Page 86 - Bridge and Highway Structure Rehabilitation and Repair

CHAPTER 2 DIAGNOSTIC DESIGN AND SELECTIVE RECONSTRUCTION 61

1. Degree of redundancy.
2. Live load member stress: The range of live load stress in fracture critical members infl u-
ences the formation of cracks. Fatigue is more likely when the live load stress range is a
large portion of the total stress on the member.
3. Fracture toughness: The fracture toughness is a measure of the material’s resistance to crack
extension and can be defined as the ability to carry load and to absorb energy in the presence

of a crack.
• FCM’s designed since 1978 by AASHTO standards are made of steel meeting minimum
toughness requirements.
• On older bridges, coupon tests may be used to provide this information.
• If testing is not feasible, the age of the structure can be used to estimate the steel type
which will indicate a general level of steel toughness.
Welding, overheating, overstress, or member distortion resulting from collision may ad-
versely affect the toughness of the steel. FCM’s that are known or suspected to have been
damaged should receive a high priority during the inspection, and more sophisticated testing
may be warranted. A bridge that receives proper maintenance normally requires less time to
inspect. Those with FCM’s in poor condition should be inspected at more frequent intervals
than those in good condition.
4. Fatigue prone design details: Certain design details have been more susceptible to fatigue
cracking. The thoroughness of a fracture critical member inspection should be in the order
of their susceptibility to fatigue crack propagation.

2.4.3 Method of Analysis and Mathematical Approach
The method of analysis is based on whether the bridge is redundant (statically indeterminate)
or non-redundant (statically determinate). The latter can be analyzed by simple laws of equilib-
rium and boundary conditions, while the former also requires stress-strain, strain-displacement
relationship, and compatibility equations. It appears that the difference in the theoretical approach
governs structural behavior and hence inspection and maintenance requirements.

2.5 THE ROLE OF GOVERNMENT AGENCIES IN MAINTAINING INFRASTRUCTURE
2.5.1 Administration of Infrastructure
1. Administrative responsibilities: Effective administration would ensure effective maintenance
and eventually prevent failures. However, bridge safety is directly linked to the basic fund-
ing required for maintenance purposes. It is also the engineer’s bread and butter to design
and maintain bridges. Failures, if correctly diagnosed, lead to improvements in design and
maintenance procedures. Besides design codes regularly being updated, it is also important
to understand the role of overseeing and to ensure that adequate funding is provided by the
federal and state agencies.
2. The federal/state role: Since potential failures are a hazard to public safety, they fall under
the jurisdiction of federal government agencies such as:
• The Federal Highway Administration (FHWA)
• The National Transportation Safety Board (NTSB)
• Federal Emergency Management Agency (FEMA).
The Federal Highway Administration (FHWA) is the main agency to oversee highway
bridges for:
– Maintenance
– Safety
– Reestablishing mobility
– Reconstructing bridges after a catastrophic failure.