the drops of water. These open joints level out the
air pressure difference between the exterior and the cavity. This joint will function well up to
Fig. 1.3.4. Hanging, strorey high façade
elelemnt
Fig. 1.3.5 Storey high, load bearing sandwich
facade elelement
Fig. 1.5.1 Joints, designed to hide dimensional
tolerances
Fig. 1.5.2 Solutions for corners
Fig. 1.5.3. Additional strips for improved
waterproofing
Fig.1.5.4. Elastic strip for rainproofing of a
vertical joint
7

The inner and the outer leaf may be produced separately and connected in a later production
stage. Then both the outer surface of the outer leaf and the interior side of the inner leaf may
be put at the bottom side of the formwork (the procedure is explained later on). Then on both
sides fine detailing is possible. With this production method it is also possible to form an open,
ventilated cavity.
Storey high element (fig. 1.3.3.)
Also with these elements, the centre of gravity makes it easy to lower the element onto the floor.
The element must be pulled horizontally between the floors. Then a wide gap is needed to ac-
commodate dimensional tolerances between the floor levels.
The connection to the floor is made with angled profiles or steel strips.
is not equal in the depth of the concrete. The inside surface temperature follows the outside
temperature with some delay.
The anchors resisting the wind load must be resistant to buckling. This requires a minimum out-
side dimension of the cross section. This must be balanced with the want for sideway flexibility.
Façade panels of concrete are heavy and therefore dangerous if they fall down.
Therefore the anchors must be resistant to the continuous attack by the exterior climate and
protected from fire.
The anchors inside the building are protected from fire by the concrete inner leaf.
However, the fire may enter the element from below and attack the anchors. If an open ven-
tilated cavity is present, the chimney effect may draw the flames into the element. To protect
the anchors and to prevent the fire from reaching the next storey, an insulation of mineral wool
or foam glass must be selected. These are not combustible and will maintain their insulating
capacity.
An insulation of plastic foam will burn or melt. This does not create a problem if the cavity is
completely filled. Because the supply of oxygen is restricted, the flames will stop after about
one meter.
Manufacturing sandwich elements without an open cavity may be done layer after layer, in one
formwork. Finally, the concrete of the inner leaf is poured on the insulating layer. Choosing an
impermeable insulation material, like foam, is then favourable.
As a result, the logical combinations are: foam insulation with no open cavity and mineral wool
with an open cavity.
Literature:
“Architectural Precast Concrete”
Prestressed Concrete Institute, Chicago, 1989
Kind-Barkauskas e.a.
“Beton-Atlas”
Verlag Bau und Technik, Dűsseldorf, 2001
Themanummer Detail 4, 2003
Publicaties van Belton
Themanummer “Schoon beton”
Cement 1999, nr 3
Bundels “Beton in beeld”
2. Exterior leaf of bricks
2.1 Introduction
A brick façade used to have a load bearing function besides separating the interior from the
exterior climate. The thickness was 22 to 33 cm.
With the introduction of the cavity wall, the brick wall was split into two layers, connected by
small steel connectors. The vertical loads from the floor rested on the interior leaf. Generally,
these were about 10.5 cm thick, but for concentrated loads they were made thicker.
Originally, the cavity had just the function to keep the moisture away from inside of the façade.
The cavity was about 6 cm deep. The exterior leaf was connected to the interior leaf with thin
bars to increase the stability of the 10 cm thick leafs and to distribute the wind loads.
From 1975 on, the cavity was filled with thermal insulation to reduce the energy losses. Over the
years, the thickness of the insulation increased to about 15 cm.
Nowadays the exterior leaf does not take any part in resisting loads from the building. It only
serves the architecture and the rain proofing. It is a cladding material like others, but with some
typical aspects, such as stiffness without strength and the way it is made.
A façade of masonry has the advantage of consisting of small elements, giving it a homogenous
surface. The surface is not split into panels with obvious joints. It can easily accommodate an
irregular façade with windows of different sizes and positioning. With special formed bricks, dif-
ferent surface textures and colours are possible and with a different style of laying the bricks all
kinds of images may be created. Also the surface treatment of the joints and their width offers
ways of achieving the desired effect (fig. 2.1.1).
Brickwork is relatively insensitive to staining because the surface in itself is already irregular. No
maintenance is required, as opposed to plate materials.
Only lightly coloured stones should be treated with care. To prevent irregular wet areas, the wa-
ter flow over the surface must be controlled. Especially areas below windows and at the exposed
top of a wall are sensitive.
An exterior leaf of masonry has a low tensile strength, making it sensitive to restricted move-
ment and forced displacement. In this it does not differ from other façade materials.
A brick wall will deform with a change in temperature. To enable an unrestricted movement, the
exterior leaf should be connected to the load bearing structure as loosely as possible. It should
also be kept free from other rigid elements, such as window frames and roofs.
The brickwork, however, must be supported by the load bearing structure to resist the wind and
gravity loads. This load bearing structure will also show deformations, caused by other loads.
These movements of the structure may not be forced back upon the brick wall.
As a result, dilatations in the brickwork are necessary.
Based on the idea that no cracks are allowed in the brickwork, guidelines were developed for
the position of dilatations. These guidelines were so strict that the façade was covered with a
lot of joints that were clearly visible, especially with lightly coloured bricks. The homogenous
surface was disrupted by dark lines. This effect eliminated one of the attractions of bricks as a
façade material.
In more recent recommendations, the general idea is to accept cracks while controlling their
width. As long as they are not visually annoying and don’t create a safety risk, they are accept-
able. In certain areas, a steel reinforcement in the horizontal joint may be a solution to spread
the extension over a large number of smaller cracks (fig. 2.1.2.).
Note:. Also in concrete technology one of the design criteria is the restriction of the width of
cracks by an evenly distributed reinforcement.
2.2. Restricted movements
The brickwork follows the average air temperature and it is additionally heated by sun radiation.
The range of the temperature is comparable with that of a concrete exterior leaf, so -20 oC to
55 oC.
To estimate the degree of unrestricted expansion, the temperature at the time of construction is
important. Starting at 5 oC, a section of 12 m1 long will expand:
Thermal coefficient of expansion (α): 6*10-6
/o
C
Rise in temperature (ΔT): 50 o
C
Length (L): 12 m
then:
ΔL = ΔT * α* L = 60*6*10-6
* 12 = 0.0036 in = 3.6mm
An open expansion joint of 5 mm is then adequate.
The sliding of brickwork over a foundation is limited to a practical maximum. The wire anchors,
necessary for the distribution of the wind forces will also be bent sideways.
With this approach, a vertical dilatation is advised for every 12 m of length of the façade. The
vertical dilatations are noticeable because they form a straight line instead of the staggered
lines of the normal joints. For a low façade, the vertical compression is less, making it advisable
to restrict the length between dilatations to 5 times the height.
