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Reinforced Concrete PipeHow to Assess the Transition from Indirect to DirectDesign Methods in Deep Cover InstallationsAdam Braun, P.Eng.Winnipeg, ManitobaCanada
Introduction, Background, andHistory Indirect Design SIDD Installations and DirectDesign Transition from Indirect toDirect Design Conclusion2Rigid Rugged Resilient
Introduction,Background &History3Rigid Rugged Resilient
Introduction Reinforced Concrete Pipe (RCP) First produced in 1896 inFrance Brought to North America in19054Rigid Rugged Resilient
Introduction Indirect Design(Marston/Spangler Analysis) Developed in the 1920’s and 30’s Empirically derived Excellent performance record in lowto moderate soil covers Direct Design (Heger/Selig) 5Developed in the 1970’s and 80’sReinforced concrete design theoryLimit states design principalsApplicable for large diameter andhigh external load conditionsRigid Rugged ResilientWhile direct design hasbeen formally adoptedby industry it does notsee widespread use
Iowa Experiment Station Anson Marston began researchinto the behavior of buriedrigid pipe in 1910 Born out of the increased useof clay and non reinforcedconcrete pipe in sizes up to 36”as both drain tile and sewerpipe The “large diameter” pipe wasprone to failure shortly afterinstallation Began with investigation intofailuresAt the time therewas no quantifiabledesign method forburied pipe!6Rigid Rugged Resilient
Original Soil Load and PipeStrength ExperimentsEvaluating Trench Loads12” Pipe Test7Rigid Rugged Resilient36” Pipe Test
Original Soil Load and PipeStrength Experiments Testing of pipe up to 42” diameter Homebuilt sand bedding testing machines Evaluation of test results to actual supporting strengthfor various installation typesTypical InstallationConditionsTests in FlatBottom Trenches8Rigid Rugged ResilientTests in ShapedTrenches
Evaluation of Testing Methods9Rigid Rugged Resilient
The Theory of Loads on Pipes in Ditchesand Tests of Cement and Clay Drain Tileand Sewer Pipe Published in 1913 bythe Iowa State Collegeof Agriculture andMechanical Arts Authors: Anson Marston A. O. Anderson New theories forcalculating soil loadson buried pipes innarrow trenches10Rigid Rugged Resilient
The Supportive Strength of Sewer Pipe inDitches and Methods of Testing SewerPipe in Laboratories to Determine TheirOrdinary Supportive Strength Published in 1917 by the IowaState College of Agriculture andMechanical Arts New theories for quantifyingthe external load capacity ofburied rigid pipe Authors: Anson Marston W. J. Schilick H. F. Clemmer11Rigid Rugged Resilient
Continued Research Further development was undertaken in the 1920’s and 30’s by: W. J. Schilick M. G. Spangler Resulted in numerous papers on an ever expanding knowledge ofsoil loads and pipe strengths Developed calculations for determining earth loads fromembankment (projection) and tunnel installations Load calculations were all empirically derived from experimentsundertaken at the Iowa Experiment Station12Rigid Rugged Resilient
Indirect Design13Rigid Rugged Resilient
Indirect Design Matches estimated trench loads to the estimatedsupporting strength of the installed pipe Installed pipe strengths estimated through the use ofempirically derived factors Pipe strengths assessed using three edge bearing(3EB) tests14Rigid Rugged Resilient
Three Edge Bearing Tests Standards: ASTM C497 - Standard Test Methods for Concrete Pipe,Manhole Sections, or Tile CSA A257 Quantifiable means of determining a pipe ability tosupport externally applied loads15Rigid Rugged Resilient
Three Edge Bearing Tests RCP is typically tested with the intent of determining thefollowing loads: Hairline Crack Service Cracking (0.01” or 0.3 mm) Ultimate Failure Typical QA/QC procedures only require testing up toservicing cracking limit with the rare test taken to ultimatefailure It should be noted that standard three edge bearing teststo do not check for diagonal tension (shear) or radialtension failure modes. Both are governing failure modes in high soil coverconditions.16Rigid Rugged Resilient
Traditional Installation Types andBedding Factors Bedding types developed to reflect pipe installationmethods at the beginning of the 1900’s Reflect installation efforts involving hand vs.machine excavation Bedding factors were developed for each installationtype reflect the increase in load a pipe can supportwhen installed vs. in the three edge bearing test The bedding factor each installation type reflects thelevel of load distribution provided Better soil support equals a higher bedding factor!17Rigid Rugged Resilient
Traditional Installation Types andBedding Factors18Rigid Rugged Resilient
Indirect Design Method Calculate live and deadloads Determine a bedding factor Determine the equivalentthree edge bearing load(divide the soil load by thebedding factor) Convert to equivalent DLoad (Divide by thediameter of the pipe) Select appropriate pipe fromthe ASTM or CSAspecification or specifyrequired load19Rigid Rugged Resilient
