From: Roger Turk <73527.1356(--nospam--at)compuserve.com>
Date: Fri, 28 May 1999 18:58:09 -0400
Bill Polhemus wrote:
. > It is typical that, after a serious storm like a hurricane here on the
. > gulf coast, local officials are pressured to revise building codes,
. > "upping" the basic wind speed criteria (usually). I know for a fact that
. > after Hurricane Fredrick struck Mobile, Alabama in 1978, the basic wind
. > speed went from 90 mph to 120 mph per the local building official.
... and then they brought in the mobile homes to temporarily house the
homeless. (Did they ever take them away or did they wait for the next
. > However, these properties were typically designed using now-outmoded
. > criteria such as those contained in UBC 1976 or ANSI A58.1.
ANSI A58.1-1972 probably had the most realistic wind load requirements of
any code or standard that I am aware of, but, boy, were they hard to use.
IIRC it was not unusual to get 100+ psf uplift at corners, roof edges and
discontinuities with that code. Also, there were some problems with sign
convention: positive wind loads were loads acting towards the building
surfaces, therefore inward acting wind loads on the outside were positive,
and internal pressures on the inside (acting outward) also were positive.
(Suction pressures on the outside were negative, and internal pressures,
acting outward were positive.) ANSI A58.1-1982 was watered down considerably
and UBC 82 attempted to upgrade the UBC wind load requirements by adopting a
"simplified" ANSI requirement in which the UBC combined the fastest mile wind
coefficient and gust coefficient into one coefficient, thereby watering the
code down even further.
The effect of discontinuities was hammered home to me when I was inspecting
damage to a flat roofed, three story motel. The roof structure was
prestressed concrete plank, similar to Span-Deck, with rigid insulation hot
mopped to the plank and built-up roofing hot mopped to the rigid insulation.
The roof was constructed to be flat and to have occasional roofing expansion
dams. Later, security lighting was mounted on the edges of the roof and
conduit laid on the roof surface connected a light on one side of the roof
with a light on the opposite side of the roof. On the lee side of the
roofing expansion dams **and** on the lee side of the electrical conduit the
roofing and rigid insulation was ripped away. On the windward side, the
roofing and rigid insulation was still in place, although damaged.
A. Roger Turk, P.E.(Structural)