Ä¢¹½tv

Safer bridges through performance-based design

Over the last three decades, the paradigm for designing buildings, bridges, and other structures has been undergoing an interesting evolution. Not long ago, virtually all buildings and bridges were designed using a simple, universal framework known as the safety margin paradigm, comprising three basic steps:

1. Determining the nature of the loads (self-weight, wind, contents, etc.) likely to be imposed on a structure during its design life, as quantified in the applicable design codes and other standards
2. Arranging a system of structural elements (beams, columns, braces, etc.) to carry those loads
3. Sizing and detailing these elements so they can safely carry the loads while recognizing the uncertainties in both the material strength and the load magnitudes as prescribed by building codes and standards

While this framework is simple and has resulted in a stock of engineered structures that have, overall, demonstrated acceptable safety, the safety margin paradigm suffers some essential drawbacks. One is that it directly addresses only preventing collapse, or partial collapse, but not other performance objectives. Robust structural design should go beyond just life-safety concerns to address the overall performance of a structure including repairs and return to service after extreme loads. These considerations are often referred to as "the three Ds" — death, dollars and downtime. These concerns have led to a new design paradigm, performance-based design (PBD), that is now spreading through many areas of structural engineering. 

Early adoption of performance-based design and quantitative risk analysis

PBD uses quantitative risk analysis to directly address the question of "How safe is safe enough?" It does this by considering multiple failure modes and ensuring the calculated failure probability remains below some pre-defined risk tolerance that considers not just collapse but also the structure's return to service after an extreme load.

For instance, the performance objective for a new bridge subject to earthquake loads might be threefold:

1. No damage in frequent, low-intensity earthquakes
2. Moderate but easily reparable damage in less frequent but more intense shaking
3. Collapse prevention (life safety) in rare, intense shaking 

While PBD has been used for nuclear power plants and other critical facilities for decades, it is now being used to design critical structures to manage the risks from earthquakes, fires, and, most recently, strong winds. 

However, most existing bridges in the U.S. predate PBD and were designed using the conventional safety margin paradigm. PBD was not initially used to protect bridge piers from ship impact. Almost all bridges in the U.S. have been designed according to a specification from the American Association of State Highway Transportation Officials (AASHTO) as amended by each Department of Transportation. Early versions of AASHTO specifications did not prescribe any requirements for protecting bridge piers from ship impacts — that was essentially left to judgment. 

AASHTO provisions are not intended to entirely preclude bridge collapses from ship impacts but rather to reduce the risk to a level generally consistent with other structural provisions for extreme loading. 

After the catastrophic collapse of the Sunshine Skyway in 1980 following a ship allision, AASHTO included a simple provision requiring that bridge piers be protected from impacts based on an "appropriate" risk analysis. While the AASHTO specification provided no specific guidance for appropriate risk analysis, the specification writer's intent is clear: Because of the massive kinetic energy of a modern ship at speed, it may not be practical to design a bridge to withstand a head-on impact while preventing collapse or otherwise maintaining life-safety protections. Other, unspecified, aspects of the risk in addition to pier design should be considered.

This allowed the owner and designer to incorporate other means (fenders, channel geometries, etc.) to achieve an equivalently low probability of an allision destroying a bridge, essentially allowing for a PBD-informed strategy around pier impacts. This represents one of the earliest adoptions of PBD outside the design of nuclear power plants.

AASHTO and risk assessments for bridges today

Current AASHTO recommendations for pier protection have expanded significantly. Today, AASHTO prescribes explicit risk tolerances and provides substantially more guidance on performing quantitative risk assessments. Designers using AASHTO specifications must explicitly account for the following when calculating the risk to bridge piers posed by aberrant vessels:

• Types and frequency of vessels using the channel
• Probability of an aberrant vessel
• Relative geometry of the bridge and vessel
• Protection provided by fenders or other blockages
• Probability of collapse given an impact (known as the bridge fragility)

AASHTO provisions are not intended to entirely preclude bridge collapses from ship impacts but to reduce the risk to a level generally consistent with other structural provisions for extreme loading. 

For new bridge design, AASHTO specifications require that the annual frequency (probability) of collapse be less than 0.1% for typical bridges and less than 0.01% for critical or essential bridges (equivalent to average collapse return periods of 1,000 years and 10,000 years, respectively). Evaluating these risks to existing bridges designed before AASHTO code changes requires specialized, multidisciplinary expertise that can account for the wide variety of factors that either mitigate or contribute to the risk. These range from structural engineering of the piers to ship dynamics and operations to data sciences and statistical modeling. 

Fortunately, the design of bridge piers that account for potential impacts from passing ships has benefited from the evolution of the structural engineering practice, which incorporates the lessons of failures into codes and standards over time. As a result, the risk of collapse of modern structures specifically designed for acceptable performance is low, leading to safer and more resilient communities and transportation networks. 

However, the U.S. has a large stock of bridges and other structures built before the advent of PBD. Evaluating those structures against modern performance requirements would necessitate careful, site-specific risk assessments followed by mitigation strategies that could involve retrofitting or full replacement.