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E.L. Robinson Engineering Co.
Structural Mechanics of Buried Pipelines
Ohio Transportation Engineering ConferenceOctober 22-23
Kevin White
Presentation Agenda
Introduction Upfront information Loads on pipelines Pipeline resistance Wrap-up Questions
Introduction
Pipelines are highly complex soil-structure interaction systems
Soil contributes to both the load on the pipe and the resistance of the pipe to the loadings
Focus on conceptual design◦ Too complex for detailed analyses
Upfront Information
Strength, modulus, stiffness◦ Strength - the strength of a material is its ability to withstand an applied load without failure
Upfront Information
Strength, modulus, stiffness◦ Young’s modulus Slope of stress/strain curve. E is a material property
Upfront Information
Strength, modulus, stiffness◦ Stiffness The extent to which an object resists deformation in response to an applied force. Depends on both material composition AND shape. Bending Stiffness Circumferential/Ring/Hoop/Axial Stiffness
Upfront Information
Rigid vs. Flexible◦ Stiffness (Axial and Bending)
RCP◦ High axial and bending stiffness
CMP◦ High axial and low bending stiffness
Thermoplastics◦ Low axial and bending stiffness
Loads on Pipelines
Live Load◦ Load distribution thru fills (Boussinesq)◦ Essentially gone within 7 ft
Highway Live LoadsCover, ft H20 H251 1800 22802 800 11503 600 7204 400 4705 250 3306 200 2407 175 1808 100 140
Loads on Pipelines
Dead Load◦ Some portion of the geostatic stress (γH).◦ Design method◦ Burns & Richard in 1960’s Modified by McGrath Ring stiffness and backfill stiffness determines load
Loads on Pipelines
Dead Load◦ Some portion of the geostatic stress. 120-140% for rigid pipe 100% for corrugated metal pipe 30-80% for thermoplastic pipes
◦ Pretty close to the historic loads
Rigid Pipe Design
Focus on RCP since it is most prevalent◦ Two main methods Indirect Design Direct Design
Rigid Pipe Design
Indirect Design ◦ Marston - Spangler◦ Three-Edge Bearing Test◦ Bedding Factors
Bedding Class Load Factor
A 2.8-3.4B 1.9C 1.5D 1.1
Rigid Pipe Design
Direct Design ◦ Heger and McGrath◦ Create a load profile for different installation qualities
Rigid Pipe Design
Direct Design ◦ Designed as any reinforced concrete structure Axial force Bending moment Shear
Flexible Pipe Design
Corrugated Metal Pipe◦ Axial Force Check versus material yield
◦ Buckling Do not allow wall stress to exceed buckling capacity
◦ Seam Strength (if present)◦ Installation Stiffness
Flexible Pipe Design
Corrugated Metal Pipe◦ Buckling Do not allow wall stress to exceed buckling capacity
Flexible Pipe Design
Corrugated Metal Pipe◦ Seam Strength (if present)◦ Installation Stiffness◦ What about deflection??
Flexible Pipe Design
Constant stress◦ Creep
Constant strain◦ Relaxation
Thermoplastics◦ The viscoelastic response of the ring makes detailed analysis extremely complex
Flexible Pipe Design
Apparent Modulus Is the pipe any less
“strong”
Thermoplastics◦ The viscoelastic response of the ring makes detailed analysis extremely complex
Flexible Pipe Design
Thermoplastics◦ Strain based design procedure◦ Maximum compressive strain limited to 5% Why not tensile strain?
Flexible Pipe Design
Thermoplastics◦ Global Buckling◦ Local Buckling Idealized profile based on Winter effective width Quality control test
Flexible Pipe Design
Thermoplastics◦ Deflection? Since equations for deflection are highly unreliable Assume 5% for calculating bending strain Use this as a field performance limit
Wrap-up
Engineers tend to have a “favorite” We’ve been working on pipes for over 100
years and there is still much we don’t know
It’s easy to find “dirty pictures” of all pipe types
All pipe types have strengths and weaknesses