Component-based fragility analysis of transmission towers subjected to hurricane wind load

Abstract

The fragility model of transmission towers is one of the key elements for the hurricane risk and resilience assessment of regional power utility infrastructure. The current methods for fragility curve assessment of transmission towers rely on either static analysis or dynamic analysis with simplified demand parameters and limit state functions. The details of tower connections, such as joint eccentricities, rotational stiffness and slippage are often overlooked in fragility analyses due to the associated complexity of modeling and large computational effort. The purpose of this study is to model the realistic performance of transmission towers under hurricanes and develop a fragility curve estimation framework balancing between accuracy and efficiency.The correlated multivariate wind load time histories on the tower are simulated, and the load transferred from the cables to the cross-arms of the tower are generated using an effective analytical model. Two types of three dimensional finite element models of transmission towers are developed in the OpenSees platform. One is a complex model with all the joint details, which requires a very large computational effort to conduct nonlinear dynamic analyses, and the other one is a simple model without all the joint details, which is more computationally efficient. A small-scale dynamic probabilistic simulation with the two models is conducted, and a method to replace the complex model with the simple model is developed and validated. Using a high-performance computing cluster, medium-scale nonlinear dynamic time history analyses are carried out to obtain the demands of individual structural members and their correlation. Component-level failures are obtained via large-scale numerical simulations and are linked to global collapse of the tower through element removal and progressive collapse analyses. A case study of two transmission towers under hurricane is carried out. The results indicate that, compared to a traditional tip displacement method, the proposed component-based method with detail driven finite element model and large-scale simulations leads to a more accurate assessment of the transmission tower fragility curve.