BRIDGE CROSSINGSWITH DUCTILE IRON PIPE

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BRIDGE CROSSINGSWITH DUCTILE IRON PIPE

IntroductionThe Ductile Iron Pipe Research

Association (DIPRA) periodically receivesrequests from engineers and contractorsconcerning recommendations on the designand/or installation of pipelines spanningwaterways, highways, and railroads.Because the variables involved in suchinstallations present numerous alternativesand challenges for designers andcontractors, DIPRA does not providerecommendations and does not assumeresponsibility for design or installationpractices on such projects. DIPRA does,however, recognize the engineeringcomplexities inherent to bridge crossingpipelines and offers information to assistthose involved with this type of installationand to point out typical design criteria whichshould be considered for bridge crossings.Adaptation of the entire pipeline as a unitapplied to a bridge structure involves closedetail to many parameters in bothstructures. The following sections coverthese parameters in detail.

GeneralDuctile Iron pipe is centrifugally cast in

18- and 20-foot nominal laying lengths.Nominal diameters range from 3 to 64inches, with a variety of pressure andspecial thickness classes. Although DuctileIron pipe is usually furnished with acement-mortar lining, optional internallinings also are available for a wide range ofspecial applications. Also, Ductile Iron pipeis normally furnished with an externalasphaltic shop coat for a “finished”appearance, although shop-applied primersfor special painting systems also areavailable.

JointsDuctile Iron pipe is furnished with

several different types of joints: push-on,mechanical, restrained, ball and socket,flanged, and grooved and shouldered joints.Typically, bridge crossings involve push-onjoints, restrained joints, or combinationsthereof.Push-on joints (see Figure 1) are

excellent for bridges with properly designedand constructed supports. Ampledeflections in these joints are possible whenproper lateral and vertical supports areprovided to resist the weight of the pipe and

its contents as well as thrust forces.Normally, expansion and contraction of thepipe due to temperature changes can beadequately provided for with such joints; ifmore adjustment is needed, expansioncouplings (see section on Expansion/Con traction Couplings) should beconsidered.Mechanical joints (see Figure 2) are

often used for fittings but are not generallyused for straight runs of pipe. Toaccommodate possible pipe movementcaused by thermal expansion andcontraction, the push-on joint may be abetter choice than the mechanical joint dueto its deeper socket depth.Since standard push-on and mechanical

joints are not “restrained,” due con -sideration should be given to proper design

and construction of supports or anchoragesto resist thrust forces, dead loads, impactand shock loads, and thermal changes.The restrained joint complements the

push-on and mechanical types bymaintaining flexibility and also by providingboth ease of assembly and a “lockingfeature” to resist pull-out. Numerous typesare available employing modifications of thepush-on and mechanical joint designs. In a pressurized system, some flexible

restrained joints are subject to significantjoint extension. Therefore, when utilizingrestrained joints, proper design andconstruction techniques normally shouldinclude provisions for extending each jointso as to engage its restraints. This may beaccomplished by extending the joints fullyduring assembly and/or by hydrostaticallytesting the horizontal portion of thecrossing separately (using restrainedclosures) before making connections tooffset bends or riser pipes. Cumulative jointextension due to thrust pressure over along crossing could result in over-deflection,excessive movements, and excessive beamloadings to fitting connections at the ends ofa crossing.

Pipe Supports/HangersTypical installations of Ductile Iron pipe

on bridges involve a “pipe on supports”approach, depending upon the placementof the pipe on the bridge and the details ofthe bridge support system. Referring tothe DIPRA publication, “Design of DuctileIron Pipe on Supports,” “the minimumpressure class in all sizes is more thanadequate to support the weight of the pipeand water it contains” with one supportper length of pipe (i.e., a span length of 18or 20 feet).*It is recommended that the saddle angle

of the support be between 90 and 120degrees. Little or no benefit is gained byincreasing the saddle angle more than 120degrees, while stresses tend to increaserapidly with angles less than 90 degrees. Itis also recommended to use one supportper length of pipe positioned immediatelybehind the bell. With these assumptions,each span can be conservatively treated as asimply supported beam.It is of utmost importance that sufficient

vertical and lateral stability be provided atsupports for both restrained andunrestrained joint pipe. When a flexiblyjoined Ductile Iron pipeline is pressurized,some thrust forces develop — even atslightly deflected joints. If not adequatelystabilized, these forces can cause the jointsto deflect to their maximum, creating a

*Note: Proprietary assemblies are available for long span(multiple lengths per support) applications. Contact DIPRAmember companies for details.