Indirect Design Method Reinforcement requirements are stipulated in ASTMC76 or CSA A257 Empirically derived in order to meet required DLoads Single factor of safety applied against ultimatefailure, ranging from 1.25 and 1.5 No differentiation between live and dead loads andtheir respective levels of uncertainty Adequate for low to moderate soil coversconsistent with those used to develop the empiricaldesign method20Rigid Rugged Resilient
SIDD Installations andDirect Design21Rigid Rugged Resilient
New Design Approach American Concrete Pipe Association (ACPA)undertook a long term research project in the1970’s and 80’s to develop a new designapproach for reinforced concrete pipe Dr. Frank Heger was engaged to develop this newdesign method based on his knowledge ofreinforced concrete design Continuation of Heger’s post graduate work atMIT Dr. Ernest Selig was engaged to providedgeotechnical expertise in development of thepipe-soil interaction models Heger’s other work included 22Rigid Rugged Resilient
New Design Theory Heger’s 1962 PhD thesis “ATheory for the StructuralBehavior of ReinforcedConcrete Pipe” strove todevelop a rational procedurefor predicting the structuralbehavior of reinforcedconcrete pipe Adapted reinforced concretebeam theory to the circularpipe wall23Rigid Rugged Resilient
New Design Theory Proposed design proceduresconsidered the following: Flexural strength Diagonal tension (shear) capacity Predicting and limiting in servicecrack widths Use of stirrups in controllingdiagonal tension Combined internal and externalloading conditions Assessed the development ofinternal wall forces under test(3EB) and in service conditions24Rigid Rugged Resilient(Heger 1962)
Imparted Wall Forces Max bending moments develop at: Obvert Invert Springline Max wall thrust: Springline Max diagonal tension: 12 to 13 from invert25Rigid Rugged Resilient
Bending MomentsCompressionSoil LoadTensionTCCTC26Rigid Rugged ResilientT
Development of BendingMoments during 3EB Test(Heger 1962)27Rigid Rugged Resilient
Diagonal and Radial Tension Radial Tension Failure: Cause by tension forces withinthe radial reinforcement These tension forces act tostraighten out curved steel,causing it to pull away from thepipe wallDiagonal TensionFailureRadial Tension Failure28Rigid Rugged Resilient
Soil Pressures and Wall Forces The interaction between buried pipe, the embedmentmaterial, and native soils is complex and dependent onmany factors Heger and Selig developed the computerized finiteelement program SPIDA (Soil-Pipe Interaction Designand Analysis) to determine the distribution of soilpressures around the pipe and wall forces developed This lead to the development of four new installationtypes to reflect modern pipe installation techniques 29Rigid Rugged Resilient
Standard Installations DirectDesign (SIDD)Type 1 InstallationQuantitativevs. QualitativeInstallationProcedureMost IntensiveInstallationRequirementsLeast IntensiveInstallationRequirements30Rigid Rugged Resilient
Standard Installations DirectDesign (SIDD)Visual Representationof Applied SoilPressures31Rigid Rugged Resilient
Heger Positive Projection Load Heger and Selig developed a simplified load calculationmethod based on the new SIDD installation types Only positive projection conditions considered Load coefficients developed with SPIDA Applied load Prism load x Vertical Arching Factor (VAF)Trench Widthsare Difficult toControl in theField32Rigid Rugged Resilient
Estimating Wall Forces Heger and Selig utilized SPIDA to assess and developload coefficients for the new SIDD installations types33Rigid Rugged Resilient
Direct Design Proposed in Heger’s 1988 paper “New InstallationDesigns for Buried Concrete Pipe” Incorporates SIDD Installations and SPIDA developedload coefficients Incorporates limit states design methodology Proposed steel reinforcement design methods tocounteract the following applied wall forces: 34Bending momentsWall thrustDiagonal tension (shear)Radial tensionRigid Rugged Resilient
ASCE 15 and PIPECAR ASCE 15 – Standard Practice for DirectDesign of Buried Precast Concrete PipeUsing Standard Installations (SIDD) Originally published in 1993 subsequentlyupdated in 1998 and 2017 Outlines SIDD installation requirementsand direct design methodology Computerized design programPIPECARTM Developed by Frank Heger for the ACPA Direct design of RCP for a myriad ofdesign and installation conditions Direct design and SIDD installationshave now been adopted by bothAASTHO and CSA35Rigid Rugged Resilient
Transition from Indirect to DirectDesign36Rigid Rugged Resilient
Limits of Indirect Design Indirect design was developed based on empiricaltesting on small to intermediate diameter pipe ( 36”)under loading conditions typical of the time ( 15’) Failure modes recognized at the time were limited toflexural failure Testing of unreinforced clay tile and concrete pipe 3EB tests do not directly assess diagonal or radialtension failure modes which govern under largeexternal loading conditions37Rigid Rugged Resilient
