Seismic Story Definition

[JAX91]

I am designing an addition to a manufacturing facility in a moderate seismic region (SDC C). The building is approximately 440'x130'. I am adding a double row of columns to break the addition into two separate structural elements. The dimensions of one of the elements is 300'x130', which is the portion of the building to which this thread refers. The northern 30' (of the 130' span) will require a mezzanine halfway between the finished floor and roof. The mezzanine will run the entire 300' of the building, and will be designed for 93 psf dead load and 75 psf permanent storage live load (actual live load rating is 150 psf). The total seismic weight of the structure is estimated to be about 3500 kip, almost half of which will be at the mezzanine level. Even though the mezzanine covers less than 25 percent of the plan area of the addition, is it correct to assume the mezzanine is the first story and the roof is the second story?

In one bay I will be running four of the columns through the roof to hold a 50'x50' equipment platform. Top of steel for the platform will be about 5' above the roof. The platform will eventually hold up to 100 kip worth of equipment. I will more than likely need to use moment connections at the platform level. This platform covers about 6 percent of the plan area of the addition. Is it correct to assume the platform is a third story?

If there are technically three stories, the horizontal and vertical irregularities would need to be considered. I don't believe the building qualifies for any horizontal irregularities, but there are several vertical irregularities which come into question. The building would probably qualify for a Weight (Mass) Irregularity and a Vertical Geometric Irregularity. Luckily I am in Seismic Design Category C, so these irregularities need not be considered. Am I missing anything else?

 

[msquared48]

Any chance of separating the Mezzanine from the rest of the structure? If you can, the problem goes away.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[JAX91]

I don't think I have the room to separate them. The mezzanine is being built over an office area. I would need to shrink up the office area enough to add two more column lines and allow for seismic drift between the two structures. I would guess off-hand the client would lose at least two to three feet of distance in their 30' direction. The office area is already tight, and I can't imagine the client would be happy with losing this area. Besides the space issue, separating the two structures would double the number of columns in the area and increase the cost of the structure.