[Univ of Cambridge] [Dept of Engineering]

Flexural behaviour of reinforced concrete flat slabs supported on non-rectangular column grid

K Baskaran


With increasing population and limited land resources, optimum use of available space in buildings is becoming important. Furthermore, architects demand to hide columns inside walls. This leads to flat slabs supported on non-rectangular column grid. Despite the long history of flat slab construction, no design or assessment method for flat slabs supported on non-rectangular column grid is available yet with sound experimental validation. To fill this vacuum, flexural behaviour of such slabs is discussed in this dissertation.

To select a design method, available design methods for flexural reinforcement are reviewed. Comparisons are made between Hillerborg’s strip method and Saether’s structural membrane approach. In both methods twisting moment is assumed zero. The validity of this assumption, and implications of any violation are checked by extending Gurley’s bimoment concepts. Furthermore, proposals are made to extend the application of Saether’s structural membrane approach to edge and corner columns.

Using Saether’s method nine flat slab panels were designed and tested in the laboratory. Instead of testing multipanel slabs, single panel specimens with edge shear applied in a proper manner to simulate the adjacent panels were considered.  To apply uniform load, a novel vacuum rig was constructed. Column layouts in the specimens were varied from parallelogram, triangle to a completely irregular layout. Further in the parallelogram column layout, direction of the reinforcement also varied exploiting Wood-Armer equations. A corner panel of a skew column grid, designed according to the proposed extension to Saether’s method, was also tested. Based on the experimental results comparisons are made with the design values to suggest improvements to the design procedure.

Punching shear failure is a common failure mode in flat slabs. Design codes don’t allow provision of shear reinforcement in the form of links or bent up bars in thin slabs. Also the method adopted to avoid punching shear failure must not interfere with flexural capacity; especially in an experiment performed with the purpose of validating a flexural design method. In the present work, spirals were introduced as punching shear resistors.

The experiments pave the way to propose some improvements to the design method and provide some valuable data to validate analytical methods. Based on the experimental observations a series of yield line patterns for flat slabs supported on parallelogram layout were identified and used to assess the experimental slabs. Further confirmation of the failure modes was obtained from observed change in deflection at the final stages of loading, crack patterns, and readings from strain gauges stuck on reinforcement bars. Two flat slabs supported on completely irregular column layout were also assessed using yield line analysis.

In flat slabs controlling deflections at service load is also a key issue in the design.   Using a commercially available finite element package (DIANA), experimental slabs were analysed and predictions were compared with experimental observations. Agreement between predicted failure loads by analytical methods (yield line analysis and finite element analysis) and experimental observations is satisfactory.


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