strongback

A strongback is a vertical or near-vertical post built or selected to carry heavy loads without significant deflection. The defining feature is a T-shaped or I-shaped cross-section, where a thick web runs perpendicular to the direction of expected bending, dramatically increasing the post's resistance to bowing. In carpentry, a strongback typically consists of a main post 4 inches to 8 inches thick joined to a perpendicular flange or tail that projects 6 to 12 inches on one or both sides, creating the T profile.

Strongbacks most commonly support temporary loads during concrete pours, formwork removal, or heavy structural changes. A strongback placed under a girder or beam can replace or assist a series of adjustable posts, particularly when the beam sags or when a concentrated load must be supported briefly. The T-tail prevents the post from bowing sideways under eccentric loads, which a simple rectangular post would do readily. The post bears vertically downward, but the tail resists the sideways stress that tends to buckle the long member.

Strongbacks are built from solid sawn timber, laminated veneer lumber (LVL), or steel channels and plates. The cross-sectional dimensions must match the load and span; a rule of thumb holds that the moment of inertia (resistance to bending) must account for both the weight of the load and the length of the unsupported post. A post 12 feet tall carrying 50,000 pounds requires substantially more section than a 6-foot post carrying the same load. Steel strongbacks withstand higher loads in a smaller footprint but require proper base plates and lateral bracing to prevent tipping.

The strongback earns its name from the structural virtue it adds: a weak post becomes strong by gaining lateral stiffness. Carpenters and formwork contractors choose strongbacks over plain posts when deflection limits are strict or when loads concentrate at midspan rather than distributed evenly across a width. A joist or beam that would sag noticeably under its own weight or under formwork can be held nearly level by a well-designed strongback positioned directly beneath the sag point.

Failure of a strongback typically occurs through crushing of the wood fibers at the base, buckling of the tail flanges if they are slender and unsupported, or tipping if the load is offset and the base lacks sufficient bearing area or lateral bracing. Checking the post for plumb and the load for centering prevents many site failures. Strongbacks are temporary or semi-permanent; once permanent steel or concrete members take the load, the strongback is removed and reused or discarded.