SAP2000 Linear Buckling Question

[jiejingzhou]

Hello Everyone,

I have a general question regarding how SAP2000 handles linear buckling.

In the buckling load case, there are two stiffness you can use, namely
(1) Zero Initial Conditions (Stiffness of Undeformed Structure)
(2) Stiffness after end of an load case, which can be analyzed using:
a. P-Delta
b. P-Delta with Large Displacement

Here is an illustrative example, say the reference loading is P;

i. If i use the stiffness of (1), then the buckling factor is 5, and the buckling load is 0 + 5P = 5P
ii. If i use the stiffness of (2a) after loading the structure with P, then the buckling factor is 3.9, and the buckling load is P + 3.9P = 4.9P
iii. If i use the stiffness of (2a) after loading the structure with 2P, then the buckling factor is 2.8, and the buckling load is 2P + 2.8P = 4.8P

This is all very consistent and meaningful, however,

iv. If i use the stiffness of (2b) after loading the structure with P, then the buckling factor is 0.05, and the buckling load is P + 0.05P = 1.05P

this is a huge different compared to the other cases.

Is this results meaningful? The beam i am analyzing does not have a huge displacement, (i.e. 10 inch deflection for a 2000 in span after loading with P).

Thank You!