SkyCiv Knee Brace Connection Calculator

The calculator covers diagonal knee bracing details built from single angles, double angles, HSS or gusset plates, and reports capacities and utility ratios for each.

Checks follow AISC 360-16, in either LRFD or ASD. The point is to get the corner detail right without working several interacting limit states by hand for every brace angle.

The joint where a short diagonal member, the knee brace, meets a primary beam and column. Instead of running diagonally across a whole bay, it cuts the corner.

It carries axial force, tension and compression both, away from the main joint. That does two things: it takes bending moment out of the beam and the column, and it raises the lateral stiffness of the frame.

The trade is geometric. You give up a triangle of space in the corner and get stiffness in return, without a diagonal blocking the bay.

• Brace member: single angles, double angles, channel sections or HSS tubing.
• Gusset plates: welded or bolted to the beam or column to anchor the brace.
• Bolts: high-strength fasteners carrying shear and tension between the plates and the members.
• Welds: fillet or groove welds securing the gussets, or joining the bracing components directly.

The whole job of the assembly is to spread a concentrated force through the beam-column corner without any one component reaching its limit first.

Wherever X-bracing or a full-height diagonal is impractical:

• Industrial warehouses and portal frames that need clear floor space or overhead clearance.
• Pipe racks and support structures in processing plants.
• Retrofits and seismic upgrades to existing frames.
• Open-air shelters, canopies and crane runway supports.

Retrofits are the case where knee bracing often wins outright: it can be added to a frame that is already built and in use, which a full-bay diagonal usually cannot.

Geometry and loading decide the arrangement:

• Gusset plate: a single or double gusset welded to the column or beam, with the brace bolted to it.
• Directly welded HSS: tube braces slotted or bevel-cut and welded straight to the flanges or to a gusset.
• Single angle: bolted through one leg to a gusset. Suits lighter wind or sway control.
• Double angle: back-to-back angles sandwiching the gusset, which removes the eccentricity a single angle carries.
• Flange-bolted: end plates on the brace end, bolted flush to the member flanges.

The single and double angle options are the same brace doing the same job, so the choice is usually about whether the eccentric load path is acceptable at the force level involved.

• Configuration and sections: the brace profile type, HSS or single or double angle, and the member dimensions.
• Fastener and weld details: bolt grade, diameter and pattern spacing, or weld sizes and lengths.
• Gusset geometry: plate thickness, yield strength and the connection work points.
• Design loads: the axial force, tension or compression, plus any shear acting with it.

Calculate, and the utility ratios come back with the capacity limits and the full check working behind them, to AISC 360 in LRFD or ASD.

A knee brace has a short load path and a lot of ways to fail: block shear, bolt shear, gusset buckling and weld tear-out all sit close together, and the work point geometry moves all of them at once.

Running the set together rather than one at a time is what makes the detail tractable. Move the work point, see which check moves with it, and the compression case stops being the one you find out about late.

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