bending stress upon lifting
bending stress upon lifting
(OP)
I have a structural member x" long of varying cross-sections. The total weight is 1970 lbf. In the middle of the member, there are 4 SAE8 grade bolts. I will be lifting one end up using a crane. I am concerned that the bolts will not be able to support the bending stresses. Any suggestions?
Thanks
Thanks






RE: bending stress upon lifting
RE: bending stress upon lifting
RE: bending stress upon lifting
you know how far apart your bolts are y", so the bolt load is 1970x/y.
RE: bending stress upon lifting
Thanks
RE: bending stress upon lifting
i don't think you need MoI as the moment is being reacted by a couple, rather than a stress distribution.
the moment at the joint is about 1970/2*x/2, and the couple is BM/(6.5/sqrt(2)) = (1970x/4)/4.5; but this is reacted by two bolts ...
bolt load is 1970x/4/9 = 55x
but i don't know what your bolt is (nor waht a grade 8 bolt means) ...
RE: bending stress upon lifting
Thank you for being so helpful. The bolts are 3/4" diameter with ultimate tensile strength of 150 ksi, yield of 130 ksi.
What is "BM"?
Thanks
RE: bending stress upon lifting
your bolts should be good for 44,000 lbs,
which means x < 800" should be ok,
but then the structure surrounding the bolts needs to be able to take this load too.
btw, how well connected are the various sections, i'm guessing they're welded. also, i'm fixating abit on the bolts, what does the section near the bolts look like ? i suspect that your sections are ok, and that you're joints are the biggest problem. in either case, you'll have a problem if the effective material (beit welds or steel) is lumped about the neutral axis (so that it can't react bending moments very well.
RE: bending stress upon lifting
Will the crane and rigging be performed in house? If you'll be using a local crane service, talk to them for rigging ideas.
Alternately, if you have material available, could you rig a temporary section to the assembly for erection purposes?
Good luck,
Daniel Toon
RE: bending stress upon lifting
I would expect that with 3/4" dia. grade 8 bolts, you don't have a problem.
RE: bending stress upon lifting
Wouldn't that be a worse case? Larger depth but only one bolt?
RE: bending stress upon lifting
RE: bending stress upon lifting
Can you explain the formula below?
BM/(6.5/sqrt(2)) = (1970x/4)/4.5
I agree that the diamond shape is less preferable.
Thanks
RE: bending stress upon lifting
This moment is reacted by a couple applied to the bolts, on a PCD of 6.5". I've assumed that the bolts are pitched on lines at 45deg to the loading line (say vertical) so that the bolt group is acting as two pairs of bolts (instead of as a diamond pattern). The distance between these two bolt pairs is D/sqrt(2) = 6.5"/1.414 = 4.5" (near enough).
So ... 1970x/4 = (2P)*4.5, P being the load in one bolt.
So the load in the bolt is (1970x/4)/9 = 55x
RE: bending stress upon lifting
Thanks.
RE: bending stress upon lifting
So just as you lift, consider it a pin/pin beam with the 1970 acting in the middle. Hence, BM is 1970x/2 in the middle. Now the bolt pattern must carry this moment and the shear (1970/2=985). Assume the bolts are centered on the neutral axis and are all the same size, then each bolt gets an even shear load (985/2) acting up and the moment puts a load acting perpendicular to a line running from center of bolt pattern to bolt. This load due to moment is BM/[(radius of bolt pattern)4]. Now vector sum the worst case location and this is your max bolt load.
Be careful with your failure mode as RB said. The bolt may be plenty strong and the beam material might fail. Check the bearing stress due to the bolt load. Also, worry about the beam if your cross section changes at the joint (maybe the flanges run out and thus decrease MOI).
Please forgive me if I'm not seeing the whole picture.