The thermal coefficient of expansion of brickwork, in vertical direction, has a higher value. For a
high façade, therefore, about every 10 m a support is needed. This equals three storeys. Below
this support a horizontal expansion joint must be designed. This joint is closed with a mastic
material against rain penetration. The compression and expansion of mastic materials is limited
to about 20%, making 15 mm the minimum width of this joint. This is more than the normal
thickness of a horizontal mortar joint, which makes these dilatations visible.
At corners, the sideway movement of one façade is pushing the other façade outwards. How-
ever, the wire anchors connecting the inner and outer leaf cannot stretch several millimetres.
To prevent damage, a dilatation is needed just around the corner. If this is unacceptable, the
anchors should be installed at a generous distance from the corner. For an exterior continuous
leaf around the corner, dilatations 1.5 m respectively 4 m from the corner may prevent cracks.
Window frames are fixed to the interior leaf to restrict, as much as possible, the movement of the
airtight joint connecting them.
Wooden frames do not expand or shrink that much, and consequently they are fixed rigidly.
To the exterior leaf they act as a rigid element. To allow for movement of the masonry, a joint
around the window frame of about 5 mm should be kept open.
Through an open joint of that size, water can penetrate the cavity. To protect the wood, flexible
slabs just behind the bricks are used. This solution is only possible when the exterior leaf is con-
structed after putting the window frame in position.
Window frames of aluminium or plastic show large thermal movement, caused by the high ex-
pansion coefficient and their small thermal mass. A larger joint is then needed.
These window frames are always installed after the exterior bricks. For wooden frames this
phase is also favourable, because it reduces the risk of damage during the building phase. With
a wider joint around the window frame, additional rain proofing strips may be used.
2.3 Forced deformation
The weight of the exterior leaf must be distributed to the foundation, but this may be done by
different routes.
Normally the masonry is carried directly by the foundation. However, above windows, at balco-
nies and around cantilevered façades the brickwork will stop at a higher level.
One way to support the brickwork is to use lintels, distributing the load sideways to the adjacent
masonry. With larger openings these lintels may be connected backwards to the interior leaf or
beams or columns of the load bearing structure (fig. 2.3.1.).
Fig. 1.6.1 Separate anchors for gravity load,
windforces and horizontal forces in the pane of
the element
Fig. 2.1.1. Brickwork with a
rough texture
Fig. 2.1.2. Reinforcement of brickwork
8

Fig. 2.3.1. Supports for brickwork.
Gravity weight transferred to the
inner leaf.
Fig.2.3.2. Support
of the exterior
leaf above and
below the window
by the inner leaf.
Besides the window
the brickwork is
supported by the
foundation.
In many cases one part is supported by floors or consoles, and the parts beside are supported by
the foundation. This always creates a different vertical deformation (fig. 2.3.2.).
Floors are generally designed for a maximum deflection of 0.002 to 0.003 times the length of
the span. Reduction of the deflection is difficult to achieve, because a floor beam may take away
too much natural light. A beam may also hinder the building process and with a tunnel form
work this is even impossible. Prefabricated floor elements are generally unsupported at the side
façade.
An exterior brick leaf supported by these floors must follow the deflection of the floor.
The deflection of the floor increases from the supports to the centre of the span. The differences
in vertical movement must be possible without overstressing the brickwork. This is possible with
vertical joints.
Prefabricated lintels made of steel or concrete have a larger thermal expansion than brickwork
(12 and 6*10-6 /oC ). Thus, lintels for large openings must be able to slide at their supports to
some extent. At the end of the lintel a vertical joint is necessary (fig. 2.3.3.).
The main structure of the building will bend sideways under wind load. This causes floors to
slide in relation to the floors above and below.
The load bearing structure also may have dilatations due to the building process or the assem-
bly of prefabricated elements. Between high and low building volumes generally a dilatation is
designed to accommodate a different settlement of the foundation.
Dilatations in the load bearing structure generally must be repeated in the façade.
Literature:
“Ontwerpen met dilataties”
Koninklijk Verbond van Nederlandse Baksteenfabrikanten, 2003
“Beheersing van scheurvorming in steenconstructies”
CUR-aanbeveling nr. 82
Fig. 2.3.3 Joint near the end
of a lintel, supported by the
adjacent brickwork.
3. Multi layer glass windows and façades
3.1. Climate windows and double-skin façades
Basically every façade in a moderate climate has three layers: an airtight inner layer, a thermal
insulation layer and a rainproof/aesthetical layer. Considerations originating from the construc-
tion process or building physics sometimes lead to an open cavity behind the rain screen.
In double glass façades this air space is made wider to accommodate an adaptable sunscreen
(louvers, curtain etc.).
The air space may be equipped with controllable openings to the outside and/or the interior. An
air flow with air from the outside or interior can reduce the temperature of the sunscreen and the
cavity. This enhances the comfort by reducing the surface temperature of the inner glass pane.
It is also possible to use exterior air, guided through the cavity, for ventilation of the room. A
wide cavity can also be useful to improve the sound insulation. A lot of combinations have been
investigated and used with a wide range of technical complexity.
The most commonly used types are known as “climate façade” and the “double-skin façade”
(fig. 3.1.1.)
The principle of the “climate façade” is generally used for windows. The air, evacuated from
the room, is guided through the cavity back to the installation. The outer window is a standard
double glass unit and on the inside a single, inward openable glass panel is added. The airflow
is controlled by the installation.
In the climate façade, the airflow is derived from the ventilation requirements for the occupants,
ranging from 30 to 60 m3/hm1 façade length.
The cavity is 10 to 15 cm deep. Since the installation ducts are generally placed above a ceiling,
the air flow is upwards.
Below the window, over the full width, a slot is left open. Above the window a plenum is needed
to create an even flow from the wide window to the duct of Ø 10 cm. Sufficient space must be
reserved for this plenum between the window and the bottom of the floor.
An open able inner glass window facilitates cleaning of the cavity and maintenance to the sun-
screen. Influx of interior air to the cavity through the joints around this window must be re-
stricted. These leaks would disturb an even air flow.
This system is possible with some relatively simple additions to the normal façade.
For the “double-skin façade” a wide variety is used, ranging from a simple extra glass panel in
front of a window, protecting the sunscreen, up to quite elaborate assemblies.
In the more complex variety, an all glass façade is achieved.
A storey high exterior glass panel is supported by a slender structure, hung from outriggers at-
tached to the floor. Generally the interior façade also contains a large glass surface. To be able to
clean the glass surfaces of the cavity without disturbing the occupants by opening large glass
doors, a wide cavity is chosen. This cavity must be 80 cm deep, at least.
