Radial Cracking in Reinforced Concrete Flat Plate Slabs

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Reinforced concrete flat plate slabs and slabs with drop panels often exhibit radial cracking in the vicinity of column supports under normal service/construction loading. This behavior has been observed in slabs in which design and/or construction errors have been identified, and in properly designed and constructed slabs. As such, the occurrence of radial cracking is not itself indicative of either design of construction errors, much less unanticipated performance. Negative flexural stresses are typically responsible for a concentration of cracking in the immediate vicinity of the columns, which commonly manifests in a star-burst pattern of radial cracks, interspersed with orthogonal or circumferential cracking. Such cracking can be found in reinforced concrete flat plate slabs in buildings that have been in service for decades, as well as in new buildings shortly after removal of shoring. While this cracking may be of potential concern to building owners, building officials, and design professionals as it may signify high flexural stresses and may result in undesirable exposure of the negative flexural reinforcing to moisture and chlorides, it is normally not indicative of a serious structural problem, and can be readily shown by analysis to occur under normal service conditions. Despite this, radial slab cracking appears to be mistaken as indicative of design or construction errors, structural overload, or punching shear failure nearly as often as it is recognized as normal behavior. Multiple case-study structures in which radial slab cracking has occurred are presented to illustrate typical in-field radial slab cracking conditions, and nonlinear finite element analyses are provided that illustrate the propensity of a variety of slab configurations to develop radial cracking when subjected only to self weight. As the case studies and analyses show, radial slab cracking is not damage at all, but rather is a predictable and expected occurrence that should be anticipated in code-compliant flat plates under service dead load, without any design errors, construction errors, structural overload, or punching shear failures. The purpose of this paper is to provide real-life examples documenting cracking in concrete floor structures, to demonstrate via analysis why the cracking occurred, to identify factors influencing the appearance of the cracks, and to quantify the loading intensities required to form a well-developed system of cracks.