Question about a peculiar elastomeric bearing

[bridgebuster]

I need a sanity check. One of my coworkers is designing elastomeric bearings to replace existing steel sliding bearings. These stringers are rolled beams. The new bearings will have sole and masonry plates. The width of his bearings are at least twice the flange widths. He was told to do this in order to keep the height of the bearings the same as the existing steel bearings; this is to avoid modifying the pedestals.

My initial thought: No good because the reactions won’t distribute uniformly over the neoprene. I can’t find any documentation that an elastomeric bearing has to be the approximate width of the flange (maybe there isn’t any because no one does this?) I did rough calculation assuming a beam on elastic foundation, which indicates unequal load distribution.

Has anyone ever seen bearings where the elastomer is twice as wide as the flange?

 

[Lomarandil]

It doesn't strike me as irregular to have a wider bearing pad, but I share your concern.. you would need to justify the uniform load distribution (or that you could live with a non-uniform distribution?)

Are stiffened or thickened sole plates an option?

 

[SlideRuleEra]

bridgebuster - I have a 1984 DuPont paper titled "Engineering Properties of Neoprene Bridge Bearings" on this page of my website: Link

The section on "Shape Factor" (page 5) appears to imply (but not state) that the full crossection of the bearing should be loaded - has to do with how the neoprene bulges at the edges under load.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[BridgeEI]

I haven't seen that before. However, would the pad work for the vertical loads if you assumed any portion of the bearing pad outside the bottom flange was "not there" or didn't contribute?

The height seems to be controlled more by expansion length with the new LRFD edition. I don't have my handy spreadsheet in front of me to verify, but that's what I seem to recall.

Just out of curiosity...are the pedestals wide enough to accommodate the much wider pad? With Our details in my part, that would very rarely be accommodated. Our pedestal aren't very wide so we usually end up rebuilding or modifying the new pads.

 

[hokie66]

Probably just a salesman who wants to sell larger bearings. The rubber outside the flange will not be doing anything except forming a receptacle for dirt and debris.

 

[Lomarandil]

Sorry, I don't deal with bearings every day, so I may have gotten some terminology wrong.

I assumed the sole plate (and masonry plate) has the same footprint as the bearing pad. Is that typically the case?

 

[bridgebuster]

Thanks for your replies.

I spoke to the department manager today. He agrees that it's out of the ordinary but it could work - uniformly distribute the load - if the sole plate can act as a rigid body. They're using a minimum 1 1/2" plate; maybe his point has validity. However, another concern I have is that these plans are going to the client in a week and a reviewer may pick up on it, which would be embarrassing since we're supposed to know how they do things. I disagree with the argument that we're saving the client money by leaving the pedestals. The concrete is 50 years old; the expansion joints have been leaking for years so the concrete most likely has a high chloride content; might as well replace the pedestals, besides trying to drill in anchor bolts with 3'-6" of headroom is no joy.

Lomarandil - I attached a copy of the DOT standard sheet for this type of bearing; you can see the terminology I'm using.

SRE - I forgot about the DuPont paper and I have a hard copy at home.

BridgeEI - I did a quick check straight compression -P/A - using your idea, some of them are OK. I'm going to check one fully.

hokoe66 - Yes - a big receptacle for dirt

 

[Lomarandil]

Ok, that's about what I thought. Thanks!

I agree with your manager that if sufficiently rigid, the sole plate can distribute the pressure uniformly (or nearly so). The question, of course, is whether the 1-1/2" plate is already rigid enough, or whether it can be made more rigid by stiffeners.