Structures subjected to startup and shutdown of rotating machinery

With the advent of fast computers and easy-to-use software, transient dynamic analysis of structures has apparently become easy to perform. The engineer usually enters the entire structure (even in three dimensions) and produces results and plots that are sophisticated in appearance. The simulation results are most probably accepted without asking the following questions: "Are the results correct?" and "What is the sensitivity of the results to the various parameters?" The approach in this paper is different and more traditional: the salient behaviour of transient machinery during startup and shutdown is presented after studying the response of a structure which has been reduced to a single degree of freedom. The two-dimensional forces generated by rotating machinery during startup/shutdown have been derived analytically. Normalised curves have been computed to show the maximum response of the structure. A wide range of damping ratios has been considered. The maximum structural deflections during startup/shutdown can be significantly greater than the steady-state response and thus cannot be ignored. The normalised curves show that common approaches to limit dynamic deflections by increasing stiffness and/or damping have to be carefully considered during the transient regime. Two examples of an unbraced and a braced portal frame have been presented to demonstrate the practical use of these normalised curves. Keywords: startup, shutdown, rotating machinery, dynamics, normalised analysis curves