Repairing and upgrading of a heavy-duty R/C slab

A heavy-duty reinforced-concrete square slab is studied in this paper based on two tests performed under the service loads, after adding a stiffening R/C beam along the only free edge. The very linear behavior exhibited during the tests under a fairly concentrated load applied in two different locations was the starting point: (a) to perform a FE analysis under the test loads to investigate the behavior of the fairly complex restraints; (b) to identify the most probable failure mechanisms within the kinematic theorem of Limit Analysis (Yield-Line Method) at the Ultimate Limit State - ULS; (c) to quantify the extra bearing capacity provided by adding the stiffening beam; and (d) to study slab behavior in fire. Slab in-plan size and thickness were 8.20x7.64 m [26.9x24.9 ft] and 35 cm [13.8 in.], respectively (side/thickness = 22-23). Two adjoining sides were supported by as many walls underneath, while the third side was partially supported and the fourth side - initially free - was later stiffened by casting an R/C beam along the extrados of the slab. The fairly complex restraints show that (a) not always the yield-line patterns and the failure mechanisms dictated by intuition are the most probable; (b) simple supports placed underneath free edges may lead to more probable mechanisms than placing stiffening beams; (c) adding a stiffening beam along a free edge may not be required at the serviceability or ultimate limit states, but by the checks in fire; and (d) adding a stiffening beam tends to shift the failure mechanism towards more heat-sensitive slab-localized mechanisms, because of the greater heat sensitivity of fairly thin slabs and of the lower heat sensitivity of solid beams.