Buckling-restrained brace frames (BRBFs), shown in Figure 1.29, have a high degree of ductility (energy absorbing capability) and good lateral stiffness, and are relatively simple to repair if need be, after a major earthquake. Unbonded brace frames, which may be considered a special class of BRBFs, consist of a steel core installed within an outer shell with mortar infill between the plate and the shell. An unbonding agent is applied to the core plate to prevent it from transmitting axial load to the buckling-restraining mechanism. The unbonded brace element, typically a diagonal member, consists of a restrained yielding segment, nonyielding restrained steel segments, and nonyielding unrestrained segments. The yielding segment commonly referred to as the core typically consists
Full depth web intermediate stiffeners
both sides for link depths ≥25 in.
Bracing
Center line of brace intersects Center line of beam at end of link or inside of link Lateral bracing
required at top and bottom
link flanges Link length = e
Beam Full depth stiffeners on
both sides
FIGURE 1.25 EBF with HSS bracing.
See Figure See Figures 1.23
1.24 and 1.25
A
B
(a) (b)
FIGURE 1.26 (a and b) Key elevations of EBF.
28 Structural Analysis and Design of Tall Buildings: Steel and Composite Construction
Center line of link Wide flange beam Link
Typ Typ
Plates with bolts as required for erection loads
Center line of brace
Wide flange brace CJP
FIGURE 1.27 Detail A.
Center line of brace WT8 at each side of brace W10 brace
PL A572 GR. 50
W16 beam
1 in. φ A325N bolts PL 3/4 in. A572 GR. 50
W.P.
1/2 in.
1 in. φ A325X bolts in std. holes
W14 column
Center line of column FIGURE 1.28 Detail B.
Lateral Load Resisting Systems for Steel Buildings 29
procured as a preassembled unit manufactured to meet the performance objectives specified by the engineer. See Figures 1.30 and 1.31 for schematic components of two types of bracing. Figure 1.30 shows a circular sleeve while Figures 1.29 and 1.31 show a rectangular HSS.
Because the braces are able to yield without buckling in compression, well-defined, stable, and symmetric hysteric loops are generated when the braces are subjected to reverse cyclic loading. This results in excellent energy dissipating characteristics.
The use of a BRBF as a seismic-lateral-resisting system is relatively new in the United States.
However, it is similar to a special concentrically braced frame in that it also has a triangulated verti-cal framework of members that resist lateral loads through axial tension and compression. The main difference is that the buckling-restrained braces achieve significantly higher ductility and energy dis-sipation characteristics by effectively eliminating buckling and the poor hysteric performance asso-ciated with it. Because of this, the tension and compression behaviors of the brace are very similar.
In buckling-restrained braces, a fairly long segment can yield in compression as well as in tension.
The yielding segment is part of an axial-force-resisting steel core. Although its effective slenderness is extremely low, buckling is not an issue due to the lateral restraint provided by surrounding casing of steel infilled with mortar. For buckling to be precluded, this casing must be kept free from axial forces. Several methods of confining the axial force to the steel core are in use in the United States.
Most of these are developed around proprietary specifications, and some are patented.
As buckling-restrained braces are typically a specification item, the required brace strengths are generally specified by the design engineer. Customarily, the manufacturer designs the braces to comply with the given requirements using the material and grade specified for the element. Since the material grade has a significant effect on the brace stiffness, the lower the yield stress, the greater the required area of steel, resulting in a stiffer brace. Decreasing the yield length concen-trates the inelastic strain, reducing the cumulative energy dissipation capacity.
Because the tension and compression strengths of buckling-restrained braces are similar, a chevron configuration (see Figures 1.29 through 1.31) does not penalize the design of the beam connected to the chevron braces.
With certain simplifications, gusset plates at the connections are designed similar to those for SCBF. However, BRBF gussets are not required to accommodate buckling of the brace; hinge zones
Coreplate
FIGURE 1.29 Main components of buckling restrained brace frames: (a and b) elevations; (c) BRBF Chevron braces; (d) bracing components. Note: K and X-braces are not viable options for BRBF.
30 Structural Analysis and Design of Tall Buildings: Steel and Composite Construction
Small eccentricities may be permissionable in the connection design if the resulting brace rotations are still within the tested limits.
The maximum connection force is calculated using the brace strength and overstrength factors β and ω determined from testing. The factor β represents the overstrength in compression (buckling-restrained braces tend to be somewhat stronger in compression than in tension), and ω represents strain-hardening within the expected deformation range. The factor Ry, representing expected yield strength as compared to nominal yield strength, is assumed not to be applicable in sizing of the braces because the final cross-sectional size of a buckling-restrained brace is typically determined considering the material yield strength as measured from coupon tests. The brace yield strength can thus be calculated without guesswork from the required strength and resistance factor.
Column design forces are determined using the special seismic load combinations specified in the Seismic Provisions. Although this can be done in a manner similar to that for SCBF, BRBF tends to have much lower overstrength. Therefore, an explicit consideration of brace capacity can usually result in lower column design forces, resulting in savings in the columns and foundations.
As mentioned previously, buckling-restrained braced frames are a special class of concentrically braced frames. As in the case of special concentrically braced frames (SCBF), the centerlines of BRBF members intersect at a point to form a complete vertical-truss system. BRBF systems have more ductility and energy absorption capacity than SCBF systems because overall brace buckling, and its associated strength degradation, is precluded at forces and deformations corresponding to the design story drift. Buckling-restrained braced frames are composed of columns, beams and bracing elements, all of which are subjected primarily to axial forces. Braces of buckling-restrained
Steel core
FIGURE 1.30 Details of buckling restrained brace: (a) partial elevation of BRBF; (b) HSS encasement and steel core; (c) section AA; (d) section BB; (e) section CC.
Lateral Load Resisting Systems for Steel Buildings 31
braced frames are composed of a steel core and restraining system encasing the steel core that pre-vents buckling.
The steel core within the bracing element is intended to be the primary source of energy dis-sipation. During a moderate to severe earthquake the steel core is expected to undergo significant inelastic deformations.
BRBF systems can provide elastic stiffness that is comparable to that of an EBF or a SCBF. The duc-tility and energy dissipation capability of BRBF is expected to be comparable to that of an EBF or SMF and greater than that of SCBF. This high ductility is attained by limiting buckling of the steel core.