Generally the cavity will be ventilated by outside air to cool the sunscreen. The air flow depends
on the chimney effect. Due to the large glass surface, a lot of sun energy enters the cavity and
this requires large openings. These will be closed in cold periods.
In spring and autumn, the temperature in the cavity may be used for natural ventilation of the
room, by opening the inner façade. In summer the temperature in the cavity will be too high and
an additional climate installation is used.
In high rise buildings the cavity is closed on every floor level, for fire safety reasons. This is limit-
ing the effective chimney height.
Fig. 3.1.1. Climate façade and second skin
facade
For lower buildings, the chimney can continue over 4 floors as a maximum. The limiting factor is
the increasing temperature of the air at the upper floors.
There are many options for the various design decisions:
− Air flow by natural convection or by ventilators
− Number of storeys along the height of the cavity
− Openings fixed or controllable
− Using double glazing for the interior side and/or the outside
− Cooling the sunscreen by interior or outside air, or making combinations possible.
− Pre-heating of outside ventilation air for the room, in the cavity
− Transporting of absorbed heat in the sunscreen to colder parts of the building.
− Expanding the cavity of a second skin façade to create a small atrium
− In a climate façade, the sunscreen can be made of a continuous porous cloth. Then the air can
be sucked through the sunscreen to the cavity between the sunscreen and the outer glass panel.
This airflow is cooling the sunscreen directly, avoiding the single glass panel on the inside.
3.2 Choice of a system
Just cooling the sunscreen will not provide enough justification for choosing a system with a
ventilated façade. Other considerations will be taken into account.
− Saving of energy by cooling the rooms at night, by natural ventilation, without the risk of
burglary
− Energy saving by the higher thermal resistance of triple glazing
− Control of cold bridges
− A wider choice of materials possible, for the inner window frame (like lower grade wood prod-
ucts).
− In refurbishing of old buildings, improving thermal performance and appearance.
However, the main reason for choosing a second skin façade is architectural. Goals are creating a
wide view to the outside for prestige and the display of the interior activity to the outside world.
This leads to large glass areas.
Large glass areas have many disadvantages for the comfort (unstable temperatures, sound
transmission) and the use of energy for cooling. These disadvantages can be partly compen-
sated by the second skin concept.
Therefore, these expensive façades are only applicable if the client wants a prestigious building.
In many cases the original idea for a transparent façade (fig. 3.2.1.) is frustrated by the necessary
technology. In fig 3.2.2 the finished building is shown.
Fig. 3.2.1. Visualisation of the intention of the
architect.
Fig. 3.2.2. The building of
fig. 3.2.1. as build.
The transparency to the interior is not achieved because the level of light in the interior is too
low, caused by the deep cavity, ventilation boxes and interior partitions. In addition, the three
glass surfaces will reflect a high percentage of the, much lighter, sky. Only at night the illumi-
nated interior will be visible.
A transparent appearance can be more easily achieved at corners, where light is transmitted
9

through the building. Another possibility is designing atriums, several storeys high, allowing
light deep into the building.
A double-skin façade is costly in building and maintenance. If the building volume is restricted
by urban planning, the loss of rentable floor area also is a major disadvantage. With some com-
promise, cheaper technical solutions may be considered.
− Natural ventilation can be achieved with separate ventilation openings, equipped with sound-
proofing. These can be used during a longer period, since the ventilation is independent of the
temperature in the cavity.
− The heating of the interior glass pane by the sunscreen can be reduced by a heat reflecting
coating on the glass surface. This will also reduce the heat losses in winter.
− A sunscreen may have a highly reflective surface. Without an air flow in the cavity the reflective
capacity is not spoiled by dust.
− An exterior sunscreen is cooled more effectively than a sunscreen in a ventilated cavity. The
extra cost for the strong structure may be less than the costs for extra glass and controllable
openings.
As a conclusion it may be stated that multi layer glass façades can give a good solution for build-
ings where a lot of requirements must be met (south façade with large glass area, individual con-
trol of sunlight and ventilation, architectural expression) and where ample budget is available.
3.3. Geometry of the cavity
The depth of the cavity has a direct relation with the size of the glass area.
With small dimensions it is possible to use inward opening windows on the inside for cleaning.
This will be necessary 2-3 times a year. This will not cause great disturbance for the use of the
room. The depth of the cavity follows from the dimension of the sunscreen and the free flow of
the air. A greater depth is unfavourable because this will be more apt to an uneven airflow.
If the glass area covers almost the entire façade, a simple openable window is not enough. Large
swing doors of glass need a lot of floor space, so all furniture must be moved 1 meter from the
façade. A sliding door is better, also for structural reasons. The cavity side of the sliding doors
can only be cleaned by a person, standing in the cavity. This enhances the depth to more than
80 cm. The cleaning can be done from an open grid of balcony at every floor level, or from a
hoist able cage.
The deep cavity of the double-skin façade has consequences for the structure of the floor.
The exterior glass panels must be connected by narrow joints to maintain a clear view to the
outside. Narrow joints are possible only when the relative movement between the glass panels
is limited. Since the glass panels are about 1.5 m wide, there must be outriggers at this distance.
These outriggers need a connection to the floor, which will not rotate. Floor beams at right an-
gles to the façade could provide enough rigidity, but their distance is generally over 3.6 m. This
means that the floor edge must be very stiff to rotation between the columns or beams. This
increases the height and depth of the floor edge.
For medium high buildings, it therefore becomes attractive to hang the complete exterior glass
façade on outriggers from the roof. There, ample space is available to make a stiff outrigger. The
mechanical connection from the exterior façade to the normal floors then only has to resist the
wind load.
Opening windows, in the interior façade of a double-skin façade, can cause hinder from sounds.
The noise will be reflected between the glass surfaces and can enter the building elsewhere.
Also smoke can be guided through the cavity.
To prevent this, the balconies in the façade can be closed, but this causes a considerable reduc-
tion of natural light in the rooms. The chimney height is also less, requiring larger ventilation
openings to get the needed air flow for cooling the sunscreen.
To prevent noise problems between rooms on the same floor, vertical separations may be need-
ed. This can also be useful to reduce wind forces on the outer façade, at the corners of the build-
ing. Overpressure in the cavity, on the windward side, should not be guided around the corner,
where negative outside pressure exists. This horizontal flow of air also prevents efficient cooling
of the sunscreen.
3.4. Air flow and cooling capacity
In a climate window the air flow is determined by the required refreshment of the interior air.
Starting with this volume per hour, and the horizontal percentage of glass of the façade, a vol-
ume for every m1 of window can be derived. In a high climate window this air will stay in the
cavity longer and consequently get a higher temperature. Air has a low specific thermal capacity
and this will limit the desired effect of cooling of the sunscreen.
