What is a Waffle Slab? Waffle slabs are a reinforced r einforced concrete footing and slab system constructed on ground. They consist of a perimeter footing (edge beam) and a series of narrow internal beams (strip footings) at one metre nominal centres running each wa y. The whole footing and slab system is constructed on top of the ground. Edge formwork makes the sides of the slab and polystyrene ‘pods’ create the formed voids between the strip footings. When viewed from underneath, underneath, the system of internal strip footings looks like a waffle – waffle – hence hence the name. Upon completion of the waffle slab house, the ground around the slab is built up by the builder to reduce the height of the slab above the the surrounding ground. Waffle slabs achieve their strength by varying their height above ground. The higher the slab above ground – ground – the the deeper the beams. The deeper the beams – beams – the the more stiffness the system has.
Characteristics of Waffle Slabs
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 pre-fabricated 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.
Waffle Slab Construction Procedure The construction of waffle slabs can be done by three ways as follows. o
In-situ
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Precast
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Pre-fabricated 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, pre-fabricated 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:
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Waffle pods
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Horizontal supports
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Vertical supports
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Wall connectors
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Cube junctions
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Hole plates
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Clits
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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.
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Benefits of Waffle Slab Construction Waffle slabs are used for larger span slabs or floors and used when there is limited requirement for number of columns.
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The load carrying capacity of waffle slab is greater than the other types of slabs.
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They provide good structural stability along with aesthetic appearance. Hence, it is constructed for airports, hospitals, temples, churches etc. The waffle slab can be made of concrete or wood or steel among those concrete waffle slab is preferred for commercial buildings and other two are preferred for garages, decorative halls etc. It has good vibration control capacity because of two directi onal reinforcement. So, it is useful for public buildings to control vibrations created by movements of crowd.
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Waffle slabs are lightweight and requires less amount of concrete, hence it is economical.
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Construction of waffle slab is easy and quick with good supervision.
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Concrete and steel volume required is small, hence, light framework is enough for waffle slab. Several services like lighting, plumbing pipes, electrical wiring, air conditioning, insulation materials etc. can be provided within the depth of waffle slab by providing holes in the waffle bottom surface. This system is called as Holedeck.
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Drawbacks of Waffle Slab Formwork tools required are very costly because of large quantit y requirement of pods and some special tools.
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The floor height should be more hence number of floors are reduced.
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The services provided in the waffle arrangement without proper maintenance ma y causes damages to the slab. Skilled workers are required during its construction. They are not suitable for sloped areas. If there is slope area, the area must be leveled with filling or by excavating. For soil filling, good soil should be used. They are not suitable against high winds or cyclonic areas because of their light weight.
A Place for Waffle Slabs
There is definitely a place for waffle slabs in the construction world. Waffle slabs work really well on sites that are almost flat, natural soils or controlled fill, that have good surface strength and where the natural ground surface falls away from the outsides of the building in all directions. They work well on non-reactive sites, slightly reactive clay sites and some moderately reactive clay sites. Waffle slabs are not recommended on highly reactive clay sites (Class H1 and H2) because the requirements for good drainage are almost impossible to achieve. There is no ‘deemed to comply’ design for waffle slabs for extremely reactive sites (Class E sites). In fact there is no deemed to comply raft footing design for Class E sit es either – the footing and slab system for a Class E site must be designed by a relevantly qualified engineer.
Are Waffle Slabs a Great Idea?
Are waffle slabs a great idea? A definite maybe. These are the sites where waffle slabs won’t work so well:
Soft ground conditions. Extra bored piers or screw piers are required so t hat the system is supported on strong ground. Sloping sites. Waffle slabs are built on flat sites. On sloping blocks, the ground has to be made level first by digging some of it out or filling some of it in. Problems arise when some of the dirt dug out is used as uncontrolled fill on the low side of the block. All houses, even waffle slabs, need firm, even support to all parts of the slab. Highly reactive and extremely reactive clay sites. These sites need stiff footing systems to span over the swelling and shrinking soils. Concrete beams get stronger and sti ffer when the depth of the concrete beams increases but waffle pod void formers tend to max out at 375mm deep (so providing 475mm deep beams and ribs with a 100mm slab). Some designers try to achieve extra slab stiffness by adding more steel reinforcement. This works but the design process becomes more complicated. Cyclonic areas and high wind areas. High winds generate a lot of pres sure on roofs which result in some very concentrated forces in modern truss ed roofs. On a regular sized 200m2 house, some truss uplift forces are as high as 5 tonnes. In a waffle slab, these forces need to be resisted only by the weight of the footing system because there is no skin friction with the ground. 5 tonnes of concrete is about 2 cubic metres of concrete. That’s a lot more concrete than is available to resist forces at the truss reaction point. The consequence? High wind forces will lift enough of the waffle slab to resist the force but this comes with deflection of the slab. In a cyclone – be prepared for your slab to lift and your walls to crack.
Buildings in the Philippines that used the waffle slab: 1. Cultural Center of the Philippines
Reference: https://www.cornellengineers.com.au/beware-waffle-slabs/ https://theconstructor.org/structural-engg/waffle-slab-construction-procedure-characteristicsadvantages/20546/
