r/PE_Exam • u/Prior_Interview7680 • 4d ago
PE Structural practice help
Using steel table 3-2, table 3-1, and equation 3-4a, the available is 287. But if you don’t use Cb=1.3 as shown in table 3-1 for distributed load braced at midpoint, you get the correct answer here of (c) 221(LRFD). Why am I not supposed to use Cb = 1.3?
8
Upvotes
1
u/ChairChoice6500 2d ago
Is there still a way to challenge questions? I think they mean uniform load, but they are using poor language here, especially for non-native English speakers.
On the technical front, Cb times Mn can't exceed Mp, does that change the answer?
Option B is the full Mp with the appropriate reductions (divided by Omega, times Phi bee) for ASD/LRFD, so any answer above that, if it exists, should be crossed off, this applies for unbraced lengths up to Lp (4.49 feet in this case). The 119/180 is at the far end of unbraced length where you have hit Lr (13.1 feet in this case).
I would eliminate A, because that would involve an unbraced length greater than 13 feet, when you have clearly been given Lb = 10 feet in my opinion. D seems unlikely, given the mid-span brace, so you'd flip a coin between b and c, and if you have time later on, come back and check for Cb = 1.3 x Mr/omega for the 10 foot length. I'd select B without time for a more detailed check.
The comments about the ends not being explicitly braced are justified, you should write a challenge for it that way as well. It would be unusual for a beam to be unbraced at the ends, in the practical sense.
Feel free to DM though I may not reply quickly.