Building Seismic Expansion joint

[Mch2018]

I am doing a building in California the will have a partial steel podium with wood on top (3 stories of wood. We are thinking we will need to have a building expansion joint were the steel stops to allow structures to move interdependently. So one side will be 4 stories of wood, the other 1 story steel with comp deck with 3 stories of steel on top. It will also have one atrium area where the steel will go up to level 3.

I was wondering if anyone had pointers on designing this, and sized the joint we need. Also if there is anything in the California building code that addresses this.

Thank you!

 

[dreber]

There's a million ways to skin a cat, but basically, calculate the deflection at each relevant location (ASCE 7-10 § 12.8.6) and make sure the gap is large enough the the two structures don't whack into one another (that's the technical term). I would plan to span a steel plate over the gap and attach the plate on one side only - make sure there is nothing structural in the gap that would prevent movement.

I'm no expert though as I've never actually done this myself. Here is a detail I like to use for accommodating inter-story drift on stairs.

 

[Agent666]

Some codes let you take the required separation/gap to be the SRSS value for required clearance. For aligned floors pounding is less of a concern than one floor pounding halfway up the columns of the other structure (you might have to allow full separation for maximum drifts in this scenario and/or some allowance for the maximum credible earthquake drifts). As damage to the column can have a detrimental effect on ability to carry the gravity loads, and the forces developed during pounding cannot be predicted.

So basically acceptance criteria might differ based on actual scenario and elements that might be able to come into contact.

Two schools of thought as well for the location of the gap, double columns vs corbel for the interface. As to which one suits better its a matter of evaluating details and constraints. If using the corbel taking SRSS deflections may not be suitable as in theory if the worst case conditions for deflection eventuate then the floor might come of the support corbel. Here in NZ we are required to factor the deflections for this scenario up to approximately MCE level so there is lower risk of loss of support. This was in response to several failures of this type of support in stairwells in the Christchurch 2011 earthquakes.

Have you considered actually tying the lighter weight wood structure into the steel/concrete structure. Seems like it would be a easy way to Brace the wood part in one direction at least.