Page 148 - Bridge and Highway Structure Rehabilitation and Repair
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CHAPTER 3                           BRIDGE FAILURE STUDIES AND SAFETY ENGINEERING           123



            •   Malfunction and locking of old bearing assemblies due to lack of maintenance; freezing
              of expansion bearings; large thermal forces causing compression and local buckling of
              truss members and fl anges
            •   Inability to prevent scour at pile top
            •   Inability to fully incorporate different types of soil interaction at abutments
            •   Lack of drainage behind abutments and pressure build-up behind abutments
            •   Inadequacy of Rankine, Coulomb or Mononobe-Okabe theories for non-homogenous soil
              conditions for wing walls and stub abutments, resulting in unstable foundation design.
        5. Providing space for bearing inspection chambers.
        6. Codes for rehabilitation of mixed (old and new) structural systems should be developed to
            enhance life and prevent early collapse.
        7. Greater vendor and construction engineer participation in revising and developing design
            codes for countermeasures is encouraged.
        8. Study of the failure mechanism of different types of structural systems.
        9. Maintaining quality control and personnel safety during construction.

        3.20.6  Steps to Avoid Substructure Failures
        1. The latest techniques of repair and rehabilitation of substructures need to be incorporated
            in codes.
        2. Fragility analysis of reinforced concrete pier to include soil-structure interaction: A proce-
            dure needs to be developed to evaluate effective soil strains, as a function of dynamic shear

            forces generated at the planar surface of a river bed due to flowing water. Uncertainties in
            soil properties can be included in the vulnerability assessment framework.
        3.20.7  Steps to Avoid Foundations and Piles Failures
        1. Pile design: The ultimate bearing capacity (UBC) of axially loaded piles must be limited to
            the compressive and/or tensile loads determined for reduced capacity for projected scour.
        2. Load redistribution must not be permitted when the axial pile capacity is reached; rather,
            axial capacity must be limited to the ultimate limit as established by L-pile analysis.

        3. Lateral soil-pile response must be determined by concepts utilizing a coefficient of sub-
            grade modulus provided or approved by the geotechnical engineer. Pile group effects must
            be considered.
        4. Substructure stabilization, repairs, and foundation improvement through underwater inspec-
            tion need to be carried out for all scour critical bridges.
        5. Use of a dynamic screening tool for pile bents: An evaluation procedure developed by the
            Alabama DOT may be employed. It is a screening tool in macro and micro fl ood charts.
        6. Preliminary or general checks such as bridge bent being located in water with scour possible,
            including:
            •   Checking bent piles for possible kick-out or plunging failure
            •   Checking bent piles for buckling failure
            •   Checking the bent for transverse to bridge centerline pushover failure from combined

              gravity and flood water loadings
            •   Foundation settlement from scour and weak soil conditions: When a pier tilts, there is

              potential for a bridge to collapse without warning. This definitely is a safety issue for
              travelers.
        7. Install countermeasures for risk reduction on the following lines:
            •   River training measures
            •   Placing guide banks to move scour away from the abutment foundation
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