rc column design. Second order analyses method: nominal stiffness vs curvature

[Jummybear]

Hi, i am trying to compere 2 methods of second order analyses: nominal stiffness and nominal curvature. I have a simple scheme: column 40x40cm, l=8,0m, Ned=2400kN. But the difference i get between two methods is: 0,96<1.0 for nominal stiffness (capacity is not verifed) and 1,44>1,0 (capacity is OK). As for me then its a bit big difference between results and as for me nominal curvature methods gives realistic results and nominal stiffness method should give aprox the same capacity or even more than for nominal curvature method.

Model included.

PS Additional question: is it possible to calculate 2 coumns but with different type of element?

PPS Model included.

[Jummybear]

ei is also a question. The formula for ei calculation is 0*l0/2, where 0 is given by the code or national annex (i didnt find the place where i can change it in Robot) and usually is 1/200, l0 - length of the column. In my case ei should be 8000/200/2=20mm and robot gives 14mm..

[Artur.Kosakowski]

Se the attached doc.

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[Jummybear]

Thank you for explanation and attached document. I found 1 question answered (about the place where i can change the 0 value in Robot), but other 2 questions still wait for answers.

1. Why in my example i get such difference in results (about 50%) between nominal stiffness method and nominal curvature method? I have such document (hope you have it also) like Autodesk Robot Structural Analyses - Verification manual for EU codes (March 2011). On pages 42-48 there is a varification example for 1 RC column. The difference between 2 methods is ~10%. Probably I am doing something wrong, but in my case the difference is much more. I leave geometry, reinforcement, forces the same and change only second order analyses method.

2. Our miserable ei value. I understand how robot calculates the ei value (by formula 5.1 in code 1992-1-1:2007). In your referred document (same as in code) chapter (7) of point 5.2 its wrote: For isolated members (as I have in my example in 1 post) the effect of imperfections may be taken into account in 2 alternative ways a or b, where a is 0*l0/2 and in my case its 8000/200*2=20mm. Formula 5.1 and result 14.142mm is also correct, but why Robot dont take the biggest value to provide safer calculation results? Moreover, in the same document (varification manual for EU codes) there is such paragraph:

*** - the authors of [1] take the value of c0 = 12 for triangular distribution of moment. In Robot program however, this value is by default assumed as c0 = 8 since the exact distribution of moment along the height of the column is not known (thus, more unfavorable case is chosen). Thus, β is taken as when the moment in the mid-height (Mc) is not fixed by the user in the load definition dialog and 2337.18/2=π1=β is assumed when Mc is fixed (i.e. when neither 5.8.7.3 (2) nor (3) can be applied). It naturally leads to the greater (in this particular case by 20%), but at the same time safer, value of total moment.

So in sometimes Robot takes some bigger factors to provide more safer results and i agree with that. Why not to do the same for ei value?

[Artur.Kosakowski]

2. Our miserable ei value. I understand how robot calculates the ei value (by formula 5.1 in code 1992-1-1:2007). In your referred document (same as in code) chapter (7) of point 5.2 its wrote: For isolated members (as I have in my example in 1 post) the effect of imperfections may be taken into account in 2 alternative ways a or b, where a is 0*l0/2 and in my case its 8000/200*2=20mm. Formula 5.1 and result 14.142mm is also correct, but why Robot dont take the biggest value to provide safer calculation results?

For the a method I think the value is 14 mm rather than 20mm

Formula (5.2): ei=0i*lo/2

Formula (5.1): 0i = 0o *ah*am

It means that 0i  is not the same as  0o

The value of 1/200 relates to q0

If you find your post answered press the Accept as Solution button please. This will help other users to find solutions much faster. Thank you.