Modelling compressive membrane action and geometrical nonlinearity in one way concrete slabs

Abstract

Increased traffic loads and ageing of concrete bridges and overpasses in the Netherlands make it necessary to reassess these existing structures. Consequently, the current condition and capacity of many concrete structures need to be evaluated. Residual capacity could be discovered during reassessments of concrete slabs due to a phenomenon called compressive membrane action (CMA). CMA is the formation of internal compressive arches caused by the lateral restraint. As a result, the load is not only transferred by bending action but also by arching action. Research has shown that the ultimate capacity can be significantly increased due to the occurrence of CMA. The goal of this study is to examine the influence of geometrical nonlinearity on this increase in capacity. Also, accurate quantification of the capacity enhancement for a variety of concrete slab variants can be scientifically useful and increases the knowledge on CMA. The study is confined to one way reinforced and restrained concrete slabs.

A new analytical model is presented to quantify the capacity enhancement due to CMA and the geometrical nonlinear (GNL) effect on the capacity. Calibration of the analytical model is performed with a finite element model in DIANA FEA. Also, the finite element model validates the analytical results and is used to study the failure mode of a restrained concrete slab in detail.

The enhancement factor – defined as the enhanced capacity divided by the conventional capacity – turned out to be varying between 1.35 and 4.7 for a large variety of restrained concrete slabs. Thus, the ultimate capacity of restrained one way slabs is significantly increased due to CMA. However, the capacity enhancement would have been even greater if geometrical nonlinearity was not accounted for. Geometrical nonlinearity reduces the increase in capacity because the formed compressive arches will tilt as a result of deflections, therefore leading to a relative decrease of the resisting arching moments. This GNL reduction effect varies between 3% and 37% according to the finite element model. The calibrated analytical model sufficiently estimates this effect with a maximum deviation of about 12%. An important finding was that the enhancement factor is larger for deep slabs than for slender slabs, while the reduction of the ultimate load due to geometrical nonlinearity is larger for slender slabs than for deep slabs.