Three prototype buildings were adapted from the SAC studies (Gupta and Krawinkler 1999) with
seismic response coefficient, C s =0.125, 0.125, and 0.106, and seismic base shear per frame,
V=3615, 7135, and 9034 kips, for the three, six, and nine story buildings respectively. For each
building height, the self-centering ratio was varied to be α sc =0.0, 0.5, 1.0, 2.0, ∞ resulting in a total
of fifteen prototype buildings. Using an assumed resistance factor of, ϕ=0.9, an SMA initial stress,
F i-SMA =138 MPa, BRB yield stress, F ysc =276 MPa, and SMA gage length equal to 60% of the brace
length, the BRB and SMA forces were proportioned using Eqs. 1 and 2. Inner, middle, and outer
tubes were selected based on a force associated with the SMA rods all experiencing a peak stress of F u-SMA =483 MPa and to provide enough clearance between elements to create a realistic and
buildable SC-BRB. The resulting SC-BRB had square outer tubes with width between 250 mm
and 530 mm. The computational model included leaning columns with gravity load applied to
simulate P-Δ effects. The same model discussed previously and shown in Figure 5a was used for
the braces. Each of the fifteen prototype buildings was subjected to the 44 FEMA P695 far field
ground motions scaled in accordance with the FEMA P695 methodology to approximate hazard
levels with 10% probability of exceedance in 50 years and 2% probability of exceedance in 50
years for a site in California, U.S.A (FEMA 2009). The 10% in 50 design spectra was anchored
with values of S DS =1.0 and S D1 =0.6, equivalent to the D max hazard definition in FEMA P695, and
the 2% in 50 design spectra was developed using the same values multiplied by a factor of 1.5.
The residual drifts for all three building heights with α sc ≥0.5 were quite small as shown in
Fig. 9a. Even at the 2% in 50 hazard level (results not shown here), there were only a handful of
ground motions that produced residual roof drift ratios greater than 0.1%. Considering a possible
limit on residual drifts of 0.2% based on out-of-plumb requirements for new steel construction in
the U.S.A. (AISC 2010b), the residual drifts for all models show that a self-centering ratio of
α sc ≥0.5 is adequate for limiting residual drifts in these configurations. The minimum required self-
centering ratio is an important result in the context of SC-BRB design because the SMA can
produce significant overstrength as shown in Figure 9b. As a result of BRB and SMA overstrength,
the SC-BRBs reached peak forces of between two (for α sc =0.0) and five (for α sc =∞) times the
design brace force meaning the surrounding columns and beams would need to be designed for
this level of overstrength. In this context, the range α sc =0.5 to 1.5 is recommended for design to
control residual drifts and produce more efficient surrounding frame design.