Page 231 - Bridge and Highway Structure Rehabilitation and Repair

206 SECTION 2 STRENGTHENING AND REPAIR WORK

• Fatigue analysis will be based on S-N curves. Maximum N value of 2 million cycles is
assumed. For steel, maximum S value will depend upon fatigue category of B or C as per
AISC Manual of Steel Construction.
• AASHTO LRFD fatigue vehicle is considered: According to a study carried out by WSDOT,

the fatigue load vehicle specified in the steel structures section of LRFD specifi cations is

more reflective of the fatigue loads experienced by highway bridges and produces a lower
calculated stress range than that of AASHTO standard specifi cations.
• Existing experimental and theoretical results from available fatigue studies on bridges can
be utilized to evaluate the effect of live load defl ection on long-term fatigue. Figure 5.7
and 5.8 shows crack location.
• AASHTO recommended 75-year life is considered for number of cycles. The highest
projected ADTT prevalent on the bridge will be used.
3. Small web gaps for fatigue prone details may result in stress concentration and subsequent
cracking, intersecting welds, lateral connection plates, longitudinal stiffeners, and cracks.
4. Notch effects, such as rivet holes and non-radius cuts, cause increase in stress.
5. All poor details, fatigue sensitive details, and stress risers of all types need to be removed.
• Rivet holes should be made round by reaming to eliminate crack initiation sites.
• Lateral connection plates should not be welded to tension fl anges.

• The stiffness of the new members should be considered and how the existing members
should be strengthened in order to carry the new loadings should be determinded.
• Major repair issues: For the alternate selected, the impact on traffi c disruption needs to
be a minimum. Items to be evaluated include crack sealing.
• Fatigue is the lower than expected performance of the member or a joint under repeated
cyclic loads when it fails at a stress level below yield stress (Figure 5.7).

• Analytical grid modeling of deck, girders, and diaphragms for fatigue.

The flow diagram of Figure 5.9 demonstrates the procedure to underline the importance of
fatigue in railway bridge girders under high fluctuating wheel loads.

5.4.3 Fracture Critical Member (FCM) and Redundancy
1. FCM is a steel member in tension whose failure is likely to cause the entire bridge to col-
lapse. These members are highly vulnerable to a fatigue type failure. The three conditions
for identifying fracture critical members are steel material, tensile stress, and vulnerability
to collapse due to simple confi guration.
2. Redundancy helps to delay fracture and increase fracture toughness. Redundancy is a
structural condition resulting when there is more than the minimum number of structural
elements for stability.

The framing or configuration of the bridge such as number of girders, number of con-
tinuous supports, or multiple numbers of members forming a connection would increase
redundancy.
• Load path redundancy: When three or more main load carrying members are present
between substructure units and if one member is close to failure, load can be safely dis-
tributed to other members.

• Structural redundancy: A configuration which provides continuity of load path to adjacent
spans due to redistribution of moments. Only internal spans, but not the end spans, will
provide structural redundancy.