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RE: ASD vs. LRFD

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Jake-

Remember that code requirements are MINIMUM requirements.  Procedure
shopping for a way to "make them work" is frequently done.  Is it good
engineering?  IMHO not by itself, particularly with wind and seismic
loadings.  I hope you tried to understand the real loadings and what they
mean to the REAL behavior of the structure before you accepted the LRFD
analysis at face value.  The structure could care less which method you
used.  It may result in a "more economical design" or it may result in one
that falls down in the next earthquake or windstorm because the design was
cut too fine.  In the latter case, the owner may have though it was
economical until it fell down, then he'll feel it is disastrous.

Both ASD and LRFD are simply tools to help us make design decisions.  They
both need to be used with good engineering judgement.  Understanding either
is not the point, understanding how the structure behaves and providing
acceptable behavior in our designs is what it is all about.

Bill Cain, SE
Oakland, CA


	Jake wrote:
	<snip>

		I am currently working on a major retail remodel in Salt
Lake.  A
	co-worker of mine did a preliminary column capacity check using ASD.

	Many of the columns failed, but only slightly.  After verifing his
	loading conditions, I did and LRFD check and many of the columns
will
	now work.  Continuing even further, his original checks on footings
used
	ACI 381-95 load factors for strength and nearly every footing
failed.  I
	then checked them using ASCE-7 load combinations (with reduced phi
	factors) and made many (not all) of them work.  It helped in this
	particular case because ACI demands a full 1.7 on snow and live load
at
	the same time.  Meanwhile, ASCE lets you reduce one of the load
factors
	to 0.70 and the other to 1.6.  This created a dramatic decrease in
	loading.

		The point: Yes ASD works, but when designing columns in
particular -
	LRFD will give you a more economicle design.  And with additional
	research, even the current load factors will change with time.  I
	believe ACI in the procedure of adopting the ASCE-7 factors. <snip>