general questions

[SoeSoe]

1) For the old-time hydraulic calc's: was it 1 foot neglected on each side of the pier to account for debris buildup, was it 18 inches??

2) What is the reason for greasing the top of an elastomeric bearing and installing a smooth plate above it: It's not a sliding bearing it's supposed to deform what would be the point of the calc's and the movement rating?

Many thanks.

 

[Qshake]

2' of freeboard has been a design criteria for a long time.

As for greased bearings I've never seen a greased elastomer bearing...but I bet I haven't seen a lot thing either...Been around since dirt though!

Regards,
Qshake

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[TTK]

Can you provide some background for your questions. Have you seen this bearing detail used somewhere?

 

[SoeSoe]

Ok Gentlemen,
I'm not talking about free board. I'm thinking of the old Railroad bridges, the kind with the concrete pier walls, the channel was analyzed assuming the the pier width plus 18" on each side of the pier and after a big storm the piers are vertical so there is indeed a big pile of debris in front of the piers taking up about an extra 18" on each side. I'm thinking around the 1900s this was a standard: I can not find it published. But, there are only a few pre WW2 books I own.

The greased bearing is used to relieve the shrinkage of the concrete, mainly in post tensioned. But of course all off this does not provide comfort: has anyone had experience where this worked over a long period of time. I haven't yet thought of anything wrong with it. I think I've read somewhere that galvanizing does not react well with concrete, but other than some vague recollection this doesn't help--suppose it could always be some other type of metal. If it works so well why isn't it used as a standard for all elastomeric bearings? Has anyone had elastomeric bearing walk away? Walk away because of loss of top fixity?

 

[Dinosaur]

Elastomeric bearings can walk out. I have never seen the elastomeric bearing detail you describe, but I can understand why someone might want to do something like this. What I have seen is a plate bonded to the top of the pad with a PTFE surface bearing against a sole plate that has a polished stainless steel plate to create a low fricting sliding surface. The reason is if you have a need for large horizontal movements, you can't get a traditional laminated pad to work. But if you can release the movement, you can make the pad work for the other design needs (e.g. rotation). The sliding surface releases the longitudinal motion, but the pad still deforms some and needs to be secured to the pier cap. If you use this type of solution you will need some type of lateral (transverse) restraint. Good Luck.