Buckling Restrained Braces (BRBs) are typically proprietary, but each brace is comprised of similar components. There is a reduced steel core over a portion of the length of the brace with significantly larger unrestrained non-yielding segments attached to a gusset at each end. It is recommended that a cross section with the area of the yielding core be assigned to the BRB in the program. However, as the steel core only extends over a portion of the full brace length (node-to-node) it is necessary that the axial stiffness of the brace be adjusted to accurately capture the behavior in the analysis. To this end an Axial Stiffness Multiplier should be assigned to the BRB (Ref 14). This multiplier can also be used to account for a steel core modulus other than 29000 ksi which is assumed for steel when using US codes.

For Example. A BRB has a yielding core that extends over 60% of the work-point to work-point distance (Lwpt) of the brace. Assume the steel core has an elastic modulus of 30,000ksi. What multiplier should be used ?

Note that axial stiffness is calculated as EA/L. Assume the brace outside the steel core is infinitely stiff under axial compression (this assumption is at engineers discretion and actual stiffness of non-yielding element can be used).

Actual EA/L of total brace (assuming rigid non-yielding section) = Stiffness of core = 30,000 x A / ( 0.6 Lwpt )

To calculate stiffness multiplier the stiffness of full node-to-node brace length (Lwpt) should equal that just calculated above:

Multiplier x 29,000 x A / Lwpt should equal 30,000 x A / 0.6 Lwpt, so Multiplier = ( 30,000 x A / 0.6 Lwpt ) / ( 29,000 x A / Lwpt )

Multiplier = 1.72

Contact the buckling restrained brace manufacturer for more accurate multiplier values.