Hydrostatic tension in elastomeric bearings 1

[OSUCivlEng]

The code and the commentary in AASHTO LFRD Sec. 14.7.5.3.3 seem to be contradicting each other. The code says "In bearings with externally bonded steel plates on both top and bottom, the peak hydrostatic stress shall satisfy:".

The commentary says "However, in a bearing equipped with external plates, upward movement of part of the plate can cause internal rupture due to hydrostatic tension. Provisions have been added to address this case."

I am designing bearing pads using Method B, and we never specify the pad to be bonded to the anchor (or sole) plate. The way I read the code, I only have to worry about hydrostatic stresses if my anchor plate is bonded to the pad. The way I read the commentary I should check hydrostatic stresses because I have an external plate.

Did they just forget to put "bonded" between external and plates or am I missing something here?

 

[BridgeEI]

First glance I assume they forgot bonded in their statement. You can always email them and they will look at it and let you know. I believe their provisions are based on an NCHRP report so you can try looking at that also. It's on my desk but I don't recall the report number.

I have my calculations to automatically check the hydrostatic stress but I have a note off to the side saying it's only required for bonded bearings. Most of the time the calcs check out and I don't need to have the bearing bonded, it's rare when I have called for it.

 

[OSUCivlEng]

I read through NCHRP Report 596. The best I can tell from reading the report is that hydrostatic stresses only need to be checked if you have an externally bonded plate. Page 14 actually states the only way hydrostatic tension can occur is if the external plate is bonded to the pad. The previous design specs didn't allow for lift off, so this was never a problem. That also explains why the bearings could get ridiculously thick (8"+).

The question remains though, why bond pads to an external plate in the first place?

 

[BridgeEI]

Expansion length dictates if it needs to be bonded. Once you have the bearing dimensions, you can calculate the total deformation (delta something) based on the bearing pad dimensions. Then you can calculate an expansion length based on your delta. if the calculated expansion length is less than the expansions length of your bridge configuration, then you need to bond the pad and check the hydrostatic stress.

I used expansion a lot so hopefully I didn't make it confusing...

 

[BridgeEI]

At least that's the way I handle it. There are a few other ways to look at it also.

 

[OSUCivlEng]

I've always designed the pad for the expansion length of the particular expansion length it is supporting (for expansion bearings).

I think a better reason to bond the pad to the anchor plate would be if the bridge had a tendency to "walk" off the bearings. I've seen this on a curved bridge before where one end of the bridge was moving off the bearings. That particular bridge had steel rocker bearings though.

I've also heard of people bonding the pad to the anchor plate when the span configuration causes uplift. I'm not sure that's how I would address uplift.