| Reaching Out
A comment was made
in the article "Steel Sector Pushes Toward Electronic Data
Interchange" that the 2005 AISC Specification for Structural
Steel Buildings represents a signal "...of the failure of
LRFD to replace ASD..." (ENR 4/18 p. 24). In my opinion, the
2005 specification represents an effort by the American Institute
of Steel Construction to reach out to practicing engineers
and assist them in making an easier transition from allowable
stress design to strength design, also called load resistance
factor design, or LRFD. The combined ASD/ LRFD design methodology
in the 2005 specification is in fact a single strength-based
design methodology. Regardless of whether a designer using
the 2005 specification is using allowable stress design (ASD)
or LRFD methodology, member strength and connection strength
are computed the same way.
Strength design methodology is
here to stay. Its been used for design of structural concrete
for over 30 years, it forms the basis of the load combinations
in ASCE 7 and its the basis on which the design philosophy
for seismic design of building structures has been developed.
While there will continue to be a place for allowable stress
design of structural steel for years to come, engineers wishing
to design the most efficient and economic building structures
will eventually lose out in the competitive marketplace to
those engineers who embrace strength design methodology.
The apparent widespread resistance
to LRFD probably has more to do with design of flexural members.
Engineers are familiar with computing section properties of
beams for ASD, specifically the section modulus, "S." Strength
design of flexural members requires knowledge of a "new" section
property, the plastic section modulus, "Z." Until engineers
understand how to compute Z, strength design will meet with
resistance.
I suspect that one of the primary,
albeit indirect reasons for the resistance by engineers to
switching to LRFD is that design fees have continued to plummet,
project schedules have been reduced to the limit and structural
engineers are often times taking on more of the responsibilities
of project coordination than was common 30 years ago. Engineers
no longer have the luxury of time and money that they may
have had to learn new design methodologies. I suspect that
if the American Concrete Institute adopted strength design
at the same time as AISC did (in 1986), instead of in the
early ’70s, it would have met identical resistance from
practicing engineers.
A misconception about LRFD is that
it is much more difficult than ASD. Fundamentally, this is
not true. Unfortunately, however, when one compares the Ninth
Edition AISC ASD Manual to the Third Edition AISC LRFD Manual,
one cant help but notice that the equations look much more
intimidating in the LRFD manual. This is not the "fault" of
LRFD design but it is a concern that needs to be addressed.
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