Section 5: Single and Multiple Opening DesignsAnchor: #i1008371
This section provides a means to establish an initial size of opening and lengths and locations of multiple openings.
- For a single opening, analyze the effect of the trial opening using the method selected from those outlined in Bridge Hydraulic Considerations. If the resulting backwater or through-bridge velocities are unacceptable, modify the opening until the estimated conditions are satisfactory for both the design and check flood conditions. The department recommends automated procedures for such analyses.
- Where a bridge must cross a relatively wide floodplain or multiple discharge concentrations, it may be necessary to design multiple openings. A multiple opening configuration usually constitutes a main channel bridge with relief openings. This type of crossing provides openings at or near the flow concentrations. The result is a reduction in along-embankment flow and backwater effects.
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.
- Assume an average through-bridge velocity (vt) that is less than the maximum allowable velocity but that is not lower than the unconstricted average velocity.
- Apply the unconstricted design water surface elevation
to the cross section, and find the area (At)
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.
- Estimate an average depth of water (Dt) in the cross section where the bridge is to be located by inspecting the section.
- Find the trial length (Lt) of the
bridge using Equation 9-13.
- Position the headers in the stream cross section (same cross section as in Step 3) so that they are approximately Lt apart and at locations that appear to maximize the through-bridge area.
- Find the exact waterway area (Aw) below the design high water within the structure limits.
- Find the average through-bridge velocity (vb)
for the actual waterway area (Aw) by using
the Continuity Equation.
- Evaluate and establish allowable maximum velocity based on individual site characteristics. If vb 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 vb 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.
- Establish a lowchord (as discussed in the Freeboard subsection of Section 3).
- 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.
- 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.
Multiple Opening Design Approach
Design multiple structures so that each structure’s carrying capacity (or conveyance) is approximately the same as the predicted discharge approaching the structure. Poorly sized structures could result in a reapportionment of the approach discharges. Reapportionment of flow, in turn, may cause excessive backwaters, unacceptable along-embankment velocities, and excessive velocities through some structures.
In addition to striving for balance in proportion (discussed in the Carrying Capacity Guidelines subsection above), satisfy average through-bridge velocity requirements. Unfortunately, widely disparate through-bridge velocities cause uneven backwaters that will likely redistribute of flow, upsetting the originally designed balance of structure conveyances. The goal is to balance conveyances and simultaneously try to assure that the resulting energy grade levels at the approach cross section (Section 4) are about the same for each bridge in the multiple opening facility. (See Bridge Sizing and Energy Grade Levels for more information.)Anchor: #i1008517
Multiple Bridge Design Procedural Flowchart
The flow chart for multiple bridge design (Figure 9-19) illustrates the steps and considerations recommended in TxDOT designs.
Figure 9-19. Multiple Bridge Design Flowchart
When estimating the design high water at a multiple structure location, the design engineer still needs to determine how the flow divides itself across the floodplain at flood stage. In the case of multiple structures, the flow division indicates the approximate portion of the total flood discharge that will be carried by each structure. One method for estimating flow division is by actually observing the flow at design discharge and design high water at the proposed site. However, the ability to make such an observation when the proper set of circumstances occurs would be rare. Therefore, use the following analytical method to determine flow distribution and establish flow division.Anchor: #i1008543
Cumulative Conveyance Curve Construction
Inspection of incremental discharges or conveyances across a floodplain cross section usually reveals the location of relatively heavier concentrations of flow. By determining these heavier concentrations of flow, the design engineer can usually find reasonable locations for each of the bridges. In some instances, the concentrations of flow and associated flow divides are quite obvious. In other cases, the distribution of flow may be subtler, and must be estimated analytically, which is most easily done with a hydraulic analysis program.Anchor: #i1008589
Bridge Sizing and Energy Grade Levels
When you have estimated relative approach discharges, you should have two, often contradictory objectives:
- Try to size the multiple structures so that they offer approximately the same relative carrying capacities as the relative flow distribution would indicate.
- To minimize cross flow, you need to obtain similar values of energy grade level at the approach section for all openings. Generally, if the relative velocity differentials are not approximately the same for all openings, head differentials develop, causing a redistribution of the approach flows.
Often, it is not possible to balance energy grade levels and conveyances simultaneously. Therefore, because of the importance of avoiding a redistribution of flow from natural conditions, place more emphasis on balancing energy grade levels by having velocity head differentials approximately the same for each of the openings.
Size the bridges in a multiple opening situation to avoid exceeding maximum allowable through-bridge velocities at any of the openings. Calculate backwater head for a multiple opening situation in the same manner as for single opening structures outlined in the Single Opening Design Procedure subsection and based on the appropriate floodplain subsection and flow apportionment. That is, consider each bridge separately using the flow apportionment and associated portion of cross section.Anchor: #i1008621
Determine the distance between the lowchord and the water surface. Then, compare the result to the recommended freeboard considerations discussed under Freeboard in Section 3.
Analysis of Existing Bridges
One-dimensional analysis of an existing bridge involves the same concepts employed for designing a new bridge: assume that the flood flow will distribute itself to attain a constant energy grade at the approach section. The existing bridge will likely redistribute flow from what the approach channel conditions might otherwise imply. The stagnation points become functions of the bridge openings and the channel conditions. Until the computed energy levels at the approach section are approximately equal, you need considerable trial and error may be needed to adjust stagnation points, determine conveyance apportionment, and analyze each opening.