Fatigue cracking of a countersunk bolt hole

[bridgebuster]

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?

 

[BridgeSmith]

The domed head on a 7/8" HS twist-off bolt is 9/16" high. Is your clearance really that tight and inflexible?

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

 

[bridgebuster]

That's a good idea, twist off head. We might be able to spare 9/16". We need to minimize jacking because the bridges have numerous utilities.

 

[BridgeSmith]

On those occasions where we've needed a few extra inches of clearance, we typically approach the road design guys and ask them if it's possible to lower the cross road under the bridge to gain the extra clearance.

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

 

[bridgebuster]

Rod,

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.

 

[dvd]

It is possible to lose tension on a fastener with countersunk head due to creep and burnishing on the limited contact area between countersink and fastener head - the angular tolerance on countersunk fastener heads and countersink seats, plus surface imperfections on the countersink almost assure line, or point, contact. Also, if hot-dipped fasteners are to be used, use caution that hydrogen embrittlement of ASTM F835 flathead fasteners needs to be thoroughly understood. Flathead fasteners are typically used in medium-strength applications, not on structural steel.

 

[MJCronin]

Post this in the "Structural Engineering General Discussion" Forum ....

MJCronin
Sr. Process Engineer

 

[bridgebuster]

dvd - I apologize for the late reply, You raise good points. It looks like we're going to drop the idea. We don't think the owner would be receptive.