A second restriction is caused by the low velocity of the air in the cavity. For the given speed, the
convective transfer of heat from the sunscreen to the air is limited.
As an indication, the next example is given:
Thermal coefficient of expansion (α): 6*10-6
/o
C
Rise in temperature (ΔT): 50 o
C
Length (L): 12 m
then:
ΔL = ΔT * α* L = 60*6*10-6
* 12 = 0.0036 in = 3.6mm
Height of the window (h): 2 m1
Surface of sunscreen (A): 2 m2
/ m2
window
Convective heat transfer coefficient (αc
|): 3 W/m2
K
Air flow through the cavity (V): 40 m3
/h
Increasing of temperature in the cavity (ΔT): 10 o
C
Temperature interior(T): 20 o
C
Specific heat air (ρ): 1000 J/kgK
Specific weight air (c): 1,2 kg/m3
For the transfer of this energy the sunscreen must be warmer than the average temperature of the air. The tem-
perature difference must be:
If the rise in temperature over the height of the window is supposed to be linear, then the average temperature
in the cavity is:
Now the temperature of the sunscreen must be:
25 + 11.1 = 36.1 o
C
With this temperature, still a large portion of the energy will pass through the interior glass panel
by radiation and convection. With a difference of 16.1 o
C , this will amount to 56 W/m2
.
For 2 m2
of window, 112 W will enter the room, compared with 133 W, removed by the air flow.
It is often suggested that a climate window removes the whole heat load from the sunscreen. In
fact it reduces the amount. A climate window is more effective than an interior sunscreen, but is
less effective than an exterior screen.
The air is generally transported to the climate installation. If this air is reused, no reduction in
cooling load is accomplished.
In winter, without sun load, the exhaust air is transported along the outside, double glazing.
Then the loss of heat energy is comparable with normal double glazing, without the advantage
of the extra cavity.
With a second skin façade, usually the option is used to get direct natural ventilation by opening
the interior window or door. However, when the sun gives a high thermal load on the sunscreen
in the cavity, generally at the same time the outside temperature is also quite high. Then the
rising of the air temperature in the cavity must be significantly restricted, if this air must be used
for the ventilation of the room.
For a modest rise in temperature a large air flow is needed. The air flow, however, is generated
by the chimney effect. The driving force is the pressure difference between inlet and outlet, be-
ing linear with the temperature difference. For the necessary large air flow then large openings
are needed.
Compared with a climate window, also more sun radiation reaches the sunscreen. This is caused
by the outside single glass pane which reflects and absorbs less light than a double glass unit.
As a result of these effects, it is not possible in summer to use natural ventilation through the
cavity. Depending on the size of the openings (0.1 to 0.3 m2/m1 façade) natural ventilation is
possible up to about 15 oC outside temperature. For the warmer periods normal air condition
units must be used. These must be designed in relation to the façade design.
Large openings must be equipped with shutters for the colder periods. These openings are vis-
ible from the outside, while making the façade less transparent. The costs of building and main-
tenance will also increase.
3.5. Balance between requirements and complexity for second skin façades
Due to the amount of design variables there is a wide choice, ranging from simple additions to
a standard façade to very elaborate and costly assemblies. The project and the location decide
which combination of qualities and cost is useful.
In figure 3.5.1 a simple solution is shown for a three storey building, with a continuous cavity.
A grating serves as a platform for cleaning of the glass and partial sunscreen. The vertical sun-
screen is put directly in front of the relatively small windows.
The exterior glass panels are supported by mullions hung from the roof. The panels are me-
chanically fixed with bolts between the panels.
In the example in fig. 3.5.2, the air in the cavity is exchanged separately
for every window. The depth of the cavity is limited, making cleaning of
the cavity possible by opening the turn and tilt window. The cavity is per-
manently open.
Fig. 3.5.1. Simple second skin
façade with ventilation
over three storeys
Fig. 3.5.2. Second skin façade
with ventilation over the height
of the window
1. Double glass unit
3. 6 mm prestressed glass
(not transparent)
5. Sun shading
7. Anchor for fixing
In fig. 3.5.3, the interior façade is alternating transparent and closed for about every 1.8 m. The ventilation open-
ings in the façade have a modest dimension. The cavity has an additional thermal mass by the concrete balconies
reducing the rise of the temperature for several hours. The sliding rigid wooden sunscreens can cover the glass
doors.
10

Fig. 3.5.3. Second skin façade in front of a
concrete interior leaf
In fig. 3.5.4., an example is given for a façade with a large glass area and enormous ventilation equipment. These
openings are the consequence of choosing a long period in which natural ventilation must be usable, increasing
cost and architectural impact. This is the façade of the building in fig. 3.2.2.
Fig. 3.5.4. Second skin
façade with large ventilation
openings.
1. Sliding doors
7. Aluminium lamellas
8. Adjustable valve
9. Engine
10. Grid as a walkway
12. Sun shading
4. Large transparent façade and roof surfaces
4.1. Introduction
For buildings, sometimes large glass surfaces in façades or roofs are required, generally at the
entrance or for atria.
In this way the building is opened from the inside to the outside. Also it is possible go get a view
to the activity inside the building to create a lively image.
On some locations, a large number of educational, cultural or commercial functions must be
accommodated. These can be put in one complex to make a more efficient use of general facili-
ties. The separate building elements may be connected by a semi public glass covered street or
a central square.
For large mono functional buildings this approach can also be used. An early example is the AMC
(university hospital) in Amsterdam. This one building volume in fact consists of separate build-
ings connected by streets and a central covered square.
Structured by their repetitive character, office buildings and hotels have a floor plan of a central
corridor with rooms on both sides. This leads to slender buildings with a monotonic façade. With
a generous entrance and reception area of glass, the desired prestige can be acquired. Structur-
ally the simplest solution is to position this volume beside the more closed part of the building.
With a sectional building, the space between the outstretched wings of the floor plan may be
closed by a glass covered space. Then the outer surface area of the building is significantly re-
duced. This is favourable for energy conservation and for reduction of noise, especially in city
centres. The extra cost of the atrium façade can partly be compensated by the inner façade
which does not have to be waterproof and needs less maintenance.
By landscaping the space with trees, gardens and benches the illusion of an exterior space, with
a moderated climate, can be created. This is necessary to prevent the offices, with only a view
into the atrium, from being too claustrophobic.
The created atrium may also be used for recreation in the lunch hour, receptions etc.
Large glass areas can also be used just to acquire prestige.
An early example of this are the façades of Hardwick Hall in England, built in 1597 (fig. 4.1.1.). The
desire to present an image and to show wealth, largely overruled the disadvantages of the cold
in winter (just open fires) and the heat in summer (no sunscreens). Glass was very expensive at
that time.
