Page 146 - Bridge and Highway Structure Rehabilitation and Repair
P. 146

CHAPTER 3                           BRIDGE FAILURE STUDIES AND SAFETY ENGINEERING           121

















        Figure 3.20  Diagram for monitoring scour critical bridges.




        4. Scour safety evaluation: The scour sufficiency rating needs to be evaluated. A scour analysis
            is carried out to evaluate scour depths based on FHWA Publication HEC-18. A scour report
            consists of a detailed field survey, substructure information, scour analysis results, hydraulics

            related findings, and countermeasure recommendations. Harnessing the river to reduce fl ood

            velocities may also be used.
            Common countermeasures are riprap, gabion baskets, concrete blocks, and sheet piling.
        3.20.4  Structural Planning and Solutions

            Planning is a major area of concern. A mistake made in planning can seldom be overcome.
        Bad planning decisions will ultimately lead to bad results. The following planning principles
        shall be observed:
         1.  The bridge footprint must be located in a suitable site ensuring stable foundations.
         2.  Sharp skew angles in the deck slab must be minimized to avoid stress concentration in the
            acute corners.
         3.  Selection of a structural system with redundancy and adequate construction materials.
         4.  The optimum number of spans, continuity over supports, and equal spans need to be con-
            sidered.
         5.  Provision for future widening of the highway below must be considered. This may require
            provision of a median, space for future lanes, an acceleration and deceleration lane, emer-
            gency shoulders, a sidewalk, and zebra crossing.
         6.  If a slab-beam bridge is selected, the number of girders and spacing shall be optimum. Girder
            spacing shall preferably be in the range of 6 to 12 feet. A higher degree of redundancy needs
            to be considered. In place of through girders that are non-redundant, a minimum of four
            girders is required. However, an odd number of girders is preferred for stage construction
            since the middle girder serves as a spine beam reducing deck defl ection.
















        Figure 3.21  Geotechnical procedures for locating unknown foundations.
   141   142   143   144   145   146   147   148   149   150   151