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If an I-shaped section is loaded in two directions (in both weak and strong axes), how can I find the maximum load-carrying capacity of them beam?

There is an interaction equation in AISC 360-10, but it's for beams which are also loaded axially. Can that interaction equation be used for only flexure in two directions, taking the axial force to be 0?

Air
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Murad Nazari
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2 Answers2

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The simple answer is: "yes, you can use the given equations with Pr=0."

There are three reasons for my answer:

  1. The code section (AISC 360, Chapter H) doesn't specifically state that Pr cannot be equal to zero.
  2. The equations in code section H1 are referenced by other sections that apply to both compression and tension (e.g. H2), so there must be some continuity to the equation when going from tension to compression.
  3. The graphs in the commentary on Chapter H show graphs that go from zero axial force with moment to axial force with zero moment.
hazzey
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This is a very interesting question. It might even be worth submitting to Modern Steel Construction's Steel Interchange. Per the AISC 360-10 user note at the beginning of Chapter F, you are allowed to use the interaction equations in Sections H1 for beams in biaxial flexure without axial load.

Interestingly, Salmon & Johnson's Steel Structures Design and Behavior 5th Edition, Section 7.11 suggests that the beam-column interaction equation is unconservative without axial load. The author suggests not relying on any plastic capacity. That is, for the yielding limit state, Mnx = FySx and Mny = FySy.

Based on my Google searches, it appears that Australian standard AS4100 Section 8.3.4 uses this approach.

CableStay
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