In this building, the transparency is only achieved from the inside to the outside. From the out-
side it is a closed building with symmetry as a main goal. The façades have false windows in
several places and all chimneys are put at internal walls.
The exhibition buildings of Crystal Palace (London, 1851) and Paris (1889) are also early exam-
ples. Very transparent examples are the shopping arcade of Milan (1867, fig. 4.2.1.2.) and the
Koornbeurs in Groningen (1865).
In some cases these structures would not be acceptable today, as a result of increased safety
standards.
Literature:
Compagno
“Intelligente Glasfassaden”
Birkhäuser, Bazel-Boston-Berlijn 1996
Oesterle e.a.
“Doppelschalige Fassaden”
Callwey, München, 1999
Danner e.a.
“Die Klima-aktive Fassade”
Alexander Koch, Leinfelden-Echterdingen, 1999
Schittich e.a.
“Glass construction manual”
Birkhäuser, Bazel-Boston-Berlijn 1999
Vakgroep Bouwtechnologie
“Façade & Klimaat”
Fac. Bouwkunde, TU Delft, 1996
v.d. Voorden en de Bruijn-Hordijk
“Geventileerde gevels”
Dictaat TU Delft, Fac. Bouwkunde, 2000
Themanummer Gevels
Bouwwereld, 2001, nr. 9
Fig. 4.1.1 Hardwick Hall
Developments since 1975 are:
− Use of double glass
− Toughened glass with point like mechanical fixing
− Support structures with prestressed cables to distribute wind load and dead load
− Glass panels hanging from cables or hanging from each other
− Glass beams as a support
In this chapter an overview is given of the principles and technical detail solutions that can be
used. These can be combined to a large number of forms. Details of realised buildings are widely
published.
4.2. Functional aspects
4.2.1. Transparency from the inside to the outside
This means that the view, looking outward through the façade, is obstructed as little as pos-
sible by the support structure. Then of course an important factor is the percentage of the area,
taken up by structural elements. However, the form and regularity of the structure is almost as
important.
The roof in fig. 4.2.1.1 has a structure that is a homogenous grid, creating the image of a net or
wire mesh. The thin elements are stabilised in their form by prestessed wires. If it would consist
of elements in only one direction, having double dimensions, the impression of enclosure would
be stronger.
Fig. 4.2.1.1 Museum in Hamburg
It is often thought that large glass panels create transparency. This
is not
ecessa
ble
Fig. 4.2.1.1 Museum in Hamburg
nrily so. Large panels must be thicker and the support structure must
be stronger and stiffer. This is because the impression of safety de-
pends on the visideformations. These depend on the absolute de-
flections and the angles between theedges of the glass panels at the
supports.
Large glass panels are not necessary; a fine meshed structure with
small panel can be very transparent (fig. 4.2.1.2.).
If the support structure is different in the two directions, the orienta-
tion is important. In the example in fig. 4.2.1.3 the vertical supports
create a column like effect for a person walking along the façade.
Fig. 4.2.1.2 Galleria Vittoria
Emanuele, Milaan
Fig. 4.2.1.3 La Bibliothèque De La France, Paris
11

By opening vertical or horizontal supports with holes or creating
trusses the transparency is increased (fig. 4.2.1.4.). In trusses, the out-
side dimension of the bars is important. Heavy, massive thin bars give
a more open and lighter image than bars made of tube section with
wider outside dimensions. Material economy is then secondary to the
visual impression.
With vertical supports minimised and at great distances from each
other, and long horizontal beams for the wind load, the horizontal
view is much more open (fig. 4.2.1.5.).
It is possible to remove the support structure from the actual glass
area. It may be separately positioned in the interior or more outward-
ly. The effect is that the supporjust look like objects without a direct
relation to the façade (fig. 4.2.1.5.). The wind load is distributed to
these supports by horizontal bars.
The transparency is often reduced by additional elements
like gangways for the accessibility for cleaning, movable
scaffolding, sun shading or ducts for warm air to control
the climate.
4.2.2. Transparency from the outside to the inside
Without adequate measures it is very difficult to look
through a glass panel to the inside of a building. This is
caused by the low level of light in the interior, compared
with the reflections from the very bright sky. Even reflec-
tions from buildings around are lighter than the interior.
If transparency to the inside is insufficient, the impression
from the outside will be a reflective enclosure of an inac-
cessible building volume.
A view to the interior is only possible with a high level of
light in the interior and a reduced reflection of the exterior
and especially the sky.
The first can be improved by a large glass area in the roof.
Reflection from the sky can be reduced by putting a hori-
zontal canopy or sunscreen above the façade. This is often
used by shops and showrooms for cars.
The reflection can also be reduced by tilting the façade
outwardly (fig. 4.2.2.1.). Then the ground level is reflected
in the glass and this area is much darker than the sky. These
visual effects can be seen at the Central Library of the TU
Delft:
− Around the corners one can look through the building
and the activities are well lit and visible.
− At the eastern side the façade is tilted outwardly, and the
roof is extended, making the interior visible for the first few
metres.
− The northern façade is tilted backwards, making the in-
terior invisible.
Please note: In the night the effect of reflection is opposite.
Then a view into the lighted building is easy. From the in-
side only the reflection from the interior is visible, unless
the exterior is well lighted by streetlamps or other lighted
buildings.
The windows of towers for traffic control at airports, bridges etc are generally tilted outwards. At
night the view outside is not hindered by reflections from the interior if the ceiling is kept dark
and the interior lamps are kept at the level of the desks.
Double glazing and coating to reduce heat radiation will enhance the reflection of light in rela-
tion to normal single glass. Infrared selective coatings are normally not visible, but some part of
the visible light spectrum is affected.
During the day there will always be some reflection. Therefore it is important for the glass panels
to be flat. A small curvature is clearly visible, especially if the panel is hollow to the outside. A
deflection of 0.005 times the length of the span creates a distance to the focus of 40 – 60 metres.
Seen from that position the reflection is highly distorted.
A curvature is often caused by the process of heat strengthening of the glass.
The supporting structure does not affect transparency from the outside to the interior to a great
extent. The observer is not enclosed by the structural elements, but is looking at the building
from a distance. The glass façade and the interior are seen as a whole picture.
4.2.3. Thermal climate
4.2.3.1. Preventing low temperatures
The level of thermal insulation is determined by the intended function of the atrium. It is also
important to establish to what extent compromises are acceptable for periods when a function
cannot be fulfilled.
Inside a single glass enclosure, especially during spring and autumn, an acceptable climate can
be achieved for recreational activities at lunchtime, receptions, exhibitions etc.
An improvement of the external climate, by eliminating wind and rain and catching the received
warmth of the sun, is enough. If a compromise is acceptable for the coldest and hottest days of
the year, with an atrium much value may be created for the building, at restricted costs.
