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Re: Uplift resistance

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Here is a detailed description of my structure and repsonses to your suggestions:

Code:  UBC 1997 w/ AISC 1997 Seismic provisions, Supplement #2.
Building:  Industrial / Office building
Seismic (WA Zone = 3) controls for lateral force system.

Overall 3 story building w/ OCBF on 1st and 2nd floor and a combination of OCBF and OMF on the 3rd floor. OCBF are 2-story X-brace with the 3rd floor frames being inverted V. The first floor is SOG and the 2nd and 3rd floors are open-web bar joists with bays at approx. 28' o.c. Decking is 20 ga deck with 4" NW conc. Building dimensions are 138'x74'. The 2nd and 3rd floor diaphragms are only resisted by one OCBF in the 74' direction creating a 138' wide diaphragm at each floor. Due to plan requirements no internal frames are possible in this direction.

I do not want to bury the footings any deeper because the TOF is what the brace frames sit on. The TOF is 12" below TOS so that the braces/columns will all be set and the SOG will be pored up against it. This will also conceal the base pl and anchor bolts for the interior columns.

I'm game to increase the size of the footings since most of them are 20' +/- o.c. The only problem is with this magnitude of uplift, the footings go from 8' sq x 15" to 14' sq x 5'-0". I only have 7 footings that are having this problem but that's still a lot of concrete. This is an option but not preferred.

Overturning force: I calculated overturning using the load combination 0.9D - E/1.4. I did not use an overturning factor of 1.5 because I could not find reference for this in UBC 97 for footing or Steel design nor general code requirement. No, this is not design build, but the owner is the contractor on this project and every item we add cost to the project we get a full discussion about keeping cost down.

I followed the path from 1605.2.2 which directed me to 1611.6 for retaining walls and 1626 for seismic. The retaining wall section requires the retaining walls to be designed for 1.5 SF in overturning which does not cover what I'm doing. Section 1626 is the overall section for seismic design. Looking in there, 1630.8 talks about overturning but states simply that you need to design it for the story shears. I also could not find any reference under section 18 for footings. If anyone has any add'l info please let me know.

We have also contacted the Geotech on the job and are awaiting a response. I'm just trying to determine what the possibilities are.

Thank you all for your responses and if you have any further insight / observations, they are greatly appreciated.

Seth W. Cutler
R. L. Morrison Engineering Co.

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