# Column Stability



## MOOK (Sep 15, 2010)

In SEAOC book V.3 page 141

To calculate the stability index "epsi sub i" they divided by 2 .

I really do not know what the 2 used for? I do not think the reason that there are 2 frames in the NS direction since in AISC 341 page 4-45, there are 2 frames in the problem but they did not divide by 2.

Any explanation?

Thanks


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## MOOK (Sep 18, 2010)

MOOK said:


> In SEAOC book V.3 page 141To calculate the stability index "epsi sub i" they divided by 2 .
> 
> I really do not know what the 2 used for? I do not think the reason that there are 2 frames in the NS direction since in AISC 341 page 4-45, there are 2 frames in the problem but they did not divide by 2.
> 
> ...



Anyone??


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## kevo_55 (Sep 18, 2010)

Mook,

I believe that they are dividing by 2 becuase the Pi term was calculated for the entire floor/roof area. Take a look on page 124 for the plan view.

Since you are calculating for the MF bays (which are on the exterior), you simply only want the Pi term to include the loads for half of the floor/roof area tributary to your MF bays.

Sorry it took me a while to get to this. Just too busy around here.

Keep up the studying!!


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## MOOK (Sep 19, 2010)

kevo_55 said:


> Mook,
> I believe that they are dividing by 2 becuase the Pi term was calculated for the entire floor/roof area. Take a look on page 124 for the plan view.
> 
> Since you are calculating for the MF bays (which are on the exterior), you simply only want the Pi term to include the loads for half of the floor/roof area tributary to your MF bays.
> ...


Thanks a lot Kevo for your answer

I was thinking in the same way you are thinking but If you check the example in AISC 341, you will find that they calculated the Pi load for the WHOLE floor (the same as SEAOC) but they did not divide by 2, even though there are 2 frames also.


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## BLMedcalf (Sep 21, 2010)

MOOK said:


> kevo_55 said:
> 
> 
> > Mook,
> ...


I do not understand why they divided by 2 either. The SEAOC material seems to be more reliable in general.


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## Hromis1 (Sep 30, 2010)

Mook, correct me if I am wrong, I am writing this from memory as my posting last night some how got lost in the internet.

I agree with Kevo, it is a result of the Quirky way the problem author gave the loads and trib areas....

The "2" is not a result of having "two" frames in the column line being considered...but is a result of the trib. area really being 29,000 sf divided by 2...not the 29,000 as they start the calcs out as...I found this confusing as well...

Although I did not have time to compare this to the 341 example.


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## MOOK (Sep 30, 2010)

Hromis1 said:


> Mook, correct me if I am wrong, I am writing this from memory as my posting last night some how got lost in the internet.
> I agree with Kevo, it is a result of the Quirky way the problem author gave the loads and trib areas....
> 
> The "2" is not a result of having "two" frames in the column line being considered...but is a result of the trib. area really being 29,000 sf divided by 2...not the 29,000 as they start the calcs out as...I found this confusing as well...
> ...


Hormis,

SEAOC calculated half of the area and then multiplied by 2 but they used the beams for only ONE Frame. In AISC 341, there is similar example, they calculated the whole area and used the beams in the TWO Frames. If you follow AISC steps you will get double the value.


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## Hromis1 (Sep 30, 2010)

Mook, I clearly see your point know and I am equally confused. Could it be that the SEAOC problem considers the columns as pinned at the base? They are checking the first floor. For example look at the note on the middle ofpage 4-6 from the AISC book.


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## kevo_55 (Oct 1, 2010)

Hromis1 &amp; Mook,

I can understand your frustration with this subject.

I wouldn't get too hung up on this factor. I too see this division by two. I do agree that it really should be the entire story weight.

All I can say is thank God for partial credit. With one month left, that is what I am banking on.


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## mjbikes (Oct 9, 2010)

Epsi-i is calculated for each direction at each story using the design story drift (relative displacement of the CM of the story) and the total load acting on the story columns. And not for each frame, bay, or grid line, etc. The SEAOSC calc is wrong in dividing by 2, and should redesign for their unstable example.


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