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RE: Rw for dual systems

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Sasha, the tip-off is that this is a historic building. If it exists on the
state registry, I would recommend that you follow the State Historic Code
for retrofit of Hazardous Buildings. If following Special Proceedure for a
URM structure, you would use a base shear as defined in the UCBC as 10% of
the structure weight for seismic zone 4. My concern is that is not a
flexible diaphragm and therefore does not conform to UCBC Appendix 1 Chapter
1 design criteria.
Is the diaphragm light weight concrete above a sheathed floor or is it a
structural slab? also is the exterior URM piers actually infills and not
load bearing? Is the exterior a concrete frame or steel frame structure with
a URM infill - in which case you would need to conform to design standards
for Infill Frame buildings.
You might consider contacting Nels Roselund SE at NRoselund(--nospam--at) who I
would consider a very well qualified expert on this type of structure. If it
is an Infill frame building, you might check with Shafat, whose firm has
also done many retrofits of this type.
Try sending Nels and shafat (shafat(--nospam--at) an email about this and see
what they think.

Dennis Wish PE

-----Original Message-----
From: Sasha Itsekson [mailto:itsekson(--nospam--at)]
Sent: Sunday, August 23, 1998 5:59 PM
Subject: Rw for dual systems

I'd like to get some opinions on the upgrade of a historic seven story
building built around 1920's in Oakland.  The building is 100 ft. by 50 ft.
in plan.

The building's frame system consists of  H columns infilled within 3 to 4
ft. wide  unreinforced masonry piers on the outside and concrete fireproofed
interior H columns and I beams. The diaphragm is 3.5 inch concrete slab. The
building back wall (not a bearing wall) is 8 to 13 inch thick brick.
The building also has both horizontal and vertical irregularities ( tall
first story with partial mezzanine and reentrant "rectangular" corner in the
middle of the building 14.5 ft. by 25 feet deep).

The building was red tagged after 1989 earthquake. The preliminary
retrofitting design scheme includes shotcreting of the back wall and
addition of either braced or portal frames on the other three sides.

I am reviewing the work of another engineer, and I am concerned about some
of the choices and assumptions that he made.  He uses Rw=8,  shotcrete the
back wall only with to the third floor, and  steel moment resisting frames.

My questions are:
1. Should we use Rw=6 with this kind of combination lateral force resisting
2. What should we limit the deflection to minimize cracking to the slender
masonry piers at the front and on the sides of the building? Is 0.005*h
3. Should we disregard these slender piers encasing steel columns in
modeling of the building?

Any other comments?


Sasha Itsekson, PE