Bearing Pad or Steel Plate for Storm Shelter

[WesternJeb]

I have a public meeting room with spans around 25' in length. This floor is framed out by steel WF beams, bearing on either CMU or Concrete walls.

This public meeting room is also serving as the roof of an ICC-500 storm shelter. There are two floors above this meeting room, which the roof of my structure must be designed for the collapse of during a tornado (not likely for a collapse in lieu of a tearoff, but code is code). I am pretty comfortable with the beam design itself using Blodgett's energy dissipation theory.

My question is, should I use elastomeric bearing pads or steel bearing plates for the beams? For the storm shelter aspect I would like to use bearing pads to help dissipate impact energy, but for the public meeting room I want to minimize any possible "bounce" from people walking that would be amplified by using a bearing pad in lieu of steel. These are also stronger and will help distribute the force better to prevent any localized cracking in the walls.

What are everyone's thoughts on this? Is the potential bouncing from the pads just a non-issue? Are there any issues with using the elastomeric pads?

 

[Struct123ure]

I have had bad experience with elastomeric pads. I would thicken and oversize the steel bearing pads to spread the load out over a larger area reducing stress.
If you do go with the elastomeric pads depends on what you choose but there are criteria that can respond to your concern. The manufacture would be your best friend, so you should reach out to them.
I would recommend leaving room for possible pad replacement in the future which is something to think about.

 

[WesternJeb]

Thanks for the reply Struct123ure. Interesting as I haven't had bad experiences with the elastomeric pads before, what have your issues been?

 

[BridgeSmith]

We use elastomeric bearing pads for highway bridge bearings all the time. They don't bounce noticeably with 70,000 lb trucks; they're unlikely to to have noticeable bounce with foot traffic.

 

[Struct123ure]

WesternJeb the issue with bearing pads as it relates to use in buildings is that what do you do if they fail.
They are used commonly in bridges as BridgeSmith mentioned, but bridge abutments are huge with lots of space correspondingly the bearing pads are huge and the loads are large but the stresses are low because of the size.
In buildings we have a 400x400mm column and we are placing a pad that's smaller then that. So the pads are stressed to like 95% of their ultimate capacity.
I've looked at a mid rise apartment building where these pads are failing and replacement is extremely difficult because the building remain operational with high loads, and little room to mauver. Bridges don't suffer the lack of space that buildings do.
I heard from some engineers that in Russia these pads are used, but they are installed with future replacement in mind meaning make the column 400x800 so that you could shove a jack or a new pad next to the exiting pad and then remove the existing.
Also there are spring bearing pads where instead of rubber it utilizes metal springs. I can't comment whether those are better, but they are pricier and sometimes you get what you pay for.

 

[BridgeSmith]

It's true bearings on bridges are more open. However, a failure of an elastomeric bearing would not be simple to fix. Maybe it's because our allowable stresses (typically 1.25 ksi) are way lower than what's allowed for buildings, but we've never had a failure of an elastomeric bearing, unless you count the ones that have walked out from under the girder because they weren't adequately restrained.

Anyway, the choice of steel or elastomeric seems like it would come down to the in-service and seismic performance.

If you're trying to use steel plates as seismic isolators, they're decent at dissipating energy without rebounding, but you're relying on friction between 2 steel plates, which is highly variable. There's also the potential issue of in-service sliding from temperature or wind forces, that typically creates very objectionable sounds.

Elastomeric bearings deform, providing some isolation, but not alot of damping, unless a lead core is added to the bearing. (ref. lead-rubber seismic isolators)

 

[WesternJeb]

I wouldn't anticipate more than a 1.25 ksi loading for the beams under typical loading. The beam bearing pockets will also be grouted full, which could inhibit any form of corrosion on the pads.

The loading of concern is for a host building collapse on the shelter roof and beams, not for seismic loading, although I am in SDC D.

 

[BridgeSmith]

So, are you looking for the bearings to provide a cushioning effect for materials impacting the roof of the shelter?

There are typically no corrosion issues with elastomeric bearings, regardless of exposure, even the reinforced bearing pads, since the reinforcement is fiberglass (FGP), cotton duck (CDP), or steel that's encapsulated in the rubber. If you're looking at reinforced pads to provide impact cushioning, FGP may be your best option. The allowable compression stress is a little lower than the steel reinforced, but they're much easier and cheaper to fabricate - they're made in large sheets and cut to size, unlike steel reinforced bearings which are molded individually. CDP has a much higher allowable compression, but they're not much of a cushion.

 

[WesternJeb]

That's correct, I am looking at more impact resistance / cushioning. Those are some wonderful tips, thank you BridgeSmith.