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Re: wind load- uplift and overturning

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Most of the old and new formulas have essentially the same numbers:

Old ASD formula:
Building must have a 1.5 F.S. vs. overturning using 90% of the dead load:
0.9 * Dead > 1.5 * Wind   ->  F.S. = 1.5/0.9 = 1.6667 = 5/3

New ASD Formula:
Building must have a 1.0 F.S. vs. overturning using 60% of the dead load:
0.6 * Dead > 1.0 * Wind   ->  F.S. = 1.0/0.6 = 1.6667 = 5/3

New LRFD Formula:
Factor of safety is built into the load factors:
0.9 * Dead > 1.6 * Wind   ->  F.S. = 1.6/0.9 = 1.7778


You question why you have to reduce the weight of the concrete by 40% when
you *know* it's there.  The reason for safety factors is uncertainty.  Do
you *know* exactly the peak wind load that this particular building will
have to resist?

Considering the guestimation and hocus pocus used to generate the wind loads
used in the codes, I don't think a 5/3 factor of safety is overly
conservative (look in the wind commentary section in the back of ASCE 7).
Hundreds of small- to full-scale tests have been performed over the years,
but did they test a building that looks *exactly* like the one you're
designing?  Maybe your building has a quirk of cladding or a wind tunnel
effect from other structures that causes an increase in wind loads over what
the codes say.

If it makes it easier to justify your possition to clients and contractors,
don't say you have to reduce the weight of concrete by 40%, say there is a
code-required 1.5 factor vs. over turning.

It's all in how you phrase it.  An analogy: If told by a doctor that 80% of
patients survive a certain operation, you might go for it if the benefits
are high enough.  If told by a doctor that 1 in 5 patients will DIE as a
direct result of this operation, few people will go for it, no matter what
the benefits.

----
Jason Kilgore
Leigh & O'Kane, L.L.C.
jkilgore(--nospam--at)leok.com
816-444-3144
816-444-9655 (FAX)
----- Original Message ----- 
From: Andrew Kester
To: seaint(--nospam--at)seaint.org
Sent: Wednesday, July 30, 2003 12:39 PM
Subject: wind load- uplift and overturning


Sorry for so many posts today, but I am ANNOYED!

An often talked about subject , which I have posted about many times, is the
use of ASD design equations for uplift and overturning. From the FBC 2001,
1609.4, Eq. 4 0.6D + W. I have interpreted this as the total dead load to be
used to resist the uplift must be reduced by 0.6. Now I have also thought
for some time that to use this for concrete footings (and dead loads that
are more assured then others of being in place) is OVER conservative, but
that is not what the code says. The FBC actually clarified this recently,
and I believe it was Chris from FL who posted this from the FBC website:


Question:
Is it the intent of Section 1620.2 to apply 150 % of overturning wind load
to the load combination of 0.6 D
+ W of Section 1609.4.1?
Answer:
No. For the High Velocity Hurricane Zones, Section 1611.1.7 states that any
conflicts between this code
and ASCE 7 the more stringent requirement shall apply. Section 2.4 of ASCE 7
requires dead load to be
reduced 40% when resisting wind loads (Basic load combination 5. 0.6D + W),
yielding overturning
stability of 167% of wind load. This load combination would apply to
overturning, as well as uplift and
sliding. Therefore, the ASCE 7 load combination would take precedent and
accomplishes the desired
excess overturning capacity.

I am assuming everyone agrees that this goes for concrete footings also.
Now, side note, I would like to see an exception in the code that allows a
less conservative reduction if the weight, as determined by the engineer, is
more likely to be in place during a wind event. I would like to see a 0.85
factor, or something similar, for reinforced concrete footings.



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