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Re: Factor of Safety against Uplift

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John,

In Saudi Arabia, by and large, ACI 318 is used for concrete structures & for lateral loads UBC is used. Sometimes BRITISH codes are also used. For Structural Steel works generally AISC is used.

But this Pre-engineered buildings subcontractor has used MBMA manuals for design while for analysis they use their own proprietory program.

And, in Saudi Arabia we don't have any local code as such, this is the problem.

Actually my concern is that the footings should be adequate enough to withstand the wind uplift & against any contingent increase of the same.

Following input from some of our learned colleagues, it is now clear that the DEAD load should be reduced to 0.85 to 0.90 & that a factor of safety of 1.5 should be used against the wind uplift.

MY QUESTION IN OTHER TERMS IS:

DOES ANY CODE ALLOW 33% ENHANCEMENT OF THE WIND REACTION, AS WE ARE REDUCING DEAD LOADS TO 0.85 TO 0.90?? IN THIS CASE, THIS WILL GIVE AN OVER ALL FACTOR OF SAFETY OF MORE THAN 2.

Any input/light on the issue shall be greatly appreciated.

Thanx in advance.

SYED FAIZ AHMAD; MENGG, MASCE
SENIOR STRUCTURAL ENGINEER
SAUDI OGER LTD
RIYADH, SAUDI ARABIA.



From: "John MacLean" <john_maclean(--nospam--at)pomeroy.ca>
Reply-To: seaint(--nospam--at)seaint.org
To: <seaint(--nospam--at)seaint.org>
Subject: Re: Factor of Safety against Uplift
Date: Thu, 8 Nov 2001 12:13:34 -0800

H. Daryl Richardson wrote:

If you were using a Canadian code you would use 0.85 as a load factor
for the weight of the foundation and 1.5 as a load factor for the wind
uplift.  Putting these together you would have a "factor of safety" of
1.5/0.85 = 1.76.

syed faiz ahmad wrote:

Iam not using 1.5, and am using 2.0 instead. caz I read somewhere, long time
ago (and have lost track as to where I read this),for wind uplift this
factor of safety should be between 2 to 3.5.

Syed:

I'm glad that Daryl mentioned limit states design for overturning which IMHO
is far superior to the allowable stress method at least for overturning
(otherwise it's mostly a pain). One other thing I take from Daryl's comments
is that your local building code should be consulted. What building code
applies in Saudi Arabia? Does it have provisions in it for factors of safety
against overturning?

The other thing you should confirm is that the code that the building
designers used is the same one you are using. This can be a problem. I have
had at least one "design the foundations for" project which was prepared in
the USA to a US model code which had no application in the area where the
building was to be erected. I believe that there is more sophistication
these days so that may not be a problem. But you may still want to request a
copy of their calculations to verify the design assumptions which they have
made. (A good way to get a short course on pre eng. building design!)

Cheers,
John MacLean



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