Structural Mechanics of Buried Pipelines

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1 E.L. Robinson Engineering Co. Structural Mechanics of Buried Pipelines Ohio Transportation Engineering Conference October Kevin White

2 Presentation Agenda Introduction Upfront information Loads on pipelines Pipeline resistance Wrap-up Questions

3 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

4 Upfront Information Strength, modulus, stiffness Strength - the strength of a material is its ability to withstand an applied load without failure

5 Upfront Information Strength, modulus, stiffness Young s modulus Slope of stress/strain curve. E is a material property

6 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

7 Upfront Information Rigid vs. Flexible

8 Upfront Information Rigid vs. Flexible

9 Upfront Information Rigid vs. Flexible

10 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

11 Loads on Pipelines Live Load Load distribution thru fills (Boussinesq) Essentially gone within 7 ft Highway Live Loads Cover, ft H20 H

12 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

13 Loads on Pipelines Dead Load Some portion of the geostatic stress % for rigid pipe 100% for corrugated metal pipe 30-80% for thermoplastic pipes Pretty close to the historic loads

14 Pipeline Resistance

15 Rigid Pipe Design Focus on RCP since it is most prevalent Two main methods Indirect Design Direct Design

16 Rigid Pipe Design Indirect Design Marston - Spangler Three-Edge Bearing Test Bedding Factors Bedding Class Load Factor A B 1.9 C 1.5 D 1.1

17 Rigid Pipe Design Direct Design Heger and McGrath Create a load profile for different installation qualities

18 Rigid Pipe Design Direct Design Heger and McGrath 4 installations

19 Rigid Pipe Design Direct Design Designed as any reinforced concrete structure Axial force Bending moment Shear

20 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

21 Flexible Pipe Design Corrugated Metal Pipe Axial Force Check versus material yield

22 Flexible Pipe Design Corrugated Metal Pipe Buckling Do not allow wall stress to exceed buckling capacity

23 Flexible Pipe Design Corrugated Metal Pipe Seam Strength (if present) Installation Stiffness What about deflection??

24 Flexible Pipe Design Thermoplastics The viscoelastic response of the ring makes detailed analysis extremely complex Constant stress Creep Constant strain Relaxation

25 Flexible Pipe Design Thermoplastics The viscoelastic response of the ring makes detailed analysis extremely complex Apparent Modulus Is the pipe any less strong

26 Flexible Pipe Design Thermoplastics Strain based design procedure Maximum compressive strain limited to 5% Why not tensile strain?

27 Flexible Pipe Design Thermoplastics Global Buckling Local Buckling Idealized profile based on Winter effective width Quality control test

28 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

29 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

30 Questions? Thank you for your time and attention

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