The forces in backprops during construction of flat slab structures

Extensive measurements were taken of the forces in the props supporting the formwork and in the backprops below the most recently cast floors during construction of a seven-storey flat slab structure at the Building Research Establishment, Cardington as part of the European Concrete Building Project. Comparison with backprop forces calculated by conventional means using the commonly adopted assumptions for backprop behaviour shows that backprop forces are significantly overestimated. This implies that the uppermost floor slab which supports the formwork and falsework for casting the next slab may be considerably more heavily loaded in practice than is normally assumed. A theoretical study showed that the errors in the calculations were due to ignoring the finite stiffness of the backprops and to ignoring the layout of the backprops relative to the layout of the props supporting the formwork. Improved calculation methods are proposed which make allowance for these factors.