Potential Errors in NCEES solution for SEII Exam

[MOOK]

Problem 250

1- In the LRFD solution

Where is the vertical effect of seismic load (0.2 SDS) in the load combinations?? any reason for ignoring it?

Problem 260

1-The solution increased the load by 15% per ACI 8.3.3.

Actually, I did not find any indication about this 15% increase in ACI 8.3.3. !!!!!!!! Can anybody tell me where exactly this issue??

Problem 450

1- Why the solution determined Tu max??

Cu factor is always larger than 1, so what is the reason behind that?

2- The problem gave us Rigidity ® for each frame but the solution ignored these values and used "R" for the OBF.

Any reason for that?

3- For part (B) in the problem, to detrmine the force on each frame

why the solution did not include the amplification factor Ax??

4- To determine the total load on Column C2

According to the tributary area, the equipment load should go completely to Column B2. There is no any equipment load should go to Column C2. The solution used part of the equipment load on Column C2, any reason???

5- To design the Pier (pedestal)

The structure in SDC (D), so the max spacing of ties in pedestal according to ACI is 6" or 6db

The solution used 12" spacing, any reason??

Problem 460

1- The solution assumed the structure is Building Frame System, why not Bearing Wall System??

2-The problem required to check the beam using LFRD load combination. Is this means that I have to study wood design in LRFD???? I know ASD wood design only.

Thanks

 

[MOOK]

No Comments from anyone !!!!!

 

[BLMedcalf]

Problem 250

1- In the LRFD solution

Where is the vertical effect of seismic load (0.2 SDS) in the load combinations?? any reason for ignoring it?

The only place the 0.2 SDS would show up in the strength combinations would be in equations 16-5 and 16-7. Since this problem is a wind example, they used equations 16-1, 16-2 and 16-4.

Problem 260

1-The solution increased the load by 15% per ACI 8.3.3.

Actually, I did not find any indication about this 15% increase in ACI 8.3.3. !!!!!!!! Can anybody tell me where exactly this issue??

The errata removed this statement. It was possibly from an older version of the code. Regardless, they corrected it now and it is no longer there.

Problem 450

1- Why the solution determined Tu max??

Cu factor is always larger than 1, so what is the reason behind that?

The only reason you would need Cu is if you calculated the T for your structure using an acceptable analysis procedure. The code caps the max at Cu x Ta (approximate period). You are authorized to just use the approximate period which is what this example does. There is no need calculate Cu in this example. I think it is a mistake.

2- The problem gave us Rigidity ® for each frame but the solution ignored these values and used "R" for the OBF.

Any reason for that?

These R values are unrelated. The R = 3 and R = 10 values shown on the plan are so you can distribute the shear and torsion to the frames appropriately. The Response Modification Coefficient, R is used to describe the ductility of a system. For an OCBF, the proper R is 3.25 which is shown in table 12.2-1 of ASCE 7.

3- For part in the problem, to determine the force on each frame

why the solution did not include the amplification factor Ax??

Ax is only used if there is type 1a or 1b torsional irregularity. I don’t think the solution checks for that, but it could be why they left it out.

4- To determine the total load on Column C2

According to the tributary area, the equipment load should go completely to Column B2. There is no any equipment load should go to Column C2. The solution used part of the equipment load on Column C2, any reason???

Some equipment load would go to Column C2. If you replace the equipment with a concentrated force at its CG, you should be able to see that the beam reaction at C2 would increase due to this point load.

5- To design the Pier (pedestal)

The structure in SDC (D), so the max spacing of ties in pedestal according to ACI is 6" or 6db

The solution used 12" spacing, any reason??

Not sure. They probably are not classifying it as a column.

Problem 460

1- The solution assumed the structure is Building Frame System, why not Bearing Wall System??

I think because it is a mix of columns and walls that support the roof. I was thinking the same way you were when I did this problem.

2-The problem required to check the beam using LFRD load combination. Is this means that I have to study wood design in LRFD???? I know ASD wood design only.

The special seismic load combinations apply to both ASD and LRFD. (16-22 and 16-23) The only difference is that for ASD, you can use a 0.7 factor as shown in ASD equation #5 on page 127 of ASCE 7. The bottom line is that the solution can be (and is..) done in ASD.

 

[MOOK]

Problem 2501- In the LRFD solution

Where is the vertical effect of seismic load (0.2 SDS) in the load combinations?? any reason for ignoring it?

The only place the 0.2 SDS would show up in the strength combinations would be in equations 16-5 and 16-7. Since this problem is a wind example, they used equations 16-1, 16-2 and 16-4.

Problem 260

1-The solution increased the load by 15% per ACI 8.3.3.

Actually, I did not find any indication about this 15% increase in ACI 8.3.3. !!!!!!!! Can anybody tell me where exactly this issue??

The errata removed this statement. It was possibly from an older version of the code. Regardless, they corrected it now and it is no longer there.

Problem 450

1- Why the solution determined Tu max??

Cu factor is always larger than 1, so what is the reason behind that?

The only reason you would need Cu is if you calculated the T for your structure using an acceptable analysis procedure. The code caps the max at Cu x Ta (approximate period). You are authorized to just use the approximate period which is what this example does. There is no need calculate Cu in this example. I think it is a mistake.

2- The problem gave us Rigidity ® for each frame but the solution ignored these values and used "R" for the OBF.

Any reason for that?

These R values are unrelated. The R = 3 and R = 10 values shown on the plan are so you can distribute the shear and torsion to the frames appropriately. The Response Modification Coefficient, R is used to describe the ductility of a system. For an OCBF, the proper R is 3.25 which is shown in table 12.2-1 of ASCE 7.

3- For part in the problem, to determine the force on each frame

why the solution did not include the amplification factor Ax??

