In the conventional design of reinforced concrete (RC) rectangular slabs in beam-slab systems subject to gravity loading, it is assumed that the code-specified moment coefficients (derived based on yield line theory, assuming non-deflecting supports at the edges) can be used, provided the beams provided at the edges are adequately stiff. Recent experimental and theoretical studies have established that such designs turn out to be over-conservative, as the assumed yield line mechanism of the slab does not occur. In general, a combined beam-slab collapse mechanism occurs at the limit state of collapse, in which the yield lines in the slab connect to plastic hinges in the supporting beams. With a proper understanding of possible collapse mechanisms and estimation of the lowest collapse load, using yield line theory, a more rational and economical design of the beam-slab system is possible. Considerable savings in steel can be achieved, while fully complying with the strength and serviceability requirements of the code. © 2020, Associated Cement Companies Ltd. All rights reserved.