Recently, I have to deal closer with
the crawlspace ventilation issues.
According to CBC/UBC 2306.3 (IBC
1203.3.1), 1 square foot of net-open area should be provided - close to the
corners - for each 150 square feet of the crawlspace. This means that for
a 1600-sq. ft (say (40x40 ft) single-story house, about 11 sq. feet of
vents (net) need to be installed, not counting the vents in the interior stem
This, in turn, means that at each
house corner there should be an opening of 20" square (or (2)
14"x14"). If distributed along the walls (although it is contrary to what
the code requires), there should be, say, (4) 6"x16" openings.
To put it mildly, in this position am
not quite understood by the contractor or even by the architect. Any
Steve Gordin SE
----- Original Message -----
Sent: Thursday, January 26, 2006 9:02
Subject: Re: Why LRFD vs ASD (was: We're
Not Getting Older, We're Getting DUMBER)
> From: Christopher Wright <chrisw(--nospam--at)skypoint.com>
I'm sure there are other reasons but I can't think of them at the
> I confess, I'm not convinced. Calculation of live loading is
> independent of the methodology used to determine response to the
> some situations--seismic response being the best example I
> of--but I've never (and still haven't) seen a persuasive
> the LRFD approach as a replacement for working stress
methods. If there
> were holes in the ASD approach they need to be
fixed, and if the
> approach itself is clearly invalid then replace the
1) more consistent "factor of safety" in ALL design
checks (e.g. ASD is
not consistent for beam vs. connection).
consistent "factor of safety" for a range of load conditions (LRFD
calibrated to ASD to produce a similar member for a very narrow
live:dead). At extremes, ASD does not really produce a
3) consistent "factor of safety" regardless of materials (e.g.
section, cold-formed, aluminum, concrete, A36 vs. A992 - see more
4) easier to apply in various construction economic situations
adjust load factors to suit socially acceptable reduced FoS to
constructed cost of materials).
5) cross-border consistency (I
think that AISC ASD is the single major
steel standard still based on ASD).
Many international engineering
offices are using an LRFD variant regularly,
anyway (sounds like US
customary vs. SI).
ASD vs. LRFD is trivial
where "service" loading controls (e.g.
vibration, fatigue, simple
deflection, etc.). For these applications,
you can call the analysis
calculations ASD but it is still covered in
LRFD as a limit condition (e.g.
load factors = 1).
Other locations are using AISC ASD or derivations of
(e.g. India is based on British WSD, as Vish noted) and
according to economic demand (e.g. when the market in a country
that it is more expensive to maintain ASD independently rather
borrowing another country's LRFD). I suspect that code and
compliance is more important than type of code in some of
There are no holes in the ASD concept. This is
reflected by the fact
that AISC is releasing the combined 13th edition. The
real debate, in
the US, is that the AISC ASD 9th has not been maintained
and no longer
represents an appropriate level of due diligence in design. A
SHOULD be doing more than the ASD 9th minimum. However, the "law"
interpreted to permit a designer to meet ASD 9th minimum, in some
to the detriment of public safety.
I'm certain that Charlie
or Scott could comment on the assumptions and
compromises that go into
developing a conveniently useable steel design
standard. Some of those in
ASD 9th may no longer be valid with the
material changes (e.g. from Tube to
HSS and from A36 to A992) that we
have seen for other economic reasons. In
this sense, ASD 9th may be
broken for some applications.
The cost of
maintaining a dual track is not insignificant for either the
writers, structure owners (generally) or government bureaucracy
(e.g. tax payers).
R. Paul Ransom, P.
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