Steel Stair Case (Lateral Design)

[chamokinawan]

I'm doing a steel stair case with the main building engineered by another consultant. Gravity load is fairly straight forward, but the lateral load... What chapter of the ASCE would I be using? How do I get my lateral load for my shear connection to the building? I assumed I would just get an Fpx using the same Cs value from the main building. Since it is a multi story, I assume I would be using vertical distribution, so basically treating it like a regular building.

 

[KootK]

If I understand correctly, the main building is the lateral system for your stair. As such, your stair would be accelerated identically to the floors of the main building. I'd expect that your seismic force at each level is W_stair/W_diaphragm x diaphragm seismic force.

That being said, I suspect that there is an ASCE section that applies. When I studied for the SE exam, I remember looking at precast wall panel attachments and there was something special for that I think. I'd expect this to be similar.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[mike20793]

Yes, like Koot said, you just need to calculate your diaphragm force at each level and design the connection for that load. If the stairs are large, it may be beneficial to try and get some frame action out of them so they can resist their own lateral loads and the connection to the structure is incidental. ASCE 7 Chapter 12 will give you the seismic force and ASCE 7 wind chapters (chapters vary depending on the year) will give you the wind load. I've found that for most of my jobs (low to moderate seismic) wind will control on these open relatively light weight structures.

 

[wannabeSE]

I beg to differ. Unless the stairs are a separate structure, I have always treated stairs as a nonstructural component and followed the provisions in chapter 13. If they are egress stairs, Ip = 1.5 per §13.1.3.1. Also, interstory drift should be considered in the design and detailing

 

[Ron]

chamokinowan....where are you located? That makes a big difference in stair design.

Most of the above posts are considering seismic. If you do not have a significant seismic consideration, I would not attach the stairs to the building at any place other than their end supports. If you do, the occupants will complain daily about noisy stair traffic.

I use a form of x-bracing on the bottom of the stringers to handle lateral loading, which is very minor unless you have a seismic consideration.

As wannabeSE notes, stairs are not considered structural steel under AISC.(AISC confirms this...) I agree with his approach.

 

[chamokinawan]

@Ron: I'm in Los Angeles, CA so seismic is definitely a big deal. Where in the AISC does it mention stairs as not structural steel?
@wannabeSE: Yes, they are egress stairs. I figured there might be provisions in Ch. 13, but could not find anything on "stairs".

Thanks guys.

 

[Wallache]

Asce 7-10, last line of Table 13.5-1 shows stair design coefficients.

 

[chamokinawan]

Thanks Wallache.
Last question: Since I am using 13.3.1 to determine my Fp, I'm sure I should use my live load in the operating weight, but I know there should be a percentage of it.Is it the 75% from the D+.75L+.525E combo?