Question: We have heard the expressions reshoring and backshoring used quite often. What's the difference between the two methods?
Answer: Peter Courtois, former chairman of American Concrete Institute Committee 347, Formwork for Concrete, gave us the following response. We'll preface the discussion by first using the definitions from an unpublished report of the American Concrete Institute's formwork committee. Shoring: System of vertical or inclined supports for forms; may be wood or metal posts, scaffold-type frames, or various patented members. Reshoring: Shores placed snugly under a stripped concrete slab or structural member after original formwork and shores have been removed from a large area, thus requiring the new slab or member to deflect and support its own weight and existing construction loads prior to the installation of reshores. Backshoring: Shores placed snugly under a stripped concrete slab or structural member after the original formwork and shores have been removed from a small area without allowing the slab to deflect or support its own weight or existing construction loads from above.
The original shores must be installed plumb and wedged securely so that each carries its share of load. When the original shores and formwork are removed, the slab or beam is required to support its own weight and any other loads that may have been applied. Reshores are provided to distribute any further applied loads among several slabs that are stronger (older), thereby helping to maximize the reuse of the formwork.When placing reshores, one must be sure not to remove the normal deflection of the floor above. The reshore is simply a strut and should be wedged only firmly enough to hold it in place. Do not set wedges with a sledgehammer, as this would remove the normal deflection from the slab above. Reshores should be located in the same position on each floor so that they will be continuous from floor to floor. Reshores must not be placed where they could cause tensile stresses where there is no reinforcement. Doing so may cause serious cracking of the concrete. Slabs should not be reshored until adjacent beams, if any, have been reshored. Any slab with a clear span of more than 10 feet may require reshoring. Proper location of reshores should be designed in advance with the approval of the engineer/architect. For flat slabs, reshores should be placed along the intersection line of the column strip and middle strip in both directions. This should be done for each bay as it is stripped before removing forms from adjacent bays.
Reshores should not be removed until the slab or beam has sufficient strength to support all loads that will be applied to it.With backshoring, so long as the first level of shores remains in place in contact with grade, each tier of shores must carry the weight of all concrete and construction loads above it. This may be the weight of several floors. When reshoring remains in place at grade level, accumulated shore loads are less because each slab has been permitted to carry its own weight before the reshores were installed.When using reshores, stripping of the formwork is more economical, since all formwork material can be removed at the same time. Reshoring generally requires fewer levels, thus freeing more area for other trades.When using backshoring, stripping of the formwork can be done at an earlier age because large areas of concrete are not required to carry their own weight.In North America, a majority of supported slabs use reshores rather than backshores.Interested readers are referred to Jacob Grossman's article, "Two-day Construction Cycle for High-rise Structures Based on Use of Preshores," in the March 1986 issue of Concrete Construction. Preshoring, a variation of the backshoring method, is a system that has been used successfully in New York City for many years.
Summary of characteristics
- Strip small areas
- Strip entire bay
- Do not let slab deflect
- Allow slab to deflect
- Install backshores before any further stripping occurs
- Install reshores without removing deflection
- Slab does not carry its own weight
- Slab carries its own weight.
- Backshores have an initial load.
- Reshores have no initial load.