From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
Date: Fri, 11 Feb 2000 18:02:39 -0500
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?<<
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
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)