Points of beam support restrained against rotation

[Deener]

Hi all,

I'm evaluating an existing pit floor for allowable forklift tire loading. This floor has been in place for quite some time. See the drawing below. I want to focus on the W10x49 beams with a midspan of 10'. Let's also ignore the cope at each end of the W10 for now. My concern is the lack of rotational restraint at the points of support. Especially since the limit state of LTB applies. I would not rely on the grating above the beam to provide torsional restraint. Is this too conservative? In the AISC on pg 2-16 under the section "Beam Ends Supported on Bearing plates" it states, It may also be possible to demonstrate in a limited number of cases, such as with beams with thick webs and relatively shallow depths, that the beam has been properly designed without providing the details described above [stiffener plates]. In this case, the beam and bearing plate must still be anchored to the support. Has anyone come across this before? I'm not sure how I would evaluate this. An engineer has previously signed off on this floor design but has long since retired. I'm designing a similar floor infill that uses a W16x57 with a 15' mid span and I can't see a way around adding stiffener plates at the points of support. I'm trying to make sure I have ammunition for the standard, "we didn't do it before so why do we have to do it now?"

 

[Lomarandil]

An old british standard (5950 maybe?) used to suggest increasing Lb for LTB checks by 2*depth for this circumstance, to account for the imperfect torsional restraint offered by a stocky web. As noted, you'd want the beams bolted or welded to the cap beams below.

A W10x49 isn't the stockiest, but it's still in the range of the lighter HP sections, so I'd feel OK about that.

I'd be most concerned about LTB and torsional restraint at the cantilever ends. There is some difference of opinion between the different codes regarding LTB capacity of partially restrained cantilevers (see https://www.eng-tips.com/threads/capacity-of-i-beam-as-a-cantilever.522305/ and https://www.eng-tips.com/threads/ltb-of-cantilevers-to-aisc-codes.235560/#post-1273316). Hopefully you're able to add some stiffener or connection there and it's not an issue.