§16.[F4] Buckling Restrained Braced Frame (BRBF)
§16.2 [F4.5] Bracing Members
§16.2a. Steel Core – The brace design axial strength, ϕPysc (LRFD), and the brace allowable axial strength, Pysc /Ω (ASD), in tension and compression, according to the limit state of yielding, are determined as follows:
Pysc = Fysc Asc
ϕ = 0.90 (LRFD)
Ω = 1.67 (ASD)
Fysc is the Fymin value specified by the user when assigning the brace.
Asc is the area of steel core of the size assigned by the user.
§16.2d. Adjusted Brace Strength – The adjusted brace strength in compression and tension are calculated as βωRyPysc and ωRyPysc respectively; where Ry is taken as 1.0.
In the Seismic Provisions check per AISC 341-10, the program calculates the β and Ω factors both at 2 times the Design Story Drift and at 2% Story Drift.
AISC 341-10 Analysis of BRBF Columns
Member code check of BRBF columns follow the requirements of Section F4.3 where braces are assumed to resist forces corresponding to their expected strength in compression or tension.
To determine Emh for BRBF Column O, the effect of braces in frame lines A-O-B and X-O-Y are independently determined according to Section F4.3. Frame lines A-O-B and X-O-Y are assumed to frame into the major and minor axes respectively of Column O. The tensile and compressive Emh values determined from the analysis are used in the load combinations for the applicable building code.
Limitation: The analysis performed does not consider the interactive effect of axial loads on the column from braces in the two framing lines. The results from corner columns or columns with framing lines into the major and minor axes should be verified by the engineer.
§16.3 [F4.6] Bracing Connections
§16.3a. Required Strength – The required strength of bracing connections in tension and compression (including beam-to-column connections if part of the bracing system) is taken as 1.1 times the adjusted brace strength in compression (LRFD) or 1.1/1.5 times the adjusted brace strength in compression (ASD).