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Re: ASD vs. LRFD

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Fountain:

I'll give you a few examples of situations when "LRFD is better."

First, consider earthquake loading.  Although our understanding (and
characterization) of earthquake loading and response are significantly
less mature than our understanding of other loading phenomena, we do
know that some measure of the "real" capacity of a structure is
essential to making correct seismic design decisions.  Because the
safety factors associated with the possible limit states vary so
widely in ASD, the designer is less able to predict whether a
desirable limit state will control.  As long as our seismic design
approach allows inelastic response, we need to think in terms of
expected capacities not allowable stresses.  The first strength of the
LRFD method is that it establishes a design framework that allows
consideration of the real, measured capacities.

Second, consider wind loading (which likely affects your Florida
practice more).  The second strength of the LRFD method is that it
forces the designer to consider possibilities that might otherwise be
ignored.  (For the following numerical example, apply force units to
interpret it as a column design problem or force per unit length to
interpret it as a roof framing problem.) D = 100; W = +/-95; L = 0. 
For ASD, the two design points are +195 and +5.  For LRFD, the two
design points are +244 and -34.  Conclusion: for members with wind
load effects that are of the same order of magnitude as dead load
effects, ASD may not identify the very real possibility of load
reversal, while LRFD does.  In this example, the result could be
column splices (or base plate conditions) that are loaded in tension
though designed for compression only or roof framing with the bottom
chord or flange unbraced for compression loads that may be
experienced.  These effects are compounded by the use of a 1/3 stress
increase and the fact that many designers "conservatively" estimate
dead loads on the heavy side.  Many experienced ASD users recognize
this weakness and either 1) always consider some small reversal of
force, or 2) perform extra calculations using a "minimum expected dead
load," but such considerations are not required by the method.  Again
the strength of the LRFD method is that it consistently quantifies
this possibility; if the LRFD load combinations don't indicate a load
reversal the possibility is arguably so remote that consideration is
not needed.  (Of course LRFD users must be "trained" to use their best
estimate of the actual dead load recognizing that the applied load
factors are intended to introduce appropriate conservatism for both
maximum and minimum expected effects.)

Many ASD users argue that structures designed using LRFD have
serviceability (deflection or vibration) problems because service
loads weren't used in the original design.  This is a red herring.  If
service conditions are correctly CHECKED in the course of design, the
performance should be acceptable.  The problem is that many designers
(using ASD or LRFD) have become lax in checking serviceability. 
Serviceability checks are essential and they become critical when
designers are working at the bounds of their past experience.  I
believe that most (if not all) service problems arise due to
conditions that were not checked.  LRFD designs that exhibit
performance problems were usually not checked for one of two reasons:
1) the experienced ASD designer, lulled into a false sense of security
by their past experience, is trying to "push the envelope" of their
strength design by using LRFD; or 2) an inexperienced designer doesn't
understand the importance of checking serviceability (they would make
this mistake using either ASD or LRFD).  The real problem is one of
consideration, not method.  Once designers are working within the
bounds of their past experience (which may require some "retooling" by
ASD users), explicit checks of serviceability conditions need not be
as frequent, but a recognition of these bounds (and some checks) must
continue.

-Mike

****************************
> From:          "Fountain Conner" <fconner(--nospam--at)pcola.gulf.net>
> But, you haven't addressed my challenge -- Show me how LFRD is better.
> ...
> I'm reasonable.  If LFRD is better, prove it to me.  Not only will I adopt
> it; I will "champion" it, embrace it, and defend it.  But don't "blow
> smoke" at me.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Michael Valley                                   E-mail: mtv(--nospam--at)skilling.com
Skilling Ward Magnusson Barkshire Inc.                  Tel:(206)292-1200
1301 Fifth Ave, #3200,  Seattle  WA 98101-2699          Fax:        -1201