415 Reduced Beam Section

Reduced beam section (RBS) or dogbone connection is a type of connection in welded steel moment frames in which portions of the bottom beam flange or both top and bottom flanges are cut near the beam-to-column connection thereby reducing the flexural strength of the beam at the RBS region and thus force a plastic hinge to form in a region away from the connection (Engelhardt et al. 1996; Iwankiw and Carter 1996; Plumier 1997). The presence of this reduced section in the beam also tends to decrease the force demand on the beam flange welds and so mitigate the distress that may cause fracture in the connection. RBS can be bottom flange cut only, or both top and bottom flange cuts. Bottom flange RBS is used if it is difficult or impossible to cut the top flange of an existing beam (e.g., if the beam is attached to a concrete floor slab). Figure 4.39 shows some typical cut geometries for RBS. The constant cut offers the advantage of ease of fabrication. The tapered cut has the advantage of matching the beam's flexural strength to the flexural demand on the beam under a gravity load. The radius cut is relatively easy to fabricate and because the change in geometry of the cross-section is rather gradual, it also has the advantage of minimizing stress concentration. Based on experimental investigations (Engelhardt et al. 1998; Moore et al. 1999), the radius cut RBS has been shown to be a reliable connection for welded steel moment frames.

The key dimensions of a radius cut RBS is shown in Figure 4.40. The distance from the face of the column to the start of the cut is designated as a, the length and depth of the cut are denoted as b and c, respectively. Values of a, b, c are given as follows (Engelhardt et al. 1998; Gross et al. 1999):

a « (0.5 to 0.75) bf b « (0.65 to 0.85)d c « 0.25bf for a bottom flange RBS

where bf is the beam flange width and d is the beam depth. Using geometry, the cut radius R can be calculated as

and the distance from the face of the column to the critical plastic section sc is given by b sc = a + -

FIGURE 4.39 Reduced beam section cut geometries: (a) constant cut; (b) tapered cut; and (c) radius cut.
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