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wind load- uplift and overturning

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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.



Case Study from a few months ago:

A GC on a metal building project that we designed the foundations for calls
up, pissed off. He hired us for the foundation design.

GC- "Why are the foundations so big? Why are they so overdesigned?"

Me- "What do you mean? Did you do calcs? What did you come up with?"

GC- "Well, you take the uplift reactions from the metal building co. and
then provide that much concrete, right?"

Me- "Kind of. You have to reduce the concrete weight by 0.6 according to
code."

Gc- Silence... "Guess some engineers are more conservative and interpret the
code differently then others."

Me- " Well, I cannot speak for other engineers, but the code is crystal
clear on this issue. I will give you that I think it is overly conservative,
but that does not help us out right now."

GC hangs up angry, probably saying we don't know what we are doing and
overdid everything, because that is what we do.



Fast forward a few months to today...


Accountant recieves 2/3 payment. Calls GC to ask where is other 1/3 ? GC
says we overdesigned it (because he is a structural engineer), and that is
too much concrete, so they have to find someone who will get them as little
concrete as possible (my version).

Our accountant explained that we cannot vouch for whatever other engineers
are doing, there are good engineers, bad engineers, engineers who make bad
decisions, etc. The building dept. will allow anything signed and sealed by
an engineer. (WE have gone through this before.)

He then made some kind of bogus bet with our accountant saying, "I tell you
what. I will get someone else to do it, and send you plans. If it is less,
you owe me $200."

Accountant- "Yeah, whatever, send the check."

Me to boss, "I hope they send us plans so we can send them in to the board
for review, because that is open and shut. It is engineers out there doing
it wrong that are putting these ideas into GC's heads that footings don't
need to be so big. Meanwhile, we do it the right way, and we get punished by
not getting jobs or being accused by GCs or clients of overdesigning."

Bottom line, it is less jobs for us (of this type, then we are better off),
and more jobs being done incorrectly, setting a poor precendent. Boss chews
me out for poor client selection. Duly noted....

Thought you guys and gals may find this story entertaining, in a sad ,
pathetic kind of way.

Andrew Kester
Longwood, FL













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