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Re: ASD vs. LRFD

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Cain, William wrote:
> 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


Mark Twain was giving a talk once and said: "Good judgement comes from experience."

Then they asked him:  "Well, where does experience come from?"

"From BAD judgement!", said Twain.

>         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>