Concrete shoring is a system of temporary vertical supports that hold up freshly poured concrete until it cures and gains enough strength to support itself. When a concrete slab or beam is poured, it starts as a heavy liquid that can’t carry its own weight. Shoring props, towers, or frames bear that load and transfer it safely to the ground or to finished floors below, staying in place for days or weeks until the concrete hardens.
How Shoring Works in Practice
The basic idea is straightforward: you build formwork (the molds that shape the concrete), then install vertical supports underneath to hold everything up during and after the pour. Those supports are the shoring system. They carry the combined weight of the formwork itself, the wet concrete, construction workers, and any equipment on the work surface.
In multistory buildings, the process gets more complex. When you pour a new floor, the shores beneath it transfer that load down to the floor below, which may itself be recently poured and not yet at full strength. A single floor often can’t handle the full construction load on its own. So shoring typically extends across multiple levels, spreading the weight across several floors at once. A five-story building under construction might have shoring in place on three or four levels simultaneously.
Types of Shoring Systems
Shoring equipment ranges from simple adjustable posts to engineered tower systems, and the right choice depends on the load, the height, and the complexity of the structure being built.
- Post shores are the most common type: individual vertical poles, usually made of steel or aluminum, with adjustable screws at the top and bottom to fine-tune the height. They work well for standard slab construction where loads are moderate.
- Frame shoring uses prefabricated steel frames that lock together, similar to scaffolding. These systems handle heavier loads and cover larger areas more efficiently than individual posts.
- Heavy-duty shoring towers are modular systems built from interlocking steel components. They’re designed for civil engineering projects like bridges, tunnel vaults, and pier caps where loads are significantly higher than in typical building construction.
Steel vs. Aluminum Shoring
Steel and aluminum are the two main materials for shoring equipment, and each has clear trade-offs. Steel is roughly twice as heavy as aluminum, which makes it harder to move around a job site but gives it significantly more load capacity. Steel’s tensile strength is about 350 MPa compared to aluminum’s 100 MPa, so steel shores can handle deeper, heavier pours and are the standard choice for large-scale projects.
Aluminum’s advantage is portability. Lightweight aluminum shores can be moved by hand or with small equipment, which speeds up setup and reduces labor costs on smaller jobs. Aluminum also resists corrosion naturally, while steel requires protective coatings to prevent rust. On the other hand, aluminum is softer and more prone to dents and surface damage, so it wears out faster under rough job site conditions.
Shoring vs. Reshoring
These two terms come up together constantly in construction, and the distinction matters. Shoring is the initial support system installed before and during the concrete pour. It stays in place while the concrete cures.
Reshoring happens after the original shoring and formwork are removed. Once a slab has cured enough to hold its own weight and has deflected (sagged slightly) under that weight, temporary post shores are installed snugly back underneath it. Reshoring doesn’t carry the slab’s dead weight the way original shoring does. Instead, it connects multiple floor levels together so they share any new construction loads applied above. Think of it as a safety net: the slab is carrying itself, but the reshores ensure it has backup support from the floors below while upper levels are still being built.
When Shoring Comes Down
Shoring cannot be removed until the concrete has gained enough compressive strength to support both its own weight and any loads placed on top of it. Federal regulations require the employer to verify this strength before stripping any formwork or shores. In practice, this determination is made either by testing concrete samples (cylinders cast from the same batch) or by following a curing schedule developed by the project’s structural engineer.
The timing varies depending on the concrete mix, ambient temperature, and the structural design. A typical floor slab might need seven days of curing before shores can come down, but in cold weather or for heavily loaded members, it could be longer. Removing shores too early is one of the most common causes of concrete construction failures, and it has been cited as a contributing factor in several major building collapses over the past several decades.
Safety Requirements
OSHA regulates concrete shoring under its standards for cast-in-place concrete construction. The requirements are detailed and specific:
- Pre-erection inspection: All shoring equipment must be inspected before it’s set up to confirm it matches the specifications in the formwork drawings.
- Ongoing inspection: Erected shoring must be inspected immediately before, during, and immediately after concrete placement.
- Damage protocols: Any shore found to be damaged or weakened below its required load capacity must be immediately reinforced or replaced. It cannot be used.
- Bracing: Sills must be sound and rigid, and each tier of shoring must be diagonally braced in two directions.
- Contact: All base plates, shore heads, and adjustment screws must be in firm contact with both the foundation below and the formwork above.
- Tiered shores: When single post shores are stacked on top of each other, the design must be prepared by a qualified designer, the erected system must be inspected by a structural engineer, and the posts must be vertically aligned and braced at every splice point.
Drawings and plans for the entire shoring layout must be available on the job site at all times. No adjustments to single post shores are allowed after concrete has been placed.
Why Shoring Systems Fail
Investigations into concrete construction collapses point to a short list of recurring problems. The most frequent causes are inadequate bracing against lateral forces, premature removal of shores or reshores, and insufficient concrete strength at the time of shore removal. In several high-profile failures, the root cause was simply not following the structural engineer’s shoring specifications.
The 1955 New York Coliseum collapse taught the industry that shoring systems need robust diagonal and horizontal bracing to resist lateral loads, not just vertical ones. Investigators concluded the collapse could have been entirely prevented with proper bracing. Two decades later, the Skyline Plaza collapse in Virginia reinforced the danger of rushing formwork removal. The investigation identified six key lessons, including that uncontrolled acceleration of formwork removal can lead to catastrophic consequences and that shoring plans, procedures, and removal schedules must be designed by professionals and enforced in the field.
Despite these lessons, similar failures have continued to occur when construction teams skip preconstruction planning, lack formal formwork removal schedules, or fail to verify concrete strength before stripping shores. Proper shoring is not just a structural convenience. It is the only thing holding up thousands of pounds of wet concrete until that concrete can hold itself.