Return to index: [Subject] [Thread] [Date] [Author]

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

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Dennis,

Is the 40 foot dimension of your mezzanine parallel to the 100ft walls or is it paralell to the 80 ft back wall?

If it is parallel to the back wall, I would minimice the load to the masonry by placing a new wood shearwall close to the exterior wall and cantilever the diaphragm to the back wall so you get less than a 5% increase in demand Maybe if you can work with the space plan of the floor you can accomodate this with a closet wall or restroom. 

If the 40 foot dimension is parallel to the 100 foot dimension, I would  do something similar to minimize the demand on that back wall.

You the 0.186w to compare the demand increase, not the increase from code now from code then. It is actually a 5% mass increase. The demand on the wall is ofcourse higher because of the new code.

When you say 2nd floor walls do not connect to the roof, is this a mezzanine with an independant ceiling/roof or is it traditional balcony type mezzanine?

Even if non-bearing, it is always good to upgrade the wall attachments to the diaphragm. Check the walls for out of plane loads with ZERO axial force from roof and with the new mezz. gravity loads and it still probably won't work. Unless you can justify it spanning horizontally between pilasters and then those probably won't figure.

Hope it helps
-gerard

>>> dennis.wish(--nospam--at)gte.net 05/16/01 12:08PM >>>
Gerry, thanks - a couple of clarifications and some additional questions:

The building is actually two retail stores - each 40X100'. The exterior
walls (80'x100') are masonry (reinf). the interior wall is wood stud
demising wall between stores. The roof is wood trusses at 15'-0" supported
on masonry pilasters with 6" non-bearing masonry walls between the
pilasters. The long walls are 100% solid. The front is a typical glass
storefront, and the mezzanine is to be constructed along the back wall at
the alley - currently about 70% solid, but will be infilled for all but the
egress an two small windows in the existing bathrooms.

The store is being converted to a night club. The mezzanine will have the
bathrooms at the first floor, and a lounge and office (and small storeroom)
at the top of the mezzanine. The stairs to the top of mezzanine is at the
open front of the new structure.

The second floor walls do not connect to the roof. The actual dimensions of
the mezzanine is less than 40x20 (one section is cut out to allow a view of
the bar below from the lounge above. Still, the new structure will probably
accommodate more than 5% of the existing area. This assumes the total
building at 8000 square feet and the mezzanine at 800 square feet (counting
only the footprint). The lateral load is less than 5% of the capacity of the
existing demand.

Two sides of the mezzanine are supported on masonry and one side on wood.

Trying to bring the entire building up to 97 code compliance seems
unreasonable as the tenant only occupies 1/2 of the building and would
impose a hardship on the other tenant if this was enforced. As you noted, I
doubt the original building was designed for more than 13.3% of base shear
and there is no way to verify without destructive testing whether the open
front is laterally braced (although I suspect it is).

There used to be a hardship clause that created a hardship if the cost to
bring the building up to full compliance exceeded something like $50,000.00
or 10% of the total construction cost. I don't remember where the provision
is for this.

Any other (more reasonable:>) idea's?

Dennis

> -----Original Message-----
> From: Gerard Madden [mailto:GMadden(--nospam--at)mplusl.com] 
> Sent: Wednesday, May 16, 2001 11:02 AM
> To: seaint(--nospam--at)seaint.org 
> Subject: Re: Adding a mezzanine to a Masonry Building in Seismic Zone 4
>
>
> Dennis,
>
> 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.
>
> -gerard
> San Francisco, CA
>
>
>
>
> >>> dennis.wish(--nospam--at)gte.net 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
> 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
>

* 
*   This email was sent to you via Structural Engineers 
*   Association of Southern California (SEAOSC) server. To 
*   subscribe (no fee) or UnSubscribe, please go to:
*
*   http://www.seaint.org/sealist1.asp
*
*   Questions to seaint-ad(--nospam--at)seaint.org. Remember, any email you 
*   send to the list is public domain and may be re-posted 
*   without your permission. Make sure you visit our web 
*   site at: http://www.seaint.org