knee

A knee is a rigid bracket or strut formed with a sharp angle, typically close to 90 degrees, used to brace and reinforce two members that meet at an angle. In timber framing, it is cut from a single piece of wood with the grain running through both arms of the angle, which gives it strength along its length. In steel fabrication, a knee is usually a plate or rolled section welded or bolted to join two perpendicular or near-perpendicular members, such as a column to a beam or a wall to a roof truss.

Timber knees rely on wood grain orientation for their load-bearing capacity. A knee cut from naturally curved timber, or from the crook where a branch meets a trunk, is strongest because the grain follows the angle. Factory-cut knees from straight stock are weaker and are reinforced with gussets, bolts, or pegs. Steel knees come in several forms: simple L-shaped angles, fabricated plates with stiffeners, or rolled structural tees and channels arranged to form a stiff connection.

Knees appear in roof trusses, where they prevent the tie beam and rafter from pulling apart under load. They are essential in ship framing, holding the ship's ribs to the keel and keelson. In building frames and machinery bases, knees resist both vertical load and lateral shear by creating a continuous load path that avoids a weak hinge-like joint. Without a knee, two members simply butted or nailed at right angles will fail under repeated stress or heavy eccentric loads.

The name comes directly from the resemblance to a human knee joint: a bend or angle in a structural member. In timber work, skilled framers could identify and select naturally curved pieces of wood that made the strongest knees. Machine shops and structural steel fabricators stock standard knee angles and gusset plates to speed assembly of frames, but custom knees are still fabricated for large industrial structures, ships, and heritage buildings where standard sizes do not fit the design loads or spacing.