Stair Requirements in Structural Models

[dreber]

Hi everyone. I don't know if this has been discussed here before. If so, my apologies. However, I wanted to point out some changes to the requirements of building models with ASCE7-16. Namely, if the stairs in your building don't have some sort of ductile elemet/slotted connection/ or similar solution to accommodate interstory drift, then you have to include the stairs in your building model and design your stairs to accommodate the resulting induced forces. See ASCE 7-16 § 13.5.10

 

[WARose]

Makes sense. For years, people (including me) have been omitting this in their models.

I guess they want people to be able to get out of the place after a quake. (As opposed to standing on the third floor and yelling "Hey, we need a ladder here!" )

 

[TLHS]

Fully restrained steel stairs also create an incredibly stiff lateral load path that can easily suck up seismic forces in a building with a moment frame lateral force system. I wouldn't consider this a change, just a codification of good practice.

 

[ENGINEER92]

I would imagine this would require two models. One model that includes the stairs, so you can design the stairs for the inter story drift. Then a second model without the stairs, so you can design your building's lateral system without the stairs taking load away from them.

 

[dreber]

Engineer92, I interpret it a little differently. I see this as a way of making it the building engineer's responsibility to ensure that code requirements for slip connections are followed.

Many commercial structures in California are designed with holes in the floor and the requirement that stairs be designed as part of a deferred submittal from the contractor. The contractor then hires an engineer, and seeking the cheapest option, pressures their engineer to omit slotted/slip connections.

With this new wording, the onus is being put on the building engineer to either make sure the stairs are designed correctly, or to INCLUDE the stairs as part of the LFRS.

 

[ENGINEER92]

Two models is not the intent of the code, that was just my first reaction. The commentary on that section gives a good explanation on the section.