For nuclear power plant life extension projects, it may be convenient and in some instances necessary to locate safety-related steel ducts and pipes outside of the main structures, exposing them to extreme environmental loads such as tornado missile impact. Examples of such applications include emergency firewater lines and Control Room vent ducts. The design of a tornado-impacted duct or pipe system requires an estimate of its dynamic response and an establishment of damage limits. These considerations are complicated by the system geometric and material nonlinearities and the local triaxiality effects on the response ductility limit. Simple, but acceptable accurate techniques for comprehensive evaluation of steel ducts under impact loads have not been established. This study presents simplified evaluation techniques for a steel duct/pipe under a heavy impact load, using fundamental concepts. The approach presented could be utilized to evaluate steel ducts/pipes against not only tornado missile impact loadings, but other types of transient loads as well. Both local and global responses of duct/pipe lines under missile impacts are evaluated. Applied impulse loadings for various types of tornado missiles are first established. A non-linear pseudo-static finite element analysis is performed in lieu of a more complicated dynamic analysis. The result is adopted for evaluation of the local ductility of ducts and pipes by using triaxial strain criteria. Global ductility and global shear checks are performed based on inelastic single-degree-of-freedom analysis methods. Pipe line design diameters and thicknesses are then determined based on the local ductility, global ductility and global shear evaluations. Accuracy of the non-linear pseudostatic analysis is discussed and a practical example using the proposed methodology is presented.

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