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A waffle slab is a type of reinforced concrete slab that features a unique grid-like pattern of concrete ribs on the underside of the slab. These ribs form a pattern similar to that of a waffle, hence the name “waffle slab.”
Waffle slab is described as:
- Waffle slab is a concrete slab made of reinforced concrete with concrete ribs running in two directions on its underside.
- The name waffle comes from the grid pattern create by the reinforcing ribs. Waffle slabs are preferred for spans greater than 12 m, as they are much stronger than flat slabs, flat slabs with drop panels, two-way slabs, one-way slabs, and one way joist slabs.
A two-way concrete joist system and a two-way slab or flat slab made up of a double system of narrow ribs or joists, usually at right angles to each other, forming a pattern of waffle-like coffers.
- A waffle slab is a type of building material that has two-directional reinforcement on the outside of the material, giving it the shape of the pockets on a waffle. This type of reinforcement is common on concrete, wood and metal construction. A waffle slab gives a substance significantly more structural stability without using a lot of additional material. This makes a waffle slab perfect for large flat areas like foundations or floors.
- The most common material for a waffle slab is concrete. These slabs are used as the foundation for many different types of buildings and structures but are most common in commercial or industrial buildings. Waffle foundations are resistant to cracking and sagging and can hold a much greater amount of weight than traditional concrete slabs.
- The top of a waffle slab is generally smooth, like a traditional building surface, but the underside has a shape reminiscent of a waffle. Straight lines run the entire width and length of the slab, generally raised several inches from the surface. These ridges form the namesake square pockets of the entire length and width of the slab.
2) Waffle Slab — continue
- Two-way waffle slabs are formed using metal or plastic pans called DOMES. The ribs formed between domes are called joists. These are two basic sizes of standard domes.
- The smaller of two forms joist 5 in. wide at the bottom on 24 in. centers, and from 8 to 14 in. deep.
- The second standard dome size produces joists 6 in. wide at the bottom on 36 in. centers and from 8 to 20 in. deep. Larger sizes are also available.
- Domes are tapered to permit their easy removal after the concrete has set. Some domes have compressed air fitting to permit easier removal.
- Waffle slabs are most efficient when allowed to cantilever at the perimeter. When cantilever is not practicable or desirable, a perimeter beam (Slab Band) must be placed there. In addition, column heads are created around the columns to accommodate the shear stresses there. Slab bands and column heads are created by omitting domes.
- Waffle slabs are constructed by first building the supporting elements, such as columns and walls, and permitting them to cure sufficiently to support the loads that will be imposed. Then the supporting structure is built for the domes. The dome should be placed on this centering and coated with a form-release agent. The steel should then be placed.
- After the steel has been installed and inspected, the concrete should be poured and the slab surface cures and is then finished. When the waffle slab has set sufficiently to support its weight, the centering should be removed, and the dome dropped from the concrete and moved to the next forming area. The formed surfaces should then be finished.
3) Characteristics of Waffle Slab
- Waffle slabs are generally suitable for flat areas.
- Volume of concrete used is very less compared to others.
- The reinforcement in the waffle slab is provided in the form of mesh or individual bars.
- Separate excavation for beams is not required in case of waffle slab.
- The bottom surface of slab is looks like waffle which is obtained by using cardboard panels or pods etc.
- The thickness of waffle slab recommended is 85 to 100 mm while the overall depth of slab is limited to 300 to 600 mm.
- The width of beams or ribs provided in waffle slab are generally 110 to 200 mm.
- Spacing of ribs recommended is 600 to 1500 mm.
- Reinforced waffle slabs can be constructed for the span up to 16 meters while beyond that length prefabricated waffle slab is preferable.
- Waffle slab is good against shrinkage and it is lower than stiffened rafts and footing slabs.
- Waffle slab requires only 70% of concrete and 80% of steel from the concrete and steel used for stiffened raft.
3) Advantages waffle slabs
Waffle slabs tend to be deeper than the equivalent ribbed slab. Waffle slabs have a thin topping slab and narrow ribs spanning in both directions between column heads or band beams. The column heads or band beams are the same depth as the ribs.
