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

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Paul,
I originally thought of this one as well. I don't want to keep the systems
independent as wouldn't I have to create a separation between structures and
account for drift of each? By doing this, I lose usable space at the lounge
and office area above. If I need to frame above stud bearing walls below, I
would also lose leasable space below and provide more than one line of
foundations. Possibly I am wrong on the assumption that the two structures
would need to be separated and drift accounted for. Could somebody educate
me on this issue?

The shear resulting from the mezzanine is relatively small - certainly less
than 10% of the total shear in the direction of the long walls which are
100% solid. Considering the infill made on the Alley wall (total length
considering two stores is 80-feet), this is the only wall that raises
concern.

If the roof contributes 50% of the roof tributary area as well as the weight
of the masonry wall itself, the existing demand to the wall including the
lateral weight of the wall(roof at say 20-psf for 8000 s.f. or 160,000-lbs
and wall at 60-psf*20-ft tall *80-ft long at roughly 13.3% gravity [original
design lateral load], the lateral load to the masonry wall
0.133*(80,000-lb+96,000-lb) would be 23,408-lbs. The mezzanine at 18.6% of
gravity would be only 0.186*(20-psf*700*s.f./2) or 1,302-lb or closer to 5%
of the demand to the wall. Even if I designed the mezzanine conservatively,
the lateral demand is still closer to only 5% of the demand in the most
critical wall.

I think it will be much easier to justify the masonry walls than to try and
keep the mezzanine independent. Any thoughts?

Dennis

> -----Original Message-----
> From: Paul Feather [mailto:pfeather(--nospam--at)san.rr.com]
> Sent: Wednesday, May 16, 2001 1:08 PM
> To: seaint(--nospam--at)seaint.org
> Subject: Re: Adding a mezzanine to a Masonry Building in Seismic Zone 4
>
>
> Dennis,
>
> 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
> new elements.
>
> Paul Feather
> ----- 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
> 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
> > design)?
> >
> > 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 ?
> >
> >
> > Regards,
> > Dennis S. Wish, PE
> >
> >
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