Fatigue cracking of a countersunk bolt hole
Fatigue cracking of a countersunk bolt hole
(OP)
I'm working on a project where we're lengthening a bridge. The plan is to splice a new stringer to the existing. However, we're looking at a non-traditional flange splice on the tension side of the beam - countersunk bolts -to avoid impacting the vertical clearance over the road below. Someone said that countersunk holes in a tension flange are fatigue prone. I couldn't find anything definitive. Any thoughts?

RE: Fatigue cracking of a countersunk bolt hole
I would think the trick to doing what you propose would be finding a supplier of high strength flat-head bolts to provide a fully-tensioned connection that won't slip under service conditions. The fatigue stress limits are much lower for nonpretensioned bolts (Category D or E', instead of Category B for pretensioned bolts).
Rod Smith, P.E., The artist formerly known as HotRod10
RE: Fatigue cracking of a countersunk bolt hole
RE: Fatigue cracking of a countersunk bolt hole
Is your splice actually over the travel lane? The clearance outside of that doesn't matter under the AASHTO spec.
The way we do things, if lowering the roadway under wasn't an option, we would likely request a design exception from the FHWA (our governing authority), and just post it for the clearance we have.
As far as the bolts, we allow a few different types for our splices, but pretty much always get the twist-off type, except occasionally if they're only doing half a dozen or less, then they might use DTIs.
Rod Smith, P.E., The artist formerly known as HotRod10
RE: Fatigue cracking of a countersunk bolt hole
changing the cross slope below will be a major problem. The road is a major highway that will be widened and reconstructed in the near future. A design exception for the clearance won't fly. We're trying to minimize the amount of jacking because of the utilities on the structure.
RE: Fatigue cracking of a countersunk bolt hole
RE: Fatigue cracking of a countersunk bolt hole
MJCronin
Sr. Process Engineer
RE: Fatigue cracking of a countersunk bolt hole