INNOVATION May-June 2016

p er spect i ve

Revisiting Ancient Engineering Principles in a Modern Construction Context

James (Jay) Drew, P.Eng. Thousands of years ago, Roman engineers built infrastructure across their empire. They used available materials such as timber, rock, earth and mortar. Timber resists tensile forces, but deteriorates in open weather conditions. The other materials are serviceable in compression, and shear, and ancient Roman engineers used them to build long-term structures that would experience limited tensile forces. The Roman arch, for example, distributes both dead and active loads through the structure to the base and tolerates seismic loading if properly designed. Some Roman structures built with stone and mortar—which is similar to concrete—remain standing today. Yet, today, it is considered acceptable to design bridges and other structures with service lives of 85 to 100 years. Some modern bridges do not last 50 years without costly rehabilitation. Even our profession, which is governed by a Code of Ethics that instructs us to protect the public interest—which includes spending client and tax dollars wisely—has come to accept limited service life spans as normal and acceptable. Why do we fall so short of our Roman predecessors? One of the main reasons is because we build with reinforced concrete. It is a fairly inexpensive building material, and engineering students are taught in their first concrete-design course that steel and concrete make the “perfect marriage” of load-bearing structures—the coefficients of thermal expansion are almost identical and, thus, the materials will not destroy each other with temperature fluctuations. What students are not told about is the nasty divorce that happens after some 85 years. The expansion of embedded rebar in a concrete matrix limits the service life of reinforced concrete structures in open weather or wet environments. Concrete is porous. It allows moisture and oxygen to penetrate to the rebar, which causes rusting and subsequent expansion. This, in turn, cracks the concrete. Students are also encouraged to minimise the dimensions of load- bearing members, which results in stresses close to the maximum allowable levels. Higher stress contributes to earlier failures. If concrete does not contain steel and is subjected to predominately compressive forces, it lasts considerably longer, even in wet environments.

As current structures are replaced, wouldn’t it be fantastic to replace them with 2,000-year structures? Although compressive designs, such as those favoured by ancient Roman engineers, are not practical for many modern structures, appropriate structures to consider include bridges and tunnels with spans of 30 metres or less, as examples scattered across Europe demonstrate. We could build highway overpasses, small bridges, railway snow sheds, wine cellars, underground homes, and so on, to last . Think of the incredible savings for residents of towns or cities that would not need to rebuild bridges for another 20 centuries or more. Because modern concrete is a relatively inexpensive building material, by increasing the size of compressive concrete members in a structural arch, two considerable benefits accrue for little additional cost. The wider load-bearing surfaces between members makes the arch more stable, and the concrete lasts longer in service at lower stress levels. Furthermore, compressive arches constructed with modular voussoirs and locking keys are more stable than monolithically cast arches without reinforcement. A one-piece arch will eventually crack and may fail if the crack occurs at the wrong angle and location. In modular arches, cracks are predetermined between voussoirs to remain stable. Keyways keep the voussoirs aligned during movement caused by differential settling, landslides, and so on. The timing for revisiting this approach is perfect. The current federal government is committed to stimulating the economy by building infrastructure. Let us, the engineering profession, take a leadership role in encouraging today’s politicians to favour designs with lower annual costs of ownership tied to extended service lifespans. I believe this approach is feasible and have dedicated the last 15 years to this goal. If we work together, BC engineers can lead in promoting the construction of long-lasting infrastructure that will benefit society in more ways than we can imagine. v Jay Drew, P.Eng., is president of Richmond, BC-based Lock-Block Ltd., makers of Lego-like, concrete retaining wall blocks. He is also an inventor, and keeps his 1938 Dodge vehicle (Shown) in excellent working condition.

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