Single Opening Design Guidelines

To establish a single structure length and elevation of lowchord, begin by estimating the design flood, obtaining accurate controlling cross sections, and determining the design and check flood water surface profiles. For complete documentation, you may need a compilation of past flood history, existing structures, and other highway crossing characteristics of the stream.
  1. Assume an average through-bridge velocity (v
    t
    ) that is less than the maximum allowable velocity but that is not lower than the unconstricted average velocity.
  2. Apply the unconstricted design water surface elevation to the cross section, and find the area (A
    t
    ) subtended by this water surface that will satisfy the Continuity Equation (Equation 9-1, reworked as Equation 9-12) for trial velocity and design discharge.
    EquationObject275329
    Equation 9-12.
  3. Estimate an average depth of water (D
    t
    ) in the cross section where the bridge is to be located by inspecting the section.
  4. Find the trial length (L
    t
    ) of the bridge using Equation 9-13.
    EquationObject276330
    Equation 9-13.
  5. Position the headers in the stream cross section (same cross section as in Step 3) so that they are approximately L
    t
     apart and at locations that appear to maximize the through-bridge area.
  6. Find the exact waterway area (A
    w
    ) below the design high water within the structure limits.
  7. Find the average through-bridge velocity (v
    b
    ) for the actual waterway area (A
    w
    ) by using the Continuity Equation.
    EquationObject277331
    Equation 9-14.
  8. Evaluate and establish allowable maximum velocity based on individual site characteristics. If v
    b
     is close to the target average velocity, the initial bridge length may be reasonable. You must usually adjust this length slightly to fit standard span length requirements. If v
    b
     is much lower or greater than the allowable maximum velocity, adjust the length as necessary, repeating steps 6 and 7. Repeat this routine until the average through-bridge velocity is close to the target velocity. To minimize the cost of the structure, it is usually desirable to adjust the bridge length so that the design velocity is at or very near the maximum allowable velocity.
  9. Establish a lowchord (as discussed in the
    Freeboard
    subsection of Section 3).
  10. For the design and 1% AEP discharges, estimate the backwater caused by the constriction of the bridge opening. Use the procedures outlined in the
    Bridge Hydraulic Considerations
    section (Section 3). You may need to adjust the bridge length to ensure that the backwater effects are not excessive and comply with FEMA NFIP criteria, where applicable.
  11. Determine the maximum potential scour envelope. The Bridge Division Geotechnical Branch is the office of primary responsibility for bridge scour. See the Bridge Division
    Geotechnical Manual
    or contact the Geotechnical Branch for bridge scour policies.