SDC Torsion Design

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Plastic Section Modulus (Zx,Zy)

The 13th and 14th editions of the AISC Steel Construction Manual were revised from previous editions to use limit states as the basis for determining the nominal flexural strength of any steel member. The nominal flexural strength (Mn) is determined by the lower value obtained according to the limit states of yielding and lateral-torsional buckling. In both cases the plastic moment (Mp) is used to calculate Mn.  Mp is determined by multiplying the yield strength (Fy) times the plastic section modulus (Zx and Zy).

Calculation of Zx and Zy is relatively straight forward for symmetrical cross sections. However, crane runway girders are usually designed with built-up, unsymmetrical sections using cap channels, web channels, or thrust plates. Since there may be no innate axis of symmetry to be taken advantage of for a numerical integration, SDC's approach is to resort to a trial and error procedure.

We have tested extensively and incorporated the solution to Zx and Zy successfully into our CRANE GIRDER PRO program. To illustrate our implementation scheme, we used the same "homework problem" that appears in our article "Warping Constant of Open Section with Arbitrary Profile Geometry - Rev. 1" as the working example. Here is SDC's paper titled "Plastic Section Modulus of Sections with Arbitrary Profile Geometry" that we share with others interested in this subject.