In my much younger years as an engineer, we would many times design continuous beams over columns and have pinned splices placed just off of the columns, perhaps 3 to 8 feet away depending on spans. The splices were typically placed near the natural inflection points. Every engineer I knew back then (1980’s) assumed that the inflection point was a brace point and designed for the negative moment at the column with an unbraced length derived from the column-to-inflection point distance.
At a seminar in Texas one year I asked Joseph Yura (a guru of AISC stability design) about this and he said as long as you use Cb = 1.0 you should be fine. I also used 1.2 x Lb just to be safer.
A short time later I was informed that Dr. Yura had revised this recommendation to say that the inflection point shouldn’t be used - rather the full span of the beam (column to column distance) and the designer should rely on the calculated Cb number to “help” the design for negative moment.
All the above is preamble to my question. We recently checked a beam on an interior mezzanine and found that, using current AISC provisions, using the full span length, and calculating out the proper Cb, we couldn’t get the beam to work under its original design live loads. Thus, technically speaking, the original engineer designed a beam that doesn’t come close to working.
In our current project with this beam, we simply added a steel angle kicker to shorten Lb and make it work. But that got me to thinking on how many beams are “out there” that technically are not sufficient…including a lot that I designed.
It made me wonder if AISC ever put out a big notice that engineers may have messed up using the inflection point as a brace. I know on our current project that we could get the original engineer’s live load to work by assuming the IP = brace.