Behaviour of RC building with beam supported slab and flat plate slab
In Bangladesh, different types of floor system, such as edge supported slabs (One way, two way), column supported slabs (Flat slab, flat plate slab), ribbed slabs, waffle slabs etc. are commonly employed in reinforced concrete building structures. This study was undertaken to carryout inelastic pushover analysis of buildings with different slab systems such edge supported slabs and column supported slabs using ETABS. The analytical results obtain from buildings having edge supported slabs compared with the same having column supported slabs and also to conduct a parametric study on the lateral strength of buildings with different aspect ratio. Eighteen reinforced concrete buildings of different numbers of stories with two different slab systems 1) Edge supported slab; 2) Column supported slab are designed model building for the study. All the buildings are designed following Bangladesh National Building Code (BNBC-1993). The buildings are designed for gravity loads at first step. The responses of the buildings are verified for lateral loads in the subsequent steps according to the code UBC-97. Inelastic pushover analysis is carried out for all model buildings to know the lateral strength of the building.
The use of inelastic static analysis in earthquake engineering is traced to the work of Gulkan and Sozen (1974) or earlier, where a single degree of freedom system is derived to represent the multi-degree of freedom structure via an equivalent or 'substitute' structure. The load-displacement curve of this substitute to the real structure is evaluated by either finite element analysis or hand calculation to obtain the initial and post-yield stiffness, the yield strength and the ultimate strength. Simplified inelastic analysis procedures for multi-degree of freedom systems have also been proposed by Saiidi and Sozen (1981) and Fajfar and Fischinger (1988).There are several publications that review the advantages and disadvantages of pushover analysis, with varying degree of success. They all, however, utilize global response parameters, namely top displacement versus base shear. Lawson et al. (1994) discuss in some detail the range of applicability, the expected realism for various structural systems and highlight the difficulties encountered. The latter study is both conceptual and applied, rendering it specifically valuable. In the course of describing recent trends in seismic design, Krawinkler (1995) discusses pushover analysis as a prelude to capacity spectrum applications. The author mentions a contentious point, which is that 'in most cases the normalized displacement profile at a first estimate of the target displacement level is utilized for these (defining the shape vector) purpose. Attempts at improving the procedure have been made, with varying degree of rigor and success. The simplest and most pragmatic of which is the work of Sasaki et al. (1998). This comprises running several pushover analyses under forcing vectors representing the various modes deemed to be excited in the dynamic response. The pros and cons of the procedure were also discussed by Krawinkler and Seneviranta (1998). They concluded that not all analysis of the same structure under a set of district earthquake records are predicted by pushover analysis, a rather obvious conclusion that did not require inelastic dynamic analysis to prove.
In order to conduct a parametric study of the lateral resistance of eight, ten and twelve storied buildings having two different types of slabs:
Flat plate slab
Beam slab
Three model buildings are adopted in this study. The buildings have three length/ breath ratios: 1, 1.5, 2.0 all the buildings are designed using Bangladesh National Building code (BNBC-1993).It is worthy to mention that the buildings are designed for gravity loads at first step, and subsequently verified for lateral loads using UBC'97.The materials, loading and the designed sections of different basic components are described in the following sections.
Pushover analysis is a static, nonlinear procedure using simplified nonlinear technique to estimate seismic structural deformations. It is an incremental static analysis used to determine the force-displacement relationship, or the capacity curve, for a structure or structural element. The analysis involves applying horizontal loads, in a prescribed pattern, to the structure incrementally, i.e. pushing the structure and plotting the total applied shear force and associated lateral displacement at each increment, until the structure or collapse condition.
By pushover analysis variation of displacement with base reaction for different slab system and different parameter of buildings are given below:
Graphical representations for different types of slab system where base reaction is varied for different story, i.e. Eight, Ten & Twelve
Graphical representations for different types of slab system where base reaction is varied for different L/B Ratio, i.e.1, 1.5, 2.0
Eight Storied Building
Fig.3. Length width ratio VS Base reaction curve for flat plate slab and beam supported sla
Ten Storied Building
Fig.4. Length width ratio VS Base reaction curve for flat plate slab and beam supported slab
Twelve Storied Building
Fig.5. Length width ratio VS Base reaction curve for flat plate slab and beam supported slab
Lateral strength of eighteen reinforced concrete buildings with two different slab systems (1) beam supported slab; (2) flat plate slab
Base reaction is increased when L/B is increased.
When L/B is 1, 1.5, 2.0 then lateral load resistance capacities at beam supported slab is higher than the flat plate slab.
When the storey height is increase the base reaction is decrease.
For plan size 18.3mx18.3m, beam supported slab has 28.8% (For 8-storied), 24.8 % (For 10 storied) and 23.1% (For 12- storied) higher strength than that of the buildings with flat plate slabs.
For plan size 18.3mx27.45m, beam supported slab has 30.64% (For 8- storied), 27.48% ( For 10- storied) and 24.4% ( For 12- storied) higher strength than that of the buildings with flat plate slabs.
For plan size 18.3mx36.6m, beam supported slab has 32.56% (For 8- storied), 30.30% ( For 10- storied) and 25.7% ( For 12- storied) higher strength than that of the buildings with flat plate slabs
An analytical study on the lateral strength several storied buildings with two different slab systems (1) beam supported slab; (2) flat plate slab is carried out.
The buildings are designed for gravity loads at first steps and subsequently verified for lateral loads. The conditions drawn in the study are as follows:
It is found that the buildings with beam supported slabs have higher lateral strength than with flat plate slab.
It is found that the lateral strength is decreased when the storey height is increased in both cases of two different slab systems.
Differences in lateral strengths are obtained for different length width ratio of the buildings.
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Engr. Md. Arifujjaman
Research Engineer
Structural Engineering and Construction
Housing and Building Research Institute