Analysis of Nonbuilding Structures Connected by Large Diameter Pipe while Subjected to Seismic Loads

It is not unusual in petrochemical plants for two adjacent nonbuilding structures to be connected together by large diameter pipe. Common examples of these nonbuilding structures include pipe racks, equipment modules, and heat exchanger structures. Typically, these structures are modeled separately from each other without considering the effects due to large diameter pipe connected to adjacent structures. The pipe is typically modeled and analyzed independent of the nonbuilding structures by considering the equipment as anchor points and the supporting structures as supports, guides or anchor points. This method of modeling nonbuilding structures connected by large diameter pipe is acceptable for static loads; however, the results can become very misleading when applied to earthquake loads. It is even more difficult to rationally apply the effects seismic displacements to these independent models. In this paper three methods of seismic analysis and corresponding results are presented using a simplified 2-D example of two nonbuilding structures connected by a large diameter pipe. The first method uses the traditional method of analyzing the two structures independently and accounting for loads due to seismic displacements from an independent pipe model. The second method utilizes a combined model which includes two nonbuilding structures connected by a large diameter pipe. The seismic load is applied statically to the structures. The third method utilizes the same combined model as the second method with analysis done using Response Spectra Analysis. The results of each method are graphically compared. The paper also includes the results and conclusions drawn from the methods which were investigated.