To: "'seaint(--nospam--at)seaint.org'" <seaint(--nospam--at)seaint.org>
Subject: RE: Who's using ASD or LRFD?
From: William Keil <WJK(--nospam--at)brph.com>
Date: Thu, 27 Jan 2000 16:57:07 -0500
I may be oversimplifying this a bit, but I consider ASD design similar to
LRFD design. I learned LRFD in school for steel and concrete, but ASD for
wood. After graduation I worked in New York City and the building code
prevents the usage of LRFD on any members involved in the lateral framing
system. So, instead of using two different methods of steel design on one
project, I used ASD exclusively.
Since I now work in Florida, I can now choose which method I want to use.
Now the decision comes down to who is going to design the connections ... if
I need to specify reactions for the fabricator's engineer to design the
connections or if I have to design slip-critical connections, I use ASD so I
only have to specify a total load instead of a DL and a LL. If the project
is simple enough that I do not need to specify reactions, I use LRFD.
In my experience, very nominal savings are achieved through the use of LRFD.
In addition, 50 ksi steel as a standard does not result in large savings
either. Yes the member is stronger, but you still need the stiffness
properties to control vibrations and building drift.
The following is my simplification to show everybody that they are already
using a form of LRFD:
For ASD Design:
One would use Actual Loads (i.e. load factor = 1.0)
One also uses an Allowable Stress (i.e. material reduction/safety factor =
For LRFD Design:
One would use Load Factors on the Actual Loads (i.e. DL factor = 1.2 & LL
One also uses a material reduction factor (i.e. phi = 0.9)
In ASD design all of the load factors and material reduction factors happen
in the allowable stress factor of 0.66.
In LRFD design DL and LL factors are assigned based on their probability of
being exceeded. The DL can be predicted more accurately than the LL, hence
a DL factor is 1.2 and the LL factor is 1.6. Also since steel can be made
with much improved consistency, a material reduction factor of 0.9 is used.
DL = 85 psf
LL = 60 psf
Factored load = 1.2(85) + 1.6(60) = 198 psf
Let's apply the allowable stress factor to the loads:
1/0.66 = 1.515
Remove the material reduction factor:
1.515(0.9) = 1.364
Equivalent factored load = 1.364(85 + 60) = 198 psf
In conclusion, instead of applying a 1.364 factor to DL and LL, LRFD
differentiates between the loads. By this example load case, ASD and LRFD
are similar but use different terminology and factors. So instead of using
different allowable stresses, various material reduction factors are used.
Instead of using actual loads, each type of load is increased by a
percentage according to its probability of being exceeded.
Of course, everything is more complicated than what is shown above. But my
main purpose was to show everybody that you are already using "LRFD." IMHO,
the steel industry shifted the "reduction" factor to a "load amplification"
factor so that the steel fabrication process can improve its image and
answer the questions "Why does steel require a 0.66 factor and concrete only
require a 0.9 factor? Isn't steel a more highly controlled material than
concrete?" Now steel and concrete have nearly identical phi factors.
Class is dismissed :o)
William J. Keil, P.E.
From: Teresa Dellies [mailto:DelliesTM(--nospam--at)c-b.com]
Sent: Thursday, January 27, 2000 3:41 PM
Subject: RE: Who's using ASD or LRFD?
Luckily, many structural programs allow an ASD user like myself to design
with LRFD without having to learn it all when a client (ie. DART in Dallas
TX) requires it. I only wish I had time to actually learn LRFD....also, the
PE exam still allows the option of either ASD or LRFD....Incidentally, the
projects that required LRFD were of the nature that the design did not end
up varying much from the ASD design (I felt compelled to check...)