36

Life Cycle Evaluation and Condition Assessment of Structures

36.1 Introduction 36-1

36.2 Safety and Risk Assessment 36-4

36.3 Failure Cost 36-7

36.4 Condition Assessment and System Performance 36-9

Series Systems • Parallel Systems • General Systems

36.5 Damage and Deterioration 36-13

36.6 Time-Dependent Reliability 36-16

36.7 Inspection 36-19

Nondestructive Evaluation Inspection • Visual Inspection — Condition Ratings • Inspection Optimization • Updating

36.8 Maintenance and Repair 36-28

Preventive Maintenance • Repairs • Expected Life Cycle Maintenance Cost

36.9 Discount Rate 36-34

36.10 Condition and Reliability Indices as Joint Performance Indicators 36-35

36.11 Life Cycle Cost Examples 36-38

36.12 Conclusions 36-45

Acknowledgments 36-45

Glossary 36-45

Notation 36-47

References 36-48

36.1 Introduction

Society relies on its engineers to design structures that are safe and perform as intended. The public wants to cross bridges, enter buildings, and live downstream of dams without having to give conscious thought as to whether there is any danger of collapse. The nation's civil engineers have a distinguished history of performing this service. As research has progressed, experience has developed and been codified, and computers have increased computational speed and ability, design methods have become increasingly sophisticated. Traditional design approaches that rely on allowable stress and factor of safety

analysis have expanded to incorporate reliability and system-based performance methods. Design optimization methods are beginning to examine the entire life cycle performance of a structure. While this is a new and evolving (and therefore incomplete) field, this chapter explains many of the principles, concepts, and issues involved in examining the performance of a structure over its entire useful life.

Design optimization research often attempts to produce the lowest cost structure that meets specific design criteria. During the 1960s through the 1990s, the objective often involved minimizing the weight of a structure and considered only the costs of design and construction (labor, equipment, materials, etc.). Over the past 15 years, research efforts have increasingly demonstrated that these initial costs are often dwarfed by the costs of repairing, inspecting, and maintaining a structure over its useful life. A truly optimum and efficient solution needs to account for these costs. In its simplest form, the difference is illustrated in the following two examples.

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