Page 101 - Bridge and Highway Structure Rehabilitation and Repair

76 SECTION 1 ADMINISTRATIVE ISSUES

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 suffi ciency
rating or coding. Scour rating and evaluation needs to be based on a more refi ned analysis.
3.1.2 Aftermath of a Bridge Failure

1. Shutdown of approaches to traffi c.
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.
6. Reconstruction.

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.

3.1.3 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 fl owed 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.

2. There is an abundance of information available about failures that are attributed to design
limitations and construction quality, in addition to those caused by extreme events.
3. Eliminating the root causes leading to failure: One of the duties of any engineer is to mini-
mize the possibility of failure. Engineering maintenance is not just patching concrete. It
means preventing failure by managing, disciplining, and applying structural mechanics to
the structural domain of bridge components, made up of single or composite materials.
It is important to understand both the mechanics and mechanism behind a failure and
the applicable theory of yielding and fracture so that structural integrity is maintained and
future designs are made safer.

3.1.4 Steps to Failures
No list can be complete since many failures or even near failures in the past have not be-
come public knowledge. A structural review of hundreds of failed bridges provides important
data for:
1. Those bridges which collapsed.
2. Those which get demolished due to their poor condition, since allowing them to continue
would result in imminent failure.
3. The wide variety of failures can be classifi ed as:
• Old bridge failures––Such failures are expected due to eventual disintegration of ma-
terials.