From: Charles Greenlaw <cgreenlaw(--nospam--at)speedlink.com>
Date: Fri, 07 Jan 2000 21:22:33 -0800
At 04:31 PM 01/07/2000 -0800, you wrote:
Case in point is the wind exposure definition of case b
>or case c which has resulted in Exposure "c" being required as the standard
>throughout Southern California. But to argue for Exposure "b" is futile
>(aerial topo maps,etc). Along with some jurisdictions requiring 90 mph wind
>speed and Exposure "c" results in designing for a Class 2? hurricane. The
>City of Big Bear Lake, which has a non-engineer building official, has
>required the following criteria for lateral design for projects close to the
>lake: 80mph Exposure "d" , 100psf snow load and no 75% reduction for snow
>loads for seismic design ( dl + 100% snow).
>So if we allow the Building Officials the choice between flexible or rigid
>diaphragms, they will always opt for the more conservative method.
Around 1979, SEAOC had a special Code sub-Committee which massaged the
then-recent ANSI Wind Code into what became the 1982 UBC Wind provisions,
with the b, c, and later, d, Exposure factors. I was on Central Section's
component of this sub committee, and Ken Luttrell was its chair and member
of the statewide SEAOC umbrella sub-committee. I understood from Ken at the
time that very unusually, there was no "bright line" division between
exposure categories. One category is not a default from the next, and the
language describing each exposure is done without coordination to the next
exposure category description in either direction. The intent is that there
are three free-standing exposure definitions, and you are expected to select
the one that most closely fits the terrain. If you failed to "meet the
letter" of exposure b, you did not necessarily fall into exposure c by
default. Your situation had to be closer to the letter of exposure c than b
to be in c.
Such a way is peculiar for sure, and little understood. Probably that way of
defining things is a lousy mechanism for codework. Ken later admitted
privately that a commentary should have been prepared at the same time. SEAW
later did up a fine Wind Commentary, as we know.
The thing that still amazes me is how the ICBO Class A voting members let
such obtusely set forth code provisions get through. They should say, "get
out of here until you have simple, unambiguous wordsmithing any fool can
understand, plus a matching commentary for the record."
A couple of months ago there was a thread about how to get a code
interpretation from ICBO. Your jobsite jurisdiction's Building Official can
get from ICBO the most thorough explanation anyone can, on these wind and
snow issues, and respectfully asking that he consult with ICBO, after you
have set forth your own version, would appear to be your most constructive
recourse. Try to improve that person's comfort with engineering issues, and
support his efforts in learning his way with these awkwardly worded code
provisions our people left to him. Always be reliable and trustworthy in
every dealing with the Building Dept.
Charles O. Greenlaw, SE Sacramento CA