The AASHTO 2005 bridge design philosophy employs the Load and Resistance Factor Design (LRFD) approach to ensure that bridges fulfill the objectives of constructability, safety, and serviceability by tackling various limit states. Each limit state emphasizes different performance characteristics of the bridge:
Strength Limit State:
This state guarantees that the bridge possesses sufficient strength and stability to endure significant load combinations throughout its lifespan. It accomplishes this by applying load factors to anticipated loads and resistance factors to material strengths, ensuring that the factored resistance surpasses the factored loads. Comprehensive structural analysis and high-quality materials are utilized to manage the maximum expected loads without failure.
Service Limit State:
This state focuses on stress, deformation, and crack width under normal operating conditions, ensuring the bridge functions effectively without excessive deflection, vibration, or cracking. The design imposes limits on deflections, vibrations, and crack widths under service loads. Regular inspections and maintenance are essential to address any issues that arise during the bridge’s operational life.
Fatigue and Fracture Limit State:
This state evaluates the impact of repeated loading and the risk of fatigue and fracture in bridge components, ensuring durability and longevity by preventing fatigue-related failures. The design considers the effects of repeated loading throughout the bridge’s lifespan, utilizing materials and methods that resist fatigue, along with conducting regular inspections to identify and address signs of fatigue or fracture.
Extreme Event Limit State:
This state accounts for rare and extreme occurrences such as earthquakes, floods, or collisions, ensuring the bridge can endure these events without catastrophic failure, thereby maintaining overall structural integrity. The design incorporates safety features and redundancies to keep the bridge stable and secure during extreme conditions, even in the event of some damage.
By addressing these limit states, the AASHTO 2005 design philosophy guarantees that bridges are safe, durable, and capable of performing their intended functions throughout their lifespan.