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Re: Adding a mezzanine to a Masonry Building in Seismic Zone 4

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A few quick comments.

I am assuming this is a one story building, not a two story with a high first story.

If you are adding more than 5% of the current demand to the walls, you are required to meet todays code for all the walls you are attaching to.

I would design the mezz. using the base shear from 97 ubc (non simplified) for the transverse eq. loading and scale up the diaphragm shears to the masonry wall at the three side by the R ratios. Parallel to the long direction, to avoid the rigid analysis, you should probably use the simplified base shear.

If you have transverse shearwalls in the mezzanine, this may cause the masonry portion to become braced and impose load into the mezzanine... try to span the diaphragms all the way across (100 ft) if the aspect ratio works. Use high-load diaphragm nailing if necessary. Otherwise, you may need to add a transverse masonry shearwall at the middle (or near) to make both your mezzanine work and to not support masonry with wood shearwalls.

If the existing building's roof is wood, the diaphragm is flexible and you should re-analyze (if necessary) using the base shear from 97 (non simplified). One thing that probably won't work, if your design is 40 years old, is the out of plane masonry wall strength.

Check the wall anchorage at the roof of the masonry building for today's code if you violate the 5% mass increase.

Also, if you are cutting any openings in the masonry walls, be sure to detail strong backing at the jambs and lintel.

... off the top of my head.

San Francisco, CA

>>> dennis.wish(--nospam--at) 05/16/01 10:38AM >>>
I need to put together a proposal to add a wood frame mezzanine to an
existing masonry building in Palm Springs. The building is more than
40-years old. The building is roughly a 3:1 aspect ratio (about 100-ft x
35-ft) and the mezzanine (wood frame) will take up only about 1/3 of the
floor space (33' of the 100-ft width).
The mezzanine will provide rooms which will be used for off space on the
first and light storage on the second floor. The mezzanine will attach at
the second floor diaphragm to the existing masonry building on three sides.
The masonry walls are 100% solid where the mezzanine occurs. One side of the
mezzanine is open at the second floor (it overlooks the first floor) and the
first floor walls will provide shear (lateral) resistance at the open side.
However, the second floor open side has no shear connection to the roof.

I would normally have added the mezzanine dead load into the existing roof
dead load (adding about 5-psf over the entire roof), and re-calculate the
lateral analysis to the masonry walls to verify that they have the capacity
to resist the additional shear. At the open side of the mezzanine, I would
have calculated only the second floor diaphragm shear transfer into the
first story walls to resist drift. Considering that there the two structures
are tied together, does this seem like a reasonable plan? The lateral load
from the roof is distributed to the four masonry walls and would include the
weight of the mezzanine - but the open front of the mezzanine is the only
side unaccounted for that I would use the first floor shearwalls.

So here are a couple of questions for Zone 4 areas;

1. Is it appropriate for me to simplify the design by using the guestimated
base shear (conservatively) at 0.186Wd to calculate the lateral load caused
by the mezzanine and verify all existing masonry walls to laterally support
the addition?

2. Would I be expected to bring the structure into compliance with the
current code (near source values, full-compliance by flexible and rigid

3. Would I be expected to use flexible design for the unsupported side of
the mezzanine based on the Simplified Static Design [0.3ICa/(1.4R)]*Wd ?

Dennis S. Wish, PE

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