Simplified and Advanced Analysis of Membrane Action of Concrete Slabs

The previous tests,4,6-8,17 however, have been limited both in number and, more importantly, in that they did not consider the use of welded mesh reinforcement or the possibility of different modes of failure for heavily reinforced slabs. The use of welded mesh reinforcement is considered important because the method developed by Bailey15,16 is used for the practical fire design of concrete-steel composite floors14 that adopts a mesh reinforcement. Therefore, this paper focuses on the membrane behavior of concrete slabs reinforced with welded mesh reinforcement. As well as comparing the new tests against the simple design method,15,16 a finite-element model18 together with the authors' purpose written subroutines were used to model the tests and predict the in-plane stress. This allowed the assumptions within the simple approach to be assessed in detail.

RESEARCH SIGNIFICANCE

Understanding membrane behavior of concrete slabs at large displacements is important when assessing structures under accidental loads. A simple design method, previously developed, uses the membrane behavior of horizontally unrestrained concrete slabs. The method was developed based on limited experimental results carried out previously by other authors and an analytical assessment of unrestrained concrete slabs at ambient temperature. The results of 14 new tests are presented, which as well as increasing the limited experimental data currently available, were designed to investigate the effects of using welded mesh reinforcement and the possibility of different modes of failure as the area, or strength, of reinforcement increases. The test results, together with predictions from advanced FEM, are compared with the simple approach to assess the accuracy of the adopted assumptions relating to stress patterns, which are used to predict the load-carrying capacity.

EXPERIMENTAL TESTS

Fourteen tests were conducted on small-scale reinforced concrete slabs at the University of Manchester.19 Seven of the tests were carried out on slabs of size 5.906 x 3.937 ft (1.8 x 1.2 m) with the other seven tests being carried on slabs of size 3.937 x 3.937 ft (1.2 x 1.2 m). Slabs M1 to M12 had a target thickness of 0.787 in. (20 mm), with Slabs M13 and M14 having a target thickness of 1.417 in. (36 mm). The actual measured thickness of the slabs is given in Table 1, with details and a photograph of the test rig shown in Fig. 2 and 3, respectively. The slabs were supported vertically around their edges by sitting on steel channels 1.969 in. (50 mm) wide. The corners of the slab were lightly clamped, with rollers between the clamp and slab, allowing free horizontal movement but restraining vertical upward movement at the corner (Fig. 4). There was no horizontal restraint provided to the slab's perimeter, although no measures were taken to reduce the friction between the slab and supporting steel channel. It was observed from the tests that the slabs were actually supported on the inside edge of the channel due to the large vertical displacements experienced (Fig. 5). This resulted in a clear span of 5.578 x 3.609 ft (1.7 x 1.1 m) and 3.609 x 3.609 ft (1.1 x 1.1 m) for the slabs (Table 1).