Lifting Beam - Load Test and Lateral Torsional Buckling 1

[RFreund]

If the contractor would like to use a beam that is 'grossly' oversized for the required picking load. Does the beam still need to be tested to 125% of its rated capacity per OSHA (I don't believe BTH addresses this)? Maybe there is a multiple of the intended picking load?

It appears that the BTH manual addresses lateral torsional buckling of lifting beams where the ends are unrestrained against twist or lateral displacement at supports (the typical case) with a Cltb factor. It appears the typical AISC LTB equations are multiplied (reduced) by this factor. Is this correct? and Is there a reference for the Cltb factor. It appears to have changed in the 2014 version.

Thanks!

EIT

http://www.HowToEngineer.com

 

[WARose]

I'm not familiar with the "BTH manual" so I cannot say......but reduction of moment carrying capacity is appropriate in a lift beam situation (as compared to a "simply" supported condition). The buckling capacity will vary based on height of cables, angle the cables are at, location of cable support, and so on. Based on these variables, the LTB capacity can be greatly reduced.

 

[dhengr]

Rfreund:
My take on the situation is that if I design a spreader beam, lifting beam/system, that I want rated at 100kips, I would load test it to 125kips, then paint 100kip cap’y. on the side of it. If I am going to make a couple lifts, and have a ‘grossly oversized’ beam in stock, which will work for the purpose, I would design the lifting lugs, etc., with the rigger’s equipment in mind, generally check the beam for other critical design considerations, and then test it to (1.25)(max. load to be lifted, plus DL’s). You might want to stencil this lighter load on the beam. I would check with OSHA to see if this is actually required if its only going to be used a couple times and then decommissioned. You should not/would not be required to test the spreader beam to (1.25)(it’s max. strength) just becuase you used an available, very heavy beam, to lift 20 tons.

 

[RFreund]

WARose - Sorry BTH is below the hook. And yes the situation you're describing is what I was getting at. Surprising there is not much information available on this. As a matter of fact the very popular AISC Engineering Journal 1991 Design of lifting beams article by Ricker does not discus this.

DH - Thanks, that makes sense. I will ask if they intend to use this again for heavy lifts, that is a good point.


EIT

http://www.HowToEngineer.com

 

[TehMightyEngineer]

Similar to what ghengr said, I would have the contractor mark a sticker with the weight of the load to be lifted, this will satisfy the OSHA requirement that lifting beams must be marked with their rated load. You then lift the load a few inches off the floor with 25% or more of dead load onto the lifting beam (sandbags, scrap steel, weights, etc.). There, load test done; "inspect" the beam (likely nothing worth seeing), and proceed with the lift. If you use your load to be lifted as part of your test load you don't even have to unhitch it. After the lifts are completed rip the sticker off and you're done (or paint the load rating on permanently, doesn't matter).

One item to watch out for is any items that are not hugely oversized should be taken seriously. We once welded a few lifting lugs on a very oversized scrap beam. Had it load tested so we could check the welds on the lifting lugs. I believe it's now back in the scrap pile where it started.

Maine Professional and Structural Engineer.
(Just passed the 16-hour SE exam, woohoo!)

 

[TehMightyEngineer]

As for the LTB factor I too have had this question of where the "Cltb" factor was derived, if you find out please post it here.

Maine Professional and Structural Engineer.
(Just passed the 16-hour SE exam, woohoo!)

 

[WinelandV]

A few things

ASME BTH-1 is meant to be used alongside ASME B30.20. B30.20 is where the 125% load test comes from, if I recall correctly.

In addition to the rated load, the Design Category and Service Class need to be marked on the lifter.

RFreund, are you the contractor's engineer for rating this beam?

A key thing about the BTH, is that it was originally written by mechanical engineers - such, all their equations will match up very well with what is in the 9th edition AISC manual, including the Cb factor. AISC 360-05 (13th edition) uses the "newer" Cb equation. The reference paper for that is:

Galambos (1998), Guide to Stability Design Criteria for Metal Structures, Structural Stability Research Council, 5th edition, John Wiley and Sons, New York, NY

The original Cb reference that the AISC commentary gives is:

Salvadori, M. (1956), "Lateral Buckling of Eccentrically Loaded I-Columns", Transactions of the ASCE, Vol. 122, No. 1

 

[RFreund]

What is Odd is that I have checked the latest edition of Guide to Stability Design Criteria for Metal Structures by Ron Ziemian (great guy that Ron). I will check an older edition if I have one. However they don't provide much information. The reference the Dux and Kitipornchai document (reference in BTH, see below)however I can't seem to find this paper, well, for free anyhow.

Here is the reference from BTH commentary:
Equations (3-10) through (3-18) are based on the
behavior of beams that are restrained against twist or
lateral displacement at the ends of the unbraced length,
Lb. Suspended beams exhibit different behavior with
respect to lateral torsional buckling (Dux and
Kitipornchai, 1990). I-shape beams show a buckling
strength less than that predicted by the standard elastic
buckling equations at proportions where (Lb /bf )/
EIx/GJ is greater than about 1.6. Tee shape beams show
reduced buckling strength at all proportions. The coefficient
CLTB in eqs. (3-16), (3-17), and (3-18) accounts for
this reduced buckling strength


Dux, P. F., and Kitipornchai, S. (1990). “Buckling of
Suspended I-Beams.” Journal of Structural Engineering,
116(7), 1877–1891


EIT

http://www.HowToEngineer.com

 

[WARose]

Dux, P. F., and Kitipornchai, S. (1990). “Buckling of
Suspended I-Beams.” Journal of Structural Engineering,
116(7), 1877–1891

I have that paper myself. It's pretty good except that in many of its graphs, it figures buckling loads in terms of a "non-dimensional" value. That makes comparison from one graph/lifting scenario to the next somewhat difficult. (I.e. is this value for the same beam from table to table?) I came away from the paper thinking the most critical scenario is the 90 degree case (i.e cables perpendicular to the lifting beam).

Probably the best paper I have on this subject is 'Distortion Buckling of Steel Beams' by: Essa & Kennedy. University of Alberta Department of Civil Engineering, April 1993'. It has a buckling formula for suspended beams (p.208-210) that I have seen used in other references (including 'Guide to Stability Design Criteria for Metal Structures').

Of course, the best approach of all is selecting a beam where LTB is eliminated (like a HSS section).

 

[boo1]

'Distortion Buckling of Steel Beams' by: Essa & Kennedy at:

http://www.civil.ualberta.ca/structures/reports/SER185EssaandKennedy.pdf