Waffle slab

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’.

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)

• 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.

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

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.

CONSTRUCTIONillustrate 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

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

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.

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.

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.

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

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.

Calculation1400 H = q·L2·e Waffle Slabs of Non-reusable

Formwork[2Ø20 for the reinforcement of

positive bending ribs]

2150 H = q·L2·e Waffle Slabs of System Formwork]2Ø25 for the reinforcement of

positive bending ribs

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 m4. Edges between (21+4) cm and (41+4) cm5. 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.

• 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.

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.

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

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.

Examplesports 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).

ExamplesArcade along a public square. Waffle slab on waffle slab to cover a space of 20x20

m. Local fire station at San Vicente (Alicante).

Adhered Tile Installation in relation to the Deformabilityof Waffle Slabs

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

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

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.

Extras• VIEWS OF THE

ANALYSIS MODEL GENERATED IN FLOOR-PRO FEATURING THE CONSTRUCTION DETAILS OF THE ROTOTYPE

Extras• TENDON

ARRANGMENT IN THE WFFLE SLAB CONSTRUCITON

Extras• The deflected

shape of the waffle slab from the analysis is illustrated in Fig