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Flat Slab Design – Should you adopt Flat slab in Seismic Zone

There are different structural systems that can be adopted in Building structures. Beam and slab system are the most commonly used structural system. A beam and slab system are one of the easiest load transfer systems in terms of structural performance as well as easiness of design.

The decision of the type of structural system is not just a structural decision these days. There are many constructions related and economy related reasons as well. Many times, architectural reasons also decide the structural system.

What is a Flat Slab?

A flat slab is a type of floor construction that is frequently used in modern buildings. It is also referred to as the flat floor, concrete flat floor, and the concrete flat sheet or flat plate. It is a floor without a frame in the building.

To explain it a bit more, A flat slab construction is where the slabs directly rest on columns, without beams. If there is no drop at the column and slab meeting point, it is a flat plate and if there is a drop, it is a flat slab.

Flat plates are possible only if the column space is smaller. If the spans are large, punching may become critical and a drop may be needed. Flat plates are generally preferred in smaller housing and apartment projects.

In India, flat plate is not yet popular for apartments. Mostly a beam and slab system are used for housing and apartment projects.

Flat slabs with drop panels are used for office buildings and shopping complexes and malls in India.

Advantages of a Flat Slab System

The main advantages of adopting flat slab system for buildings are listed below.

Reduction in Total Height of the Building

Flat slab reduces the total height required for each floor of the building. This allows to accommodate a greater number of floors in a building for a given height. Many times, the total height of the building will have a restriction in a given location. An example is building near an airport. The building regulatory authorities would have limited the maximum allowable height in the flight path. Adopting a beam and slab system increases the floor height because of the larger beam depth. This reduces the number of floors possible to be built in a given height and the builder will lose a floor to sell. If there are multiple blocks in a project, you can imagine the profit a builder can make by selling more, if he adopts a flat slab system.

Avoiding Sleeves through Beams

By adopting Flat slab system, we can avoid sleeves passing through the beams. Many times, the builder would like to keep the floor heights less as mentioned in the previous section. If the services like electrical or plumbing needs to pass pipes, it has to cross through beams. If we take it under the beams, the floor clear height will reduce and will not be ok functional point of view. This forces to raise the total height of the building. A 150mm difference in a floor makes it 3000mm in a 20-floor building.3m is a floor in fact. So, builder loses 1 floor to sell. I have explained about this in another blog. See the blog here to understand how sleeves are provided and how it affects the design and strength of the beam.

Easy Accommodation of Vertical shafts /Ducts

The above point on sleeve is about horizontal pipes and shafts. In buildings, we need vertical shafts too, to take many services like plumbing lines. In a beam and slab system, the beams come in the way of possibility of ducts in certain location. In flat slabs, it’s much easier to provide these vertical shafts. Of course, the critical locations should be avoided even in flat slabs but then it is a bit easier in flat slabs.

More Day Light in the Building

Beams are hindrance to light and air movement in a building structure. A beamless construction will allow more uniform lighting inside the building. Also, more airflow in the building without any hindrance is possible with beamless design. Flat slab is a beam less construction and is useful for having better day light and air movement. Architects and clients would love to have this system though it is a structural challenge.

Room layout Flexibility

The partition walls can be laid out anywhere. So, the owner has more flexibility in changing the room layouts. Vey much suitable for stores and malls.

Easy Reinforcement Placement

The rebar arrangement in a flat slab is simpler and hence it can be laid out much easier than in a conventional beam and slab system.

Faster Construction

Since it is easier to install form works and reinforcement, the flat slab construction can be faster than conventional beam and slab.

Disadvantage of Flat Slab System

The major disadvantage of flat slabs and any beam less system like flat plate is their lack of resistance to lateral loads like wind loads and seismic forces. Due to this, we may have to adopt a single system where the entire lateral forces are dumped in to the shear walls. Because of the higher challenges due to these, it is not recommended to use flat slab systems in high rise constructions or in highly seismic areas.

Critical Design Forces in Flat Slabs

Generally, a flat slab should be designed for the moments, shear and the punching shear.

In modern and large buildings, it is ideal to use an analysis tool like ETABS for the structural analysis. You can zone the forces in ETABS and design it manually by a few quick steps or use the strip method of design. The same strip method of design is available in SAFE as well.

FAQs on Flat Slabs

Can we design mid rise buildings using flat slab system and design for gravity alone?

No. All buildings in India needs to be designed for certain seismic force. The whole of the sub-continent is seismically active. The needed reduction for less active zone is applied in the code as a zone factor.

Is it ok to use Flat slabs in higher seismic zones?

Ideally, No. Flat slabs are poor in seismic resistance. In any seismic zone, the lateral system may have to be single and at the same time, the clause 7.2.2 of IS 1893:2016 has to be met so that each column has the prescribed minimum lateral capacity.

Can we use a wide beam like a strip beam (wide and shallow connecting columns)?

Yes. However, one need to meet the Cl. 6.1.4 of IS 13920: 2016 where the width of the beam is restricted in relation with the supporting column.

Is it ok to avoid edge beams or peripheral beams in flat slab system?

It will be a little easier to meet the seismic demand if you have edge beams. Better to have it.


Though flat slab is a great structural system with respect to its aesthetic and functional use, it needs to be carefully chosen only in non-seismic zones. In India, it shall be used only in seismic Zone II as much as possible and not in higher zones. If ever we choose to use flat slabs, it needs a careful attention in the scheme to take care of the lateral load transfer. The structural design points mentioned in the FAQs section needs to be strictly implemented.

If you have any structural question on flat slab design using ETABS, leave a comment below. Also like and share the blog for a better reach to other civil engineers and structural engineers.

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