Continuing Education

Streamlined Design of nonprismatic I-Section Members

In metal building frame design, significant design economy is commonly gained by varying the web depth along I-section member lengths, as well as by stepping the web and flange plate thicknesses, and in some cases the flange widths, at selected positions along the member lengths. Similarly, design economy is achieved in bridge I-girder design by stepping the cross-section geometry at selected positions along the member lengths. In addition, for continuous-span bridge I-girders with spans beyond approximately 200 ft, significant design economies are commonly realized by using a continuously varying web depth, e.g., haunched girders, within negative moment regions. All of these nonprismatic member designs vary the member capacities largely to better fit the required elastic moment envelopes obtained from a structural analysis. Efficient routine design of nonprismatic I-section members can be achieved by a straightforward adaptation of the AISC (2016) and/or AASHTO (2017) design provisions for prismatic I-section members. This paper first focuses on explaining the theoretical and conceptual basis for these adaptations, which are captured within the AISC/MBMA Design Guide 25 (2019) recommendations, as well as in draft updates to the AASHTO LRFD rules. Secondly, the paper focuses on the streamlined manual calculation of design quantities, particularly on methods of estimating lateral-torsional-buckling (LTB) resistances for various nonprismatic member geometries. Lastly, the paper compares the resistance predictions to results from experimental tests and test simulations.

  • Date: 4/2/2019 - 4/5/2019
  • PDH Credits: 0


Ryan Slein and Donald W. White; Georgia Institute of Technology; Atlanta, GA

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