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RE: Anchor bolt holes[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: <seaint(--nospam--at)seaint.org>
- Subject: RE: Anchor bolt holes
- From: "Chris Towne" <ctowne(--nospam--at)chaptech.com>
- Date: Mon, 19 Sep 2005 14:28:14 -0400
Chris Towne, P.E.
Chapman Technical Group
200 Sixth Avenue
St. Albans, WV 25177
From: Michael Hemstad [mailto:mhemstad(--nospam--at)mbjeng.com]
Sent: Monday, September 19, 2005 2:10 PM
Subject: Anchor bolt holesChris,I have a couple of opinions.1. The longitudinal forces you refer to are either due to some theoretical (read: overstated) code requirement or are from thermal expansion and contraction. Yes, I know you are required to follow AASHTO for the loads; but calculate your dead load and multiply by a reasonable coefficient of friction, and see whether the 50 psf wind forces (!) or outlandish traction/braking forces required are enough to break those bearings loose.2. Thermal forces are self limiting; the bearings move, the forces go away. Why are you so eager to lock those forces in? It won't hurt the bridge seat to have the "fixed" bearings slide 4 times a year. Having tight anchor bolt holes only increases the forces. If your bridge was 9 feet shorter, the codes says you could neglect thermal movement entirely. How much will it mess up the expansion joint on the fixed end of the bridge if something moves 1/2 inch?3. There are basically two ways these anchor bolts are installed: they're either cast in, and the bearings set down over them; or the bearings and beams are set and then the holes are drilled and bolts grouted. In either case, a half inch of oversize is really pretty tight. I would recommend more, and so does AISC. When I started designing bridges 20 odd years ago in Iowa, the favored detail was to weld large bars to the tops of the bearing plates, extending back into the abutment backwall. The beams were then set on the bearings and the backwall cast around the bars. Neither here nor there to your question, but it shows how far engineers will go to try to hold the bearings in place. I think it's a mistake.HTHMike Hemstad, P.E.Meyer Borgman JohnsonMinneapolis, MinnesotaChris Towne wrote:"It does matter in this case. These anchor rods were designed to transfer
longitudinal loads from the superstructure to the abutment. During design,
I didn't think about how much the holes would be oversized. Now, I think I
should account for the possible movement of 0.5" for these "fixed" bearings.
Is this too conservative to do this? As I'm sure you are aware, there will
be significant differences in the elastomeric bearing with movement as
opposed to being fixed.
Chris Towne, P.E."
- Anchor bolt holes
- From: Michael Hemstad
- Anchor bolt holes
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