Return to index: [Subject] [Thread] [Date] [Author]
Re: Lift Lug
[Subject Prev][Subject Next][Thread Prev][Thread Next]- To: "?" <seaint(--nospam--at)seaint.org>
- Subject: Re: Lift Lug
- From: Christopher Wright <chrisw(--nospam--at)skypoint.com>
- Date: Fri, 9 Feb 01 17:26:16 -0600
>The design, while technically correct, seemed odd because of a large hole to > accommadate a smaller pin. Am I missing something? I think so. I also think your lug is probably overstressed, although I haven't seen your numbers. You should be especially careful because lifting lugs are subject to fatigue failure since the loading tends to be cyclic and dynamic. The AISC requirements obscure the fact that the load at the tension section comprises both a moment and a direct load. (Verify this for yourself by making a free body diagram of the segment of the lug between the tension section and the symmetry centerline. The reaction is offset from the applied load resulting in a bending moment equal to half the applied load times the average of the hole radius and the lug radius.) The maximum stress occurs at the hole ID and is the sum of the bending stress and the direct stress figured in the usual way. And don't forget that the curvature of the lug wants to increase the bending stress over and above the value calculated by Mc/I. The bottom line is that the load is carried by an area fairly close to the hole. By increasing the hole diameter you increase the bending stress because you increase the offset. You don't want to do this. The apparent intent of the AISC rule is to keep the bending stress to a minimum and 'simplifying' the calculation by specifying appropriate details to cover the moment loading so we can pretend it doesn't exist. The requirement for a 1/32 in diametral clearance has nothing to do with rattling, by the way--it's intended to minimize the bending moment. This is a good example of basic engineering mechanics obscured by simplification gone wild, not unlike all the bugger factors and provisos and such applied to seismic design. It's also a good example of what happens when we violate the Einstein principle ("Things should be made as simple as possible, but not any simpler.") Christopher Wright P.E. |"They couldn't hit an elephant from chrisw(--nospam--at)skypoint.com | this distance" (last words of Gen. ___________________________| John Sedgwick, Spotsylvania 1864) http://www.skypoint.com/~chrisw
- Follow-Ups:
- Re: Lift Lug
- From: Padmanabhan Rajendran
- Re: Lift Lug
- Prev by Subject: Re: Lift Lug
- Next by Subject: RE: Lift Lug
- Previous by thread: Re: Lift Lug
- Next by thread: Re: Lift Lug
- About this archive
- Messages sorted by: [Subject][Thread][Author][Date]