Seismic Joint Bearing Condition

[jdgengineer]

We have a situation for a single-family residence where we are trying to extend a steel beam (approximately 20' long) which is serving as a front entry bridge on top of a concrete retaining wall at a daylighting basement condition. We are trying to allow some movement at this interface to serve as a seismic joint and for temperature expansion. Our detail right now has a steel embed plate cast into the top of the concrete wall with some threaded rods sticking up for attachment. The steel beam is sitting on a bearing plate that has slotted holes in the direction of the movement.

In these situations, is it common to put some type of spacer between the steel bearing plate and the embed plate to minimize friction between the two surfaces to allow for movement? We are hoping to provide 2" of separation movement. It is an exterior condition with the steel serving as the front entry bridge. Teflon coated washer between the surfaces? Or do you think this is overkill and to just make sure the nuts are not too tight to allow for some movement. I know in bridge construction it may be common to put in an elastomeric pad (neoprene or sim) but not sure if that would be common in a building application.

 

[BridgeSmith]

You only need to put in a spacer for one of 2 reasons - 1) The retaining wall or the connection at the other end of the beam is not adequate for the friction force that may be generated when it moves during a seismic event, or 2) It will move, even slightly, under wind or other loading under normal service conditions. Metal sliding on metal can make for some very annoying squeaks.

For seismic events, friction is generally a positive thing, as it provides a damping effect to the oscillations. As long as the movement is limited so there is no unseating, and the force generated does not exceed the capacity of anything in the load path, it shouldn't be a problem.

That said, I would still provide a thin elastomeric or cotton duck pad to preclude any noise issues. A 1/4" or 1/8" thickness should do it.