Concentric Braced Frames

Just about any system that has identifiable load paths for gravity and lateral loads is permitted without any height limits for buildings in regions of low seismicity or assigned to SDC A, B, or C. These may be designed by using the provisions of American Institute of Steel Construction (AISC), Load and Resistance Factor Design (LRFD), Allowable Stress Design (ASD), and Hollow Structural Section (HSS) specifications, all of which have been adapted by provisions of the American Society of Civil Engineers (ASCE) 7-02, National Fire Protection Association (NFPA) 5000, and International Building Code (IBC) 2003.

The design of braced frames in regions of high seismicity, or those assigned to SDC D, E, or F, is performed according to the provisions of AISC 341-02, "Seismic Provisions for Structural Steel Buildings," commonly referred to as AISC-Seismic. A brief discussion of the seismic provisions of this publication, including the salient characteristics of braced frame design, are as follows:

• A variety of braced frame configurations are permitted by AISC-Seismic. Some of these are shown in Fig. 3.40a.

• AISC-Seismic permits seismic design of braced frames either as an ordinary concentric braced frame (OCBF) or as a special concentric braced frame (SCBF). The only difference between the two is in detailing of the connections and some prescriptive requirements for SCBF intended to enable them to respond to seismic forces with greater ductility.

• Both the V- and inverted V-braced frames, often referred to as chevron braces, have been poor performers during past earthquakes because of buckling of braces and excessive flexure of beam at midspan where the braces intersect the beam. Buildings with single or multistory X-braces or V-braces with zipper columns are deemed better performers and hence, should be considered for braced frame configurations in high seismic zones.

• Braced frames with single diagonals are also permitted by AISC-Seismic. However, there is a heavy penalty since the braces must be designed to resist 100% of the seismic force in compression, unless multiple single-diagonal braces are provided along a given brace frame line.

• A preferred but difficult-to-achieve behavior in an SCBF is the in-plane buckling of the brace. Given a choice, a brace would buckle out-of-plane rather than buckling in the plane of the braced frame. This is so because the in-plane buckling is inhibited because: 1) placement of braces in a flat position is generally not permitted for architectural reasons; and 2) the presence of infill metal studs above and below the braces adds considerable in-plane stiffness to the braces. Recognizing these features, both UBC 1997 and AISC-Seismic permit out-of-plane buckling of braces, provided an uninterrupted yield line can develop in gusset plates at each end of the brace connection. This is achieved by prescribing the following detailing requirements:

1. Provide a minimum of 2t and maximum of 4t offset from the end of brace to the yield line, where t is the thickness of gusset plate.

2. Provide a 1-in. minimum offset from the brace to the edge of gusset plate.

3. Isolate gusset plate yield line from the floor slab.

X-bracing

Two-story X-bracing

Two-story X-bracing

Chevron inverted V-bracing
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