Seismic Analysis of Fossil-Fuel Boiler Structures

Seismic analysis has been performed for a 600-MW and a 1200-MW steam generators and their supporting structures in this paper using realistic three-dimensional finite element models. The 600-MW power plant is located in Zone III of the earthquake-risk map whereas the 1200-MW plant is located in Zone I. Because of greater possibility of an occurrance of an earthquake in Zone III, the power plant was designed according to the specifications of the Uniform Building Codes. Each suspended boiler is analyzed by an analytic model as well as by a finite element model with four hanger rods. For each plant, twelve natural frequencies and associated mode shapes of the combined boiler and structure finite element model have been obtained. For the 600-MW boiler structure, the spectra of modal responses of the relative displacements of the joints and the axial stresses in the structural members are obtained based on El Centro earthquake. It is found that the axial stresses exceed the yield stress and Euler buckling stresses in a minority of members. Small damping coefficients reduce the response displacements significantly.