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<br />C. CHECKLIST OF DESIGN CONSIDERATIONS <br /> <br />TABLE 3.1 CHECKLIST OF DESIGN CotlSIDERATIONS <br /> <br />General <br /> <br />1. Raise the bridge superstructure elevation above the general <br />elevation of the approach roadways wherever practicable, <br />This provides for overtopping of approach embankments and <br />relief from the r.ydraulic forces acting at the bridge. <br />This is particularly important for streams carrying large <br />amounts of d!briE which could clog the waterway of the <br />bridge. <br /> <br />2. Superstructures Ehould be securely anchored to the <br />substructure if buoyant, debris, and ice forces are <br />probable. Further, the superstructure should be shallow <br />and open to minin:ize resistance to the flow where <br />overtopping is likely. <br /> <br />3. continuous span bridges withstand fcrces due to scour and <br />resultant foundation movement better than simple span <br />bridges. continLous spans provide alternate load paths <br />(redundancy) for unbalanced forces caused by seLtlement <br />and/or rotation of the foundations, This type of <br />structural design is especially recommended for bridges <br />where there is a significant scour potenLial, <br /> <br />4. Local scour holes at piers and abutments may overlap one <br />another in some instances. If local scour holes do <br />overlap, the scour can be deeper. The top width of a local <br />scour hole ranges from 1.0 to 2,75 times the depth of <br />scour. <br /> <br />5. For pile and dri:led shaft designs subject to scour, <br />consideration should be given to using a lesser number of <br />longer piles or "hafts as compared with a greater numbei of <br />shorter piles or shafts to develop bearing loads. This <br />approach will provide a greater factor of safety against <br />pile failure due to scour at little or no increase in cost. <br /> <br />6, At some bridge s~tes, hydraulics and traffic conditions may <br />necessitate consideration of a bridge that will be <br />partially or even totally inundated during high flows. <br />This consideration results in pressure flow through the <br />bridge waterway, Since this consideration has received no <br />attention relative to estimation of bridge scour, there is <br />no recommendation for determination of scour pending future <br />research. <br /> <br />28 <br />