Exterior stair tower independent gravity system 1

[doejohn]

We're looking at replacing an exterior steel stair tower at an industrial facility. The stairs serve as a means of emergency egress for 4 elevated floors of an industrial-use building (not open to public, IBC occupancy Group F). The tower is non-enclosed, non-combustible construction, and fire-separated from the building by the building exterior wall/doors. Primary gravity framing for the stair tower uses HSS columns at the 4 corners of tower, keeping the stair gravity system independent of the building. The columns at the 2 "inboard" corners (next to the building) are less than 1 foot from the building exterior wall. Structurally, it would have been viable to omit these 2 inboard columns and attach the horizontal framing to the building, which we are considering doing for the replacement tower.

Question: Is there an advantage code-wise, from a fire-protection or some other perspective, to keeping the stair gravity system independent of the building? Applicable (US) standards include IBC, NFPA, and OSHA.

 

[Ron247]

I cannot speak about NFPA or OSHA but other than having to check the building for the added loads from the stair if the current one is independent and yours will be dependent I guess Seismic is the next thing to consider. Since I do so little heavy seismic, I will leave that to others.

 

[milkshakelake]

Not sure about OSHA/IBC but when I worked with power plants, we used to separate stairs going from one area to another with free ends or slip joints because they thermally expanded/contracted independently from each other. There were boilers and heat exchangers everywhere. I designed several exposed stair towers in this way. The walkway between the stair tower and power plant would be free to rotate at each end, or in one case, it was cantilevered with an inch of clearance.

An idea why the stair tower was constructed independently might be because it was modular. Sometimes, they prefabricate the whole thing and ship it to the site in one piece. It might have holes for crane rigging at the top.

 

[JLNJ]

We did a recent delegated exterior stair design because the architect thought it seemed simpler to require the stair guy to make his tower independent. That way there is no confusion about who is leaning on whom. Plus, the contractor building the building wasn't tasked with coordinating with the stair guy.

I say "seemed simpler" because the winning bidder for the stair didn't know anything about structural analysis. Or snow drift. Or the concept of tributary area. Or lateral forces. Or load path.

 

[racookpe1978]

Differential settlement of the stair tower and building is "acceptable" if they are on different foundation pads. Freezing/ice/snow buildup on the stairs and landings? Will they be exposed to weather, or be walled in? Leaves and debris if near trees become ugly, mildewed messes in corners.