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Chapter 10

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BRIDGES ARE relatively expensive but often arethe most desirable stream crossing structurebecause they can be constructed outside of

the stream channel and thus minimize channelchanges, excavation, or placement of fill in the naturalchannel. They minimize disturbance of the naturalstream bottom and they do notrequire traffic delays onceconstructed. They are ideal for fishpassage. They do require detailedsite considerations and specifichydraulic analyses and structuraldesign.

The bridge location and sizeshould ideally be determined by anengineer, hydrologist, and fisheriesbiologist who are working togetheras a team. When possible, a bridgeshould be constructed at a narrowchannel location and should be inan area of bedrock or coarse soiland rock for a bridge site with goodfoundation conditions. Manybridge failures occur due tofoundations placed upon finematerials that are susceptible toscour.

“Bridges -- usually the best, but most expensive drainagecrossing structure. Protect bridges against scour.”

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Bridges should be designed to ensure that theyhave adequate structural capacity to support theheaviest anticipated vehicle or posted for load lim-its. Simple span bridges may be made of logs, tim-bers, glue-laminated wood beams, steel girders, rail-road car beds, cast-in-place concrete slabs, prefabri-

Photo 10.1Photo 10.1Photo 10.1Photo 10.1Photo 10.1 Culverts, fords, or bridges may be used for streamcrossings. Use bridges to cross large permanent streams, minimizechannel disturbance, and to minimize traffic delays. Use an openingwide enough to avoid constricting the natural channel. Locate thebridge foundation on bedrock or at an elevation below the antici-pated depth of scour.

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Brow Log

Native Log(typ.)

Shot RockFill Liner

Brow Log

19mm Ø Cable

Native Log Stringer Bridge

Curb

Untreated Timber Running Surface

Curb

Steel Beam

Treated Timber Decking

W Shape(typ.)

Stationary DiaphragmMovable Diaphragm

Stationary Diaphragm

Hamilton EZ Bridge (Modular)

Curb Curb

Metal Diaphragm(typ.)

Untreated Timber Running SurfaceTreated Timber Decking

Glulam Beam(typ.)

Treated Timber Glue Laminated Bridge

Curb Curb

PrestressedConcreteDouble-T(typ.)

Prestressed Concrete Single or Double-T Bridge

FFFFFigurigurigurigurigure 10.1 e 10.1 e 10.1 e 10.1 e 10.1 Cross-sections of typical types of bridges used on low-volume roads.

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cated concrete voided slabs or “T”beams, or using modular bridgessuch as Hamilton EZ or BaileyBridges (see Figure 10.1). Manytypes of structures and materialsare appropriate, so long as theyare structurally designed (Photo10.1).

“Standard designs” can befound for many simple bridges asa function of bridge span and load-ing conditions. Complex struc-tures should be specifically de-signed by a structural engineer.Bridge designs often require theapproval of local agencies or gov-ernments. Concrete structures aredesirable because they can be rela-tively simple and inexpensive, re-quire minimal maintenance, andhave a relatively long design life(100+ years) in most environ-ments. Log bridges are commonlyused because of the availability oflocal materials, particularly in re-mote areas. However, keep in

Photo 10.2 Photo 10.2 Photo 10.2 Photo 10.2 Photo 10.2 A bridge structure typically offers the best channelprotection by staying out of the creek. Use local material forbridges as available, considering design life, cost, and maintenance.Inspect bridges regularly, and replace them when they are no longerstructurally adequate.

mind that they have relativelyshort spans and they have a rela-tively short design life (20-50years) (Photo 10.2).

Foundations for bridges may

Photo 10.3 Photo 10.3 Photo 10.3 Photo 10.3 Photo 10.3 Scour is one of the most common causes of bridgefailure. Use an opening wide enough to minimize constriction of thenatural channel. Locate the bridge foundation on bedrock whenpossible, or below the depth of scour, and use stream bank protec-tion measures such as riprap.

include simple log sills, gabions,masonry retaining walls, or con-crete stem walls with footings.Some simple bridge foundationdetails are shown in Figure 10.2.Deep foundations often use drilledpiers or driven piles. Most bridgefailures occur either because of in-adequate hydraulic capacity (toosmall) or because of scour and un-dermining of a foundation placedupon fine soils (Photo 10.3).Thus, foundation consider-ations are critical. Since bridgestructures are typically expensive,and sites may be complicated,most bridge designs should bedone with input from experiencedstructural, hydraulic, andgeotechnical engineers.

