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Elastomeric bearing - suspect

Elastomeric bearing - suspect

Elastomeric bearing - suspect

(OP)
Hi

is the first time that am using a elastomeric bearing for an skybridge. After the installation it looks very very suspecious. According the calculation the pressure should be 5N/mm² (by full self weight of the skybridge) now during the installation we have just 25% of the load and the bearing is already so squezed out. Can this be?

By the way the bearing capacity is specified for 15N/mm²

the free overhang edges coudl be also cut off, or?

Is this maybe why it looks suspect or is there really something wrong?

What can go wrong here?

Lolobau

RE: Elastomeric bearing - suspect

Based on the photo, this doesn't appear to be an expansion joint. Rather, it looks like a bearing assembly intended to accommodate rotation and smooth out the delivery of the load to the supporting structure. Is that correct?

Are we looking at the end of your bridge or a mid-span support? Does the bearing pad look equally compressed on the side that we don't see? If this is an end support, you can pretty much expect the bearing to take on a profile matching the slope of the bridge structure at that location.

What benefit do you hope to accrue by cutting of the overhanging edges? The only thing that I can think of is that it might reduce the extent that water will make its way under the bearing pad. And even that is dubious.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: Elastomeric bearing - suspect

(OP)
Hi Koot

yes you right. This is the images of basically "the fixing" end of the 15mm long sky bridge. On the other side we have more less the same scenario but just with slotted holes.

Here at the fix end I specified also the same elasto metric bearing just to accommodate the slope of the bridge. There is a slightly slope. Slotted holes were here not really needed since the bridge can expend to the other side.

To reduce the water guidance is the one thing but there other one is the Architect and his aesthetic issues.

RE: Elastomeric bearing - suspect

It is possible that the elastomer is not as hard as specified. Have the hardness checked. It also looks like the bolts have been tightened. There are marks on the nuts that indicate perhaps an impact wrench was used. Make sure the bolts are not tightened beyond what you have specified.

RE: Elastomeric bearing - suspect

Normally in bridge work we use thinner layers of elastomer separated by 3 mm steel plates to avoid this type of problem.

RE: Elastomeric bearing - suspect

Bearing pad material should have load deflection curves you can use to check if your deflection is correct. Like Ron said, maybe the hardness isn't correct or as specified.

Bearing pad capacity is affected by the extension of the bearing pad material beyond the loaded area. If you trim the edges flush, you may limit the pad capacity (think of concrete pedestals that are larger in area than the baseplates they support).

Have you contacted the bearing pad manufacturer?

Architects and their aesthetic issues??!! I guess failed skybridge bearings are quite attractive to them?

Bob

RE: Elastomeric bearing - suspect

(OP)
Bob hahaha smile you right

I spoke to the supplier today he said that it was the specified hardenes and type but he just refuses to give it to me in writing...hhmm...

actualy there is not much what coudl happen or?

Is there a difference between elastometric bearing and just an rubber bearing?

Lolobau

RE: Elastomeric bearing - suspect

Hey... If he refused to give you anything in writing on the product, go to another supplier or manufacturer. This should not be on your nickel for the liability here.

Mike McCann, PE, SE


RE: Elastomeric bearing - suspect

I'd expect to see stiff plates above and below the rubber, cut a little bigger than the rubber itself, so as to distributte the load even when the rubber swells laterally under load.

As shown, the gross active area of the rubber that's actually being used is the product of the flange widths, so you're using maybe half of the rubber you paid for. ... and it's probably stressed twice as much as you calculated.

The bolts are way too short. I'm guessing the nuts were cranked until the bolts were flush to the nuts' distal faces. ... so the deflection may be due to bolt preload rather than structure weight.

Mike Halloran
Pembroke Pines, FL, USA

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