“Which version of the steel manual did you learn on?”
On its face, that’s a simple question. But last week, when I asked Providence’s 50+ structural engineers, I got more than just simple answers. Through a series of enlightening conversations, a story with color, conflict and compromise was revealed to me.
A Colorful History:
The American Institute of Steel Construction (AISC) has been publishing the Steel Construction Manual for more than 90 years. The 1st edition was published in 1923 with a black cover and approximately 500 pages. The 15th edition, published in 2017, has a teal cover and approximately 2,000 pages. The editions in between include an increasing amount of detail and a variety of cover colors.
When you ask an engineer which version of the steel manual they learned on, the simplest response is “the red book” or “the green book”. This shorthand quickly highlights which edition an engineer is most comfortable with…and it may indirectly highlight the era he or she graduated from school (see above).
However, this simple response also captures, but does not blatantly highlight, an important philosophical shift that occurred within the world of structural engineering.
LRFD vs. ASD:
For almost 50 years, AISC published the steel manual using the philosophy of Allowable Stress Design (ASD). This philosophy appeared in editions 1-9. Then, in 1986, AISC issued its 1st edition Load and Resistance Factor Design (LRFD) manual—the blue book.
The LRFD 1st edition proposed a new philosophy for steel design. Evidence showed that LRFD produced structures with a more consistent factor of safety than structures designed using ASD. However, in most cases, the differences between ASD and LRFD were not significant.
Both ASD and LRFD were acceptable under current building code, and ASD had been successfully used for decades. So, as you might expect, the academic community readily adopted the new LRFD design philosophy, but the engineering profession did not.
This led to an undesirable situation, which lasted from 1986 until 2005, where an engineering student was primarily learning LRFD, but upon graduation, they were primarily using ASD.
Finally, in 2005, AISC published a unified specification (the most recent “black book”). This black book was not a combination of the old ASD and LRFD provisions, but rather a new approach intended to utilize the best provisions from each philosophy. The 14th and 15th editions of the steel manual have built upon this unified approach.
Today’s engineers continue to have the opportunity to apply judgement in choosing the approach best suited for their steel design. Current versions of both LRFD and ASD are still acceptable under current building code, and both philosophies still have their advocates and opponents.
If you’re interested in learning more about these two design philosophies, or which one your structural engineer prefers, I suggest you ask “which version of the steel manual did you learn on?”