Figure 1

Figure 2

PUSH-ON JOINTS

Fastite® Joint

Tyton Joint®

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“snaking” of the pipeline and possibly evenseparation of unrestrained joints.Supports must be designed to carry the

weight of the pipe and its contents, and otherapplicable loadings and conditions, such asthrust forces, seismic loadings, trafficvibrations, and insulation in cold climates.The location of the pipeline on the bridge

will, in many cases, dictate the type ofsupport to be used. Because of the manytypes of bridges and placements for thesepipelines, specific design details for hangersand/or supports for all possibilities cannotbe presented; however, three commonlocations for pipe placement are inside aconcrete box utility corridor, underneath thedeck between girders, and on the exteriorside. Each location may require differenttypes of support; here is a look at each.

• Inside a concrete box utilitycorridor

Support of the pipeline inside a box utilitycorridor is normally provided by the ceilingor the floor. A universal hanging detail fromthe ceiling is shown in Diagram A.Lateral movement is restricted by

bracing against the side of the box corridor.Long rod lengths are not advisable due tothe possibility of twisting or buckling andthe lack of proper support resulting fromcritical movements.Supporting the pipe from the “floor” of

the box corridor may be accomplished inseveral ways. One way is shown in DiagramB. Straps should be used to secure the pipeto the support, with each length of pipe“strapped down” immediately behind thebell (the bell should not rest on the floor).

• Underneath the deck betweengirders

Because of the unlimited configurations

in types of bridges and structural supports,it is impossible to address all thevariations. In many cases, the pipe can besupported from the concrete deck as inDiagram A, with sufficient vertical andlateral support from the bridge under-structure. Where structural steel beamsare used throughout the bridge, beamclamps present an easy solution to hangingthe pipe; however, the pipe will alsorequire adequate vertical and lateralsupport. Many bridges have steel beamsrunning laterally under the bridge whichmay serve to support hangers or cradles.

• On the exterior sideThe structural material of the bridge

exterior and its design configuration willgenerally dictate the arrangement forinstalling the Ductile Iron pipe. Examples ofbridges with concrete or steel exteriors areshown in Diagram C. For bridges withwooden exteriors (see Figure 3), drillingand tapping the supports are done easily inthe field and generally, the spacing ofsupports will accommodate the 18- to 20-foot lengths of Ductile Iron pipe. Thespecific design of the supports is unlimitedand open to the discretion of the engineer.The design choices for any placement

location are many; however, these specificpoints need to be observed: •At least one support per length of pipe is required (unless “long span” pipe is utilized).

•The minimum pressure class of Ductile Iron pipe is sufficient in all sizes to support the weight of the pipe and its contents.•Proper lateral and vertical support is needed to prevent “snaking.”

Expansion/ContractionCouplingsConditions of fluid flow inside a Ductile

Iron pipeline — as well as ambienttemperature changes throughout the year —will affect expansion and contraction of thepipeline with respect to the bridge. Attentionshould be drawn to the fact that bridgeexpansion could differ from that of the pipebecause of (1) differences between bridgeand pipe temperature, with pipe temperaturebeing affected by the temperature of itscontents, (2) differences in coefficients ofthermal expansion,* and (3) locations ofbridge expansion joints which mayconcentrate movement relative to the pipe.Also, the engineer needs to consider theinteraction of expansion/contractioncouplings with thrust restraint systems,especially when restrained joints are used. Expansion and contraction in con junc -

tion with thrust movement could introduceexcessive stresses in the Ductile Iron pipe,its joints, or its structural supports. The number and location of expansion/

contraction couplings, if required, aredetermined by the length and design of thebridge in consideration of the maximumanticipated temperature differential. Thesecouplings are normally utilized for longercrossings and bridges with multiple spans.Typically, expansion/contraction couplings inthe pipeline will be located adjacent tobridge expansion joints. Since these devicesoccasionally require maintenance, theyshould be located in an area that is easilyaccessible.

*Coefficients of expansion:Ductile Iron 0.0000062 inch/inch Fstructural steel 0.0000065 inch/inch Freinforced concrete 0.0000070 inch/inch F

ADJUSTABLE CLEVIS

ROD

PIPE

STRAP

STRUCTURAL BRACE

Diagram A

SUPPORT

STRAP

BRACKET

CONCRETE GIRDER

OR STEEL BEAM

PIPE

SUPPORT

STRAP

PIPE

CONCRETE GIRDER OR STEEL BEAM

CONCRETE OR STEEL BEAM

Diagram C

Figure 3

PIPE

STRAP

SUPPORT

FLOOR

Diagram B

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Basically, there are two types ofcouplings: single-end and double-end (seeFigure 4). Manufacturers’ recommendationsfor installation and proper use should befollowed. The type of joints used for theDuctile Iron pipe will dictate the possibleneed for an expansion/contraction coupling.Generally speaking, push-on joints,

strapped down behind the bell, are adequatefor most short crossings without specialexpansion/contraction devices, assumingcare is exercised in the installationprocedure of these joints. Specifically, push-on joints should not be assembledcompletely “home” if installed in the winter,due to the expansion caused as the weatherbecomes warmer. This temperature changecould cause a reaction to occur at each joint,“kicking” it out of alignment, creating a“snaking” action.

