2. Voided slabs in reinforced concrete
Lightweight reinforced concrete slab:
The necessity of decreasing the weight of RC slabs has various reasons:
ARCHITECTURE
* Obtain large spans, with fewer columns or walls;
* Avoidance of drop‐beams.
ENGINEERING
* Self‐weight reduction of the slab in order to:
‐ reduce its deformation
‐ reduce the weight (loads) transmitted to the foundation
‐ reduce the oscillating mass, thus the movements during an earthquake
The solution :
to hollow out the slab
3. Precast concrete elements
The use of precast elements has certain constraints:
TRANSPORT A flatbed truck must be used even for
small quantities or single parts. This
may cause difficult access to jobsites in
city centres and with heavy trafic
LOGISTICS Precast slabs occupy a lot of space on
site. A crane is mandatory for placing.
In case of polystyrene void formers the
precast parts must be protected from
rain.
USE The precast parts are handled with
some difficulty above the fourth floor.
Drop‐beams are often required.
5. Advantages
The advantages of the new Nautilus void former are numerous:
TWO‐WAY STRUCTURE: a two‐way slab will distribute
loads on 4 sides (instead of 2 for one‐way slabs),
reducing the maximum loads on beams or mushroom
posts.
LIGHT: the concrete not essential for the structure is
eliminated. The self‐weight of the slab is limited,
reducing the loads transferred to the foundation;
deformation of the structure is reduced.
FLEXIBLE: it will allow to eliminate drop‐beams and
create flat‐soffit slabs without interruptions of large
surface.
6. Advantages
The advantages of the new Nautilus void former are numerous:
QUAKE RESISTANT: the voids reduce the self‐weight of
the slab, reducing the seismic mass.
LARGE SPANS: larger spans between columns are
possible. The number of columns is reduced, the result
are archutecturally more manageable volumes.
COST‐EFFECTIVE: a slab with new Nautilus with the same static and
seismic characteristics consume a smaller amount of concrete and
steel than the full‐concrete equivalent slab:
‐ up to 35% slab weight reduction
‐ up to 50% fewer columns
‐ combined saving effect: 5 to 10% cost reduction potential
7. new Nautilus single
The new Nautilus void formers are available in various heights, all measure 520x520 mm in
plane. The “feet”, are spacers which determin the thickness of the lower slab, and are available
in height between o and 100 mm.
H16
Geoplast Nuovo Nautilus h16
Geoplast Nuovo Nautilus
H20
Geoplast Nuovo Nautilus h20
H24
Geoplast Nuovo Nautilus h24
8. new Nautilus double
The new Nautilus “single” void formers can be combined in a “double” configuration to allow
larger void‐formers.
Geoplast Nuovo Nautilus h20 Geoplast Nuovo Nautilus h24
Geoplast Nuovo Nautilus h16 Geoplast Nuovo Nautilus h20
Geoplast Nuovo Nautilus h16
H32 H36 H40 H44 H48
9. Installation
Installation of the new Nautilus void formers is very simple and fast:
1. The new Nautilus void formers don’t have an orientation. Installation is fast
and does not require any special care or attention.
2. The spacer strip makes spacing control between void formers simple and
accurate.
3. It is possible to tread on the new Nautilus void formers. This keeps the laying
of the upper slab armature really simple.
The new Nautilus caissons resist up to 1500 N
pressure on an 80x80 mm footprint for safe job‐
site application.
10. Steps 1-2
INSTALLATION
1. Prepare a slab formwork; lay the welded mesh on spacers.
2. Install the new NAUTILUS void formers, spacing between caissons as per drawing. Use the spacer strip to
check correct distance.
11. Steps 3-4
INSTALLATION
3. Install all required extra reinforcement (shear‐ and moment‐reinforcement); lay the upper slab
welded mesh.
4. First pouring phase: pour concrete to fill the lower slab, starting from the ribbing, up to the lower side of
the new Nautilus void formers. Vibrate the concrete immediately.
