Return to index: [Subject] [Thread] [Date] [Author]

Re: ASD vs. LRFD

[Subject Prev][Subject Next][Thread Prev][Thread Next]
Christopher Wright P.E., wrote

> I think it's fair to remind everyone that Fountain's original question 
> was 'show me it's better,' not 'explain why LRFD isn't wrong.' The thread 
> is moving noticeably toward and 'LRFD good, ASD bad,' format when we 
> should be discussing the original point--is LRFD demonstrably better 
> athan ASD? 'I like (ASD)(LRFD) better, so up yours,' isn't an answer. 
> (LRFD)(ASD) (provides better service performance)(results in 
> economies)(provides client savings) and is also a more cost-effective 
> engineering tool _is_ an answer.

	I agree, this is not a personal issue.  It appears to me that this has
boiled down to two points: 1) Tradition, 2) Comfort Zone.  There is
nothing wrong with eithor one.  But always staying within both is a sure
way to get into trouble.  Most ASD people have no fundamental arguments
regarduing LRFD.  (Yes I have been reading the list - no one has yet to
say don't use (LRFD or ASD) for it is wrong!)  They are simply using
what they know and feal comfortable using. So here is a case study I
believe will help clarify the issue.

	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

	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.  So I think
this illustrates the most important point of this discussion - or
profession is not static.  As much as everyone would like to graduate
from college with a complete body of knowledge regarding design, we all
know that new research will shed new light on virtually every issue. 
This is painfully evendent reqarding the Pre-Northridge steel moment
connection.  Both styles learned from mistakes.  However, most of the
new information will be incorperated in LRFD desing, not ASD.  Disagree
with me, agrue with me, do what you will - it will not change any of
these points.  

	Furthermore, it sounds like all major structures currently being
desinged are using LRFD.  You ask why?  One simple reason - MONEY.  LRFD
will give you a more economical design.  What I don't understand is why
only large projects?  Is money not as imporant in small projects as it
is in large ones?  Or does it only matter if it is in millions of
dollars and not hundreds?  So Fountain did I answer your question this
time? Or would you like another responce?


Jake Watson
Salt Lake City, UT