Transporting Stress Limits & Loads Specification

[BridgeEI]

Is there a guide by AASHTO or other agency that defines loads/stress limits/etc on items that are being transported? For instance, tensile stresses/lifting points in a pcb or steel girders? I figure fabricators, or their engineers, use something but wasn't sure of a publication or if there are rules of thumb.

 

[bridgebuster]

None that I'm aware of. I have seen DOT shipping requirements.

 

[EQguys]

I have not seen guidance on stress limits for transportation and erection of girders. For Prestressed girders, my experience with Contractors has been that they prefer two-point pickup without a spreader beam with the sling angle at 45 to 60 degrees. I have seen DOT details on tie-down anchorages for Prestressed girders as well. For steel girders, transportation lengths are based on either erection crane pick capacity, available maximum plate lengths for girder fabrication, or maximum length of the girders that can be transported without using specialty trucks or permits. I have not seen tie-down details for steel girders.

 

[Lomarandil]

For prestressed concrete girders, check out the PCI Recommended Practice for Lateral Stability of Precast, Prestressed Concrete Bridge Girders

https://pci.imanuscript.com/ProductDetails.aspx?productID=194

That covers both handling during transport and lifting.

(It's based largely on the work published by Mast in the 1990s, which you can probably find for free online if you want a preview).

For short precast girders, you can get away with a one crane pick and inclined rigging. For longer spans and more slender beams, we usually see two crane picks, with one rigged to each end. Especially as precast girders get more and more slender, the lifting case is actually more critical than the case when the girder is in it's final condition -- even with the similar span lengths. Concrete tension in the top corners usually governs that.

AASHTO says that stresses during transport and erection need to be considered, but I'm not aware of anything more specific being formally codified. Helwig from UT Austin had done some research on stability and adequacy of steel girders (especially curved girders). Like EQ mentioned, steel girders are often governed by other considerations.



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The name is a long story -- just call me Lo.

 

[BridgeEI]

Thanks for the responses. I assumed that's what I was going to hear, but you never know. Thanks all!

 

[gwynn]

CSA-S6 (the Canadian Highway Bridge Design Code) limits stress in precast girders during transport and erection to 0.5fcr in tension and 0.6f'c in compression. The PCI manual methodology mentioned above for evaluating pre-stressed concrete girders during transport and erection is also accepted for I girders.

Stress limits in steel girders during transport generally only govern when girders are shipped on their side. I don't think there are definite standards for the limiting stresses in this case, but standard practice here is to complete a dynamic analysis of the girders subjected to both period and step excitations. The amplitude and period of the periodic forcing function is not codified, but is based on an expected out of roundness of tire, maximum rutting depth on roadway, tire size and expected transport speed range. The step function is based on the expected drop from running over a curb or similar. While this may not be what you are looking for, I think it was important to bring up. I see around one case per year where steel girders are shipped on their side and require reinforcing in order to transport.