Backshoring vs. reshoring in drop-head slab formwork systems

Two new resources outline potential hazards and how to prevent them.
WorkSafeBC
November 21, 2017
By WorkSafeBC
November 21, 2017 — Backshoring and reshoring systems both work to support the weight of newly poured concrete slabs and look extremely similar, but using the wrong one can lead to structural collapse.

WorkSafeBC has recently published two safety bulletins: an overview that describes how drop-head slab formwork systems work, the hazards of mixing them up, and how to prevent overloading reshoring systems; and a detailed backgrounder that outlines the key similarities and differences between the systems, provides a list of questions to ask contractors, and details the requirements under the Occupational Health and Safety Regulation.

In backshoring systems, temporary post shores are installed snugly below a recently stripped concrete slab in a way that prevents the slab from bending (deflecting) under its own weight. The weight of the slab is therefore transferred down through the post shores rather than to the structural columns and walls of the building. In drop-head slab formwork systems, the drop-head post shores are typically retained as backshoring posts.

In reshoring systems, temporary post shores are installed below a recently stripped concrete slab after the slab has deflected under its own weight. As a result, the weight of the slab is transferred to the structural columns and walls of the building. In some cases, the drop-head post shores are sometimes retained as reshoring; in other cases, the drop-head post shores are replaced with different reshoring posts.

The forces resisted by post shores and cured slabs are typically much greater for backshoring than for reshoring, because the weight is not transferred to columns and walls. If post shores and cured slabs are not designed for higher loads, they may fail, causing the slab formwork and falsework at the top of the structure to fail. Any of these failures could result in serious worker injuries and/or fatalities.

For more information, please visit worksafebc.com.

WorkSafeBC is an independent provincial statutory agency governed by a Board of Directors appointed by the provincial government. The organization serves approximately 2.3 million workers and 231,000 employers throughout British Columbia. In administering the Workers’ Compensation Act, the organization is accountable to the public through the provincial government.    

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