Page 229 - Bridge and Highway Structure Rehabilitation and Repair

204 SECTION 2 STRENGTHENING AND REPAIR WORK

• Alternate single longitudinal T-girder analysis: A simplified approach will be used to

compare/check results from three-dimensional model.

• Field study of bridges damaged by large deflection: Explanation of damage in the light
of vibration and fatigue studies may be required.
• Correlation between deflection and vibration: Correlation between vibration model results

and field measurements may be carried out.

5. Boundary conditions may be fixed, pinned, or free.
6. For pedestrian bridges, moving load patterns of varying numbers of people on a bridge
can be applied. Alternate ways to model them will offer different results. For example, one
person walking across the bridge will cause minimal vibration. However, several individuals
jumping on the deck can create higher oscillations.
Following the failure of Tacoma Narrows Bridge under wind, bridges with vibration effects
have been retrofitted with stiffening decks. Although wind analysis is seldom a primary consid-

eration, it can cause an increase in vibrations when combined with live load.
Member sizes allow designers to predict the dynamic behavior of a footbridge to a certain
extent. Computational analyses for live loads and lateral forces for pedestrian bridges needs to
be carried out.

5.3.11 Parameters Affecting Vibrations
1. Structural system: Through girders or multiple composite girders.
2. Truck load, lane load, or combination with dynamic load allowance (impact).
• All design lanes loaded.
• Increase in vibrations and fatigue may result for certain types of load patterns.

All beams may be assumed to deflect equally with CG of live loads to coincide with CG of
dead loads.

3. More advanced finite element responses (deflections and stresses) will be compared to
design values. This will help with better identifying the effect of spatial stiffness and load
distribution on bridge response.
Furthermore, dynamic analysis will be performed to investigate vibration characteristics.
Damping effects: These depend on a number of parameters such as:
• Random live loads and modeling.
• Materials used.
• Complexity of the structure.
• Type of surfacing.
• Bearing conditions.
• Weight of railings and parapets.
Bridges designed for equestrian loads are subject to galloping horse loads and impact and
need to be checked for vibration. For damping, use of a timber deck supported on lightweight

aluminum or steel floor beams of pony or through truss is preferred.
5.4 FATIGUE AND FRACTURE

5.4.1 Fatigue Analysis
Old steel bridges to be rehabilitated require a fatigue analysis of all existing steel members
to provide an estimate of the remaining fatigue life. The analysis shall be performed in accor-
dance with the:
1. Latest edition of the AASHTO Guide Specifications for Fatigue Evaluation of Existing Steel

Bridges.