Main concepts of waffle slab structure system

1. Waffle slab

2. Defination
• According to engineer Florentino
Regalado[1], ‘the waffle slab belongs
to the family of reinforced concrete
slabs that are non-homogeneous,
lightened, and reinforced in two
orthogonal directions, forming a
ribbed plate’.

3. Defination
• Waffle slab characteristics are defined by the following parameters:
• Total slab edge (H)
• Height of the lightening formwork or lightening blocks (h)
• Spacing between rib axes (e)
• Basic thickness of the ribs (b), though in waffle slabs with system formwork they
have a truncated pyramid-shaped core of variable thickness
• Thickness of the compression layer (c)

4. • This type of slab is able, very
adequately, to support distributed and
point vertical actions, and to a lesser
extent also horizontal actions. They
are bi-directional or two-way slabs,
because of the two orthogonal
directions of their reinforcement, and
slab bending can be broken down and
analysed according to those two
reinforcement directions.

5. benefits
• Flexible
• Relatively light, therefore less foundation costs
and longer spans are economic
• Speed of construction
• Fairly slim floor depths
• Robustness
• Excellent vibration control
• Thermal mass
• Good for services integration
• Durable finishes
• Fire resistance

6. CONSTRUCTION
• Waffles are generally limited to the
interior of a slab, leaving one or two of
the forms out to create a solid fill around
the supports. The solid fills provide the
strength required for shear transfer to
the supports. The fills also reduce the
compression stresses at the soffit of
thefloor around the supports, thus
avoiding the necessity of bottom
reinforcement in this region.

7. CONSTRUCTION
illustrate typical waffle constructions
using unbonded tendons. A light
top mesh over the waffles is
generally the only top reinforcement
at the interior of the floor panels

8. CONSTRUCTION
• With larger loads and
longer spans, such
as is common in
department stores a
heavier solid slab
band between the
supports
accommodates the
overage of
reinforcement from
the individual waffle
stems in each
direction

9. CONSTRUCTION
• Where aspect ratio of a panel exceeds 2 (ratio of
one side over the other), it becomes more
economical to use a joist slab construction.
• In joist slab constructions, such as the
• one shown in the figure, each joist is typically
provided with one or more strands. The joists
• in the transverse direction serve to distribute the
loads among the primary joists in the short
• direction. It is not economical to post-tension the
longitudinal joists.

10. CONSTRUCTION
• The post-tensioning required to account for
the strength of the structure is limited to the
slab bands along the two long sides of the
structure shown in the figure, where
profiling of tendons between adjacent
supports can best serve the in-service and
safety of the structure. Top mesh in the slab
is used to address the shrinkage and
temperature considerations that are genially
accounted for by precompression in slabs
with smaller aspect ratios.

11. Example
• waffle slabs with lightening formwork
that provides features other than
purely resistant properties. For
example, polystyrene formwork for
better thermal insulation, formwork
with special fire-resistant fibres, with
aesthetic finishes, etc.

12. Uses
• Public buildings .
• Commercial building.
• Residential buildings.

13. range of spans
• A waffle slab with a (40+5) cm edge
may reach spans of 11±1 m for
service overloads between 3 and 4
kN/m2, this being brought back to 8 m
if the service overload increases to 10
kN/m2.
• for service overloads that do not
exceed 5 kN/m2, range from 5 to 12
m.

14. Calculation
1400 H = q·L2·eWaffle Slabs of Non-reusable
Formwork
[2Ø20 for the reinforcement of
positive bending ribs]
2150 H = q·L2·eWaffle Slabs of System Formwork
[2Ø25 for the reinforcement of
positive bending ribs

15. Calculation
• The calculation example takes into consideration:
1. 3 points on each storey, coinciding with the centre box, a
perimeter centre box, and a corner box, on the second and
fourth storey
2. Cross-sections of the columns between 25x25 cm and
75x75 cm
3. Spans between 5 and 8 m
4. Edges between (21+4) cm and (41+4) cm
5. Considering, in all cases, 10-cm-thick basic ribs, this being
a more unfavourable hypothesis than the reality of waffle
slabs with system formwork, in which the compression
layer is 5 cm thick and the ribs have a truncated pyramid-
shaped cross-section with a 12-cm base.

