Bucklingrestrained Braced Frame

Unbonded Brace System

Buckling-restrained brace frames (BRBFs) have a high degree of ductility (energy absorbing capability) and good lateral stiffness, and are relatively simple to repair after a major earthquake. Unbonded braced 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 (see Fig. 8.36a). The unbonded brace element, typically a diagonal member, consists of a restrained yielding segment, nonyield-ing restrained steel segments, and nonyielding unrestrained segments (see Fig. 8.36b.). The yielding segment commonly referred to as the core typically consists of a steel plate. The nonyielding segments are typically of cruciform shape. The entire assembly is generally procured as a preassembled unit manufactured to meet the performance objectives specified by the design engineer.

Figure 8.36a. Main components of unbonded brace.

Because the braces are able to yield without buckling in compression, well-defined, stable, fairly symmetric hysteretic loops are generated when the braces are subjected to reversed cyclic loading, resulting in excellent energy dissipating characteristics.

Use of a buckling-restrained braced frame 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 vertical 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 dissipation characteristics by effectively eliminating buckling and the poor hysteretic performance associated with it. Because of this, the tension and compression behaviors of the brace are very similar.

Restrained nonyielding

Restrained nonyielding

Figure 8.36b. Components of unbonded brace. (1) buckling restrained brace; (2) core; (3) sleeve; (4) section. The yielding of core plates in compression without buckling results in a stable hysteretic loop with excellent energy-dissipating characteristics.

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. Its effective slenderness is extremely low due to the lateral restraint provided by a 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.

Since BRBFs are a recent development, they are not yet addressed by building codes such as IBC-03 or AISC seismic provisions (AISC 341-02). Therefore, to facilitate wider use of this system, the Structural Engineers Association of Northern California (SEAONC) has developed, in conjunction with the SEAOC seismology committee and AISC TC9, a set of design provisions. Their work has resulted in a document, Recommended Provisions for Buckling-Restrained Braced Frames (SEAONC 2001). The provisions are currently under review for inclusion in future building codes and seismic provisions.

The design approach for this system typically follows the same force-based approach that is used for other types of braced frames. The method for computing the base shear is similar to that for the special concentrically braced frame, with differences in the values of certain seismic coefficients because BRBFs are more flexible than SCBFs. Consequently, buildings with BRBFs with longer periods may warrant use of a larger value of the coefficient CT in the calculation of base shear. Similarly, use of larger values of the response reduction coefficient R may be appropriate in the design of BRBFs. SEAONC 2001 proposes a value of 8.0, while a value of 7.0 is being considered for inclusion in the 2003 NEHRP provisions.

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 concentrates 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 Fig. 8.36c) 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 are therefore not required, nor are the gussets required to have flexural strength in excess of those of the brace. Small eccentricities may be

Figure 8.36c. Buckling-restrained brace frame, BRBF, with chevron braces. Since the braces yield both in tension and compression, a chevron configuration does not penalize design of the beam connected to the braces.
Figure 8.36e. Unbonded brace frame connection (photographs courtesy of Edwin Shlemon, S.E., Associate Partner, ARUP Partners, Los Angeles; CA).

permissible 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 over-strength factors b and w determined from testing. The factor b represents the overstrength in compression (buckling-restrained braces tend to be somewhat stronger in compression than in tension), and w 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 codes. Although this can be done in a manner similar to that for the method presented for SCBF, BRBFs tend 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.

Figures 8.36d and 8.36e show an unbonded braced frame elevation and connection.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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