ACI 347.2R

Abstract

<strong>INTRODUCTION</strong> <br>In multistory cast-in-place concrete building construction, freshly cast floors are placed on formwork that is temporarily supported by a system of shores and reshores until the concrete has the ability to be self-supporting. Construction loads, imposed by the shoring system on the slabs below, may be significantly larger than the design loads of those floors. Furthermore, the concrete of slabs below may not have attained sufficient strength before the construction loads are applied. As a result, it is critical to determine the early-age load strength of the floor slabs, including punching shear strength, to avoid the possibility of partial or total failure of the structural system due to construction overload. To reduce and distribute the large construction load on the floor immediately below, to several lower floors, it is important to add reshores on lower levels. Therefore, an engineering analysis that considers both the construction load distribution and the early-age loadcarrying capacity of the concrete slabs should be performed before shoring/reshoring operations begin. <br>Formwork failures and failures caused by improper reshoring or premature removal of supports and inadequate lateral bracing, have periodically occurred throughout the history of concrete construction. Premature removal of shores and reshores can contribute to construction failures or defects such as permanent excessive deflections (sagging) or cracking in the completed structure. Also, if over-loaded prematurely, time-dependent deflections under load (creep) will be larger, and sagging is more likely to be both noticeable and objectionable.