PE Civil (Structural)- NCEES problems 508 and 534

To all:

Problem 508: To calculate negative moment on the slab using the ACI 318-05 approximate method (Section 8.3), why is Ln is taken as center to center of span (see solution)? Per Section 8.3.3. it is clear that we should be using average of the adjacent CLEAR span lengths.

Problem 534: To calculate the effective span length for a deck slab of a steel girder bridge, why is the answer based on AASHTO Section 9 (Ans = 7.5 ft)? I thought the answer is 8 ft (Center to center spacing) per Section 4.6.2.6 (Interim 2008).

Am I misunderstanding the above questions, please help.

Thanks.

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Tam said:

To all:
Problem 508: To calculate negative moment on the slab using the ACI 318-05 approximate method (Section 8.3), why is Ln is taken as center to center of span (see solution)? Per Section 8.3.3. it is clear that we should be using average of the adjacent CLEAR span lengths.

Problem 534: To calculate the effective span length for a deck slab of a steel girder bridge, why is the answer based on AASHTO Section 9 (Ans = 7.5 ft)? I thought the answer is 8 ft (Center to center spacing) per Section 4.6.2.6 (Interim 2008).

Am I misunderstanding the above questions, please help.

Thanks.

Click to expand...

Do you have the book on PDF? i just realized i don't have the most up to date NCEES sample test and i tried to order it and it's not going to get here in time. Trying to find someone that has it on PDF.

PE Civil: Structural Sample Questions and Solutions

No. It is only available in print format :-(.

Tam said:

No. It is only available in print format :-(.

Click to expand...

The Codes ACI and AASTHO should be followed if the NCEES shows wrong approach there should be an errata.

I was confused by these two problems as well. However, I think the difference is that the beam spacing (S) is the center to center distance between beams, and the effective deck span (Ln) is "the distance between flange tips, plus the flange overhang"(AASHTO 9.7.2.3).

DJsigma and Steel man: Thank you for your reply.

Since the effective span length is asked, I'm okay with using AASHTO 9.7.2.3. Any thoughts on problem 508

Thanks.

DJsigma said:

I was confused by these two problems as well. However, I think the difference is that the beam spacing (S) is the center to center distance between beams, and the effective deck span (Ln) is "the distance between flange tips, plus the flange overhang"(AASHTO 9.7.2.3).

Click to expand...

Tam said:

To all:
Problem 508: To calculate negative moment on the slab using the ACI 318-05 approximate method (Section 8.3), why is Ln is taken as center to center of span (see solution)? Per Section 8.3.3. it is clear that we should be using average of the adjacent CLEAR span lengths.

Problem 534: To calculate the effective span length for a deck slab of a steel girder bridge, why is the answer based on AASHTO Section 9 (Ans = 7.5 ft)? I thought the answer is 8 ft (Center to center spacing) per Section 4.6.2.6 (Interim 2008).

Am I misunderstanding the above questions, please help.

Thanks.

Click to expand...

I was wondering the very same thing on question 508. I think it is a example of a very poorly worded problem on an exam (The test witer realy should have provided a diagram for this question). I'm in agreement with you that the CLEAR span should have been used for the calculation and the wording of the problem is such that the CLEAR span is equal to 9'. But for whatever reason, they used 10' for the solution?

Another problem on the Structural sample exam that I haven't 100% figured out. # 528. This is a question showing a steel angle fastened on one leg to concrete with a expansion anchor. The other leg supports a point load P. You are to assume the anchor bolt is sufficient and the concrete is sufficient. Find the maximum load P the angle can support. Load geometry and a actual angle size are given.

The solution given treats the leg of the angle as a 'beam' and the allowable moment is calucated for the leg of the angle (M=S/Fb). I follow all of the ASD solution except their choice of the allowable stress in the leg of the angle of Fb=0.75x36ksi. Where does the 0.75 factor come from? In my mind, I want to say the allowable bending stress is Fb=0.67x36ksi. What am I missing?

I don't know, i'm confused too.

Thanks.

rkelachim said:

Another problem on the Structural sample exam that I haven't 100% figured out. # 528. This is a question showing a steel angle fastened on one leg to concrete with a expansion anchor. The other leg supports a point load P. You are to assume the anchor bolt is sufficient and the concrete is sufficient. Find the maximum load P the angle can support. Load geometry and a actual angle size are given.
The solution given treats the leg of the angle as a 'beam' and the allowable moment is calucated for the leg of the angle (M=S/Fb). I follow all of the ASD solution except their choice of the allowable stress in the leg of the angle of Fb=0.75x36ksi. Where does the 0.75 factor come from? In my mind, I want to say the allowable bending stress is Fb=0.67x36ksi. What am I missing?

Click to expand...

One more problem.

