Application of AISC Design Provisions for Tapered Members

Abstract

The use of tapered structural members having tapered depths and/or widths was first proposed by Amirikian for reasons of economy. Typically, tapered members are used in single story structures of one or more bays (Figs. 1a and 1b) and in cantilevered sections of buildings (Fig. 1c). Some low-rise structures also contain tapered members (Fig. 1d). In view of the lack of basic understanding of the behavior of tapered members, as well as design criteria, a joint WRC-CRC subcommittee was established in 1966 to coordinate research studies. In addition, a subcommittee of the MBMA Technical Committee was established in 1973 to facilitate the development of design guides for tapered members. Two previous papers containing design recommendations were a result of the technical guidance of these subcommittees. The major thrust of these papers deal with the formulation of allowable axial stress, allowable bending stress, and combined axial-flexural compression for tapered members. These design recommendations have been incorporated into the AISC Specification as Appendix D. The examples presented here serve to illustrate the use of the design specifications for tapered members as required by AISC Specification Appendix D. There are still several unanswered questions regarding the rational design of tapered member. Three topics of current interest are (1) local buckling limitations, (2) tension flange support requirements,6 and (3) the Cm factor for beam-columns in unbraced frames. The designer must still use experience and sound engineering judgement regarding design procedures not provided by Appendix D of the Specification.