By: William Whidden, PE

Keeping up with the latest advancements in technology, materials, and construction is critical to maintaining the continued transport of safe drinking water and the reliable collection and treatment of waste-water. For this reason, the American Society of Civil Engineers (ASCE) presents an updated manual of practice (MOP) specifically for flexible steel pipe.

Released in August 2009, Manual of Practice 119, Buried Flexible Steel Pipe: Design and Structural Analysis was prepared by the society's Buried Flexible (Steel) Pipe Load Stability Criteria and Design Task Committee with input from a joint committee of the Steel Tank Institute and the Steel Plate Fabricators Association. The technical publication is structured to guide the reader, whether an engineering student or design professional, through the design or analysis process. Read on for a preview of topics discussed.


What precisely constitutes flexible pipe has long been the subject of some confusion. Complicating the situation further, terms such as “flexible,” “semi-flexible,” “semi-rigid,” and “rigid” have been used.

Essentially, engineers can choose from two basic philosophies of pipe design: rigid and flexible. What distinguishes the two is the method of analyzing resistance to internal and external forces.

In rigid pipe design, the forces are additive: Internal and external loadings must be analyzed in combination as part of an evaluation of the stress in the pipe wall created by both thrust and bending forces. The wall of a rigid pipe is then structurally designed to resist these forces.

In flexible steel pipe design, internal and external pressures are evaluated independently because a combined analysis would show a reduction of the stress to be resisted. Therefore, independent analyses produce a more conservative design for flexible pipe than would a combined stress analysis. Flexible pipe deflects and conforms with soil embedment as soil is compressed, therefore flexible pipe wall should be designed to resist excessive deflection and buckling.

Designers of steel pipe must consider more than simply the thickness of the steel cylinder. For example, the type of coating and linings to be applied and the type of joint configuration to be used also must be considered. Simply stated, certain coatings and linings that work well in some installation conditions may not work well in others. The same holds true for the different joint configurations.


A contemporary practice manual may seem like an odd place for a history lesson, but engineers can benefit from understanding the role that pipelines have played throughout history. Knowing how design and construction techniques have evolved in response to differing circumstances can enliven and improve efforts to install new pipelines today.

Consider bamboo pipes in China. Clay pipes in Mediterranean countries. Rock-lined tunnels in Persia. These approaches were used to transport drinking water in the ancient world, sometimes across distances that remain impressive today.

Although iron had been used for making weapons and other implements for thousands of years, it was not used to make pipes until the 19th century. Stronger but costlier than other pipe materials, iron first was used for gas pipes in England. Within decades, inventors had devised methods for making steel in large quantities, and the era of the steel pipeline began. The impact of this innovation was felt most in cities, where growing populations benefited from increasingly sophisticated distribution systems.

First made in New York in 1905, lock-bar pipe was the next innovation. So named because it consisted of two semicircular pipe halves fitted together and held in place by two longitudinal lock bars with an H-shaped cross section, lock-bar pipe had a smoother interior and a greater carrying capacity than riveted pipe. As a result, it was used much more often than riveted pipe between 1915 and 1930.

Subsequently, developments in the relatively new field of automatic electric welding prompted new innovations. The massive demand for welding during World War II led to numerous improvements in technique, which helped to increase maximum pipe size during the 1950s. During the 1960s, experimentation began on the use of mortar lining and coating to increase the ring stiffness of steel pipe. In this way, steel was better able to maintain proper shape during installation.

With increased testing and field experience, engineers developed a design approach based on realistic performance limits, including stresses that exceed yield strength, excessive deformations, and leaks. They also developed advancements, such as cathodic protection and coatings, that combat corrosion.


Although people have been constructing buried pipelines for centuries, the field of pipeline design didn't begin until nearly 100 years ago, when Anson Marston became the first dean of engineering at Iowa State College.

Prompted by a desire to improve the condition of roads in Iowa, Marston led an effort to bury drainpipes along roadsides, eventually developing the first engineered design of buried drainpipe. In addition to devising an equation for soil load on pipes, he developed the first performance specification for rigid pipes, which required manufacturers to produce pipes that could withstand the “Marston load” in a three-edge bearing test.

Meanwhile, corrugated steel flexible pipe manufacturer Armco Co. sought to use its pipe as culverts.

To assist with these efforts, M.G. Spangler, another Iowa State College faculty member, studied the interaction of steel pipe and soil, revealing that flexible pipe deflects under soil load and develops horizontal support from soil on the sides of the pipe. Spangler and Reynold Watkins — a student with whom Spangler was collaborating and now an emeritus professor of engineering with Utah State University — developed what became known as the Modified Iowa Formula, which predicts the amount of deflection that will occur with flexible pipe under a certain vertical soil load. Published in 1958, the formula helped usher in widespread use of buried steel pipe.

Since then, however, engineers have frequently misapplied the formula, using it for a purpose other than what it was intended. Rather than simply using the equation to predict deflection, some pipe designers employed it to determine pipe thickness. In many cases, engineers using this approach have “over-designed” their pipes, making the wall thicker than necessary.

The manual was created, in part, to address the long-standing confusion regarding the proper use of the formula. Subsequent research by Watkins has found that soils surrounding a buried pipe provide most of the support needed by the pipe. For this reason, engineers using the formula should design the type of soil support they want for a buried pipe, rather than designing the pipe itself. In this way, engineers can conserve resources for their clients by avoiding the unnecessary expense associated with over-designed pipelines.


The manual also addresses such key concepts as pipe mechanics, soil mechanics, the interaction of pipes and soil, and design analysis. Special considerations, including parallel pipes in a common trench and trenches in poor soil, also are discussed. Appendices cover such topics as soil slip analysis, external fluid pressure, ring analysis, and impact factors in soil.

Over the past century, buried steel pipe has become a critical feature in countries around the world. Many pipelines installed decades ago remain in service; undoubtedly, many more will be installed, ensuring the continued use of this vital infrastructure component well into the future.

It's imperative that engineers and others involved in the design and construction of buried steel pipelines further their understanding of this technology. ASCE's manual of practice provides a foundation for ongoing efforts to further refine the design and construction of this key resource.

— Whidden ( is a senior engineer IV in the Orlando office of consulting engineering firm PBS&J. As chair of the American Society of Civil Engineers' Buried Flexible (Steel) Pipe Load Stability Criteria and Design Task Committee, he was editor of the Manual of Practice 119, Buried Flexible Steel Pipe: Design and Structural Analysis.