THE ARCH ADVANTAGE

For thousands of years, arches have been an important structural form in buildings, tunnels, and bridges.  Notwithstanding their antiquity, in many applications their governing mechanics are can be very complex and difficult to quantify analytically, even with today’s advances in mathematics and structural analysis. The design of early arches was based upon experience and experimentation.

An arch behaves much like a hanging cable.  Because the cable is unable to resist bending moments it takes a form that will result purely in tensile forces under an applied load.  An efficient arch follows the same principle; inverting this shape to result in pure compression.  Unreinforced masonry and concrete are very weak in tension, and therefore are able to withstand only minimal bending moments under loading.  A well designed arch will be shaped such that the majority of the force is carried by compressive forces in the arch.

Figure 1.1 – Common arch types are a) semicircular, b) segmental, and c) elliptical

Figure 1.2 – Arch terminology

Some of the most famous historic structural sites in the world owe their longevity to the fact that they were built using arches.   The Pont du Gard, in Nimes, is one of the most frequented tourist stops in France (see Figure 1.3).  Built by the Roman Marcus Vipsanius Agrippa, it stands as a classical example of early semicircular arch construction.

The multiple tiers of arches carry the Roman aqueduct over the Gardon River.  The Pont du Gard is constructed without mortar.    In 1998 major flooding occurred on the river, destroying many upriver structures.  This structure, however, sustained only minor damage.

Figure 1.3 – The Pont du Gard in Nimes, France

An example of early segmental arch construction is the Anji Bridge, in the Hebei Province of China (see 1.4).  The Anji Bridge was built by Li Chun between the years of 595 – 610 AD.  It is the oldest and most well preserved open spandrel arch bridge in the world and is still in service today.

Figure 1.4 – The Anji Bridge in the Hebei Province of China

The Sounding Arch Bridge in Maidenhead, England is particularly inspiring because of its large span-to-height ratio (see Figure 1.5).    It is built of unreinforced brick, and at the time of its construction in 1838, I.K. Brunel, its designer, was criticized, however, it remains in heavy use as a railway bridge to this day.

Figure1.5– The Sounding Arch Bridge in Maidenhead, England

Arches not only serve an important role above ground, as bridges, they also are an important feature of many tunnels and drainage systems.  The most well known network of arch tunnels is in the Paris, France sewers. The sewer system was begun in 1370 by Hugues Aubriot and was expanded sporadically for the next five centuries by various others. The 1,300 mile long system of tunnels and culverts has a “clean” service history and is still used today as Paris’s primary sewer.

Figure 1.6– A sewer tunnel in Paris, France

Masonry arch construction diminished with the invention of reinforced concrete in the late 1800’s.  Arch structures lost their heavy Romanesque look and began to take on lighter, more graceful forms.  With the ability to now resist tensile forces, and thus bending moments, through reinforcing steel, arches became flatter and more economical. 
Daniel Luten was one of the most successful arch engineers of the early 20th century.  His studies and experiments led to innovations that greatly reduced the amount of steel and concrete in arch bridge design (see Figure 1.7).  With greater economy, guaranteed serviceability, and a patented design, Luten began to take over the small- to medium-span bridge market.

Figure 1.7– A 1926 Luten bridge in Washington County, Arkansas

The main uses of buried arches today are as drainage systems, small bridges, storage facilities, and tunnels.   Arch shapes, in general, take an elliptical form because height restrictions limit the use of semicircles and segmental shapes are inefficient.

In 1988, Wolf Michelson began Hydro-Arch, in Henderson, NV.  Hydro-Arch specializes in cast-in-place concrete arches (see Figure 1.8), primarily for use in drainage systems, tunnels, small bridges, and storage facilities.  The majority of our business is in the Nevada/Southern California region.  We have created an efficient construction process that rivals anyone in economy and reliability.  Hydro-Arch has played an important role in gaining greater acceptance of arches, as well as in producing a more efficient design methodology.

Figure 1.8– A Hydro-Arch Bridge in Las Vegas, Nevada