In-Plane Properties and Modeling of Reduced Beam Sections

In the seismic design of steel frames, one of the objectives is to provide ductile beams capable of energy dissipation through the formation of plastic hinges (or "fuses"). With wide-flange shapes, it is desirable to force these hinges away from the brittle zones near the column faces by reducing the beam section at some specified distance from these faces. Obviously, the cuts change the overall elastic properties of the reduced beam sections (RBS). However, the cuts also affect the fixed-end actions used in the elastic analysis of the structure. The first objective of this paper will be to compute not only the stiffness properties of RBS, but also the fixed-end actions. Most structural analysis programs found in design offices do not let the designer input stiffness matrices and fixed end actions. Input of beam elements is usually limited to prismatic or tapered beams. The second objective will be to show how this difficulty may be overcome by modeling a RBS with three prismatic beam elements of suitable properties. Some very simple frames will be used as examples to compare interstory drifts, and examine the Q33 method proposed by Iwankiw and Mohammadi in an earlier paper. Coincidentally, these calculations lead to the observation that beam shear deformation is not always negligible.