# Theoretical fork supports vs Simple connections in practice

QUESTION
Being a relatively new structural engineer, I am trying to get a feel of when differences between theory and practice are ignored / deemed as unimportant by structural engineers. Although I have an idea about the answer, I wanted to ask you about an aspect related to lateral torsional buckling. Most elastic critical moment formulas assume the ideal fork supports at the ends of a beam. In reality, connections neither fully restrain or allow a certain motion, but somewhere in between. I guess the same applies to torsion at the end of a beam. I have seen that differences in critical moment between when a beam is restrained from rotating about its longitudinal axis at one or both ends and when it is free to twist are quite large. This of course is intuitive. Since designers often go for simplicity where possible, I have seen many instances of common, simple beam to beam connections that do not seem to offer a proper torsional end restraint for the supported beam. Do you ever check or think about this aspect when designing?

REPLIES

MacGruber22
Are you asking about designing an end connection for torsion due to a torque being placed on the beam, or are you talking about resisting end rotation for stability at a simple connection? If it is the latter, it is super easier to design a simple connection to prevent excessive out of plane rotation for reaction stability. Clip angles or single shear plates should be nearly full-depth, i.e. between web to flange fillets in an I-section. Other simple connection configurations have different ways to handle that stability.

When modeling in many 3D analysis programs, ends of beams in many models may need to be torsionally restrained to maintain global or local stability of the frame. Of course, this fixity can attract torsional moment in the beam. How much depends on the configuration of the frame, loads, and the torsional rigidity of those members. Often times you will find that when you check the analysis output, the tabulated torsional moments are very small or near zero. It is up to the engineer to decide when those torsional moments are appreciable enough to warrant an actual moment connection. Usually, the most cost-effective way is to use kickers, etc. to make the torsion in the beam go away so no moment connection is required or to prevent beefing up the beam section. Spandrel/fascia beams are a typical example when you have hanging facade from the beam. Without kickers, or some sort of horizontal member lower to the beam, the torsional moments would lead to uneconomical sections.

The AISC Steel Manual has examples and commentary regarding simple beam and torsional stability. Not near mine, or else I would point out the sections. The only other thing I was thinking (but don’t know the answer) is whether the design codes have empirical components within the LTB equations in order to account for some reasonable minimal support restraint. The code specifically requires this stability detailing at every beam support no matter what. However, I know for a fact that many engineers design loose laid W-sections bearing on masonry and concrete without proper support torsional restraint. I see home builders making the same mistake regularly.

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