Difference between revisions of "Bridge: Stream Crossings"

From MDOT Policy Manual
Jump to: navigation, search
Line 19: Line 19:
 
=Discussion=
 
=Discussion=
  
−
==Curved Horizontal Alignments==
+
==General==
 +
This guidance covers bridges and small structures that carry streams or other features that have been labeled as Waters of the U.S .  Bridges are defined by FHWA  as structures with clear spans of 20 ft. or greater.  Small structures are defined as having a clear span length or diameter of 5 ft. up to 20 ft.  Small Structures are also included in this guidance if they have a clear span length or diameter of 3 ft. up to 5 ft. with the fill over the structure being less than the clear span of the structure.  For smaller culverts or culverts that carry drainage, refer to Culverts in the Roadway section.
  
−
If possible, avoid locating bridges on curved horizontal alignments, since curved structures are more expensive to fabricate and are more difficult to construct than straight structures.  If a bridge falls within a curved roadway [[File:Curved_Roadway_Alignment_Ordinate.jpg|600px|right]] alignment, see if a tangent (straight) section can be introduced into the curve that encompasses the bridge limits.  If the curve radius is large enough, consider constructing the bridge with straight girders and deck and variable width shoulders.  To determine if this is feasible, determine the ordinate between the inside curve gutter line and the chord created by connecting the gutter line location at the bearing center line at each abutment.  If this value is less than 1 ft. 0 inches, the structure should be laid out with straight girders and deck.  
+
==Stream Crossings==
 +
Where possible, bridges should cross a stream at a straight section, instead of at a bend.  When a bridge is built over a stream bend, the substructure is susceptible to attack as flood flows can be directed at an abutment or pier, increasing scour potential, and the foundation costs.  The bridge may also need to be lengthened to move the substructure units out of the way of the flood flows.  In general, a bridge that crosses a straight section of a stream will result in a shorter and less costly bridge.
  
−
==Cross Slopes==
+
==Box Culverts==
 +
When possible, box culverts should be used for small stream crossings, since they can be constructed quickly and are virtually maintenance free.  Often environmental entities object to box culverts because they do not provide a natural stream bottom, but lowering the bottom of the culvert to allow for 2 ft. or 3 ft. of siltation can address this concern.  Consider precast box culverts when feasible, in order to speed up construction.
  
−
The preferred minimum cross slope on a bridge deck is 2 percent, which will facilitate travel lanes drainage and help avoid an icy bridge deck.  Constant cross slopes are preferred across the entire bridge length to reduce complex deck construction operations. It is possible to accommodate transitional cross slopes in a bridge deck; however, the transition from normal crown (2 percent in each direction from the crown) to plane incline requires a longitudinal joint at the crown/pivot point and complicated camber in the girders to account for the unusual deflections that will occur. It is also possible to have transitioning superelevation across a bridge, which would occur if it was in a reverse curve. This should be avoided when possible, since it will result in a flat area on the deck when the superelevation is 0 percent, which does not facilitate the travel lanes water drainage and can lead to safety and maintenance issues.  If it is not possible to adjust the alignment so the bridge is not within the area of transitioning superelevation, scuppers should be placed on either side the flat area to capture bridge deck drainage before it reaches this point.
+
==Pipe Culverts==
 +
Pipe culverts for stream crossings should consist of concrete pipes, not metal pipes, since the metal pipes have a history of deteriorating and will require major maintenance such as installing liners or paving the inverts.  Since these culverts are carrying streams or Waters of the U.S.,  stream diversions will be necessary to construct them, requiring permitting, which adds cost to a future replacement project.  Concrete pipes also have the added advantage of needing minimal cover as compared to metal pipes.  
  
−
==Sumps in Roadway Profile ==
+
==Piers==
 +
For structures with short spans, piers in a stream or other waterway should consider piles with protective jackets and concrete caps above the water line.  This will eliminate the need for a cofferdam or dewatering to construct the pier.  This may not be possible for longer span bridges, which will have piers that experience larger longitudinal forces, requiring footings with multiple rows of piles and cofferdams for construction.
  
