|
The program NSPECTRA is a computer program for obtaining the response spectra of nonlinear systems. The program can perform the analysis for a set of ground motions (maximum 200 ground motions), and average them. The program can also print out the time history response for a particular pendulum in the spectrum.
For calculating the restoring forces in the SDOF system, the program NSPECTRA uses a smooth hysteretic model; a generic model developed by the authors based on the same concept as the popular Wen-Bouc model. In this model, the restoring force in the SDOF system is given in terms of a differential equation independent of time. This model has been developed at the University at Buffalo to incorporate strength and stiffness degradation, pinching, and gap opening and closing effects that allows to model the behavior of different type of structural materials (e.g. structural steel, reinforced concrete).
The program uses a time-stepping scheme to solve the equation of motion of the SDOF system. The integration scheme is based on the Newmark’s method assuming a constant acceleration during the step that makes the program computational stable. A constant-size integration step is used. The use of a constant-size step can lead to unacceptable inaccurate results. In order to minimize this error, the Newton-Raphtson iterations method is implemented in the computer program NSPECTRA.