In the entrance zone of the building, low temperatures are no problem.
To argue in favour of an atrium, in the design stage, there is a tendency to increase its usability
for more activities and functions. However this goes along with higher standards for the climate.
For a longer stay in a restaurant or reception area in winter, heating will become necessary. Then
for economy the thermal insulation must be improved by double glazing, even with infrared
coating, reducing transparency.
The double glass panels accept less bending than single glass, because bending leads to shear-
ing forces at the edge connections. These forces are acceptable to a certain extent and over-
loading may cause leaks, with condensation in the cavity as a result. The consequence is a stiffer
support structure, again reducing the transparency.
In summer the extra thermal insulation leads to higher temperatures. These must be tackled by
sunscreens, extra ventilation or even a cooling system.
If the atrium is additionally heated, the surface temperature of the glass on the inside is still
colder than the air temperature in the atrium. The inside air is cooled close to the glass and flows
downward. The speed of this flow increases with the height of the surface. This cold draft is very
uncomfortable and precautions may be needed.
By adding heat to the glass façade this draft can be avoided. The simplest way is warm air blown
to the glass. The low heat capacity of air makes large volumes necessary, resulting in wide ducts
to distribute the heat evenly over the large glass area. By using a transport system with water,
with a much higher heat capacity, smaller dimensions are possible. Locally, a much smaller air
flow can transport this heat to the neighbouring glass surface.
The conclusion is that it is possible to make the climate in an atrium controllable in winter, to
facilitate a wide range of functions. This, however, requires a lot of costly additional elements,
hampering the transparency.
4.2.3.2. Preventing high temperatures
Through the large glass area, much light comes in and is partially absorbed by the floor, the
walls and interior objects. This light is transformed into heat. Heat radiation is long wave, and
this radiation cannot pass through the glass, backwards to the outside. This leads to increased
temperatures of the air and all material in the atrium.
To reduce the rise in temperature, a light coloured floor helps by reflecting most of the light. The
light is a short wave radiation, and this energy may escape to the outside.
The part of the energy, absorbed by the floor, should be transported from the surface to deeper
layers. If this transport is fast, the temperature of the surface stays cool.
A floor material with a low heat resistance and a high thermal capacity in a thick layer is favour-
able. The heat is quickly transported inside the material and a high capacity can take several
hours of this heat without rising too much in temperature. An example is white natural stone
tiles on a concrete floor. The absorbed energy can be drawn out of the floor during the night, by
natural ventilation.
Water has a large thermal capacity. This capacity is equal to that of steel and double this value
for concrete (for a given volume). A small basin can stabilise the temperature and an additional
fountain can absorb much heat by evaporation.
Additional measures cannot be avoided if the temperature must be restricted to about 25 oC on
the hottest day, with the maximum sun load.
Light reflecting coatings on glass are unacceptable because they would make the atrium a mir-
ror like block, generally offensive to the environment. A reflective roof is also to be avoided
because, seen from the inside, it would differ very much from the façade.
Fixed or adjustable louvers on top of the roof or in front of the façade are an effective solution,
but these have much influence on the architecture.
For the façade only removable sunscreens are possible because otherwise the whole idea of
transparency is lost.
An alternative are dots or stripes printed on the glass. If they are white on the outside, the ab-
sorption is restricted. From the inside, fine black dots are not visible separately.
Sometimes electrical sun cells are used to reduce the heat load.
An internal sunscreen must have a light colour. The variant of the climate window, where the
exhaust air from the atrium is sucked through the mesh like sunscreen, taking away part of the
heat, may also be used.
Like in winter, to make the climate fully controllable, a great many additional elements will ham-
per the transparency.
4.2.3.3. Ventilation
For incidental activities in a large atrium, the need for fresh air will be limited. Per hour, only a
small portion of the atrium volume must be exchanged.
Generally the volume of ventilation is determined by the need to control the temperature in
summer. Sometimes used air from the building is led through the atrium, or ventilation air for
the building is drawn from the atrium.
An exhaust opening in the roof or high up the façade must be combined with openings that are
situated as low as possible. However, the openings should stay several meters above ground
level, to avoid draft over the floor, incoming dust and intrusion.
The chimney effect is the driving force. On the hottest days, however, the cooling capacity must
be at maximum, while the available margin between interior and outside air temperature is
minimal. Therefore the ventilation openings must be quite large (about 5% of the floor area) for
this extreme situation. In normal situations they must be smaller and consequently they must
be adjustable. In some cases the whole atrium roof can be lifted to create an opening along the
perimeter.
The idea to cool the atrium at night is attractive. Then for safety, grids must be placed in the
lower openings.
In case of fire, ventilation smoke must escape through the ventilation openings. An automatic
opening mechanism is needed. The smoke must be kept high enough above floor level to keep
this free from the poisonous gasses. Generally the atrium is also the entrance of the building,
and a natural escape route.
4.2.4. Maintenance
A glass façade and roof is expected to be transparent. This is only true with regular cleaning.
For cleaning, easy and safe access is conditional, meaning hanging scaffolds, catwalks or lift-
ing platforms. All of these are hindered by structural elements, expanding from the inside or
outside.
A hanging scaffold needs rails and a stiff support along the roof, creating a strong accent.
If the façade is supported by wide horizontal trusses, these can serve as a catwalk if an open
grating is added.
In most cases, however, a lifting platform is used. For this equipment, about 3 m1 from the in &
outside of the façade must be kept free of landscaping. A flat and hard surface is needed for the
stability of this equipment.
In some projects the glass is so poorly accessible that, due to staining, the intended prestige is
lost.
Fig. 4.2.1.4 Exposition hall in the Castle
Garden in Arcen
Fig. 4.2.1.5 Atrium of the Centraal
Beheer building in Apeldoorn
Fig. 4.2.2.1.
Western Morning News,
Derriford (GB)
12

Replacing mastic joints is part of the maintenance.
Mastic joints are often used to create a flat exterior of the glass area, without cover strips at the
joints. The service lifespan of this material is limited to about 20 years, due to hardening and loss
of adhesion.
The quality of these joints depends on the working conditions in the building stage. Dirt or
damp edges of the glass, for example, may prevent a good adhesion. An earlier replacement is
necessary sometimes.
In a glass façade or roof, a leaking mastic joint seldom leads to extended damage, because the
leak is easily detected and the materials around the joint are not affected by moisture.
4.2.5. Safety
Glass is a brittle material; it breaks suddenly without plastic deformation. It breaks at spots
where micro cracks or scratches appear on the surface. The material also contains internal re-
sidual stresses from the solidifying during the manufacturing (see also chapter 7). The material
has a high stiffness (about 1/3 of steel), meaning modest deformation before breaking.
