huangyhg

p-delta analysis and sap2000
hi,
please do correct me if i'm wrong, to my knowledge, p-delta anlysis could be summerized in 4 parts, pseudo displacement approach, pseudo load approach, non-linear static (newton raphson) analysis and two-cyclic iterative method.
i have difficulty finding the type of p-delta analysis used by the software sap2000 as i have not access to the manual of the software to refer to. i was wondering what approach does this software take in p-delta analysis. it seems to me that the software uses the incremental step by step analysis as this could be seen while the software runs the analysis. (on the green page). does anyone reckon methods such as the newton-raphson method is used to solve the non-linear equations or not.
cheers.
any suggestions?
the last time i checked, sap had two basic kinds of second order analysis.
for small displacements, the program uses an element geometric stiffness matrix approach. it's iterative because the force in each element can change, so the geometric matrices change. the joint coords do not change in this type of analysis and i'm pretty sure that n-r iteration isn't used. it uses consistent geometric stiffness matrices rather than linearized ones, so elements don't need to be subdivided as much to ensure accuracy. this type works very well for almost anything other than a cable structure.
for larger displacements, i'm pretty sure that it also uses the consistent geometric element stiffness matrix approach, but the nodal coords can change. i'm also pretty sure that it uses n-r iteration or something similar, but i can't remember if it iterates on tangent stiffness or secant stiffness. of course, this one should be used for cables, but the last time i tried it, it gave weird answers, so i don't trust it yet. cable net form finding must be used to find the initial shape and i don't think sap was doing this or doing it right.

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