This paper reviews the relationship between yield line theory, developed for reinforced concrete slabs by Johansen in the 1930s and 1940s, and limit analysis, formulated by the Hodge–Hill–Prager school around 1950. Recalling work from the 1970s, it is shown that not only is yield line theory fully compatible with limit analysis, but the Johansen ‘stepped’ yield criterion is in fact the only yield condition for the slab that allows coinciding upper and lower bounds in the sense of limit analysis to be derived by yield line theory. The yield condition for an arbitrarily reinforced slab is reproduced in closed form, obviating the need for coordinate transformations, and allowing the permissibility of stress states to be checked for anisotropic slabs with non-coinciding principal directions of top and bottom reinforcement.

Research in concrete technology has served the interests of durability, competitiveness and sustainability of concrete construction through development of mix design. Work in this field is mainly aimed at making efficient use of aggregate and developing binders with optimum cement content and industrial by-products as binder components, for example, pulverised fuel ash and ground granulated blast furnace slag. Compared with the traditional Portland cement concrete, concrete elements made with such mixes are expected to have enhanced durability that is attributable to improvement in their microstructure, reduction in voids and increase in resistance to ingress of moisture, gases, etc. This paper examines whether such attributes of different concretes could also improve their engineering properties, e.g. increase in the ratio of flexural tensile strength to compressive strength, better bond between steel and concrete and, in the end, enhanced shear capacity. It is also proposed to examine whether flexural tensile strength should be used as a parameter in the design rules to obtain realistic estimates of shear capacity, in place of the compressive strength. This approach could promote the use of modern mix design techniques and it may have some positive influence on economy of concrete construction, particularly flat slabs, where depth of a concrete member is often governed by contribution of concrete to the shear capacity.

The influence of elastic deformations on compressive membrane action (dome effect) in restrained slabs is considered. The elastic-plastic circular slab is analyzed, using a flow-theory approach. The inplane and boundary flexibilities are lumped and an initial elastic deflection is assumed. The first part of the predicted load-deflection curve is compared with the results of a test series and reasonable agreement is found, especially with respect to the peak load.

SYNOPSIS Using the theory of plasticity, coincident upper and lower bounds for the ultimate shear load are determined. The materials are assumed to be rigid, perfectly plastic, the tensile strength of the concrete being neglected. The unrealistic assumption of unlimited concrete ductility is amended by introducing an empirical effectiveness factor on the compressive strength. The solution, which gives the shear strength as a function of the shear-span ratio and the longitudinal reinforcement strength, is compared with the results of a test series, and excellent agreement is found. The dependence of the effectiveness ratio upon various factors, principally the concrete strength, is discussed by comparing it with a number of shear tests reported in the literature.