Problem 102: Asking for the elevation of the TOP of the pipe (60 in. O.D. concrete pipe with 6 in. wall thickness). I got 623.06 ft (invert elevation) + 5 ft (pipe OD) = 628.06 ft but the solution is 623.06 ft (invert elevation) + 4.50 ft (OD - wall thick) = 627.56 ft. I'm not sure why they added 4.50 ft instead of 5 ft. I'm confused.

rkelachim said:

Another problem on the Structural sample exam that I haven't 100% figured out. # 528. This is a question showing a steel angle fastened on one leg to concrete with a expansion anchor. The other leg supports a point load P. You are to assume the anchor bolt is sufficient and the concrete is sufficient. Find the maximum load P the angle can support. Load geometry and a actual angle size are given.
The solution given treats the leg of the angle as a 'beam' and the allowable moment is calucated for the leg of the angle (M=S/Fb). I follow all of the ASD solution except their choice of the allowable stress in the leg of the angle of Fb=0.75x36ksi. Where does the 0.75 factor come from? In my mind, I want to say the allowable bending stress is Fb=0.67x36ksi. What am I missing?

Click to expand...

Back in the old days, the ASD 9th edition (the green book) had a 0.75Fy allowable stress for sections undergoing weak axis bending.

I hope this helps.

Tam said:

One more problem.
Problem 102: Asking for the elevation of the TOP of the pipe (60 in. O.D. concrete pipe with 6 in. wall thickness). I got 623.06 ft (invert elevation) + 5 ft (pipe OD) = 628.06 ft but the solution is 623.06 ft (invert elevation) + 4.50 ft (OD - wall thick) = 627.56 ft. I'm not sure why they added 4.50 ft instead of 5 ft. I'm confused.

Click to expand...

The 4.5' is the 5' O.D. minus the 6" thickness of the pipe.

I got it. This what i was missing "The invert elevation of a pipe is measured at the flow line which is the bottom INSIDE of the pipe"

Thanks.

DJsigma said:

Tam said:

One more problem.
Problem 102: Asking for the elevation of the TOP of the pipe (60 in. O.D. concrete pipe with 6 in. wall thickness). I got 623.06 ft (invert elevation) + 5 ft (pipe OD) = 628.06 ft but the solution is 623.06 ft (invert elevation) + 4.50 ft (OD - wall thick) = 627.56 ft. I'm not sure why they added 4.50 ft instead of 5 ft. I'm confused.

Click to expand...

The 4.5' is the 5' O.D. minus the 6" thickness of the pipe.

Click to expand...

Tam said:

To all:
Problem 508: To calculate negative moment on the slab using the ACI 318-05 approximate method (Section 8.3), why is Ln is taken as center to center of span (see solution)? Per Section 8.3.3. it is clear that we should be using average of the adjacent CLEAR span lengths.

Problem 534: To calculate the effective span length for a deck slab of a steel girder bridge, why is the answer based on AASHTO Section 9 (Ans = 7.5 ft)? I thought the answer is 8 ft (Center to center spacing) per Section 4.6.2.6 (Interim 2008).

Am I misunderstanding the above questions, please help.

Thanks.

Click to expand...

I found the ACI reference problem 508 uses to solve the problem. (I only have the ACI 318-08, so section #'s may be different). The solution for 508 uses center to center spacing becasue of ACI section 8.9 Span length. Paragraph 8.9.2 says 'continuous construction for determination of moments, span lenght is taken center to center of supports" Becasue the slab is one monolithic pour, it is continuous construction. So the span for moment determination is taken center to center of supports.

Gotta love when they test on really obscure code references!

rkelachim said:

Tam said:

To all:
Problem 508: To calculate negative moment on the slab using the ACI 318-05 approximate method (Section 8.3), why is Ln is taken as center to center of span (see solution)? Per Section 8.3.3. it is clear that we should be using average of the adjacent CLEAR span lengths.

Problem 534: To calculate the effective span length for a deck slab of a steel girder bridge, why is the answer based on AASHTO Section 9 (Ans = 7.5 ft)? I thought the answer is 8 ft (Center to center spacing) per Section 4.6.2.6 (Interim 2008).

Am I misunderstanding the above questions, please help.

Thanks.

Click to expand...

I found the ACI reference problem 508 uses to solve the problem. (I only have the ACI 318-08, so section #'s may be different). The solution for 508 uses center to center spacing becasue of ACI section 8.9 Span length. Paragraph 8.9.2 says 'continuous construction for determination of moments, span lenght is taken center to center of supports" Becasue the slab is one monolithic pour, it is continuous construction. So the span for moment determination is taken center to center of supports.

Gotta love when they test on really obscure code references!

Click to expand...

In ACI 318-05 it's Section 8.7.1, but I still think it's a typo, if you look at Pg 22 of 318-05 is clearly says Ln=clear span. I'm sticking with clear span on the test, and writing to NCEES incase it needs to be in the errata