Questions on NCEES Structural Sample Questions

[Will.I.Am PE]

This information may be buried in this forum, somewhere. I couldn't find it, though. 

I'm looking at the NCEES Sample Questions and Solutions, c.2008 and I'm wondering about two of the morning breadth sample problems that both focus on structures. (I'm taking the geotech depth, but I took a couple of structures classes in my bachelor's and master's degrees, so I'm somewhat versed in more complex details.)

Question 111 asks what the effect would be of changing the yield stress (Fy) of a steel beam from 50 to 36 ksi, while keeping all other factors the same. The answer choices are essentially, would the deflection increase, decrease, remain the same, or there isn't enough information given. I knew it had to be one of the latter two, given the available information. I'm under the impression that they implicitly assume the deflection must be elastic in the solution, because the "most correct" answer is that it remains the same. However, if you don't make that assumption and the stresses in the beam fall between 36 and 50 ksi, the beam would experience plastic deformation and defections would increase. No information is given about the loading or stress states, which leads me to believe that, in reality, not enough information is given. Is this problem slightly bogus, or am I missing something?

The very next problem, question 112, asks what the most efficient steel section would be for a beam with a span of 20 feet and an unbraced length of the compression flange of 20 feet. The answer choices are a square HSS, a channel, a wide flange, and a double-L/T section. When I read the question, I figured that it had to be alluding to a more complex concept, but had no idea what it might be. After I saw the solution, it was obvious that, unbraced length of the compression flange was the indicator that this was, indeed, a lateral torsional buckling problem. However, I really don't know how I would have recognized that from the outset. Are there any tips out there for recognizing the key words and phrases that indicate an implicit concept that's being tested?

This all ended up being kind of wordy, but I hope it's decipherable. Thanks!

 

[DoctorWho-PE]

This information may be buried in this forum, somewhere. I couldn't find it, though. 

I'm looking at the NCEES Sample Questions and Solutions, c.2008 and I'm wondering about two of the morning breadth sample problems that both focus on structures. (I'm taking the geotech depth, but I took a couple of structures classes in my bachelor's and master's degrees, so I'm somewhat versed in more complex details.)

Question 111 asks what the effect would be of changing the yield stress (Fy) of a steel beam from 50 to 36 ksi, while keeping all other factors the same. The answer choices are essentially, would the deflection increase, decrease, remain the same, or there isn't enough information given. I knew it had to be one of the latter two, given the available information. I'm under the impression that they implicitly assume the deflection must be elastic in the solution, because the "most correct" answer is that it remains the same. However, if you don't make that assumption and the stresses in the beam fall between 36 and 50 ksi, the beam would experience plastic deformation and defections would increase. No information is given about the loading or stress states, which leads me to believe that, in reality, not enough information is given. Is this problem slightly bogus, or am I missing something?

The very next problem, question 112, asks what the most efficient steel section would be for a beam with a span of 20 feet and an unbraced length of the compression flange of 20 feet. The answer choices are a square HSS, a channel, a wide flange, and a double-L/T section. When I read the question, I figured that it had to be alluding to a more complex concept, but had no idea what it might be. After I saw the solution, it was obvious that, unbraced length of the compression flange was the indicator that this was, indeed, a lateral torsional buckling problem. However, I really don't know how I would have recognized that from the outset. Are there any tips out there for recognizing the key words and phrases that indicate an implicit concept that's being tested?

This all ended up being kind of wordy, but I hope it's decipherable. Thanks!

The first question is remain the same, as the deflection equation does not include yield stress.  It is dependent on the  shape and elastic modulus of the material.

 

[Will.I.Am PE]

The first question is remain the same, as the deflection equation does not include yield stress.  It is dependent on the  shape and elastic modulus of the material.

Yes, that's true ... if you assume the loading remains in the elastic range for both materials. The simple deflection equations that I know about (like Appendix 44.A of the CERM), which depend on elastic modulus, are only valid for elastic deflection, as far as I know. 

 

[DoctorWho-PE]

On the civil morning... they are not likely to get into anything outside of elastic.  Perhaps in the afternoon for structural, although in my experience, not even there.

 

[Stardust]

1. AM Structural deflection questions are unlikely to get more complex than the formulas in the Steel manual.

2. A closed section is best against torsion, and the square HSS is the only closed section in the answers.

Generally, there's no need to overthink the morning.

 

[civilized_naah]

These are both poorly posed questions, as pointed out by the OP.

For the first, the deflection equations are indeed valid for for elastic behavior only. So, faced with such a question, especially in the AM, one would be forced to play mind games and second guessing such as 'Could they be really getting into the elastic to plastic transition for a mere AM question'?

I always thought the explanation to the second question was half-baked. Lateral torsional buckling is not pure torsion, it is only named so (poor choice) because when the compression flange wants to buckle sideways while the tension flange doesn't, it looks like the beam section is subject to a torsional moment. The argument 'a closed section is more resistant to torsion', while true, is not a good justification for their chosen answer. 

 

[Will.I.Am PE]

These are both poorly posed questions, as pointed out by the OP.

For the first, the deflection equations are indeed valid for for elastic behavior only. So, faced with such a question, especially in the AM, one would be forced to play mind games and second guessing such as 'Could they be really getting into the elastic to plastic transition for a mere AM question'?

I always thought the explanation to the second question was half-baked. Lateral torsional buckling is not pure torsion, it is only named so (poor choice) because when the compression flange wants to buckle sideways while the tension flange doesn't, it looks like the beam section is subject to a torsional moment. The argument 'a closed section is more resistant to torsion', while true, is not a good justification for their chosen answer. 

Thank you.

What adds even more doubt to the first problem is that the parameter in question is literally yield stress... The point at which materials transition from elastic to plastic behavior. 

I feel like I could have at least come up with the answer they're looking for if the question was "Which of these sections is most resistant to lateral torsional buckling?" As it's stated, I think it's just too cryptic, particularly for an AM question.