A strut is a structural member designed to resist compression, meaning it’s being pushed inward from both ends. Think of it as the opposite of a cable or chain: where those handle pulling forces, a strut handles pushing forces. You’ll find struts in roof trusses, deep excavations, bridges, and mechanical support systems throughout modern construction.
How a Strut Works
Every structure deals with two fundamental types of force: compression (pushing) and tension (pulling). A strut is a member that works in pure compression. When load pushes on both ends of a strut, the strut pushes back equally on both sides, transferring that force through the structure to the ground or to other supporting members.
The simplest way to picture this: imagine placing a wooden dowel between two walls and pushing the walls toward each other. The dowel resists being crushed and keeps the walls apart. That dowel is acting as a strut. In real construction, struts do this same job at much larger scales, bracing walls apart, keeping rafters from collapsing inward, or holding excavation walls in place while crews work below ground.
The counterpart to a strut is a tie, which is a member that resists tension (being pulled apart from both ends). In many structures, struts and ties work together. A roof truss, for example, uses diagonal struts to handle compressive loads while ties handle the pulling forces along the bottom chord. Understanding which members are in compression and which are in tension is foundational to how engineers design safe structures.
Struts in Roof Trusses
One of the most common places you’ll encounter struts is inside a roof truss. In a traditional king post truss, struts extend diagonally from the central vertical post outward to the principal rafters. Their job is to prevent those rafters from bowing or collapsing inward under the weight of roofing materials, snow, and wind. Without struts, the rafters would need to be much heavier to span the same distance.
These truss struts are typically made of timber or steel, depending on the building. Research from MIT confirms that wood performs very well under compression, which makes it a natural fit for strut members in trusses. In many existing buildings, you’ll see timber used where there’s compression (the struts) and thinner steel members used where there’s tension (the ties). This combination takes advantage of each material’s strengths while keeping the structure lightweight.
Struts in Excavation and Shoring
When construction crews dig deep into the ground for basements, tunnels, or subway stations, the surrounding soil and water pressure want to push the excavation walls inward. To prevent collapse, engineers install a braced excavation system that uses internal framing rather than external anchors. Struts are the key compression members in this system.
In a typical braced excavation, horizontal beams called walers run along the excavation walls, distributing soil pressure evenly. Struts then span between opposing walers, pushing back against the inward pressure from both sides. These struts are often large steel pipes or box sections, and they’re installed progressively as the excavation deepens. Each new level of struts braces the walls at a greater depth. The same principle applies in tunneling, where struts or temporary ribs brace tunnel linings against the surrounding earth.
Common Strut Materials
The material a strut is made from depends on the loads involved, the environment, and whether the strut is permanent or temporary.
- Steel struts are the most common choice for heavy-duty applications like excavation shoring and large building frames. Steel handles both compression and tension well, and it can be fabricated into pipes, I-beams, or box sections to suit the load requirements.
- Timber struts are widely used in roof trusses and lighter framing. Wood is excellent in compression and has a much lower carbon footprint than steel, making it the preferred choice for structures where tension isn’t a concern.
- Concrete struts appear in permanent underground structures, retaining walls, and foundations where long-term durability matters more than ease of installation.
- Hydraulic struts are adjustable and used in temporary shoring. They can be extended or retracted to fine-tune the bracing force, which is especially useful as excavation conditions change.
Cross Struts and Diagonal Bracing
Not all struts run horizontally. Cross struts are diagonal members placed between vertical columns or posts. They prevent the structure from racking, which is the tendency for a rectangular frame to lean sideways and collapse into a parallelogram shape. You’ll see diagonal cross struts in everything from scaffolding to steel building frames to transmission towers. Those crisscrossing arrays of diagonal struts visible on antenna towers and bridges are all working in compression to keep the structure rigid under lateral forces like wind.
Horizontal struts, by contrast, are commonly placed between retaining walls or across excavations. The soil pushes inward on the walls, and the strut transfers that load from one wall to the other so the two walls effectively brace each other.
Strut Channel Systems for Building Services
There’s a second, very different meaning of “strut” in construction that you’ll encounter on job sites: strut channel, often called by the brand name Unistrut. This is a standardized steel framing system, a C-shaped metal channel with slots or holes along its length, used to mount and support mechanical, electrical, and plumbing systems throughout a building.
Strut channel is used to hang pipes and conduits from ceilings, support HVAC ductwork and rooftop equipment, create ceiling grids for lighting and plumbing, and mount electrical panels. The open channel design allows fittings to slide in anywhere along its length, making installation fast and adjustable without drilling new holes. While this type of strut doesn’t serve as a primary structural compression member, it’s one of the most frequently referenced “struts” on commercial construction sites.
Safety and Installation Standards
Because struts are compression members, their failure mode is buckling: bowing outward under load and suddenly collapsing. This makes proper sizing, bracing, and installation critical. A strut that’s too long or too thin for its load will buckle before it reaches its material strength limit.
OSHA requires that structural stability be maintained at all times during the erection process. For temporary struts used in shoring or steel erection, a competent person (someone qualified to identify hazards and authorized to correct them) must determine when bracing equipment is needed and approve its removal. Plumbing-up equipment and temporary struts must be in place before the structure is loaded with construction materials like bundles of decking or joists. For larger projects like highway bridges, federal regulations often require a registered engineer to prepare and seal working drawings for temporary support systems including struts.
On excavation sites, strut installation follows the dig. As each new level is excavated, a new row of struts is installed before digging continues. Removing struts in the wrong order, or before permanent structure is in place to take over the load, is one of the most dangerous mistakes in deep excavation work.