Using yield line theory for interior panels of slabs

This paper describes some yield line collapse patterns for interior flat slab panels supported on a parallelogram column grid. These patterns were used to analyse six model slabs tested at the Cambridge University Engineering Department as part of ongoing research. Further confirmation of the failure modes was obtained from observed changes in deflection at the final stages of loading, from crack patterns, and from readings from strain gauges stuck on reinforcement bars. For the experimental slabs, the calculations were done using the work equation. In this paper it is demonstrated that by drawing a few yield line patterns on a reinforcement detail drawing, and sketching the corresponding hodographs, ultimate load calculations can be performed on a piece of paper or a spreadsheet. Predictions from such calculations are accurate enough to avoid over-design and save valuable time and effort in assessing the ultimate collapse load for flat slabs supported on a non-rectangular column grid.