WorkSafe Bulletin

Backshoring vs. reshoring in drop head slabformwork systems — Overview

When working with drop head slab formwork systems, it’s important not to confuse backshoring and reshoring. Doing so could create life-threatening hazards for workers. This bulletin gives a brief overview of drop head slab formwork systems and highlights the key differences between backshoring and reshoring. It also describes the hazards of mixing up backshoring and reshoring, and outlines how to prevent overloading of reshoring systems.

How drop head slab formwork systems work Drop head slab formwork systems are becoming increasingly prevalent on construction projects across British Columbia. The primary reason: they’re fast to install and remove.

Here’s how drop head systems differ from more traditional slab formwork. In drop head systems, the formwork beams bear on special ledges on the sides

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4a

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of the post shores. In a traditional arrangement, these beams sit on top of the post shores. In addition, the drop heads have a feature that enables the special ledges to be lowered by about 100 mm (4 in.) once the concrete slab has developed sufficient strength to span between the drop heads. Once the ledges have been lowered, the formwork beams and panels can be removed (or stripped) while the post shores continue to support the slab.

Drop head system before (left) and after (right) ledges are dropped

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Concrete slab

Drop head

Formwork panel

Special ledge in the:

“up” position 4a

4b “down” position

Formwork beam

Post shore

Approximately 100 mm gap due to special ledge being dropped

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4b

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WS 2017-11 Rev. Sep. 2019

WorkSafeBC Prevention Information Line: 604.276.3100 or toll-free 1.888.621.SAFE (7233)

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Backshoring Concrete slab is prevented from deflecting (bending slightly) under its own weight by the post shore; slab weight does not transfer to structural columns and walls.

Reshoring Concrete slab is allowed to deflect under its own weight (the deflection above has been exaggerated for image clarity); slab weight transfers to structural columns and walls.

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3a

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Hazards of mixing up backshoring and reshoring Backshoring and reshoring look very similar, but the forces in backshoring systems are typically much greater than the forces in reshoring systems. If post shores designed as reshoring are inadvertently installed in the field as backshoring, the following could occur:

• Buckling or failure of post shores

• Structural failure of cured concrete slabs

• Failure of slab formwork or falsework

• Serious worker injuries or fatalities

With drop head slab formwork systems, you must take special care to ensure that post shores designed as reshoring are actually installed as reshoring.

How to prevent overloading of reshoring systems Responsibilities of formwork contractors:

• Ensure that you follow the instructions provided by the formwork engineer for stripping of slab formwork and installation/modification of post shores for reshoring.

• Ensure that workers and supervisors understand the differences between backshoring and

Backshoring vs. reshoring

1 Top of post shore.

In backshoring, top of post shore is not lowered after formwork is removed; weight of slab is carried by post shore and slabs below.

1a

1b In reshoring, top of post shore is lowered after formwork is removed; weight of slab is not carried by post shore or slabs below

Recently stripped concrete slab.

Post shore supporting the formwork

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3 is left in place to support the slab as:

Backshoring 3a

3b Reshoring

4 Top of slab before formwork is removed.

reshoring, and the hazards associated with mixing up these systems.

Responsibilities of prime contractors:

• Confirm that the formwork engineer has provided clear instructions for stripping of slab formwork and installation/modification of post shores for reshoring.

• Confirm that the formwork contractor understands the differences between backshoring and reshoring, and the hazards associated with mixing up these systems.

Responsibilities of formwork engineers:

• Provide clear instructions (typically on the reshoring drawings) on the specific sequence that must be followed during stripping of slab formwork and installation/modification of post shores, to ensure that the post shores act as reshoring rather than backshoring.

• Verify that the above instructions have been followed as part of your inspection of the reshoring [per section 20.26(1)(b)(ii) of the Occupational Health and Safety Regulation].

For more information For more information and OHS Regulation requirements, see the companion publication, Backshoring vs. reshoring in drop head slab formwork systems — Detailed background.

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