Construction loads on shores and stability of horizontal formwork

A lot of formwork accidents and collapses occur when the concrete is poured, and one of the causes is overloading of the shores. One common type of formwork is the one here referred to as the traditional one, in which the load due to the newly poured concrete is transmitted to the foundations via the soffit formwork and a falsework structure consisting of joists, stringers and shores. This thesis discusses the load on shores and the stability of such a structure. The sizes of the loads used in designing horizontal formwork vary both internationally and nationally, even between different sites of the same company for similar structures. The object of this thesis is to study the sizes of shore loads in conjunction concreting in a number of different structures. Shore loads were measured when bridges and residential buildings with slabs of varying thicknesses were concreted. A method of calculating the design shore loads is presented. The method produces shore loads in better agreement with the real loads, irrespective of the thickness of the concrete. Existing design methods with variable loads yield too low shore loads for structures with large slab thicknesses. The object of this thesis was also to study the stability of Horizontal Formworks. Thirty full scale models of shore-stringer constructions of timber were loaded to failure. The full scale models tested in many cases failed before the load reached that permitted in the codes of most countries. This clearly shows that stringers on shores cannot be designed safety using the permissible values specified in codes. The tests showed that stringers supported on shores, without any restraint against overturning of the stringer, will buckle in a knee when loaded to failure. This phenomenon is referred to here as knee buckling. Cover plates of the type which are commonly used do not prevent buckling of the shore-stringer construction. Some tests on cover plates on one side only showed that the resistance was increased only by about 20%. The ultimate load for these constructions is considerably lower than had been assumed. In some cases the resistance was only one third of the critical load for the shore.