SAP2000 Buckling, P-delta, P-delta+ large different results

[TLycan]

Dear all,
I've posted this thread in "STR engineering and other topics" and I had no Response
So Iam posting it again here since this forum is dedicated for SAP2000; hopefully I might get any response.

I have the following problem. I am trying to find the buckling load for the beam in attached SAP file. The following are the strange results I’ve got:
Eigen Value buckling (buckling analysis) = 39X1 tons
P-Delta Non-linear analysis = 9 tons
P-Delta + large displacement Non-linear analysis = 72 tons
Though I know that the buckling load is near the 39 tons , the more important question why is there so much difference between the difference analyses, and even more important why is there a difference between P-delta and P-Delta + large displacement

Note: the file may take a bit while running also refer to the plot function of joint 393

 

[stressed]

TLycan, the 3 load cases in your model use different load patterns and factors. Your Eigen buckling case is the only 1 of the 3 load cases which doesn't include the "lateral" load pattern as part of the load case, which could definitely affect buckling.. and the load amplification factor is 5X for P-Delta and 40X for the P-Delta with large displacements.

Please explain your criteria for determining buckling with the P-Delta and P-Delta with large displacement cases.

Also, I believe your "Axial" load is 4.4 Kips, not 1 ton, so your Eigen buckling factor would be 39 X 4Kips, not 39 X 1 ton. You appear to have double-assigned joint loads for the Axial load pattern. Your joints have two load assignment for the same load pattern. When you assign loads, you have choices to Add, Replace, or Delete existing loads

 

[TLycan]

Stressed

thank you for your response


and This is embarrassing, you are right about entering the axial load. I don't know how it slipped.

The Axial load then becomes 1.999 tons.

Which now , Wow, makes the Eigen -Value results equal to the P-delta+ large displacement i.e. 39*1.999 = 78tons

the problem then seems to be be with the P-delta only.
to answer your questions , its as follow:
1-I tried the Eign analysis with the lateral load (actually as rest of the cases 0.01 * lateral) it did not affect the buckling analysis much.

2- The difference between the two scaling factor between the P-delta and P-delta + large Is

if I assigned the 40 scale factor for the P-delta, the analysis runs forever (i.e cannot find an answer) that is the the buckling or convergence load is small that the load step cannot capture it

If I assigned the 5 scale factor for P-delta + large, the analysis does not reach the buckling load and the plot function is still linear

therefor I had to make trial to reach the convergence or instability (buckling) load

3-for the joint I've answered it above

the problem now is with the P-delta analysis results