Section 7: AppurtenancesAnchor: #i1010175
The type of railing used on a bridge is as much a hydraulic consideration as one of traffic safety and aesthetics. This is particularly true in instances where overtopping of the bridge is possible. The two types of rail discussed here are:
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- Solid bridge railing -- Solid bridge rail should be used only where the bridge superstructure is in no danger of overtopping. A solid type of rail (e.g., a parapet wall) is useful from a safety standpoint but constitutes a significant impediment to flood flow. Anchor: #FLPWPGGN
- Open bridge railing -- The most desirable type of rail for accommodation of flood flow offers the floodwater an opening. An open slender type of bridge railing has a lower backwater and reduced lateral forces than a more solid type. A TxDOT research project was initiated to determine which of the standard TxDOT rails are the most hydraulically efficient. Results from this project are documented in a report, Hydraulic Performance of Bridge Rails ( 0-5492-1).
Effective deck drainage is necessary to minimize the possibilities of vehicular hydroplaning and corrosion of the bridge structure. Generally, it is more difficult to drain bridge decks than approach roadways for several reasons. Deck drainage can be improved by any of the following:
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- providing a sufficient gradient to cause the water to flow to inlets or off the ends of the bridge Anchor: #PQPKAHXE
- avoiding zero gradients and sag vertical curves on bridges Anchor: #GWNOOGSD
- intercepting all flow from curbed roadways before it reaches the bridge Anchor: #BOUETXGG
- using open bridge rails without curbs, where possible.
Currently, there is a trend toward using watertight joints and carrying all deck drainage to the bridge ends for disposal because of changes in environmental regulations.
Deck drains should be located so that water does not drain directly onto the roadway below. (See Ponding Considerations in Chapter 10 and Bridge Deck Drainage Systems, FHWA-SA-92-010 ( HEC-21) for more information.)
When using downspouts, splash basins should be provided to minimize erosion or tie the downspouts into the storm drain conduit. Drainage should not be allowed to discharge against any part of the structure.
Where practicable, the need to suspend a conduit collection system on the superstructure should be avoided. Collection systems should be designed with cleanouts at all bends, runs as short as practicable, and sufficient gradients provided to minimize problems with debris.
Because of the vulnerability of approach roadway shoulders and foreslopes to erosion from concentrated flow, should be provided sufficient inlet capacity off the bridge ends to intercept flow from the bridge. A closed conduit is often preferable to an open chute down the foreslope because it controls the water in a more positive manner, is aesthetically more pleasing, and is less susceptible to damage by maintenance equipment.
When bridge end drains are not provided with the bridge construction, temporary provisions for protecting the approach fill from erosion should be utilized until permanent measures are installed and functional.