Optimal design of redundant structural systems: fundamentals

TL;DR

This paper explores the fundamental aspects of designing redundant hyperstatic structures, emphasizing the impact of progressive collapse and uncertainties, and highlights the importance of latent failure probability in optimal structural design.

Contribution

It introduces a comprehensive analysis of redundant system design considering progressive collapse and epistemic uncertainties, emphasizing the role of latent failure probability.

Findings

Latent failure probability is crucial in determining optimal designs.

Redundant structures are necessary when latent reliability is below target.

Reliability depends on nonstructural factors and uncertainties.

Abstract

In spite of extended recent interest in System Reliability-Based Design Optimization (System RBDO) and life-cycle cost or Risk Optimization (RO), there is a lack of published studies on optimal design of redundant hyperstatic systems with objective consideration of (a) progressive collapse and (b) the impact of epistemic uncertainties. This paper investigates the fundamental aspects of the problem, by addressing the optimal design of simple two-bar active and passive redundant systems. Progressive collapse is objectively addressed, differentiating consequences of direct collapse of statically determinate structures, and progressive collapse of redundant, statically indeterminate structures. It is well known that reliability of a structural system also depends on nonstructural factors, or factors beyond structural design, such as unanticipated loading, manufacturing quality, quality of workmanship and human errors. These factors can be taken into account in an encompassing risk analysis, which accounts for physical and epistemic uncertainties, and which contributes a fixed latent failure probability to the structural optimization problem. Results presented herein show that the latent failure probability is the single most important parameter in determining optimal solutions, in System RBDO and in RO solutions. When the latent failure probability is smaller than target (System RBDO) or optimal (RO) failure probabilities, there is an equivalence between redundant and non-redundant (hyperstatic and isostatic) designs. However, when the latent reliability is smaller than target or optimal reliabilities, optimal designs become necessarily redundant (hyperstatic); as the only way to make system reliability larger than the latent reliability is by making structural systems redundant.