12. Step 5
INSTALLATION
5. Second pour phase: after completion of the first pour phase, when concrete has achieve
some strenght, fill the ribbings and complete the top slab as required by the project.
13. TEST RESULTS
Cross‐section of actual pour:
concrete type: S3 poured in two lifts as per
instructions, vibrated with eccentric poker.
NEW NAUTILUS
Single H16, lower slab thickness 50 mm
No concrete ingress ;
lower slab completely filled
Typical cross‐section:
NEW NAUTILUS
Double H32, lower slab thickness 60 mm
14. Calculation
STUDY The new NAUTILUS caissons create voids in a RC slab poured in situ.
The condition in shich the new Nautilus void former gives the most advantage is in a two‐way
slab configuration. In order to have a two‐way behaviour the ratio between the sides of the
slab must be between 1.7 and 1: beyond this ratio the behaviour will become one‐way, and
other methods to reduce slab weight may be more advantageous.
SLAB THICKNESS
The first step in the study of a full concrete
slab is to formulate an hypothesis of the
indicative thickness.
This thickness depends from the type of
structure being studied:
Slab on columns d = L / 25
Slab on beams d = L / 30
Waffle slab on columns d = L / 20
15. Inertia
STUDY Once the hypothesis of the minimum slab thickness has been formulated, the voided slab with
equivalent charactesistics of resistance and deformation must be identified.
A full concrete and a voided slab are compared based on their inertia.
The inertia of the full concrete slab must be calculated, and compared with the voided slab solution.
1 I
Ifull = ⋅ 100 ⋅ H2 Iall = full
12 B + 52
The inertia of the voided section is calculated according to the span of the void formers.
Based on the inertia values of the voided section is is possible to calculate the thickness of the slab, and
consequently choose the size of dimensions S1, S2 and h.
16. Full vs voided slab
STUDY Here below the comparison between a full concrete slab and an equivalent voided slab.
I=225000cm^4/m I=153626cm^4/i
I=225920cm^4/m
I=422108cm^4/m I=304140cm^4/i
I=422417cm^4/m
The thickness of a voided slab is slightly greater that the one of the equivalent full concrete
slab.
17. Slab support
STUDY Once the thickness of the voided slab has been established it is possible to calculate the steel
reinforcement.
Loads will be divided in two directions: this can be calculated with the Grashof formulae.
This calculation considers also the conditions
at the limits.
q ⋅ l4 q ⋅ l4
qx = y
qy = x
k ⋅ l4 + l4
x y
k ⋅ l y + l4
4
x
18. Reinforcement
STUDY The reinforcement of a voided slab with new Nautils is typically composed by:
‐ a welded mesh in the lower slab, with spacers to assure the required concrete cover;
‐ additional reinforcement (bars or grids);
‐ a welded mesh in the upper slab, laid directly on the void formers (which include ribbing on
their upper surface that serve as spacers).
Geoplast Nautilus h24 Geoplast Nautilus h24 Geoplast Nautilus h24 Geoplast Nautilus h24 Geoplast Nautilus h24
Example of additional reinforcement between the two welded meshes.
19. Slab profile close to support - beams
STUDY In the case of a slab and beam system there are typically rather high values of shear stress and
negative moment.
To manage these stresses is possible to use void formers of lower height close to the supports
in order to increas the resistant section.
20. Slab profile close to support - columns
STUDY In the case of a voided slab without beams it will be necessary to create a full concrete zone
around the top of the columns (“mushrooms”).
The armature must be properly calculated in order to manage shear‐stress and negative
moment.
Geoplast Nautilus h20
21. New Nautilus projects
New Nautilus projects
realised or in development
realised or in development
24. Typical project with new Nautilus + Modulo
Modulo used to fill the gap
between the slab and the
beam.
new Nautilus to decrease
the self-weight of the slab
PROJECT: DIGITEO LAB
and allow a wide span.
SITE: SACLAY, FRANCE
PRODUCT: new NAUTILUS®
SURFACE: 38.590 m2
