Some quick thoughts...
I would look at providing the mezzanine with an independent vertical and
lateral system if possible. If you increase the seismic load to any
existing wall by more than 10% (varies depending on jurisdiction, see
Chapter 34 volume 1 "lateral loading to existing elements does not exceed
their capacity", and Appendix 34, repairs to existing) you will need to
bring the existing structure into compliance with current code provisions.
Watch out for change in occupancy requirements, or you may have to bring the
structure into compliance regardless.
If you tie the mezzanine to the existing walls you are essentially bracing
the existing walls at the mezzanine level, your new lateral elements for the
mezzanine will be loaded by the masonry (i.e. no wood shearwalls resisting
loads from masonry).
Another alternative would be to provide new masonry or steel braced frames
below the mezzanine and design for the mezzanine to brace the ext. walls.
Provide ledgers and anchorage to the existing walls. The seismic loads
could be balanced where the new loads imposed on the existing walls at the
perimeter would be offset by the reduction in existing demand taken by the
----- Original Message -----
From: "Structuralist" <dennis.wish(--nospam--at)gte.net>
To: <aec-residential(--nospam--at)polhemus.cc>; "SEAINT Listservice" <seaint(--nospam--at)seaint.org>
Sent: Wednesday, May 16, 2001 10:38 AM
Subject: Adding a mezzanine to a Masonry Building in Seismic Zone 4
> 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
> The masonry walls are 100% solid where the mezzanine occurs. One side of
> mezzanine is open at the second floor (it overlooks the first floor) and
> first floor walls will provide shear (lateral) resistance at the open
> 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
> 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
> 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
> 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
> base shear (conservatively) at 0.186Wd to calculate the lateral load
> by the mezzanine and verify all existing masonry walls to laterally
> 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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