I'm using RAMSS (v15.04) to design the backspan of a small extension type cantilever (see snip) with the "use inflection point" and "perpendicular deck bracing the top flange" settings turned on. The backspan beam is being designed as a composite beam as the cantilever is not too significant. The pre-composite stage checks seem to use the expected unbraced length upto the inflection point. However, the critical "1.4DL" combo for the composite stage seems to be using the full 30' length of the backspan as the unbraced length instead of the expected length upto the inflection point for that combo. Any reason why the program is treating the unbraced lengths under different combos differently?
Check the moment diagram for that load combination, perhaps there is no inflection point. Also note, our latest version is 17.03.
Thanks Seth. FYI, I've uploaded the file if you want to take a look. While getting to the latest version might take a while with the rollout, just wanted to confirm if there is something funky going on with how RAM 15.04 is considering the unbraced length.The moment diagram does have an inflection point here:
The new version is the same. Take a look at the program manual for details on the treatment of the inflection point as a brace point (section 9.11 Unbraced Length). In short, the inflection point is considered as a brace point for the bottom flange (for negative bending) only if there is some other bracing of the bottom flange as well. Since your beam has no other bottom flange brace points anywhere (except the supports) it uses the full span unbraced length. When it comes to positive bending the beam is fully braced. This case is a bit confusing because the distance to the inflection point is nearly equal to the cantilever length and the positive back-span moment is nearly equal to the negative moment at the support.
Thanks Seth, that was my understanding after looking at the technical notes. However, we do see reduced lengths being used as the bracing lengths for the pre-composite (PreCmp-) and the post-composite (Max-) negative bending per the snip in the original post. I believe these should be 30'-0" as well as the bottom flange would not be braced throughout the length of the beam.
Hi Seth, can you please let me know if I'm missing something here?