Membrane action in thickness design for ground slabs

The existing methods of ground slab analysis either provide designs which are too conservative or have other inadequacies. The two main inadequacies are that frequently they do not show good agreement with test results and they do not provide a rational basis for determining the radius of circumferential cracking around a localised load. It is suggested that the reason for these shortcomings is the absence of membrane action consideration in those methods. Membrane action needs restraint from surroundings which small slabs do not provide and therefore has not warranted much attention. It is only now worthwhile considering membrane action owing to the growing use of large area, jointfree, slab construction processes. Recent research is described in this paper showing the effect of the stiffness and the strength of the slab areas surrounding a mechanism resulting from a local overload on a ground slab. Research which was intended to identify rationally the value of the self-selecting radius of a rigid-plastic yielding mechanism revealed that a migrating system operates with a moving hinge shifting the mechanism radius during failure. The result, which needs further experimental work and adaptation for design use, promises to provide more economic designs through its consideration of membrane action and also a more faithful analysis of the mechanics.

John Eyre, BA, BSc, PhD, RIBA
Department of Civil & Environmental Engineering, UCL