1) Thickness of flat slab:- The thickness of flat slab shall be generally controlled by considerations of span to effective depth ratio.
2) Drops:- The drops when provided shall be rectangular in plan, and have a length in each direction not less than one third of the panel length in each direction. For exterior panels, with drops at right angles to the non continuous edge and measured from centre line of the columns shall be equal to one half the width of drop for interior panels.
3) Column heads:- When column heads are provided, that portion of column head which lies within the largest circular cone or pyramid that has vertex, angle of 90 and can be included entirely within the outlines of the column and the column head, shall be considered for design purposes.
Failure of RCC slab during severe earthquake has lead to wide spread rejection of flat slab. Many of the existing flat slab buildings may not have designed for seismic forces. Hence it is important to study their response under seismic conditions and to evaluate seismic retrofit schemes. Inorder to study the response of flat slab under seismic conditions and to evaluate seismic retrofit schemes push over analysis is performed and by conducting push over analysis, we can know the weak zones in the structure and then we will decide whether particular part is retrofitted or rehabilitated according to the requirement. The retrofitting can be done by
1) Column jacketing
2) Addition of beams at floor
3) Column jacketing and addition of beams
Column jacketing and is a good cost effective technique but it is adequate only when seismic deficiency is small.
The beam retrofitting reduces the sagging hinges significantly. Increasing the number of storey of retrofitting by either column retrofitting alone or beam retrofitting alone doesnot improve the behavior significantly. When column jacketing and addition of beams are adopted simultaneously one or more number of stories, large increase in lateral strength and stiffness can be achieved. The pushover analysis of a structure is a static non-linear analysis under permanent vertical loads and gradually increasing lateral loads. The equivalent static lateral loads approximately represent earthquake induced forces. A plot of the total base shear versus top displacement in a structure is obtained by this analysis that would indicate any premature failure or weakness. The analysis is carried out upto failure, thus it enables determination of collapse load and ductility capacity. This type of analysis enables weakness in the structure to be identified. The decision to retrofit can be taken in such studies.
Conclusion
Under the pressure of recent developments, seismic codes have begun to explicitly require the identification of sources of inelasticity in structural response, together with the quantification of their energy absorption capacity. Many existing buildings do not have been designed for seismic forces. It is important to study their response under seismic conditions and to evaluate seismic retrofit schemes. Hence push over analysis is gained importance for the strengthening and evaluation of the existing structures. By conducting the pushover analysis on flat slabs, we need to decide whether to perform rehabilitation or retrofitting depending upon the seismic zone of the existing structures and came to conclusion such as
a) Flat-slab systems with columns only can be applied under conditions buildings with a small number of storeys.
b) Flat-slab structural systems with perimetric beams supported only by columns, in that case big cross sections for the columns is needed.
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