Without plastic deformation, a redistribution of forces after local overstressing is not possible.
Therefore, stresses must be predictable and not be influenced by tolerances and movements
of building parts. A statically determined fixing is the simplest way to create a reliable solution.
This applies not just to the connection of anchors to the structure, but even more so to the con-
nection between anchors and the glass. A rigid connection of an anchor to the glass could easily
lead to unintended, secondary bending moments or sliding forces. Especially around holes in
the glass for a bolt connection, bending moments create a risk.
In early examples, designers often chose for complex ball joints in the centre of the thickness of
the glass. This safeguards free rotation at the support if the panel is bent. The glass panels were
always supported at the corners.
Later, after more experience was gained and better modelling of the behaviour was possible,
more rigid connections were used. The position of the support also was chosen more freely,
away from the perimeter. By accepting bending moments at the fixing points, deflection is re-
duced, or wider spans are possible with the same glass thickness.
Glass has mechanical qualities that makeI it not very suitable for distributing mechanical loads.
However, with the right precautions it can handle these loads.
Modelling glass structures is not possible in the same way as modelling structures of steel or
concrete. There, redistribution of stresses is incorporated before the ultimate limit state arises.
Also, glass does not have a fixed material strength. The “strength” of glass is determined by the
number and nature of faults in the surface. Small pieces of glass have less cracks from the shrink-
ing during solidifying than large pieces and consequently the material seems stronger.
For large assemblies of glass it can prove to be advantageous to incorporate plastic behaviour in
the connection points to reduce forces due to local collapse. It is also possible to construct con-
nections that are stiff in normal conditions but are much more flexible in an overload situation
(see also 4.6.4.).
If normal glass breaks, the large sharp peaces form a grave risk to passers-by. This risk can be
reduced by hardening the glass, where high internal stresses are generated. In the event of lo-
cal breakage, the equilibrium of the internal stresses will be disturbed. The glass panel will then
break in a large number of small pieces that are less dangerous. This can be a good solution for
vertical windows at low level. For horizontal glass panels over a circulation zone, hardened glass
is still a major risk.
Another way to reduce the risk following breakage is laminating two or more layers of glass, with
a plastic foil in between. Then the broken pieces are kept together. A precondition is that the
broken, more or less flexible panels, stay in their position. This can be achieved by clamping the
glass panel strongly to the supports.
Gluing the glass is also possible.
A wire mesh encapsulated in the glass also keeps the glass together.
The biggest threat to large glass areas is equipment of people working on a glass roof. Also un-
predicted movement of the structure and temperature effects cause breakage.
Vandalism also forms a risk, but experience shows that large glass panels are less inviting.
Large glass panels are relatively safe. Large panels will generally be made of hardened glass to
obtain a higher breaking strength for the same thickness. The stiffness, however, is not affected.
Then the deflection before breakage is quite large. The bending radius for hardened glass at the
breaking stress is about 870 times the thickness of the panel. For a panel of 2 m1 span and 6 mm
thick this means a deflection of about 10 cm. Such a deflection will be very noticeable.
4.2.6. Sound
Reducing the heating of the air in the atrium with a large heat capacity leads to floors covered
with tiles. In combination with the glass walls and roof, there is hardly any sound absorbing sur-
face in the atrium. The volume of the space is also quite large. The reverberation time depends
on the relation between volume and absorption surface. To reduce the noise nuisance, addition-
al absorption panels can be used. Hanging objects and landscaping with trees may also be used.
4.3 Fixing of glass panels to the structure
The glass is held in position by a supporting structure, to which it is connected in different ways.
The following ways of fixing may be chosen:
− External cover strips on four sides This technique has the visual disadvantage that the glass
area is divided into small sections. On angled roofs also water is trapped above the horizontal
cover strips. After evaporation dirt stays behind. This way of fixing, however, is simple and reli-
able.
− External cover strips on the vertical sides and a glued connection on the backside on the hori-
zontal side. This is attractive for roofs. The horizontal joints, formed by mastic joints and flush
with the surface, will prevent the problem of staining.
− Glued connection on four sides. From the outside, the glass façade appears as one unbroken
surface. The glue technique is quite reliable, although some municipalities demand additional
mechanical connections that become active in case of failure.
− Visible point like connections around the edges. These connections are hardly visible, but
locally high tensions appear at the edges. The closing of the joints near the connections is dif-
ficult. This fixing method is frequently used with atria where single glass and open joints are
acceptable.
− Bolt connections through the glass. The bolts are hardly visible if they are kept flush with the
surface. In double glass units the bolt can also just penetrate the inner glass panel. To restrict the
secondary bending moments in the glass around the holes, hinges or flexible connections can
be put close to the surface. A large number of systems is available.
− Point like glued connection on the backside. Here the glass has no drilled holes, which reduces
some of the problems with tolerances, compression forces in the glass, etc. The risk, however,
is increased. The stress on the glued connection is very high. A rotating connection can only be
placed at a distance from the surface. Therefore bending moments will always appear in the
glue. Not one of the four connections may fail, because this creates a large increase of the forces
on the other points. Also the small area of glue, compared with a linear gluing along the perim-
eter, reduces the margin for error. Because the thermal behaviour of the anchor is different from
that of the glass (absorption of sunlight, thermal mass), the glue must have some flexibility. At
the moment of this writing just one example is known, the atrium in the Prinsenhof in Delft (fig.
4.3.1.). For the vertical panels additional mechanical support is used to prevent the continuous
gravity load on the glue.
At the bottom of this list, the visibility of the fixing is reduced. The susceptibility to errors in de-
sign and realisation, however, increases. This makes strict quality control essential.
A point like connection is only useful in combination with a very transparent support structure.
The only continuous lines are formed by the joints of about 20 mm wide and thin cables or bars.
If the support is more massive, with wider members, then a continuous glued connection is
more logical. This is simpler, less sensitive and a thinner glass panel is possible.
Glued fixings are not always allowed. For example in Germany and Rotterdam additional me-
chanical fixing is compulsory above a certain level.
4.4. Wind load
4.4.1. Introduction
The size of the glass panels is limited by the accepted deflection and the steep increase of the
price of glass with thickness (fig. 7.4.1.1.). Also the equipment in the industrial process of harden-
ing or laminating limits the size.
Generally, the wind load is dominant, leading to a maximum of about 2 m1 for the shortest side
of a panel.
For horizontal panels, smaller dimensions are favoured. In laminated glass, necessary for safety,
the glass layers do not act together for a permanent gravity load. This means extra thickness.
To combine the small glass panels to a large glass façade, a support structure must be used. For
the purpose of transparency, this must be as unobtrusive as possible. To reach this goal, several
techniques are available. These may be combined for the primary and secondary structural ele-
ments.
