Guide for Shoring/Reshoring of Concrete Multistory Buildings

Description

In multistory cast-in-place concrete building construction, freshly cast floors are placed on formwork that is tempo-rarily supported by a system of shores and re shores 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 shearstrength, to avoid the possibility of partial or total failure of the structural system due to construction overload.

To reduce anddistribute the large construction load on the floor immediatelybelow, to several lower floors, it is important to add reshores onlower levels. Therefore, an engineering analysis that considersboth the construction load distribution and the early-age load-carrying capacity of the concrete slabs should be performedbefore shoring/reshoring operations begin.Formwork failures and failures caused by improperreshoring or premature removal of supports and inadequatelateral bracing, have periodically occurred throughout thehistory of concrete construction.

Premature removal ofshores and reshores can contribute to construction failures ordefects such as permanent excessive deflections (sagging) orcracking in the completed structure. Also, if over-loadedprematurely, time-dependent deflections under load (creep)will be larger, and sagging is more likely to be both noticeableand objectionable.Decisions regarding the removal of forms and relocationof the shores are too often made without the benefit of aproper analysis of the structural effects, or in many cases,without any analysis at all. Still, there is no commonlyaccepted method considered as the proper analysis in theconstruction industry.

To ensure satisfactory performance and structural safetyduring construction, a thorough understanding of constructionloads applied to the slabs at early ages is necessary. Equallyimportant is knowledge of the behavior and the strength ofearly-age concrete members that support their own weightand construction loads.