2.1 HISTORY OF DISASTERS AND SAFETY MANAGEMENT
2.1.1 Unexpected Material Deterioration and Failure
Engineering is usually about avoiding failures and investigating why failures occur and ways to fix the problem. There is a need to understand the conditions giving rise to past failures and ways to avoid such failures so that loss of life can be minimized. Historical events and selected case stud-ies demonstrate the causes of each type of failure. Future design codes can make use of the deficiencies identified in order to develop guidelines for safe practice. If failures are interpreted correctly, a great deal of information for correct analysis, anticipated behavior, detailed design, and construction can be obtained to help formulate accurate design guidelines.
Failures occur in different forms in a material. Physical forms of failure can be seen as infinitely large deformation and metallurgical disintegration of elements. It can be localized cracking without collapse or discontinuity or total separation in a component.
At failure, critical sections for plastic hinges are located at the midspan of beams or under the concentrated load where deflection or bending moment is highest. It can also be at a support where shear force, reaction, or negative bending moment is the highest.
Failures are encountered on construction sites and are not just confined to the collapse of structures. Deaths and injuries to construction workers by far exceed the number of fatalities of the bridge users in failure events. Structural design methods related to construction loads and equipment need to be refined.
Physical causes are varied such as erosion, reversal of stress, impact, vibra-tions, wind, and extreme events. Usually, it is a combination of dead load stress
combined with one or more external transient forces resulting in a compound critical stress. If dead load stress is already high and approaching the elastic limit of members, any applied force or stress will exceed the allowable limit and lead to failure. Scour evaluation reflects the scour sufficiency rating or coding. Scour rating and evaluation needs to be based on a more refined analysis.
3.1.2 Aftermath of a Bridge Failure
1. Shutdown of approaches to traffic.
2. Emergency relief work by police such as calling hospital ambulances and helicopters.
3. Provision of a detour or alternate route.
4. Emergency repairs and retrofits, if applicable.
5. Forensic engineering to resolve litigation issues.
7. Deficient bridges need to be replaced by efficient bridges designed by using the latest criteria. Issues include:
• Solving design challenges
• Availing of benefits provided by new materials
• Deploying new techniques such as extending the span length by beam splicing and post-tensioning
• Rapid reconstruction.
2.1.3 Studying the Reasons for Failure
1. Past failure studies have shown that failures occur due to a variety of reasons. The primary causes of failure and the numerous secondary causes contributing to failure need to be investigated. Primary effects may not all be dangerous by themselves, but when combined with secondary effects, their cumulative action can trigger a collapse.
Lessons need to be learned from each failure. It appears that much water has flowed over the bridge since the disaster at Schoharie Creek in New York State. If such failures can be prevented or even minimized, the engineer has done his duty for the community. Failure of a bridge due to flood is shown in Figure 3.1.