Brace Overstrength Factors
For the BRBF system, the brace is designated as the “fuse” element and all other parts of the frame and connections are designed to remain elastic. As the BRB brace engages in a seismic event, the steel core is designed to yield and then to strain harden. This process will require the beams, columns, and connections to be designed for these higher, strain-hardened brace forces. The increase in the brace force in tension is represented by the factor ω, while the increase in compression is represented by the factor βω. Using the AISC341 (US) test interpretation methods, factors can be determined from the results by use of a “backbone curve” drawn through the resulting peak values of the test data. Again, these factors vary by brace manufacturers and even by brace connection type. The engineer need not calculate these, as they can be provided by the manufacturer. These values are determined from the expected elongations in the braces. These may be determined by the BRB designer from the engineer’s drift analysis or simply through use of the design factors used by the design engineer. Following the drift analysis, the engineer may provide the BRB manufacturer with work-point to work-point elongations at each frame based on the final governing drift profiles of the frame. If these profiles include possible inelastic mechanisms, indicate this to the BRB manufacturer. Elongations determined directly from the amplified elastic deflections may require an additional factor of 2 to be applied to the brace strains, while elongations determined from possible mechanisms or those determined from inelastic analysis should not require this factor. Note that the use of inelastic drift alone to determine brace elongations may be overly conservative, since there are many contributors to drift from all the members of the frame (column elongation/shortening, beam distortion, etc) and the commonly used formulas to calculate brace elongation from drift assumes all the members in the frame besides the braces are infinitely rigid.