NONLIN is a computer program that has been specifically designed to enhance learning in graduate university classes and continuing education seminars that cover subjects related to structu ral dynamics and earthquake engineering. The program is highly interactive, graphically intensive, and user friendly. This paper describes the basic features of NONLIN, and provides several suggestions for use in an academic environment. The latest version of NONLIN may be obtained at no cost by sending an e-mail request to the principal author of this paper. Programming Philosophy NONLIN was programmed using Microsoft Visual Basic version 6.0. All of the problem input is provided interactively. Th e program has a variety of error and data completion checking procedures. For example, the program will not allow the user to attempt an analysis unless all of the data has been provided, nor will it allow access to previously computed results if system p arameters have changed. NONLIN is graphically intensive in both the input and the output. The various graphs displayed by the program may be viewed on the screen, printed, or saved in tab delimited Microsoft Excel format for future processing. The basic principle in programming NONLIN is “minimum click design”. Whenever possible, an intuitively labeled button is provided to access a feature instead of requiring the user to navigate through a hierarchy of menus. For example, a ground motion acceleration record may brought into a model with only two mouse clicks (one to select the ground motion tool, one to select the earthquake). Two additional clicks provide a series of response spectra for the ground motion (one to select the tool, one to plot the spe ctra). Once a response history has been computed and plotted the Fourier amplitude spectrum of the response may be computed by a single mouse click. Fourier spectra may be obtained for input signals (e.g. accelerograms) in the same manner. The Fourier Tool is described in more detail later in this paper.
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