- Relatively light, therefore less foundation costs and longer spans are economic
- Speed of construction
- Fairly slim floor depths
- Excellent vibration control
- Thermal mass
- Good for services integration
- Durable finishes
- Fire resistance
- Savings on weight and materials
- Long spans
- Attractive soffit appearance if exposed
- Economical when reusable formwork pans used
- Vertical penetrations between ribs are easy.
Two-way Waffle Slab
- Longer 2-way spans and heavier load capacity.
- Usual spans : 25’ to 40’./
- Standard pan sizes : 20” to 30” square with other sizes available. Standard pan depths 8” to 20” in 2” increments.
- Usual maximum ratio of long to short side of bay is 1.33.
- Typical ratio of span to depth: 25-30
4) Disadvantages of Waffle Slabs
Waffle slab was not used in conventional construction projects.
It is expensive, so it is only economical when large scale production of similar units is desired.
Construction requires rigorous supervision and skilled labour.
- Depth of slab between the ribs may control the fire rating
- Requires special or proprietary formwork
- Greater floor-to-floor height
- Large vertical penetrations are more difficult to handle.
5) Waffle Slab Construction Procedure
The construction of waffle slabs can be done by three ways as follows.
In-situ waffle slabs are constructed by pouring concrete in the site or field with proper arrangements. In case of precast waffle slab, slab panels are casted somewhere and they are joined together with proper reinforcement and concrete is filled.
The third case, prefabricated waffle slab is costliest than the other two methods. In this case, reinforcement is provided in the slab panels while casting with some tension. Hence, they do not need internal reinforcement in the site.
To construct a waffle slab in-situ conditions, formwork should be necessary to support the slab. But some special tools are required for the form work in case of waffle slab.
Formwork tools required in the construction of waffle slab are:
- Waffle pods
- Horizontal supports
- Vertical supports
- Wall connectors
- Cube junctions
- Hole plates
- Steel bars
Horizontal support and vertical supports are arranged first and they are fixed in position by the connectors. At the edges wall connectors are used to provide connection between wall and slab. The horizontal beam supports are connected by small beam connectors which form square like shape in which pods are going to be placed.
The pods are generally made of plastic, and they are available in different sizes and different shapes. Size selection of pod depend upon the requirement and span length. For longer span large number of pods are required. Same size should be used for one complete slab.
Similarly, beam connectors and cube junctions are also available in different sizes based on the suitability of pod sizes.
Cube junctions are used to fix the corners of pods with the framework. After fixing the formwork, reinforcement is placed in the two directions of the slab and then concrete is poured in the gaps which are called as ribs after hardening.
Thin concrete slab is provided on the top and after its hardening pods and frameworks are removed from the bottom. Thus, the waffle like shape appears at the bottom surface.
- 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 the floor around the supports, thus avoiding the necessity of bottom reinforcement in this region.
- A light top mesh over the waffles is generally the only top reinforcement at the interior of the floor panels. The number of strands in each rib is typically limited to one or two, with bottom rebar not exceeding two (#5; 12mm). Where design requirements demand more reinforcement that is generally assigned to a typical interior waffle stem, solid strips along the lines of supports is used to accommodate the excess of reinforcement.
- 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.
- 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, 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.
- 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 pre compression in slabs with smaller aspect ratios.
7) Structural modeling design constructions
- The principal advantage of a waffle slab or joist constriction is the composite interaction of the stem and the topping slab. The relative position of one with respect to the other provides the stiffness characteristic of a ribbed slab. Moreover, contributing moment from a post tensioning tendon relies primarily to the position of a tendon with respect to the centroidal axis of the construction.
- In summary, substituting a ribbed geometry by a simplified model invariably results in losing some of the advantages that led to the selection of a waffle/joist construction in the first instance. In summary:
- Smearing the geometrical properties of a waffle/ribbed slab, such as its second moment of area into an equivalent slab of uniform thickness might be an approximation acceptable for deflection analysis of conventionally reinforced concrete but does not work for post-tensioned floor systems.
