ASD-LRFD for Wood Construction (Provisions for Wind and Seismic)
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Transcript of lrfd vs asd
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Michelle Mentzer English 202C.24 February 22, 2001
Should Structural Engineers Adopt A New Design Method?
LRFD vs. ASD
The question of what method to use when designing structural members for
buildings seems like it should be purely a technical one, but when it affects the standards
to which professionals in the field are accustomed, it becomes much more involved. The
current standard for industry use in steel design is the Allowable Stress Design (ASD).
However, the future of the field seems to be headed in the direction of the Load and
Resistance Factor Design (LRFD). ASD has successfully served the profession since
1923, so this leaves many established structural engineers wondering why a change is
necessary. While their apprehension is understandable, movement toward LRFD will
only benefit the field in the future.
To better understand the controversy surrounding the proposed switch from ASD
to LRFD, it is helpful to examine some of the history behind the two design methods.
The ASD method began in the 1920s and has been continuously updated and used
effectively ever since then. [3] The industry has made many advances in material since
ASD was originally developed. At that time, most materials behaved elastically, but now
many common materials behave inelastically or plastically. Engineers have developed
ways of looking at the new situations through this method. They developed safety factors
to ensure that the structure will still be safe if the members do not behave elastically.
However, the engineers are limited in how well they can incorporate these new properties
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into the ASD calculations. Since ASD was not initially developed with these types of
behaviors in mind, the results, while on the safe side, are not the most accurate. The
safety factors generally overcompensate for the difference in strength, so the members
are overdesigned. These limitations were the incentive for developing a new method of
steel design, LRFD. LRFD is being developed by the American Institute of Steel
Construction (AISC) as a replacement for ASD, which is becoming outdated. LRFD
takes material differences into account and is much more accurate. While it is still safe, it
is more economical than ASD. Currently, the AISC board is continuing to develop
LRFD while maintaining ASD for use in the field.
The controversy stems from the fact that any time a change of any kind is
proposed, there is strong opposition from those who have grown accustomed to the old
way and feel that it is still adequate. Many established Structural Engineers fall into this
category. These people have successfully performed their job for years, so they see no
reason for change. They look at the new design method as an inconvenience, an
unnecessary complication to a procedure that has become second nature to them. While
the ASD method is simple to anyone who just finished school and learned steel design
using LRFD, the methodology of LRFD is difficult to grasp for those accustomed to
ASD. The older professionals may also fear that younger engineers just entering the field
may have an extra edge over the experienced engineers if LRFD is adopted.
Additionally, it appears that, at least in the short term, there is no problem with
continuing to use ASD. The reason for adopting LRFD is to allow further development
in the design of steel structures. However, this may not seem like an appealing idea to
the older professionals in the field either. Since people always tend to resist change,
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especially once they get comfortable with the old way of doing something, they have no
desire to change the design method simply the overall design can be changed.
Despite the resistance of many structural engineers, there is still a strong group
pushing for the adoption of LRFD in place of ASD. Educators and researchers lead this
group. They are the people who have the greatest interest in seeing improvements in the
field. No one in this group would deny that a lot has been accomplished with ASD, but
they see that in order for design progress to continue, some fundamental changes must be
made. Innovative progress is required for continued success in any industry. An
example of an industry in the U.S. that tried to stay competitive without this kind of
progress and failed is the automobile industry. Since then, many important and
innovative changes have continuously been made in that industry to accommodate
changing needs. Examples include seat belts, fuel injection, antilock breaks, and
airbags. [3] When ASD was developed, only elastically behaving materials were
considered. Since that time, new materials have been developed many of which behave
inelastically. LRFD offers a solution to inelastic design that ASD is incapable of
providing. Instead of addressing the root of the differences in material, ASD uses a
general safety factor that has little to do with the actual properties of the material. While
this is suitable for current building design, it does not leave room for progress in the way
that buildings are designed. [2] Topics such as allowable stress and seismic design are
not addressed sufficiently in ASD. LRFD takes care of this already and still is being
developed and improved. [1]
The conflict between the structural engineers who are set in their ways and those
who see the need for continued progress to keep the field competitive and successful has
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a possible solution in sight. Those who wish to make LRFD the industry standard are as
determined to see that happen as others are to resist the change. Since LRFD is backed
so strongly by educators, they will do whatever they can to make the transition easier.
They have scheduled conferences around the country to present LRFD through a three-
day tutorial. Professors in favor of the change are willing to train all interested structural
engineers in the field so that they are as familiar with the method as they would be if they
had just graduated from school. [4] Experienced engineers may fear that this will cause
them lose their edge over new engineers. The fear stems from the fact that an engineer
just out of school would not generally be as comfortable with designing steel as someone
who has several years of experience. However, in most cases the values given by the
LRFD calculations will be very similar to those given by ASD, so an experienced
engineer will still know if an error has occurred. [2] Therefore, the experienced
engineers learning the new method will still have the benefit of their time in the field.
Adopting the new method will not jeopardize their seniority. This similarity of the values
produced by the two methods is why many people say that there is no need for the
change. It is important to realize that although the design changes have little impact on
current traditional designs, the only way for the field to progress is to adopt a more
accurate design method. In addition to educating practicing engineers in LRFD, that is
the method that most universities are currently teaching. The new engineers entering the
field will be qualified to use this method but will most likely be forced to use ASD since
most companies have not yet switched to LRFD. Eventually, when older engineers learn
the new method, the younger engineers will be well prepared for the change. Therefore,
it is in the engineer’s best interest to take advantage of the conferences and learn LRFD
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as soon as possible so that when their company finally decides to adopt the change, they
will not be left behind.
While steps are being taken in the right direction, eventually firms are going to
have to take it upon themselves to adopt a change from ASD to LRFD. ASD has served
the field well in the past, but it has reached its limit of development. Most people agree
that the future of the field lies with LRFD; the conflict is in the short term since either
method appears to be acceptable. Experienced engineers are never going to be happy
about changing their procedures, but at some point the change will have to be made.
This is the only way that the field of structural design in this country can continue to have
the success that it has always known in the past. Resources are currently available to
educate engineers in the new method and make the switch. The question then arises as to
when it would be appropriate to adopt the change. Why delay the progress of structural
design rather than take just three days to learn a new method?
Works Cited [1] American Institute of Steel Construction. Load & Resistance Factor Design: Manual
of Steel Construction. Vol. 1 Chicago: AISC, 1998 ed. [2] Carter, Charles J. “What’s Up With the New Design Procedures?: LRFD and the
Steel Detailer.” Modern Steel Construction. May 2000. [3] Carter, Charles J. “The Better (Not Bitter) Truth.” Steelstuff. February 1997.
Accessed 2/15/01. <http://www.engr.psu.edu/ae/steelstuff/lrfd.htm> [4] Melnick, Scott. “No Gamble Involved, LRFD Is Here To Stay.” Modern Steel
Construction. January 2000.