I-Beam Bending Strength 1

[XPDQ]

I am currently working with a Pillar Type Jib Crane. It appears that its cantilevered S-Beam was damaged at some point in time by a load that was applied to the end of the beam and at the bottom flange, perpendicular to the length of the beam. The beam has yielded from this torsional force and it appears the web has bent to make a radius (on one side of the web the flanges are closer together, on the other side of the web the flanges are farther apart). This deformation is tapered down the length of the beam and the fixed end of the beam has minimal deformation.

Is there a method to determine how much the bending strength of the beam about its strong axis has been reduced?

Is there a way to technically justify that this beam needs to be replaced or its use be continued?

Any help would be greatly appreciated.

 

[Lion06]

Check if it is within the tolerances per AISC.

 

[XPDQ]

The S-beam is out of tolerances per AISC. However AISC gives no indication to what degree these out of tolerances affect the strength of the beam. In fact the AISC gives no technical reasoning what-so-ever for the numerical value of these tolerances. The AISC simply states that these permissible variations are "Standard Mill Practice." These values have allowed me to determine that the beam was indeed plastically deformed from use/misuse and not as original from the manufacturer, but they give no guidance as to reduction in strength.

This particular Jib Beam seems to have been built with a high Factor of Safety with respect to bending among the strong axis of the beam, for which its use was intended. Replacing the Beam seems very impracticable if the bend of the beam only slightly effects its strength.

 

[IFRs]

You might consider proof loading it.

 

[kelowna]

Is there a method to determine how much the bending strength of the beam about its strong axis has been reduced?

IFRs is right, proofloading would be the best and less controversial way to prove it still works. I can not think on any theoretical approach that would work.

Is there a way to technically justify that this beam needs to be replaced or its use be continued?

I can not think on a way to justify its use technically, replacement yes, it is out of tolerance, it defformed plastically, out!

Another thought. Have you considered heat straightening? Have you considered adding stiffeners to maintain flange distance?

If it was my crane though, proof load or replacement

 

[kslee1000]

You have two problems here:

1. Depends on the rotation, the hoist may not be working with lift weight.
2. Torsion to both the arm (beam) and the support connection.

I wouldn't allow the operation without any corrective measures. Replace the beam seems a small price to pay.

And, yes, lifting devices are usualy designed with amber safety factors build-in, check with ASME code, as it governs the jib crane design.

 

[kslee1000]

Wrong word - "amber" should be "ample".

 

[Ron]

OSHA requires that crane parts not be loaded above their rated load capacity. This one obviously has exceeded its rated load capacity. Since this is a potential safety issue, I would replace any plastically deformed part.

 

[csd72]

Buckling failures can occur suddenly and with little warning - replace the beam!

 

[msquared48]

csd works in the Beltway where buckles fail a lot.

Trust his judgment.

Mike McCann
MMC Engineering

 

[rb1957]

the failure you're describing sounds abit fishy to me ... if i applied a lateral force at the end of a beam, i wouldn't get a torsion failure at the end where i'm applying the load. i also wouldn't expect to get only one cap deformed.

is this a welded section ?

 

[XPDQ]

The failure I am describing is a bit "fishy." It is unknown how the failure exactly occurred. Of course no one individual will fess up to allow me to know how or even when this occurred.

Maybe my description of how the deformation occurred is not completely accurate. Ignore that because we really don't care how the deformation occurred, rather if the beam is usable or not.

I believe this beam is as rolled.

 

[rb1957]

the safest thing is to proof load the beam, maybe by lifting something, maybe as an isolated part. remember (and this may be "preaching to the choir") that the proof load is higher than the maximum service load.

i suspect that some mgmt type is leaning on you to "show it good", and it probably is good ! (which the test would show). Analytically it sounds like a I-beam with one flange rotated ... something that is easy enough to analyze (re-calc I). but is the beam still straight ? what internal stresses result from the "accident" ?

 

[cvg]

contact your insurance carrier and ask for their opinion...

 

[XPDQ]

What is the best course of action in terms of heat straightening?

 

[Ron]

There are several crane inspection agencies that will inspect and certify the crane. You might want to call one.