The capacity of both new and old bridges to withstand scour at their foundations and any other flow phenomena that could lead to failure needs to be examined. The problem is somewhat different in the two cases because a new bridge should be designed for the maximum flood to be expected, and there is ample opportunity in the design process to suggest foundations--or even bridge configurations--that may lead to safer, less costly bridges. Making existing bridges less vulnerable is likely to be difficult, awkward, and costly. However, even old bridges can be valuable--as can be discovered after they are lost--but the cost of remedial measures for the maximum expected flood may be more than can be justified. The prediction of scour at bridge foundations is a three-step procedure: (a) the establishment of the flood magnitude-frequency relationship, (b) the conceptualization and analysis of the flow characteristics of floods that might occur during the life of the bridge, and (c) the prediction of scour. The first step needs evidence of the maximum flood that should be expected; the second step is the most difficult as a general rule; the third step is likely to raise questions about scour that have not yet been answered adequately. As a result of the Silver Bridge failure, visual examination of bridges for structural integrity has become routine. Despite occasional spectacular failures like the Interstate bridge in Connecticut, there are probably more bridges lost in floods than from structural inadequacy. The assessment of the vulnerability or existing bridges to floods is also needed and would pay dividends. Recent research, sponsored by the Arizona Department of Transportation and the FHWA, has resulted in relationships for predicting the scour at the toe of a vertical wall and at the toe of a sloping sill. On the basis of the depth of scour, the structural form, and the ease of adding to the sill structure if need be in the futue, the sloping sill is the preferred solution. Recent unsponsored student research indicates that the previous solution for sizing riprap was too conservative. Both of these studies are aids to the engineer seeking ways to make existing bridges less vulnerable. (Author)

Media Info

  • Media Type: Print
  • Features: Figures; References;
  • Pagination: pp 222-229
  • Monograph Title: Second bridge engineering conference. volumes 1 and 2
  • Serial:

Subject/Index Terms

Filing Info

  • Accession Number: 00390833
  • Record Type: Publication
  • ISBN: 03090036593
  • Files: TRIS, TRB
  • Created Date: Dec 30 1984 12:00AM