frame partial fixity springs in SAP2000 or ETABS
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Author: ralarcon
Last Post: parhyang
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frame partial fixity springs in SAP2000 or ETABS
#1
Hello!

I want to simulate a metalic structure with intermedium values (p.e. 70% or 50% of stiffness) on the joints in SAP2000 or ETABS using:
Assign menu > Frame/Line > Frame Releases/Partial Fixity .

How can I find 'k' values that could simulate my reality?

Thank you
#2
I am supposing that you want to alter the stiffness of your current system. As such you would be interested in the active stiffness of this system. And by active I mean the effects your system feels at current state. As such if you want to find out the rotational stiffness of your beam, as part of a frame, at the left hand support then you need to extract the rotation of the node in question and then divide it by the equivalent moment of the beam at hand at the same node. The reciprocal of this ratio is the active stiffness at hand. Multiplication of this number by 0.7 would then yield the partial fixity for 70% stiffness level. A word of warning regarding the units of all the above numbers. Hope this helps.

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#3
fully rigid connection have stiffness k=lamda*4*E*I/L, where lamda > 100

if you want to define partial connection just multiple coefficient with these values.

gold rules, the units should be consistent.
#4
Thanks for the answers but I believe the correct solution is as explained in McGuire Matrix Structural Analysis chapter 13. this is not so simplified.


#5
yes i do make simplified as proposed by Romstad (1970),

Sc = n/(1-n)*4EI/L

you can verified by make simple example and play a role with fixity factors says 0.0 trough 1.0 (increment 0.2) and plot bending moment result, and you'll get clear.

about references you prefer, how do you apply alpha_1 and alpha_2 (p.394 of the books) in SAP2000. did you ever verify?
add shoot,

[Image: 54938593242537551120.png]
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#6
Could you tell me where I get the book where it appears the explanation of Romstad (1970)?

thanks
#7
i don't have original papers/report, i was taken from:

[Image: 73340394620611866297.jpg]

you can found these books in this tread,

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btw, it's implemented too by MULTIFRAME developers.
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take attention in SAP2000/ETABS steel design routine since moment magnification with "k" factor method is not appropriates for these type of connection, you should perform 2nd order p-delta analysis and follow DAM-AISC 2005 procedures.
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