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

Upfront Information

Rigid vs. Flexible

Upfront Information

Rigid vs. Flexible

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

Pipeline Resistance

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 ◦ Heger and McGrath◦ 4 installations

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◦ Axial Force Check versus material yield

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

Questions?

Thank you for your time and attention