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RE: Unreinforced Masonry Building

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Dennis Wish wrote:

>>If you are in a seismic zone Zero, why worry about retrofitting the
building. The masonry mass should be sufficient to resist the wind loads -
which if at 80mph is not very much (we design for 70-80 mph in Exposure B
and C in our area's. I suspect that the wind load is resisted well by the
weight of the masonry alone as it has throughout California.
What is the reason for retrofitting the buildings?<<

Dennis,

I had to inspect wind damage to a 3-story masonry bearing wall building that 
used precast, prestressed concrete plank for the roof and the floors.  
Although the wind was strong, it was nowhere near the 75 mph fastest 
mile design speed that applies in Tucson.  Exposure might be considered B as 
the building was in downtown Tucson and there was no quarter that qualified 
as Exposure C.  The wind came from the west, the one area where there were no 
obstructions and in which there was probably a half-mile of "open" space.  
One east-west wing had an exit stairway protruding from the west end of the 
wing.  At one corner of the stairwell, 3-4 courses of cmu had separated from 
the rest of the stairwell walls, probably the depth that anchor bolts 
extended into the walls.

Wind loads at corners and discontinuities are *tremendous* and the current 
UBC coefficients and ASCE 7 procedures come nowhere close to describing their 
true effect.  The coefficients of the 1982 UBC were closer to the truth, but 
wind loads from ANSI A58.1-1982 probably are closer to reality.  Using either 
the 1982 UBC or ANSI A58.1-1982, it was not unusual to get wind uplift loads 
of 100 psf or more at corners and at discontinuities.

40 to 50 mph winds will strip roofing (and some roofs) off of buildings here 
in Tucson; a 60 mph wind stripped metal sheathing from an open pre-engineered 
metal hanger and caused 2 of the three rigid frames to have their bottom 
flanges fail in lateral instability.  (The hanger was at the airport where 
the weather bureau is located, so the wind speed could pretty well be 
determined.)

While arithmetic may show that the weight of building materials is more than 
the wind uplift, it must be remembered that the code coefficients do not 
represent reality.  There should not be an inch of unreinforced masonry 
constructed under any circumstance, and the minimum reinforcing should be the 
.0020*A(g) that the UBC now has applicable only to seismic zones 3 and 4.

A. Roger Turk, P.E.(Structural)
Tucson, Arizona