−
Bridges should not be located in a sump (low point) in the roadway profile.  Bridge deck sumps will result in an area where water will collect and can lead to safety and maintenance issues.  Even when the sump is moved off the bridge, the roadway elevations should be checked to ensure the bridge deck is not essentially flat as you approach the sump.  If it is not possible to avoid a bridge sump, scuppers should be placed at and on either side of the sump to ensure sufficient drainage.
 
  
 
|}
 
|}
Line 36: Line 40:
  
 
=See Also=
 
=See Also=
−
*[[Roadways: Roadways:_Vertical_Alignment|Roadways: Vertical Alignments]]
+
*[[Roadways: Culverts|Culverts]]
−
*[[Roadways:_Vertical_Alignment|Roadways: Vertical Alignment]]
 
 
*[[Practical Design Implementation Guidance|Practical Design Implementation Guidance]]
 
*[[Practical Design Implementation Guidance|Practical Design Implementation Guidance]]

Revision as of 14:22, 2 November 2016

Stream Crossings

Primary Guidance

  • This guidance covers bridges and small structures that carry streams
  • Stream crossings should be at a straight section of a stream, instead of at a bend
  • Box culverts should be used for small stream crossings whenever possible
  • Pipe culverts should consist of concrete pipes, not metal pipes
  • For structures with short spans, piers in a stream should consist of piles with a concrete cap
  • For Design Storm criteria, refer to Drainage/Hydraulics – Design Storm
  • For Scour evaluation of bridges and bottomless culverts, refer to Drainage/Hydraulics – Scour

Discussion

General

This guidance covers bridges and small structures that carry streams or other features that have been labeled as Waters of the U.S . Bridges are defined by FHWA as structures with clear spans of 20 ft. or greater. Small structures are defined as having a clear span length or diameter of 5 ft. up to 20 ft. Small Structures are also included in this guidance if they have a clear span length or diameter of 3 ft. up to 5 ft. with the fill over the structure being less than the clear span of the structure. For smaller culverts or culverts that carry drainage, refer to Culverts in the Roadway section.

Stream Crossings

Where possible, bridges should cross a stream at a straight section, instead of at a bend. When a bridge is built over a stream bend, the substructure is susceptible to attack as flood flows can be directed at an abutment or pier, increasing scour potential, and the foundation costs. The bridge may also need to be lengthened to move the substructure units out of the way of the flood flows. In general, a bridge that crosses a straight section of a stream will result in a shorter and less costly bridge.

Box Culverts

When possible, box culverts should be used for small stream crossings, since they can be constructed quickly and are virtually maintenance free. Often environmental entities object to box culverts because they do not provide a natural stream bottom, but lowering the bottom of the culvert to allow for 2 ft. or 3 ft. of siltation can address this concern. Consider precast box culverts when feasible, in order to speed up construction.

Pipe Culverts

Pipe culverts for stream crossings should consist of concrete pipes, not metal pipes, since the metal pipes have a history of deteriorating and will require major maintenance such as installing liners or paving the inverts. Since these culverts are carrying streams or Waters of the U.S., stream diversions will be necessary to construct them, requiring permitting, which adds cost to a future replacement project. Concrete pipes also have the added advantage of needing minimal cover as compared to metal pipes.

Piers

For structures with short spans, piers in a stream or other waterway should consider piles with protective jackets and concrete caps above the water line. This will eliminate the need for a cofferdam or dewatering to construct the pier. This may not be possible for longer span bridges, which will have piers that experience larger longitudinal forces, requiring footings with multiple rows of piles and cofferdams for construction.


See Also

Retrieved from "http://policymanual.mdot.maryland.gov/mediawiki/index.php?title=Bridge:_Stream_Crossings&oldid=3676"