Day: October 1, 2010

Composite Materials in Large Civil Engineering Structures – Design Optimization

Friday, October 1, 2010 3:00 pm – 4:00 pm
Room 610, M&M Building

John Pilling
Technical Director, Electric Park Research

Abstract

The choice of materials for use in load bearing civil engineering structures are often determined by costsimply because of the large volumes of materials involved.  One instance of the large scale use ofpolymeric materials is in the rehabilitation of cracked, corroded or collapsed pipes that were originallyinstalled under most of the large American cities during the late 1800s. In many instances it is extremelycostly or impossible to dig up and replace the existing pipes. Rehabilitation by lining the pipes with apolymeric material is common practice. HDPE is currently favoured for small diameter internallypressurized pipes such as water and gas mains and is usually pulled into and through the existing pipes.However, this is not practical for many of the waste and storm water pipes that are either non-circular orlarger than about 24” in diameter as the pipe wall becomes excessively thick in order to support theimposed loads without buckling or fracture.  Combining micromechanics of materials, elasticity theory oflaminates, and a geotechnical analysis of the loading of buried pipes, it is possible to design compositestructures that can support all the imposed loads and be easily installed in the existing collapsed pipe.The design process involves a geotechnical analysis of the imposed soil, water and rolling loads (vehicle,rail or aircraft) to determine the imposed pressure on the pipe. The pressure that a given pipe wall willsupport depends on the flexural rigidity of the  pipe wall (EI), its strength (s)  and a critical designdimension (D), usually the diameter of the pipe, but can be a critical radius of curvature or the length of astraight section when the pipe is non-circular. The actual equations used to determine the pressure thatcan be supported depend on the shape of the pipe, the type of loading and the country in which the pipeis to be installed (National Design Codes).  An “Ashby” type analysis is then completed in which therequired pipe thickness to support the imposed loads is determined as a function of the internal structureof a laminated composite given that the mechanical properties of the pipe wall are themselves functionsof thickness. The cost of the design is then calculated.  Typical microstructural variables include the typeof reinforcing fibre, the fibre spacing (volume fraction), resin type and fibre position within the laminatedstructure, i.e thickness.  A numerical solution method is employed to determine the combination ofmicrostructural variables that produce the composite with the minimum cost. Each rehabilitation projectproduces a unique composite microstructure which can be easily manufactured, on demand, usingtechnology currently deployed in the textile industry.  Typical municipal rehabilitation projects cangenerate material costs savings in the millions of $ range over conventional monolithic materials.  Thispresentation explains how micromechanics and elasticity theory are combined with typical civilengineering design codes to produce cost minimized structural composites. Examples of pipes andinstallations will be included.