a case in calculation of lateral stiffness !! 2

[Alireza1945]

I have posted same problem in Seismology department here , but i think i explained in a not clear way , i got no reply ,,
well , the problem is , i am calculating lateral stiffness for frames , steel frames , and to reach story lateral stiffness and CR point .
problem is , i have an oblique grid line due to architect plan , and i dont know what to do with the oblique frame lateral stiffness ,
if i neglect it , i have to deal with great deal of torsional moment due to eccentricity ,,,
plz guide me !

 

[jayrod12]

Post a sketch of the plan view in order to aid the smart fellows in this forum in helping you

 

[Alireza1945]

OKey-doke , and yeah , this question is well defined under seismology , i dont know why ....

here is the pic :

 

[jayrod12]

I'm assuming you mean the frame line from D1 to F4.

Then don't neglect it, however it will cause a different type of torsional moment due to it's orientation.

But it's likely this torsional moment will be easier to deal with then if you neglected it altogether.

 

[KootK]

If you're concerned with how to handle the frame for hand calculation, I believe that you can introduce a pair of fictional orthogonal frames representing the EW and NS stiffness components of the actual oblique frame.

As for preliminary member sizes for the frames, I usually do this by concentrating on limiting drift and second order amplification effects. It's pretty involved reall, especially by hand.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Alireza1945]

jayrod12@ thanks , i neglected it so far , but it produces a great deal of eccentricity , and torsion , as u said ,

KootK@ i dont get you , by fictional frames ,, i mean , i only need to calculate in NS direction , and in that direction under E and F we have only one column , do u mean i add imaginary columns under E and F ?

what about calculating the lateral stiffness of D1 TO F4 , and then divide it into horizontal and vertical components of stiffness , does it make sense at all?

Tnx guys

 

[KootK]

All I see in your sketch is columns. It would be useful if we could see your proposed frames. Also, is it correct to assume that you're doing hand calculations?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Alireza1945]

yeah , i am doing hand cal instead of letting Etabs ... , i like to understand things , i really have no experience ,
okay , i will attach the actual plan here :

 

[KootK]

yeah , i am doing hand cal instead of letting Etabs ... , i like to understand things

Kudos to you. I like to understand things too. Is it correct to assume that you have moment frames around the perimeter of the building and not in the interior?

KootK@ i dont get you , by fictional frames ,, i mean , i only need to calculate in NS direction , and in that direction under E and F we have only one column , do u mean i add imaginary columns under E and F ?

With hand calcs, we basically convert all of our lateral resisting vertical elements into equivalent lateral springs. What I was proposing was to take the oblique lateral spring, representing your oblique moment frame, and convert it into equivalent N-S/E-W springs using trigonometric projection.

what about calculating the lateral stiffness of D1 TO F4 , and then divide it into horizontal and vertical components of stiffness , does it make sense at all?

I believe that this is precisely what I have proposed.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Alireza1945]

first , thanks for your attention ! and time !
umm , as the specifications tell me , in the NS direction my lateral bearing system is moment frame , yeah , but that is not the case for the EW directiom , i assumed that bracing system will act on that direction only , and frame is simple ,,
....
so , i take your advice and will divide the stiffness of the oblique frame into NS EW directions ,
it is simply interesting how trigonometry involves everywhere )) !

 

[Alireza1945]

oh , by the way , for frame transformation to spring , i know nothing about it , i have to read about it first , tnx for mentioning ))

 

[wannabeSE]

Don't expect this method to give results identical to ETabs or other software. With north-south lateral forces, the oblique frame can move the diaphragm to move east-west. This east-west movement will be resisted by the other east-west frames.

FYI: For Seismic Design Category D and higher, ASCE-7 does not allow equivalent lateral force analysis for buildings with this type of irregularity unless it is risk category I or II and two stories or less. I encourage you to do the hand calcs, but the code may require a different analysis

 

[KootK]

Don't expect this method to give results identical to ETabs or other software

I believe that the non-orthogonal effects can be captured in the hand analysis so long as the frames in all directions are represented in the analysis.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.