# Problems 450 and 460 in NCEES SEII



## MOOK (Sep 6, 2010)

Problem 450:

1- What load combination is this 1.2D+1.0E+1.0L besides they added both Roof live load "Lr" and snow load "S" even though you have to use only one of them in any load combination.

In the load combination they used 0.9D+1.0E, the dead load and seismic load supposed to be subtracted from each other not added to each other.

2- Why they did not include the SDS any load combinations?

In Foundation design:

1- No load combinations used even though there are 3 load combinations shall be used

2- The pedestal was not designed according to the SDC "D" , the hoops shall be completely different

Any Explanation.

Problem 460:

1- Why in the solution, the chord force was magnified by omega node? As far as I know Omega nods is used with drag struts not collectors, any explanation?

2- Why the solution did not consider at line "C" to determine the chord force, is this because there is no shear wall at that line?

Thanks


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## BLMedcalf (Sep 7, 2010)

MOOK said:


> Problem 450:1- What load combination is this 1.2D+1.0E+1.0L besides they added both Roof live load "Lr" and snow load "S" even though you have to use only one of them in any load combination.
> 
> _ I think it should be: (1.2 + 0.2SDS)D + rhoQE + f1L + f2S, and the equipment should be treated as dead load. SDS = 0.733_
> 
> ...


See my response to the first part above.. Still thinking about the rest..


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## MOOK (Sep 12, 2010)

Thanks BLMedCalf

Please add this question to problem 450.

Why they did not consider the amplification factor "Ax"?


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## mjbikes (Oct 9, 2010)

MOOK said:


> Problem 460:
> 1- Why in the solution, the chord force was magnified by omega node? As far as I know Omega nods is used with drag struts not collectors, any explanation?
> 
> 2- Why the solution did not consider at line "C" to determine the chord force, is this because there is no shear wall at that line?
> ...


1) I think collectors are drag struts. They drag the diaphragm shear into the diaphragm A-C @ line 2 (if there is continuity btwn the BM1s).

2) The problem asked to check a beam at a certain location probably to check for your ability to determine chord forces at locations other than the middle of the diaphragm. You would use the chord force @ C for that connection.


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## MOOK (Oct 9, 2010)

mjbikes said:


> MOOK said:
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Thanks mjbikes

1- Sorry it's my bad. I should say I know Omega nods is used with drag struts not CHORDS instead of collectors. So, my question is still valid why they used omega node to design the chord.

2- What I mean by my question why in the analysis the solution did not consider a support at line C. They solved the diaphragm as one span not two spans with support at line C. This is what I mean.


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## mjbikes (Oct 10, 2010)

MOOK said:


> mjbikes said:
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1) THe chord tension/compression load is generated from Eh. since it is a collector (no shear wall supporting it) the overstrength factor needs to be applied.

2) There is no shear wall on line C. Vs is the reaction to the lateral loading at line J. Ws is the per foot lateral loading for diaphragm C-J only. The diaphragm moment at 81ft from line J is 19k x 81ft - moment from the uniform load Ws over the 81ft. Drawing the beam diagrams will help.


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## MOOK (Oct 10, 2010)

mjbikes said:


> MOOK said:
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Thanks again mjbikes

1) This chord is working as collector in E-W direction but in the problem they neglected the seismic in E-W direction and they designed for N-S direction only. This chord can be designed using omega node if it is designed to resist the collector force in (E-W) direction not the chord force in the (N-S) direction.

In my opinion, the chord should be designed without omega node in (N-S) direction and with omega node in (E-W) direction and since the problem ignored (E-W) direction; the chord should not be designed using omega node.


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