Steel Abutment Cap Design

[sdock]

I am working on performing some design calcs on a steel abutment cap for a bridge. Where I work, a common type of abutment is constructed of a steel beam sat on top of steel h pile. The bottom flange of the cap is welded to the steel H pile. So essentially the steel cap is laterally supported on the bottom flange where they are attached to the piles. The tip flange of the cap is not laterally supported anywhere. I am confused on what to use for the unbraced length when performing lateral torsional buckling calcs.

 

[BridgeSmith]

I would say if you have stiffeners welded to the flanges at the piles, it's braced for LTB at the piles. Most W-beams are probably stiff enough in the web to be considered braced for LTB even with out stiffeners, but that's just a guess on my part, since I've never really looked at it. We only do steel caps for pile bents, and even that is rare (abutment caps and nearly all bent caps we do are concrete). When we do a cap-on-piles abutment or bent, there's typically a pile under each girder, so LTB wouldn't need to be considered, since the bending load on the cap is nearly zero.

 

[Lomarandil]

If in some odd case you don't have stiffeners, there's an old BS code which recommends taking the LTB length as the points of support plus a beam depth "d" at each end to account for the flexibility of the web.

 

[AS_PEng]

I would think you'd need stiffeners at the pile bearings. Have you checked for shear capacity at pile locations? If you have stiffeners, then it's braced.

 

[BridgeNerdGuy]

Hard to imagine this cap without stiffeners at each pile. In that scenario the cap is braced by each pile.