Periodic bridge inspection(every 2-4 years) and maintenanceis needed to ensure that the struc-ture is safe to pass the anticipatedvehicles, that the stream channelis clear, and to maximize the de-

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sign life of the structure. Typicalbridge maintenance items includecleaning the deck and “seats” ofthe girders, clearing vegetationand debris from the stream chan-nel, replacing object markers andsigns, repairing stream bank pro-tection measures, treating dry andchecking wood, replacing missingnuts and bolts, and repainting thestructure.

RECOMMENDED PRACTICES• Use an adequately long

bridge span to avoidconstricting the naturalactive (bankfull) flow chan-nel. Minimize constriction ofany overflow channel.

• Protect the upstream anddownstream approaches tostructures with wing walls,riprap, gabions, vegetation,or other slope protectionwhere necessary (Photo10.4).

• Place foundations onto non-scour susceptible material(ideally bedrock (Photo10.5) or coarse rock) orbelow the expected maxi-mum depth of scour. Pre-vent foundation or channelscour with the use of locallyplaced heavy riprap, gabionbaskets, or concrete rein-forcement. Use scour pro-tection as needed.

• Locate bridges where thestream channel is narrow,straight, and uniform. Avoidplacing abutments in theactive stream channel.Where necessary, place in-channel abutments in adirection parallel to thestream flow.

• Consider natural channeladjustments and possiblechannel location changesover the design life of thestructure. Channels that aresinuous, have meanders, orhave broad flood plains maychange location within that

area of historic flow after amajor storm event.

• For bridge abutments orfootings placed on naturalslopes, set the structure intofirm natural ground (not fillmaterial or loose soil) atleast 0.5 to 2.0 meters deep.Use retaining structures asneeded in steep, deep drain-ages to retain the approachfills, or use a relatively longbridge span (Figure 10.2).

• Design bridges for a 100- to200-year storm flow. Expen-sive structures and struc-tures with high impacts fromfailure, such as bridges,justify conservative designs.

• Allow for some freeboard,typically at least 0.5 to 1.0meter, between the bottomof bridge girders and ex-pected high water level andfloating debris. Structures ina jungle environment withvery high intensity rainfallmay need additional free-board. Alternatively, a bridgemay be designed for over-topping, somewhat like alow water ford, and elimi-nate the need for freeboard,but increase the need for aerosion resistant deck andapproach slabs (Photo 10.6).

• Perform bridge inspectionsevery 2 to 4 years. Dobridge maintenance asneeded to protect the lifeand function of the struc-ture.

PRACTICES TOAVOID

• Placing piers or footing inthe active stream channelor mid-channel.

• Placing approach fillmaterial in the drainagechannel.

• Placing structural founda-tions on scour susceptiblesoil deposits such as siltsand fine sands.

• Constricting or narrowingthe width of the naturalstream channel.

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Typical Log Bridge InstallationEnsure that the bridge has adequate flow capacity beneath the structure. Keep fill mate-rial and the abutments or footings out of the stream channel. Set footings into thestreambank above the high water level or below the depth of scour if they are near thechannel. Add protection against scour, such as riprap, gabions, or vegetation.

Set the stringers or deck slab at least 0.5 to 2.0 meters above the expected high waterlevel to pass storm flow plus debris.

Bridge Abutment DetailSet bridge foundation (gabion abutment, footings, or logs) into rock or firm, stable soil.Set footings 0.5-2.0 meters into firm material.

Timbersill

Install bottom of stringersat least .5 m above high water

LogAbutmentGabion

Abutment

0.3-1.0 m widebench into firmsoil (not fill) infront of abutments.

1 m x 1 mGabionsor masonryabutment

Slab or Girder

Bridge Deck Roadbed

Set into firmsoil 0.5 to 2.0 m

1-2 m

Install bottom of stringers atleast 0.5 m above high water

Logs

FFFFFigurigurigurigurigure 10.2 e 10.2 e 10.2 e 10.2 e 10.2 Bridge installation with simple foundation details.

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Photo 10.4Photo 10.4Photo 10.4Photo 10.4Photo 10.4 Concrete struc-tures have a long design life andare typically very cost-effectivefor long bridge spans. Medium-length structures often combinea concrete deck placed uponsteel girders. Use bank protec-tion, such as riprap, to protectthe entrance and outlet ofstructures.

Photo 10.5 Photo 10.5 Photo 10.5 Photo 10.5 Photo 10.5 Locate the bridgefoundation on bedrock or onnon-scour susceptible materialwhen possible. When it isnecessary to locate the bridgefoundation on materials suscep-tible to scour, use a deepfoundation or design the bridgewith scour protection.

Photo 10.6 Photo 10.6 Photo 10.6 Photo 10.6 Photo 10.6 Here is a well builttreated timber bridge with goodstreambank protection andadequate span to minimizestream channel impacts. Thefreeboard is marginally ad-equate.