- There are two reasons. why the approximation is not acceptable. First, the presence and contribution of axial pressure from tendons require that in the analysis, in addition to the second moment of area and total depth, the value of the cross-sectional area to be retained.
- Second, the necessity of the three parameters, namely: second moment of area, total depth, and cross-sectional area in each of the two orthogonal directions, make it impractical to find a substitute slab of uniform thickness for analysis. Hence, the necessity of modeling the structure in its true geometry.
- Topping slab and stems should be modeled and analyzed as a contiguous medium, in order to properly represent the stiffness of the structure. Disjointed modeling, meaning, modeling the stems as standalone beams not monolithic from the topping. Slab does not adequately represent the mechanical properties of a stemmed floor system for analysis and design.
- The structural modeling used in most commercially available software, where stems are considered as disjointed from the topping slab does not deliver the advantages associated with waffle slab construction.
- More importantly, where the stems are shifted in the simplified analyses to have the stems’ centroids line up with the topping slab grossly misrepresent the effects of post tensioning in the structure.
- Tendons in each stem should be modeled with their actual eccentricity with respect of the combined centroid of the stem and topping slab, in order to adequately represent their impact on the design of the structure.
- To capture variation of actions in each waffle stem, the finite element cells generated for the analysis should represent each stem separately, as opposed to larger cells across two or more stems.
8) The time saving system
- The Waffle Raft Slab System will save time and money on sites containing reactive soils, sand, rock and water tables. In addition, the Polystyrene Pod Systems are waterproof enabling work to be undertaken during wet weather conditions.
- Waffle Raft Pod slabs should be designed and approved by a civil and structural engineer,
9) Waffle Raft Pod standard sizes
- 1090 x 1090 x 175 mm
- 1090 x 1090 x 225 mm
- 1090 x 1090 x 300 mm
- 1090 x 1090 x 375 mm
10) Multi purpose beam spacers
- Multispacer 110 series – forms 110 mm internal beams and 300 mm perimeter beam.
11) Construction of Waffle Slab System
The purpose of this design is to create a more efficient use of material and reduce the weight of the slab without compromising its strength. The waffle slab can be used for a variety of building types, such as residential, commercial, or industrial buildings. It is often used for large spans, as it is a cost-effective solution for creating a structurally sound floor or roof.
In addition to being structurally efficient, waffle slabs can also provide improved acoustic performance and thermal insulation, making them a popular choice for modern buildings. The grid-like pattern of the ribs also allows for easier installation of services, such as electrical wiring or plumbing, since the recesses between the ribs provide ample space for these services.
- Ensure building site is level and all plumbing pipes are fixed in place as per plan.
- Spread a layer of sand or metal dust approximately 50 mm thick over building area and screed level. Place 200µm plastic sheeting and tape laps
- Set up formwork, set out Waffle Raft Pods in a grid pattern using multi spacers provided which form all beams.
- Cut the Waffle Raft Pods to fit around plumbing pipes etc. using sharp handsaw or power saw. Place reinforcing bars in multi-spacers positioned between void formers.
- Place steel mesh on top of Waffle Raft Pods and chair steel mesh at a rate of 4 bar chairs per Waffle Raft Pod.
- Pour concrete, part fill the ribs to hold the Waffle Raft Pod in position before pouring 85 mm thick slab with 25 mm minimum cover over reinforcing mesh (or as per your engineers instructions).
12) Waffle Pod Slab System Benefits
- The major benefits of the Waffle Raft Slab System over conventional slab systems are Clear savings in time – money – effort
- Reduced building costs on sites containing: reactive soils, sand, rock, water tables
- Class S sites reduce construction time by 1 day
- Less use of concrete as no footings required
- Eliminate concrete quantity overruns
- Greater strength and stiffness over conventional slab systems
- Savings in construction time over conventional slab systems
- Installation during inclement weather
- Insulation benefits
- Effective termite protection
- Design flexibility
Overall, the waffle slab is a versatile and cost-effective solution for creating a structurally sound and efficient floor or roof in a variety of building types.