Diagonal and Radial Tension Pipes designed to withstand applied flexural stressesmay not contain sufficient reinforcing to withstandimparted diagonal tension forces If not confirmed, mobilization of tensile steel underhigh loads may result in radial tension failureUnder 3EB test conditions largediameter pipes can fail underdiagonal or radial tension prior toexperiencing flexural failure38Rigid Rugged Resilient
Reinforcing for Diagonal Tension Diagonal tension is resisted by: Placement of additional radial reinforcing steel (to a point) Placement of stirrupsStirrups placed tointercept theshear plane(Brzev et al 2006)39Rigid Rugged ResilientHeavily reinforced 10’diameter concrete pipe,designed using direct designmethods
Reinforcing for Radial Tension Stirrups must be employed to overcome radial tensionforcesStirrups used toanchor the innerreinforcement intothe pipe wall40Rigid Rugged Resilient
Transitioning from Indirect toDirect Design Determine the transition from a flexural controlledfailure to diagonal and/or radial tension controlledfailure Requires the direct design methodologies (i.e. handcalcs or PIPECARTM41Rigid Rugged Resilient
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Verification Governing failure modes Bending moment capacity Diagonal tension capacityDiagonal Tension Failure 1050 mm Class V RCP43Rigid Rugged Resilient
Verification Governing failure modes Bending moment capacity Diagonal tension capacityRadial Tension Failure 1650 mm Class V RCP44Rigid Rugged Resilient
Verification Bending moment and shear capacity estimated usingHeger’s 3EB wall force coefficients Actual bending moment and diagonal tension capacityin excess of predicted capacities Result of: Increased material strengths Limit state design factors45Rigid Rugged Resilient
Conclusion46Rigid Rugged Resilient
Conclusion Indirect design has a long and successful servicerecord for lower soil covers where diagonal and radialtension to not govern the design The use of indirect design for large diameter pipelinesexposed to high external loads can ignore governingfailure modes This exposes the Engineer, owner, and contractor toadditional risk relating to failure and/or serviceabilityissues47Rigid Rugged Resilient
Conclusion All large diameter pipes should be checked forgoverning failure modes regardless of pipemanufacturing methods (C76 or C1417) Available radial reinforcing should be confirmedwhenever diagonal tension governs the design Direct design should be employed anytime stirrupsare required48Rigid Rugged Resilient
Conclusion Use in indirect design in deep cut applications willresult in a pipe that’s vulnerable to: Excessive cracking, exceedingservice cracking criteria Premature degradation Ultimate failure49Rigid Rugged Resilient
References: ACPA (1998), Concrete Pipe Handbook – Fifth Printing (1998), American Concrete Pipe Association (ACPA),Irving, TX, USA. ACPA (1993), Concrete Pipe Technology Handbook, American Concrete Pipe Association (ACPA), Irving, TX,USA. ASCE 15-17 (2017), Standard Practice for Direct Design of Buried Precast Concrete Pipe Using StandardInstallation (SIDD), American Society for Civil Engineers (ASCE). Brzev, S., Pao, J. (2006), Reinforced Concrete Design: A Practical Approach, Pearson Prentice Hall, USA. Heger, F. (1962), A Theory for the Structural Behavior of Reinforced Concrete Pipe, Massachusetts Instituteof Technology. Heger, F. (1963), Structural Behavior of Circular Reinforced Concrete Pipe – Development of Theory, Journalof the American Concrete Institute, November 1963. Heger, F. (1988), New Installation Designs for Buried Concrete Pipe, American Society of Civil Engineers(ASCE), Pipeline Infrastructure Conference, June 6-7, 1988. Marston, A,. Anderson, A. (1913), The Theory of Loads on Pipes in Ditches and Tests of Cement and ClayDrain Tile and Sewer Pipe, Iowa State College of Agriculture and Mechanical Arts. Marston, A., Schlick, W., Clemmer, H. (1917), The Supportive Strength of Sewer Pipe in Ditches andMethods of Testing Sewer Pipe in Laboratories to Determine Their Ordinary Supportive Strength, IowaState College of Agriculture and Mechanical Arts. Simpson Gumpertz & Heger Inc. (1992), SIDD – Version 2.0 User Manual, American Concrete PipeAssociation (ACPA).50Rigid Rugged Resilient
Questions?Adam Braun, P.Eng.AECOM Canada Ltd.Winnipeg, Manitoba, Canadaadam.braun@aecom.com51Rigid Rugged Resilient
New Design Approach American Concrete Pipe Association (ACPA) undertook a long term research project in the 1970's and 80's to develop a new design approach for reinforced concrete pipe Dr. Frank Heger was engaged to develop this new design method based on his knowledge of reinforced concrete design
