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Flat Slab vs Conventional Beam Slab

Flat Slab vs Conventional Beam Slab

Dear All,

For a high seismic activity zone, what frame system for a multi storey RCC building should be proposed.

Two options being;

1. Flat Slab supported directly on columns
2. Conventional Beam Slab supported on columns.

Recommendations requested.


Engr. Shakeel Ahsan
Director Engineering
CMEC - Pakistan

RE: Flat Slab vs Conventional Beam Slab

I would say beam-column is my choice. The chance to survive after a major seismic event looks better than flat slab.

RE: Flat Slab vs Conventional Beam Slab

Could you please give a description of how the structure responses better with a conventional frame system than a flat slab?


Engr. Shakeel Ahsan
Director Engineering
CMEC - Pakistan

RE: Flat Slab vs Conventional Beam Slab

I would say both would work favorably as long as you provide lateral force resisting system, say, concrete shear walls. Your choice may be driven by whether cost or aesthetics is critical. with flat-slab you have a flat ceiling finish as compared to exposed beam which may require additional ceiling.
I expect beam-column option could be cheaper.

RE: Flat Slab vs Conventional Beam Slab

My call was based on concern over crashing force due to stress reversal around columns. Stress redistribution is likely for beam-slab (also higher reserve capacity) systems rather than flat slab.

RE: Flat Slab vs Conventional Beam Slab

Beam systems if with moment frames may offer some seismic advatages, lower R. Otherwise flat plate with shearwalls generally will have cheaper forming cost.  

RE: Flat Slab vs Conventional Beam Slab

There are several ACI documents and there are also papers related to flat slab connections to shear walls and columns. Some engineers will use the slab to resist seismic moments....kind of like connecting the shear wall to the exterior frame to load share (semi-dual system). It is up to you to decide wether you want to take the slab or slab/beam stiffness into account for the structural system...or merely use it as mass....different design philossphies that will have impact on the overall response. I would say envelope the cases.

Others have studied PT slabs and the ability to prevent crushing at column and shear wall interfaces.

Like anything else, you can always design and detail the connection.

In Terms of concrete systems, see ASCE 7-05. In California we use Concrete Moment Frames, Concrete Shear Walls, and Dual Systems. System selection depends on height, arch requirements, and what space is available for a seismic system etc... High rises lend themselves to dual systems....but some have designed with shear walls alone (tube structures). Pauley & Priestly have a good discussion on the topic of system selection.

RE: Flat Slab vs Conventional Beam Slab

Would that be a good idea to set different fc' for Columns and other structural members.

We have set fc' for columns at 3750 Psi and for raft, retaining walls, shear walls, beams and slabs at 3000 Psi.

How this helps increasing stiffness at joints?


RE: Flat Slab vs Conventional Beam Slab

It is prone to causing mistakes in design and construction. Except the floor slab, and non-structural walls, try to maintain consistent fc' to simplify the processes.  

RE: Flat Slab vs Conventional Beam Slab

I dislike the use of flat slabs for lateral resistance in high seismic regions because there's no sensible way to provide shear resistance without relying on the concrete shear strength.  That makes it tough to ensure a ductile flexural failure mode.  Shear studs do help with this to some degree.

It's also very difficult to design flat slab / column joints such that they can sustain repeated, inelastic rotations without resulting in bond stress problems.

RE: Flat Slab vs Conventional Beam Slab

How helpful is stiffness at Column - Beam joints?


Engr. Shakeel Ahsan
Director Engineering
CMEC - Pakistan

RE: Flat Slab vs Conventional Beam Slab

Why not to make a simple example for comparison. Say a 20'x20' one story building with 4 columns and 2' overhang each side. One with flat slab, the other with beam in between columns. A day's work, worth a lot.

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