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Re: Factor of Safety against Uplift

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Syed,

	Do not use a 33% enhancement of allowable stresses if you are using any
limit states type of code.  The results could be very bad.  Please
accept the following as a somewhat speculative explanation as to why
this is so.

	Prior to the development of limit states codes working stress methods
of analysis provided safe designs by using an allowable stress that was
less than yield by a chosen safety factor.  It was, however known that
there was a significant strength reserve after yielding started and
before the plastic moment was reached.  It was found that this reserve
could be accessed if one was willing to accept some damage (yielding),
but not failure of the section.  One easy way to do this was to allow a
33% increase in allowable stress for load cases involving wind or
earthquake.

	With the development of higher yield strengths, lighter, more efficient
shapes, and better methods of performing calculations, the arbitrary use
of 33% increase is not a very accurate method of assessing the actual
strength of a section.  In some cases there may actually be less than
33% reserve, hence, safety may actually be compromised.  A better, safer
way is to use a load reduction factor for unlikely load combinations
(such as {dead + live + wind}, BUT NOT {dead + wind}, which is not at
all unlikely) and to use a better way of calculating the strength of the
section.  This also permits the utilization of the strength reserve for
unlikely load combinations that don't necessarily involve wind OR
earthquake.  Hence, the development of limit states codes.

	The major differences between working stress codes and limit states
codes is that the limit states codes allow access to the structural
reserves for unlikely load cases and they provide a more reliable
assessment of the actual strength reserves of the structure.  Limit
states codes also permit the use of very thin sections (like cold forms,
but not necessarily cold forms) which would not have been permitted
under earlier codes.

	I hope you haven't found this too boring.

				Best regards,

				H. Daryl Richardson

syed faiz ahmad wrote:
> 
> 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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