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Anchor bolt holes
- To: seaint(--nospam--at)seaint.org
- Subject: Anchor bolt holes
- From: Michael Hemstad <mhemstad(--nospam--at)mbjeng.com>
- Date: Mon, 19 Sep 2005 13:09:32 -0500
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
Mike Hemstad, P.E.
Meyer Borgman Johnson
Chris Towne wrote:
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."