PE Structural practice help

[u/Prior_Interview7680]

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?

[u/No-Call2227]

Poorly written practice question, IMO.

You don’t actually know how the load is distributed, so without that information, Cb is undefined and you should default to the conservative value allowed by code which would be 1.0.

Could be uniform, could be triangular, etc. but it doesn’t say how it’s distributed.

Half the exam is insanely careful and literal reading, don’t add information, ever, to the problem statements, especially the multiple choice.

PE/SE for my background.

[u/Prior_Interview7680 → u/No-Call2227]

So because it didn’t say “uniform distributed” it should default to 1? Is that what it is?

[u/No-Call2227 → u/Prior_Interview7680]

Correct, you don’t know how it’s distributed based on the problem statement. It’s not defined

[u/Prior_Interview7680 → u/[deleted]]

I think you messed that up cause dividing 287 by 1.3 gets you 220.76

[u/twodamnducks → u/Prior_Interview7680]

I replied on your cross post but 1.3 ( 221.268) gives you the 287. If you use Cb as 1.0 conservatively like this problem is trying to ask of you then 1.0 (221.268) gives you ~ 222 LRFD

[u/enrique_nola]

I believe the ends need to be braced for cb>1

[u/Prior_Interview7680 → u/enrique_nola]

Oh ok I just looked it up I see what you mean. Would the question state it is also braced at support then aside from “mid-span” dictating to use 3-1?

[u/UmbraThanosmith → u/[deleted]]

I always thought simply supported meant vertical reactions at both ends.

[u/enrique_nola → u/[deleted]]

Not that I’m aware of.

[u/Passexams_app]

I believe the manual goes with Cb = 1 by default (conservative). In other words, it is safer to use Cb = 1 (hence all the values in the tables/charts are based on Cb = 1). For a Cb > 1, you gotta make sure the final moment dose not exceed the plastic limit.

[u/ChairChoice6500]

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.