If you want to start a lively discussion among new engineers or contractors, ask them about the differences between shoring, reshoring, and backshoring. They all sound similar, but in reality, perform different jobs at different stages of the construction process. More importantly, the number of shores, reshores, and backshores are project-dependent. They also vary wildly according to climate and speed of construction. So, what are each of these things?
Understanding the Different Types of Shores
According to Peter Courtois, a former chairman of the American Concrete Institute's Formwork for Concrete Committee, there are a few key differences between the different types of shoring. He defines shoring as a system of supports for forms that are vertical or otherwise inclined. These shores may be wood, metal, frames that look like scaffolds, or other patented special materials. Shores are installed prior to pouring the concrete slab.
Reshoring comes into play after the concrete slab has been stripped from its formwork. Reshores are placed beneath the slab or another structural element. Reshores make the slab deflect and support both its own weight and the weight of previously constructed loads. For reshores to function correctly, they should be designed by a qualified engineer.
Backshores are the opposite of reshores: they are installed after a slab is stripped of its formwork but do not allow the slab to deflect. This means the slab or other structural member does not yet support its own weight or previously constructed elements. Backshoring is a delicate process that should be used in conjunction with shoring and reshoring, with input from an experienced structural engineer.
The Difficulty with Installing Reshores
Reshoring is a more delicate process than either initial shoring or using backshores. When constructing reshores, it is important to remember that the shores used should only be tight enough to be held in place. They cannot remove the normal deflection from the slab or whatever structural member is being reshored. Additionally, reshoring on a slab cannot be started until any adjacent beams have also been reshored.
If that was not enough hassle, reshores must be placed in the exact same location on each slab they are used on. Lastly, reshores cannot be placed in any location that will cause tensile stress to the slab when there is no other reinforcement. Bear in mind that these details are not located on typical construction documents. This is one reason the locations of reshores should be approved by an engineer or architect.
Just as installing reshores can be a difficult process, so is removing them. Concrete gains strength over time, so depending on the quality of concrete placed and weather conditions, you may not be able to install reshores relatively quickly. You should make sure the concrete has cured to an appropriate strength so that it can support itself and the loads of weight that will be applied to it once the reshores are put into place.
The Importance of Reshoring Engineers
There are many complex calculations that go into figuring out where reshores should be placed. Concrete strength, load from other levels, tensile strength, and other elements must be considered before reshoring operations should begin. An experienced reshoring engineer will take these into consideration, and develop a plan that minimizes the amount of equipment necessary to support the structure. These calculations should always be run by a qualified, experienced construction engineer for the safety of all workers.