Local Tension Stresses in Web of I-beam

[polskadan]

Hi all,

I have a question regarding a local check that I need to perform for a spreader beam. For this given application the lifting points on the beam are directly over the loading points, so in theory this means no shear is in the member , rather there is tension flowing through the web of the beam (although I also do have compression as well as flexure due to the eccentricity of the lifting point and angle of lift). My question is what is the effective area that I should consider as resisting the tensile stress? In a column it is as simply as P/A but in this case my 'A' becomes a local issue and thus an effective area is needed. Does anyone know of a reference for this effective area? Right know I am leaning towards using b_effective as the length of the lug plate however would like to know if this stress distribution has been referenced before in literature. Thank you ahead of time for your help!

 

[desertfox]

Hi polskaden

Can you provide a sketch? I can't follow how you have no shear in the beam.

desertfox

 

[polskadan]

Here is a very quick sketch up done in MS_paint of the scenario

 

[desertfox]

Hi polsadan

This site might help with the pure bending analysis

http://emweb.unl.edu/NEGAHBAN/Em325/11-Bending/Bending.htm

, as for the lifting lugs I would stress the welds based on there length and throat area as per this site:-

http://www.pdhonline.org/courses/s106/s106.pdf

 

[dhengr]

Polkadan:
Why not put your lifting points and load points on the same plate, and use a 6" or 8" pipe for the spreader, since it is a more effective column section, with no weak buckling axis? The pipe is slotted at each end, wide enough to fit over (around) the end plates (1" in the example below); with simple fillet welds for whatever compression and bending forces are imparted to the beam/column. The end plates might be shaped, t&b as you show, but one continuous plate 12" wide x 24" high x 1" thick. Then you add doubler doughnuts, fillet welded all around, at the holes to improve pin bearing area and match the shackles (shackle throat opening) you are going to use. You can then bore/drill the holes in the plates prior to welding the plates to the pipe column. Now, is there any question about the load path? Just common sense engineering judgement. Nice clean welding details, with no stress raisers, are an important part of this design.

 

[polskadan]

Desertfox, I appreciate the links, the second one is a good summary for anyone who is designing lifting lugs and I have followed the procedure on several lifting beams in the past.

dhengr, I understand what you are saying, however I am limited in my materials to an I-beam as the client has expressed that desire due to availability.

I have decided I will use plate stiffeners between the lifting points in order to lower the tensile stress going through the web. I was just hoping though that there was some literature on an effective width when a web experiences tension as shown in my previous attachment as i am not aware of any. O well, I appreciate the effort all! Thank you!

 

[DamsInc]

What is the plate thickness and strength of the lugs compared to the beam web? I would think that the design of the lugs with a hole in it (and connection) would be critical and the capacity of the web would be much greater than that. Would it not be conservative to take the effective length as the length of the lug?

Either way, I would doubt that the web would be a critical factor in the design, depending on your dimensions.

Check out "Design and Construction of Lifting Beams" by David Ricker, published by the AISC. I believe it was posted on this forum some time ago.

 

[dhengr]

Polskadan:
I rarely try to go out of my way to argue with a client, but I do sometimes try to educated them to the fact that rusty beam availability does not always lead to good clean design and detailing and reduce the complexity of the fabrication, in trying to use that rusty beam out in the yard. Consider the complexity of trying to reduce the web tensile stress. Load from the lug, through the two fillet welds (full pen. welds if you wish, likely not needed); into the flange, in bending and with cross grain tension, probably not an issue; and then through the web/flange fillet and some more welds and partly into the existing web and partly into the added web stiffener plates. Think about that split and mixed up load path for a few minutes. Remember the potential of high tri-axial stresses at each of these transition points, changes in direction and the roots of welds. And, remember that at each one of those transition points you might be introducing stress raisers, weld roots with tension across the root, undercuts, plate corner/edge undercuts or notches, etc.

So then, cope the flanges on the I beam back, and clean up the web at the web/flange fillet. Use a spacer plate the same thickness as the I beam web thickness. Weld that 180° on the top to two side plates. Then use two side plates 12" x 24" x .5" each, etc. etc. and slide these over the I beam web and weld for compressive load and moment. You will be limited by the weak axis buckling of the I beam.