How to accommodate large bearing rotations during construction?
How to accommodate large bearing rotations during construction?
(OP)
For composite steel bridges, the bearings undergo relatively large rotations during construction (i.e. the weight of wet concrete, any superimposed dead loads, creep and shrinkage in the deck).
For long spans, this rotation could be close to the rotation capacity of the bearing, e.g. ~0.015 radians for elastomeric bearings. This would leave no further ability to rotate and limits the allowable vertical loads on the bearings. In fact, it is a requirement in my local code that the faces of the elastomeric bearing are parallel at the completion of construction.
In my case, elastomeric bearings are really preferable because they keep the superstructure fairly well isolated seismically, which improves the design. Spherical bearings, while having a large rotation capacity, are probably not an option in this case because they will attract enormous earthquake forces which would have to be designed for.
I am wondering what options there would be to avoid over-rotating the elastomeric bearings during construction, such that they end up essentially at 'zero rotation' by the time construction has finished? I suppose using temporary bearings during construction and installing the permanent bearings after most of the rotation has occurred would be one option, but not sure how practical this is. Any other ideas?
For long spans, this rotation could be close to the rotation capacity of the bearing, e.g. ~0.015 radians for elastomeric bearings. This would leave no further ability to rotate and limits the allowable vertical loads on the bearings. In fact, it is a requirement in my local code that the faces of the elastomeric bearing are parallel at the completion of construction.
In my case, elastomeric bearings are really preferable because they keep the superstructure fairly well isolated seismically, which improves the design. Spherical bearings, while having a large rotation capacity, are probably not an option in this case because they will attract enormous earthquake forces which would have to be designed for.
I am wondering what options there would be to avoid over-rotating the elastomeric bearings during construction, such that they end up essentially at 'zero rotation' by the time construction has finished? I suppose using temporary bearings during construction and installing the permanent bearings after most of the rotation has occurred would be one option, but not sure how practical this is. Any other ideas?
RE: How to accommodate large bearing rotations during construction?
Thanks,
RE: How to accommodate large bearing rotations during construction?
The National Steel Bridge Alliance (NSBA) has a lot of great resources available for free. An updated version of the bearing document is coming out soon.
https://www.aisc.org/nsba/design-and-estimation-re...
Also, time dependent effects usually are not explicitly included in most composite steel girder bridge design. It is accounted for in AASHTO by using "short term" and "long term" section properties.
RE: How to accommodate large bearing rotations during construction?
I guess a potential (but more costly) solution would be to jack the bridge to reset the bearing.
RE: How to accommodate large bearing rotations during construction?
jorton & kewli,
Thanks to you both. It seems obvious now that you mention it that the girder could be landed with existing rotation at the bearings so that it ends up flat by the time construction is completed.
RE: How to accommodate large bearing rotations during construction?
Elastomeric bearings "typically" won't over-rotate during deck placement. They might slide a little until the DL & SDL camber is released. The way to deal with that is to reposition them and then weld them to the girders after the deck is placed.
gusmurr - one thing you didn't mention is skew. If the skew is zero, all four corners of each bearing should deform vertically by the same amount. If you have a sharp skew, the deck placement can do funny things to the bearings depending on how the steel was erected -it could make things worse - and if the thermal design length didn't account for the skew.
A few years back I had to investigate bearing issues on a new bridge with 30-degree skew. The contractor erected the steel vertical and didn't run the finishing machine along the skew. The girders were out of plumb and numerous attempts at resetting the bearings were futile. It didn't help that the designer didn't account for the skew in the length used for thermal expansion.
RE: How to accommodate large bearing rotations during construction?
Rod Smith, P.E., The artist formerly known as HotRod10
RE: How to accommodate large bearing rotations during construction?
As an interesting side note, yesterday I was visiting with a fabricator and asked why there were holes in the bottom flange at the end of the girder. A particular DOT requires the anchor rods to go through the bottom flange and not through the sole plate on each side of the girder. The problem was if the anchor bolts were preset, the end rotation of the girder due to dead load camber would not allow the anchor bolts to fit through the holes in the flange, even with oversized holes (the girder was 9 feet deep).
Don't ask me about the holes in the bearing pads for the anchor bolts. I seem to remember reading that was a bad thing.
RE: How to accommodate large bearing rotations during construction?
Three reasons we don't put holes or slots in bearing pads:
1) There's a reduction in the stiffness (shape factor) of the bearing pad that's a PITA to calculate,
2) the pads are more difficult to fabricate, and
3) If the anchor bolts go through the pad, it makes it a much more difficult process to replace the bearing pad.
Number 3 is the primary reason we don't put holes or slots in the bearing pads.
Rod Smith, P.E., The artist formerly known as HotRod10