Ax is only used if there is type 1a or 1b torsional irregularity. I don’t think the solution checks for that, but it could be why they left it out.

4- To determine the total load on Column C2

According to the tributary area, the equipment load should go completely to Column B2. There is no any equipment load should go to Column C2. The solution used part of the equipment load on Column C2, any reason???

Some equipment load would go to Column C2. If you replace the equipment with a concentrated force at its CG, you should be able to see that the beam reaction at C2 would increase due to this point load.

5- To design the Pier (pedestal)

The structure in SDC (D), so the max spacing of ties in pedestal according to ACI is 6" or 6db

The solution used 12" spacing, any reason??

Not sure. They probably are not classifying it as a column.

Problem 460

1- The solution assumed the structure is Building Frame System, why not Bearing Wall System??

I think because it is a mix of columns and walls that support the roof. I was thinking the same way you were when I did this problem.

2-The problem required to check the beam using LFRD load combination. Is this means that I have to study wood design in LRFD???? I know ASD wood design only.

The special seismic load combinations apply to both ASD and LRFD. (16-22 and 16-23) The only difference is that for ASD, you can use a 0.7 factor as shown in ASD equation #5 on page 127 of ASCE 7. The bottom line is that the solution can be (and is..) done in ASD.

Thanks BlMedCalf for your comprehensive answers

I still have a note about Problem 450

4- To determine the total load on Column C2

According to the tributary area, the equipment load should go completely to Column B2. There is no any equipment load should go to Column C2. The solution used part of the equipment load on Column C2, any reason???

Some equipment load would go to Column C2. If you replace the equipment with a concentrated force at its CG, you should be able to see that the beam reaction at C2 would increase due to this point load.

How do you guess the beams layout??

For instance, if we assume that the beams layout are spanning from east to west (no beams in North to South direction), there will not be any load on column C2 from equipments. I still see that since the problem did not provide any beam layout, I would determine Column load by Tributary area which lead to no load on column C2 from equipments.

 

[BLMedcalf]

Thanks BlMedCalf for your comprehensive answersI still have a note about Problem 450

4- To determine the total load on Column C2

According to the tributary area, the equipment load should go completely to Column B2. There is no any equipment load should go to Column C2. The solution used part of the equipment load on Column C2, any reason???

Some equipment load would go to Column C2. If you replace the equipment with a concentrated force at its CG, you should be able to see that the beam reaction at C2 would increase due to this point load.

How do you guess the beams layout??

For instance, if we assume that the beams layout are spanning from east to west (no beams in North to South direction), there will not be any load on column C2 from equipments. I still see that since the problem did not provide any beam layout, I would determine Column load by Tributary area which lead to no load on column C2 from equipments.

Fair enough. I see your point. I think that the solution must assume that beams supporting the equipment must frame into the beam that ties into Column C2.

 

[bdd]

... since the problem did not provide any beam layout, I would determine Column load by Tributary area which lead to no load on column C2 from equipments.

You may need to revisit your idea of tributary area, it is a helpful concept if you dealing with uniform loading. As soon as the load is non-uniform or a point load you must consider that any load between two or more supports will be supported to some degree by the surrounding supports. Regardless of which way the beams are running the column at C2 would experience an increase in load from any load added anywhere south of grid B. Certainly the direction of framing and location of the load would determine how much is received by the column... but you could not say that it adds no additional load to that column.

In this very special case, not a coincidence, the layout of the equipment load in problem 450 would contribute the same amount of load to column C2 regardless of the framing direction.

Assume beams are simple span.

Therefore, only be concerned with the equipment load south of grid B.

Consider the equipment load as an equivalent point load at the center of the 15'x15' shaded area.

First assume intermediate framing spans E-W.

Based on the point load's location, 3/4 of the load will be transfered to the beam along grid 2 between B & C.

Then, based on the location of the load on that beam only 1/4 of that load would be carried by column C2.

Therefore Equipment Load at C2 = Equipment Point Load x 3/4 x 1/4 = 3/16 of full equipment load.

Next assume intermediate framing spans N-S.

1/4 of the equipment load would be carried by the beam along grid C between 2 and 3.

3/4 of that load would be carried by the column C2.

Equipment load at C2 = Full Load x 1/4 x 3/4 = 3/16 of full equipment load.

Does that make sense?

 

[MOOK]

You may need to revisit your idea of tributary area, it is a helpful concept if you dealing with uniform loading. As soon as the load is non-uniform or a point load you must consider that any load between two or more supports will be supported to some degree by the surrounding supports. Regardless of which way the beams are running the column at C2 would experience an increase in load from any load added anywhere south of grid B. Certainly the direction of framing and location of the load would determine how much is received by the column... but you could not say that it adds no additional load to that column.
In this very special case, not a coincidence, the layout of the equipment load in problem 450 would contribute the same amount of load to column C2 regardless of the framing direction.

Assume beams are simple span.

Therefore, only be concerned with the equipment load south of grid B.

Consider the equipment load as an equivalent point load at the center of the 15'x15' shaded area.

First assume intermediate framing spans E-W.

Based on the point load's location, 3/4 of the load will be transfered to the beam along grid 2 between B & C.

Then, based on the location of the load on that beam only 1/4 of that load would be carried by column C2.

Therefore Equipment Load at C2 = Equipment Point Load x 3/4 x 1/4 = 3/16 of full equipment load.

Next assume intermediate framing spans N-S.

1/4 of the equipment load would be carried by the beam along grid C between 2 and 3.

3/4 of that load would be carried by the column C2.

Equipment load at C2 = Full Load x 1/4 x 3/4 = 3/16 of full equipment load.

Does that make sense?

Thanks bdd

Actually, it does not make any sense. What are your bases for all these assumptions??