16. • The foregoing empirical formulas
have been envisaged to deduce
the allowable limit span for a
given edge, inter-axis, and load.
Since it is not easy to find non-
reusable lightening formwork
higher than 30 cm, and the
service loads, column distribution,
and allowable maximum
deformations are conditioning
factors, humbler values for spans
in waffle slabs [6±1 m] and edge
columns set back between 1.5
and 2 m, as shown in the
following drawing.

17. Spans form 15 – 20
Meters
• A waffle slab with 40-cm-high system
formwork, edge of (40+ 5) cm or (40
+10) cm, and prestressed ribs with two
non-adhering stranded wires of 0.5 or
0.6 inches, could allow spans of 15 to 20
m, while simultaneously minimizing the
problem of deformations. It suffices for
the deviation forces to oppose each
other and cancel out the loads of the
own weight to assure a horizontal mass
concrete slab with zero deflection.

18. taking into account the
following
• The directives and requirements established by
the instructions and standards.
• The quality of the materials (concrete and steel)
• Structural dimensioning in project designs and the
most recent advances
• The influence of envelopes and partition walls
• Integral management of the deformations, from
the approaches in the design phase to the actual
construction process, including the ‘impositions’ of
the Develop

19. Examples
• he following have been taken as
minimum references in drawing up the
synoptic chart set out below: edges of
(31+4) cm and columns with a 30x30
cm cross-section, considering
modular rigid flooring of commercial
areas, offices, and services buildings,
in which large spans are not needed,
but also assuring stability in apartment
buildings.

20. Examples
ports centre with a waffle slab roof of (20+5) cm,
with a surface area of 52x34 m. San Vicente
(Alicante).
High-rise buildings resolved with waffle slabs
and vertical screens (to absorb the horizontal
thrusts).

21. Examples
Arcade along a public
square.
Waffle slab on waffle slab to cover a space of
20x20 m. Local fire station at San Vicente
(Alicante).

22. Adhered Tile
Installation in
relation to the
Deformability
of Waffle Slabs

23. Maximum Stability or
Minimum Deformability
• Conditions of Maximum Stability or Minimum Deformability,
compatible with a rigid or little deformable adhered
installation, without any constraints regarding the selection
of the technique and the materials.
• Minimum slab age: 6 months, with maturing under standard
temperature and relative humidity conditions
• Minimum age of envelopes and partitions: 2 months
• With maximum spans of 7 m for minimum edges of (31+4)
cm, with formwork of 80x80 cm [minimum ribs of 10 cm] and
minimum column cross-sections of 30x30 cm
• With partition loads no larger than 5 kN/m and envelopes
with loads no larger than 7 kN/m
• With service overloads no larger than 5 kN/m2
• With active deflections below 5 mm

24. Intermediate Stability or
Intermediate Deformability
• Conditions of Intermediate Stability or Intermediate Deformability,
compatible only with a deformable or very deformable direct
adhered tile installation, open joint, constraint on tile size, and
correct distribution of the movement joints.
• Minimum slab age: 4 months [or 2 months with maturing under
conditions of high relative humidity (RH>75%]
• Partitions and/or envelopes installed 1 month before the flooring
• Spans between 7 and 8 m, for minimum edges of (31+4) cm,
formwork of 80x80 cm (minimum ribs of 10 cm) and minimum
column cross-sections of 30x30 cm
• With envelope and partition loads, and service overloads no larger
than the ones reviewed in the foregoing section
• With active deflection larger than 5 mm and smaller than 10 mm

25. Low Stability or Low
Deformability
• Conditions of Low Stability or Low Deformability, which
require the design and installation of a resistant self-
bearing floating floor screed prior to the installation of
modular rigid flooring.
• When one or more of the intermediate deformability
conditions are not met
• When edges are smaller than (31+4) cm for spans
between 5 and 7 m
• With service overloads larger than 5 kN/m2 and/or
point loads larger than 7 kN
• Spans larger than 8 m, with edges of (31+4) cm or
larger, and column cross-sections of at least 30x30
cm.

26. Extras
• VIEWS OF THE
ANALYSIS
MODEL
GENERATED IN
FLOOR-PRO
FEATURING THE
CONSTRUCTION
DETAILS OF THE
ROTOTYPE

27. Extras
• TENDON
ARRANGMENT
IN THE WFFLE
SLAB
CONSTRUCITON

28. Extras
• The deflected
shape of the
waffle slab from
the analysis is
illustrated in Fig