Cain, William wrote:
> 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:
> 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
> now work. Continuing even further, his original checks on footings
> 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
> the same time. Meanwhile, ASCE lets you reduce one of the load
> to 0.70 and the other to 1.6. This created a dramatic decrease in
> 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>