H3.1 Scope
Structural steel eccentrically braced frames (EBF) have been extensively tested and utilized in seismic regions and are recognized as providing excellent resistance and energy absorption for seismic loads as discussed in Commentary Section F3. While there has been little use of composite eccentrically braced frames (C-EBF), the inelastic behavior of the critical steel link region should be comparable to that of steel EBF, and inelastic deformations in the encased composite or filled composite columns should be minimal as well as in the structural steel or filled composite braces. Therefore, the and values and usage limitations for C-EBF are the same as those for steel EBF. As described below, careful design and detailing of the brace-tocolumn and link-to-column connections is essential to the performance of the system.
H3.2 Basis of Design
The basic design requirements for C-EBF are the same as those for steel EBF, with the primary energy absorption being provided by the structural steel link.
A small eccentricity of less than the beam depth is allowed for brace-to-beam or brace-to-column connections away from the link. Small eccentricities are sometimes required for constructability reasons and will not result in changing the location of predominate inelastic deformation capacity away from the link as long as the resulting secondary forces are properly accounted for.
H3.3 Analysis
As with EBF, satisfactory behavior of C-EBF is dependent on making the braces and columns strong enough to remain essentially elastic under loads generated by inelastic deformations of the links. Because this requires an accurate calculation of the shear link nominal strength, it is important that the shear region of the link not be encased in concrete.
H3.6 Connections
In C-EBF where the link is not adjacent to the column, the concentric brace-to-column connections are similar to those shown for C-OB F in Figures C-H1.1 through C-H1.3. An example where the link is adjacent to the column is shown in Figure
C-H3.1. In this case, the link-to-column connection is similar to composite beam-to-column moment connections in C-SMF covered in Section G3 and to steel coupling beam-to-wall connections addressed in Section H5.
H3.6a Beam-to-Column Connections
While the majority of the energy dissipation is anticipated to occur at the link, beam-to-column connections in C-EBF are anticipated to go through large rotations as the system undergoes large inelastic deformations. The maximum inelastic deformations are anticipated to be on the order of 0.025 rad, resulting in the requirement that when simple beam-to-column connections are used that they be capable of undergoing this rotation demand. Alternatively, fully restrained, ordinary moment connections can also be used since they have been shown to accommodate this rotation demand. See Commentary Section F2.6b for further discussion.