AbutmentsInstallation of a pipeline on a bridge is

complicated by the transition fromunderground to the bridge itself. Thiscritical point in design and operation of thepipeline is dependent upon the type ofjoints, the amount of vertical and horizontaldisplacement, and the physical structure ofthe abutment. If a change in direction existsat this transition, it is good design practiceto restrict the fittings to as small an anglechange as possible.Thrust restraint is of primary

importance at this transition section if thereis a change in direction (see Figure 5).Thrust restraint also has to be consideredfor all possible changes in direction or otherpoints of thrust in the pipeline. If restrainedjoints are utilized on the bridge structure,the thrust can be carried through thistransition section back to the underground

piping on each side of the bridge. Theequations for calculating the thrust forcesare covered in the DIPRA manual “ThrustRestraint Design for Ductile Iron Pipe.” Ifdesired, a concrete block can be poured toanchor the elbow or bend at the end of thebridge. Sometimes the abutment of thebridge or separate piles can safely be usedto “tie-down” the pipeline and/or bends andrestrain the thrust forces.Where a Ductile Iron pipeline is to be

installed on a new bridge, coordination withthe contractor can facilitate this transitionsection by leaving an opening in theabutment wall for the pipeline.

Air ReleaseDuctile Iron pipe installed on a bridge may

need an air release valve assembly to purgethe pipeline of any accumulated air (seeFigure 6). The size of the valve and assemblyis governed by the pipe size and expectedaccumulations of air. Some restrictions mayapply from the governing authority, such asthe State Highway Department. However,an air release valve assembly generally isrequired at a high point in the pipeline, suchas could occur on a bridge due to the changein elevation from the underground piping upto the supports on the bridge, or on a longvertically arched bridge.

Seismic ConditionsSpecial design considerations are warrantedwhen seismic conditions might beencountered. The approach piping to thebridge (specialty joints) and the pipeconstruction (bracing) on the bridge need tomeet particular design criteria. The approachpiping is subject to great joint deflectionsduring earthquakes as well as elongation andcontraction as the earth moves. Likewise, thepiping on the bridge needs adequate supportsto maintain integrity during shaking.

External ProtectionIt has been found that standard Ductile

Iron pipe exposed to the atmosphere doesnot require special corrosion protectionmeasures in most cases. In areas subject tocorrosive atmospheric conditions,appropriate protection should be providedfor the pipe as well as the steel hangers,clevises, threaded rods, etc.In underground installations where

Ductile Iron pipe is subjected to corrosivesoil conditions, polyethylene encasement isgenerally recommended in accordance withANSI/AWWA C105/A21.5 Standard. Forthese installations at bridge crossings, thepolyethylene sleeve should extend aminimum of 12 inches beyond the soil at thesoil/atmosphere transition and be taped inplace to the pipe.

Freezing ConditionsIn the northern parts of the country as

well as mountainous areas, freezing of aDuctile Iron pipeline on a bridge is apossibility. This is more likely to occur forsmall-diameter pipe with low flowvelocities. Where freezing conditions areanticipated, consideration should be given toinsulating the pipeline.The amount, type, and application of

insulation depends upon the conditions.Other measures used to retard freezinginclude the use of a heat tape wrappedaround the pipe underneath the insulation.This, of course, requires a source ofelectricity. During periods of inactivity,system drains or blow-offs on a raisedbridge pipeline may be employed to emptythe pipeline.

Basic Design ApproachesFrom a basic design standpoint, there are

some generally accepted principles for theoverall design of a Ductile Iron pipeinstallation on a bridge. Generally, they aredependent on the bridge structure, the typeof thrust restraint to be used, and on themethod of pipe support.The use of restrained joints and fittings

allows the following:• Axial thrust forces can be resisted.• Movement of the pipe due to thermal expansion/contraction may be trans- mitted to expansion/contraction couplings without joint separation.

• Thermal expansion/contraction may be accommodated without joint separation and without pipe expansion/contraction couplings.

• Extra security is provided in the event of failure or damage of a support.

• The pipeline is effectively “separated” from the bridge structure (in conjunc-

Figure 4

Figure 5

Figure 6

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tion with rollers or other pipe supports.)• An easy means is provided for testing the bridge pipeline separate from the main pipeline without external thrust restraint (using restrained joint test closures). When using unrestrained push-on or

mechanical joint pipe and fittings, somegeneral guidelines are appropriate:• Unrestrained pressure pipelines should not be installed on hangers with little or no lateral support. This also applies to flexible restrained joints.

• Unrestrained Ductile Iron pipe should be clamped securely to the supports or structure with at least one support per length of pipe.