For a systematic approach they are presented separately. In projects, however, all kinds of com-
binations may be seen.
In fig. 4.4.1.1 a part of an atrium roof is shown as an example.
For downward loads the primary structure is a truss with thin diagonals, fit only for tension. For
the upward loads the diagonals are ineffective, and the truss acts as a frame truss. The slen-
der trusses are stabilized by prestessed diagonal cables. As secondary beam, a laminated glass
beam is used. The glass panels are fixed to these beams with point like mechanical connectors.
N.B. There is no mechanical cooperation between glass panel and beam to form a T-shape. This
has been experimented in the laboratory, but is not yet a reliable technique.
Fig. 4.3.1. Roof over the interior square of the
Prinsenhof, Delft
13

To minimize the structural elements, these are designed to distribute the maximum load on
the material, meaning large deformations. Especially in the building stage, different loads will
appear if not all panels are fit. The increased deformations make the building process more dif-
ficult. In a partially closed façade uneven wind loads may appear.
Because hardly any unnecessary material is used that could cope with the various situations, this
building stage must be taken into account in the design.
Transparency from the inside of the façade is greatly influenced by the structural elements. The
transparency from outside is mainly influenced by the level of light in the atrium and the reflec-
tion from the sky. In the following paragraph the structure is always considered from the inside.
4.4.2. Making the structural elements less visible
The following measures may be taken to make elements
less obtrusive:
− By giving the interior side of the element a light colour,
there is less contrast with the clear outside.
− To enhance this effect, the sides of the member may be
angled. These sides are still lighted by the sky, but they
reduce the width of the darker interior side. By a curved
side the reduction of light is gradual. The sides can even
come together, avoiding an unlit backside (fig 4.4.2.1. and
4.4.2.2.).
− The beam in the façade can be made of glass, avoiding
even further a dark backside.
− Already mentioned was the orientation: horizontally a
wide span with deep members to facilitate an unobstruct-
Fig. 4.4.1.1 Main Office Proctor & Gamble,
Weybridge (GB)
Fig. 4.4.2.1 Mullion and transom with tapered
interior side
Fig. 4.4.2.2 Visual effect
ed view in the horizontal direction instead of deep vertical members.
− A truss without diagonals has fewer elements and gives a more open view than a truss with
diagonals (fig. 4.4.2.3.). The need for more material can be neutralised by more massive sec-
tions with small exterior dimensions. The goal is visual transparency; material efficiency comes
second.
− Round or oval tubes are visually thinner than rectangular or square tubes. H-type sections are
worst.
4.4.3. Reduction of visual surface of the members.
For transparency it is essential to reduce the exterior dimensions of the members in the plane of
the façade. Here, the parts resisting compression forces, are the most important. To resist buck-
ling, they must have a minimal bending stiffness. Therefore it is useful to try to avoid compres-
sion forces in beams and trusses. This can be achieved by prestressing where the compression
forces are overruled by an external tension force. The principle is explained here.
− A post or beam must resist wind pressure and wind suction. With wind pressure, compression
forces appear on the outside and with suction, the inside takes the compression. This means
that on both sides of a truss, a wide outside dimension is needed. This is illustrated in fig.4.4.3.1.
Shown are the forces in unloaded and loaded situation. For clarity, the connecting bars and
diagonals are not shown.
− By prestressing the system of fig. 4.4.3.2, both tension bars are under tension and the central
compression bar under compression (figs. 4.4.3.3. and 4.4.3.4.). With a bending moment, one
as a central compression bar. To prevent buckling for larger glass
panels, glass ribs can be added. This solution has been applied
on some small projects. The central compression bar may be
eliminated if the pre-stress cables are put into an external frame
of support structure (fig. 4.4.3.6.). The total amount of forces and
bending moments increases drastically, but the accompanying
material is kept out of the area where it matters. This solution
is very attractive if the frame around the façade is already pres-
ent, for example trusses for a roof or other parts of the building.
The structure will show deformations and also the cables of the
façade structure will shrink and expand by thermal movement.
The thermal mass is small and the cables receive a lot of sun en-
ergy. For this reason the cables are sometimes put in series with
a flexible spring (fig. 4.4.3.7.). This spring has shown a large de-
formation to obtain the required pretension. Due to the small
stiffness, a small displacement will not change the pretension to
a great extent.
− If the tension cables are put in two directions, the ends of the
cables may be spread around the frame. Numerous configura-
tions are possible (fig. 4.4.3.8.). Double curved nets are also used
for façades.
− If large deflections are acceptable (0.01 to 0.02 times the
length of the span) with the maximum load, a completely flat net
or cable system is possible. The pretension must be very high,
since the reactive bending moment is equal to
the product of the pre-stress and the displace-
ment. Displacements may go up to 50 cm, and
the joints must be able to accept angled defor-
mations, especially along the perimeter of the
façade.
The given principles may be used for primary,
secondary structural elements or both.
4.4.4. Reducing the number of elements in a sup-
port structure
The support structure can become more trans-
parent by omitting certain supports or by mak-
ing linear members non continuous.
By supporting glass panels only on two sides,
only horizontal or vertical lines remain (4.2.1.5.).
In the other direction only a narrow weather
tension bar gets double tension and the other one almost zero tension. The amount of compres-
sion is unaffected. The bending stiffness is formed by the area of the tension bars and the full
height. Because of the height and the double surface, the stiffness is doubled in comparison
with 4.4.3.1.
− To gain a flat exterior tension bar, lying in the face of the façade, the geometry may be bent
Fig. 4.4.2.3
John Moores University (GB)
Fig.4.4.3.2 Forces in the tension bars and the
central compression member (non prestressed
system), without and with wind load
0 = no force
++ = tension
--= compression
Fig.4.4.3.3 Forces in the tension bars and the
central compression member (prestressed
system), without and with wind load
0 = no force
++ = tension
--= compression
Fig. 4.4.3.5.
− To reduce the number of compression bars, 4 or more tension
cables can use the same compression member. A spatial truss is
then formed. Again, the outside dimension of the compression
member can stay the same, only the area of the cross section
increases. It is also possible to use the glass panels of the façade
Fig. 4.4.3.4 Horizontal truss system with pre
stressed cables and a central compression bar.
The vertical cables only support the truss and
not the glass panels.
Park André-Citroën, Parijs
Fig. 4.4.3.5 Truss system like
fig. 4.4.3.3. but with a curved
compression bar.
Fig. 4.4.3.6
Atrium Parc de
la Vilette, Paris
Fig. 4.4.3.7. Spring structure at the bottom of a
cable truss to absorb movement and maintain
the pre stress.
Fig. 4.4.3.8 Cable structure in
two directions.
14

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