Seismic Desgin of platforms 1

[carnahanad]

Hello,

We have a building in a Site Class F Design Category E location so we have special concentrically braced frames on our main structure. We are clear on the code for that. However, where we get foggy are platforms. There are platforms that are only 3 feet to about 8 feet tall inside of our main structure which don't connect to any of the braced frames or other main structure supports. We know they will need a lateral system to resist the sesimic forces, but we can't figure out if these platforms will also need the heavy duty SCBF or moment frame detailing. We'd like to just design simple braces for these short platforms if possible. I've been looking through ASCE 7 Chap 15 and I can't really find what I'm looking for.

Any thoughts?

Any thoughts?

 

[tai99]

Without resorting to code, my take is to look into the importance of each platform, from the point of operation and/or worker's safety. Categorize the platform as essential, or non-essential facility, then design each accordingly.

 

[JoshPlumSE]

I don't have the code in front of me. But, if they are relatively seperate from the rest of your structure, then the easiest thing to do might be to design it as a piece of equipment that is attached to your structure.

You'll have to design for some higher forces, sure. But, you might not have to follow all the seismic detailing provisions for each connection. Sometimes those requirements / restrictions can be too onerous for these little things attached to your structure.

 

[DHKpeWI]

I would consider designing it as a non-building structure per chapter 15 of ASCE-7, 2005.

 

[JLNJ]

We just had this discusion the other day.

I agree with DHK - non-building structure. A better option might be to call up the plans examiner to get the tentative OK prior to starting any real design.

 

[carnahanad]

Thanks for the input everyone. I think it's reassuring to see that other engineers have similar thought patterns when it comes to these platforms. I think we are going to do do a little more research and follow up on some of the leads you guys have given us.

Thanks again!

 

[DHKpeWI]

An additional thought: per Table 15.4-1 for concentrically braced frames you could use and R of 1.5 and an omega of 1. You would not need to use the Seismic provision of AISC 341. If the platforms are not very heavy using an R of 1.5 will provide larger loads but less detailing hassle.

 

[wannabeSE]

I would use chapter 13, Nonstructural Components, unless the platform has it's own foundation or the platform is over 25% of the combined mass of it and the building. Platforms are similar to penthouses and access floors that can be designed as components (see table 13.5-1).