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Good morning,
 
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 walls. 
 
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 opinions? 
 
Steve Gordin SE
Irvine CA
 
----- Original Message -----
Sent: Thursday, January 26, 2006 9:02 AM
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
> > moment.
> I confess, I'm not convinced. Calculation of live loading is
> independent of the methodology used to determine response to the loads.

> some situations--seismic response being the best example I can think
> of--but I've never (and still haven't) seen a persuasive argument for
> 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 whole thing--no

1) more consistent "factor of safety" in ALL design checks (e.g. ASD is
not consistent for beam vs. connection).
2) consistent "factor of safety" for a range of load conditions (LRFD
was calibrated to ASD to produce a similar member for a very narrow
range of live:dead). At extremes, ASD does not really produce a
consistent "factor of safety".
3) consistent "factor of safety" regardless of materials (e.g. mill
section, cold-formed, aluminum, concrete, A36 vs. A992 - see more below)
4) easier to apply in various construction economic situations (e.g.
adjust load factors to suit socially acceptable reduced FoS to reduce
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 earlier ASD/WSD
(e.g. India is based on British WSD, as Vish noted) and will update
according to economic demand (e.g. when the market in a country decides
that it is more expensive to maintain ASD independently rather than
borrowing another country's LRFD). I suspect that code and material
compliance is more important than type of code in some of these
situations.

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 designer
SHOULD be doing more than the ASD 9th minimum. However, the "law" is
interpreted to permit a designer to meet ASD 9th minimum, in some cases,
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
standard writers, structure owners (generally) or government bureaucracy
financers (e.g. tax payers).

Regards
Paul

--
R. Paul Ransom, P. Eng.
Civil/Structural/Project/International
Burlington, Ontario, Canada
<mailto:ado26(--nospam--at)hwcn.org> <http://www.hwcn.org/~ad026/civil.html>

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