P-Delta plus large Displacements

[h-badawy]

Dear experts

in case of define the nonlinear analysis type , i want to know the different between the two type of nonlinear geometric analysis (P-delta and P-delta plus large displacement) and what is the effect of the two option on the analysis.

 

[JoshPlumSE]

I would go through their help files or their web FAQ's for the most accurate description.

I can guess at what it means. And, I feel pretty confident that my guess is mostly correct.... But, it is just a guess in that I'm not really a SAP expert (I'm really a RISA guy).

For SAP2000, I believe PDelta means that they make a single geometric adjustment to the stiffness matrix based on the presence of axial load.... essentially softening the lateral stiffness of the structure. But, then they use this same stiffness matrix regardless of how much lateral load is applied or how much deflection is experienced. Essentially a linear analysis, but with a lower stiffness.

A large displacement analysis, on the other hand, implies (to me at least) a sequential non-linear analysis. Something along the lines of apply 10% of the load to the undeflected structure with the original stiffness matrix. Then calculate the stiffness matrix for the new deflected shape of the structure and apply the next 10% of the load.... Continue iterating until all the load has been applied. Then the final result is a superposition of forces and displacements from each increment.

Alternatively, they could just be doing a multi-step geometric adjustment to the stiffness matrix. That's really just a slightly more advanced form of P-Delta analysis and would be a bit of a misnomer (IMO) to say that it accounts for large displacements.