Optimization of the machine foundation using frequency constraints

Abstract

Faced with the constantly increasing requirements for machine efficiency and reliability, designers have to pay attention to every detail, including machine foundation. Possible outage costs due to foundation damage can easily exceed the costs of design and construction of the foundation. This paper presents a novel and systematic procedure for finding an optimal design of reinforced concrete columns of the spring mounted machine foundation subjected to dynamic loads. The proposed approach assumes considerably decoupled natural modes of the critical structural members which can be optimized as simplified analytical models. It includes the following steps: FE modal analysis of the complete structure, identification of expected dynamic excitation, determination of the critical structural members, optimization of the corresponding analytical models and verification by FE modal analysis of the complete optimized structure. Turbine-generator-foundation structure was employed as an illustrative example. Operating speeds of the turbine and the generator and their higher order harmonics are taken as a critical dynamic excitation. Therefore, avoiding possible resonance region became the design objective, while the design variables were dimensions of the foundation columns cross-section. Lower and upper bounds of the design variables are determined on the basis of structural stability and space limitations regarding condenser and other auxiliary components of the turbine-generator system. The results of the analyzed example confirm that the optimization was successfully performed, since the optimized dimensions of the columns yielded natural frequencies outside the critical frequency range.