Stepped Beam with Multiple Load Combinations


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Title: Stepped Beam with Multiple Load Combinations

Author(s): Matthias F. Andermatt and Adam S. Lubell

Publication: Special Publication

Volume: 273


Appears on pages(s): 1-22


Date: 9/20/2010

A simply supported stepped beam having multiple disturbed regions is designed. The beam, consisting of a 3’-5” (1041 mm) cantilever span and a 16’-10” (5131 mm) back-span, is required to support an intensive green roof on the tenth floor of a 14 storey office building. The cantilever and part of the back-span support the roof. The remainder of the back-span is 16” (406 mm) deeper and supports the office floor. The beam is subjected to tributary area loads from the green roof and the offices. A spandrel beam is indirectly supported on the cantilever section of the stepped beam. Since the disturbed regions at the column supports, indirect support, and at the step render the majority of the beam as disturbed, the entire beam is designed using strut-and-tie models in accordance with Appendix A of ACI 318-08. This design example considers multiple load combinations resulting in different strut-and-tie truss analysis models.