Seismic slip joints at stairs in non-combustible buildings

[wikidcool]

I've been called on to engineer steel stair systems for the non-combustible portions of mid-rise buildings in USA IBC governed seismic regions. They are usually 3-6 stories, mixed-use with residential on top and commercial on the bottom. There have been a couple pushing 10 stories which were parking garages/apartments. These buildings usually have a certain number of the lower non-combustible levels made of concrete walls and elevated post-tension concrete slabs. On occasion, I see the building EOR call for a slip-joint where the bottom of the stair stringer sits on the podium slab, so that it can slip horizontally 1/2" or so, but not vertically. Other times, I see no such requirement.

Can anyone enlighten me as to when this is needed? Is it a code requirement or an engineering judgment call? And who is responsible for deciding if it's needed - the building EOR or the stair engineer?

These stairs are usually in concrete walled shafts with concrete floor diaphragms. Seems unlikely (to me) that enough story drift would occur to ever need to accommodate it in the stringer connections.

 

[BridgeSmith]

I would think it would be related to the expected story drift, and whether the stair can accommodate that amount of movement as internal deformation, without exceeding any stress limits. If it can't, it has to be allowed to slip. As such, that would make it the stair designer's responsibility to decide, based on the expected drift, whether the slip joint is needed.

 

[JLNJ]

I believe the code has moved toward either designing the stairs as part of the structural system or isolating the stairs so that they don't add stiffness and attract load. This makes sense in that you want the stairs to remain reasonably functional, not attract load up until they fail at which point the main LFRS takes up the task.

I'm guessing that the guy who specified the 1/2" wanted the stair to float and not add to the stiffness of the upper section. That said, 1/2" doesn't seem like enough. If the drift is just 1/2", the stair could probably just deform and stay usable.

It sure seems like it's the EOR's responsibility to determine how the components work together. I don't see how you can expect the stair delegatee to determine the relationship of the stair (or the break in the stair at the podium level) to the whole lateral force resisting system.

 

[BridgeSmith]

If there's code provisions that require the stair to be either isolated or considered as part of the LFRS, then I would change my view of the responsibility and agree with JLNJ that it would necessarily fall to the EOR to specify if the stair is to be isolated.

If it is not isolated, I would think the EOR would have to specify what deformation it must be capable of accommodating, and possibly the minimum and/or maximum resistance to that deformation is required. It would then be up to the stair designer to meet the deformation and stiffness requirements. Since that would require some collaboration, isolation is probably a popular choice.