CHARLOTTE, N.C. - You pick up a few things when you’re immersed in structural steel design for half a century.
Louis F. Geschwindner, PE, PhD, is one of the few people to have that experience. He’s a professor emeritus in architectural engineering at Penn State University, a retired American Institute of Steel Construction vice president, and a senior consultant at Providence Engineering in State College, Pa.
He shared the wisdom of that unique perspective with a crowd of participants at NASCC: The Steel Conference on Thursday. The goal: Give engineers something to think about as they use the information they already have--and what they learn--to design tomorrow’s structures.
“All that we accomplish as structural engineers is, in some way, based on what those who came before us have accomplished,” he said, be they mentors, colleagues, teachers, or others engineers know only by name (or not at all!).
After tracing the history of engineers’ understanding of bending; load and load combinations; and safety reliability, Geschwindner reviewed the differences between 1986 AISC Specification for Structural Steel Buildings, which introduced load and resistance factor design, and the 2005 unified Specification, which combined LRFD with the existing allowable stress design approach.
LRFD is both a probability-based specification and a limit states-based specification; Geschwindner noted that the latter was also true for ASD since 1961. The 2005 Specification combined the best aspects of both approaches to determine a single resistance for each limit state. That number is then modified by a resistance factor in LRFD or a safety factor in ASD.
Ted Galambos, to whom the 2022 edition of the AISC Specification is dedicated, summed it up this way in 1988: “The job of the design engineer is not to slavishly follow the specification but to think through the complete structural action under load.”
So where does that leave today’s engineers? Geschwindner boiled his advice down to a few simple points for engineers designing steel their way: default to LRFD and use the direct analysis method for stability analysis (only use the effective length method for braced frames).
“As structural engineers, the most valuable thing we can bring to our clients is our ability to apply engineering judgment,” Geschwindner said. That judgment is based on previous generations’ discoveries, and therefore a basic understanding of history is a vital tool for any structural engineer--a tool with which all engineers in that Steel Conference session on Thursday are now equipped.