Page 142 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 3 BRIDGE FAILURE STUDIES AND SAFETY ENGINEERING 117

2. As discussed earlier, construction difficulties, wind, hurricane, tornado, flood, support settle-
ment, earthquakes, and tsunamis are some of the major environmental forces responsible
for failures. Physical causes are varied, such as vibrations, wind, extreme events, reversal
of stress, impact, erosion, and violent shaking during earthquakes.

3. Wind, earthquakes, and floods are acting at random in multi-dimensions. In the mathematical
model, they are resolved in three directions at right angles. The vertical component of wind
or an earthquake may be smaller than the two horizontal components, but can cause uplift
of a bridge deck over the bearings and therefore is important. The magnitude of a seismic
vertical component will depend upon the distance from the epicenter of the earthquake.
Seismic forces acting at right angles to each other in a given plane can be resolved with
maximum stress occurring in the resultant direction.
4. For modifi cations to existing structures, specifi cations used for original design need to be
checked against the latest LRFD specifi cations.
5. Studies have revealed the following causes (also addressed in Section 3.4):

• Construction problems and difficulties seem to be the biggest issue.
• Lack of timely inspection, maintenance, or neglect: The expected life of 75 years or more
for modern bridges and their components may not be achieved without effective inspec-
tion, structural evaluation, and timely rehabilitation.

• Design deficiencies, bridge design code violations, and in-depth analysis.
• Truss types and non-redundant structural systems are most vulnerable.
• Use of inferior material in some cases such as cast iron when wrought iron was available.
Today, HPS 70W steel plates fabricated with high strength welds are available.
• Acts of man, such as vandalism.
• For bridges located on rivers, impact from ships and soil erosion from floods account for

most failures.

• Acts of God such as accidents, fire, explosions, and extreme weather events.
6. Any one of the above factors may contribute to bridge failure or may trigger a collapse.
However, failures can only occur due to a combination of loads of which the principal or
additional cause can be one of those listed above. The load combinations have been defi ned
by AASHTO LRFD Bridge Design Specifi cations, 2004. AASHTO load combinations do

not include accidents, fires, or vandalism. However, AASHTO rating specifi cations address
details of inspection and the Manual on Rehabilitation addresses repair and maintenance.
3.20 STEPS TO AVOID FAILURES
3.20.1 Signs and Events Leading to Failure

In situations where considerable uncertainty exists in the ability to reliably inspect critical

details and in the predictionof accumulated damage, preemptive retrofit strategies appear to be
highly desirable.
1. It is too late when collapse and loss of life have already happened (replacement required).
2. Appearance of minor cracks and signs of possible incremental collapse (immediate action
required such as bridge temporarily closed for investigation or partial lane closure).
3. Damage and unserviceability (bridge is closed for major repairs or retrofi t).
4. Identification of any structural deficiency from inspections (action is required).

Figure 3.16 shows the role of extreme events, overload, or accidents, etc. in evaluating the
vulnerability ratings of bridges. The rating shows the extent to which a bridge is vulnerable to
failure. The likelihood of unexpected failure needsto be minimized.