• Fittings must be externally thrust anchored and/or blocked.

• If crossings are to be tested independently of and without attaching to the main pipeline, test closure pieces must be externally restrained and/or anchored.

Each individual unrestrained joint shouldbe brought fully home and then, dependenton the ambient temperature conditions atthe time of installation, backed out slightlyto provide for anticipated or possiblethermal expansion of the pipe lengths. Ifcounted on for thrust restraint, restrainedjoints normally should be fully extended toremove slack, engage the restraint, andminimize movements under pressure.If the pipeline is attached to the

structure, particularly on long crossings,pipe expansion / contraction couplingsmay be required at the bridge expansionjoint locations. If such couplings arerequired, the adjacent pipe generally mustbe firmly attached to or anchored to thebridge to prevent joint separations as thebridge and pipeline expansion mech -anisms do their jobs.Governmental regulations and/or

construction codes may affect the pipelinebridge crossing design/construction andshould be adhered to where applicable.

ConstructionThis article is not intended to give step-

by-step, detailed instructions for the designor installation of any particular Ductile Ironpipeline on a bridge; however, there arecertain principles and application techniquesof which the installer and the designer/engineer need to be aware:1. The design location for placement of the pipeline on the bridge will dictate, in many cases, the choices available for handling and installing the pipe. For example, if the pipe is to be installed in a box utility corridor, the

option of installing the pipe from below the bridge is eliminated.

2. From a performance standpoint, one support per length of pipe is sufficient. However, two supports per length may facilitate the actual construction of the pipeline.

3. To eliminate over-deflected joints and misalignment, vertical as well as horizontal bracing must be properly designed and installed.

4. For any joint that needs to function as an expansion joint, a suitable water-insoluble pipe lubricant (such as an underwater lubricant or the like) may be preferable over regular water soluble joint lubricant or soap for exposed pipe on the bridge.

5. The designer/engineer needs to consider the ambient temperature conditions with regard to assembly and service of the joints and expansion/contraction devices.

6. Where necessary, appurtenant items such as air-release valves, drain blow-offs, insulation, etc., should be designed and constructed to facilitate any future maintenance.

7. Support and hanger installations are critical to the integrity of the pipeline and as such should be properly designed, installed and maintained.

8. In the design of a new bridge, consideration should be given to provision of support locations and insertions of anchors, openings in the abutment and/or beams, etc., to facilitate pipeline installation.

9. Thrust restraint for a pipeline on a bridge is critical and needs to be properly considered. All thrust restraint mechanisms designed for the pipeline on the bridge should be installed with proper care and attention to detail. The use of restrained joints should be coordinated with the manufacturer to ensure proper assembly and installation practices.

SummaryThe design and construction of a

Ductile Iron pipeline on a bridge structureis unique from the standpoint of itsplacement outside the confines of a typicalunderground installation, its transitionfrom underground to the bridge structure,and its susceptibility to the variousprevalent loadings and conditions. Evenso, with proper design approaches andcon struction techniques, bridge crossinginstallations can be easily accomplished.Further design and installation informa -tion is available through DIPRA and/or its

member companies.

ReferencesBonds, Richard W., P.E., Design of DuctileIron Pipe on Supports. Birmingham,Alabama: DIPRA, 1995.

Thrust Restraint Design for Ductile Iron Pipe, Fourth Edition. Birmingham,Alabama: DIPRA, 1997.

Manufactured from recycled materials.

BC/8-01/4MPublished 12-96Revised 02-12

DIPRAMEMBER COMPANIES

American Cast Iron Pipe CompanyP.O. Box 2727Birmingham, Alabama 35202-2727

Atlantic States Cast Iron Pipe Company183 Sitgreaves StreetPhillipsburg, New Jersey 08865-3000

Canada Pipe Company, Ltd.1757 Burlington Street EastHamilton, Ontario L8N 3R5 Canada

Clow Water Systems CompanyP.O. Box 6001Coshocton, Ohio 43812-6001

Griffin Pipe Products Co.1011 Warrenville RoadLisle, Illinois 60532

McWane Cast Iron Pipe Company1201 Vanderbilt RoadBirmingham, Alabama 35234

Pacific States Cast Iron Pipe CompanyP.O. Box 1219Provo, Utah 84603-1219

United States Pipe and Foundry CompanyP.O. Box 10406Birmingham, Alabama 35202-0406

An association of quality producers dedicated to highest pipestandards through a program of continuing research.245 Riverchase Parkway East, Suite OBirmingham, Alabama 35244-1856Telephone 205 402-8700 FAX 205 402-8730http://www.dipra.org

Copyright© 2001, 2000, 1998, 1996 by Ductile Iron Pipe Research AssociationThis publication, or parts thereof, may not be reproduced in any form withoutpermission of the publishers.