The Importance of Compression Part 3

Bridge 99, Cowley County, K.S.

In the first post in this series we discussed arch bridges and how they worked. In the second we investigated slab bridges. In this post, we will compare slab, truss, and arch bridges.

The Fatigue Problem

Steel, whether in the form of rebar or simply as a structure in its own right, fatigues over time, though in reinforced concrete typically the cement itself disintegrates before the rebar wears through. Continual up-and-down flexing causes wear in which the steel softens, wears out, tears, and breaks. Even if not wearing out, steel can deteriorate from the weather, causing it to break. This can be a big problem for bridges, especially steel bridges, but also reinforced concrete, especially once cracked. And really, corrosion aside, even materials strong in tension, like steel, can wear out over time with the continual flexing between self-weight and the intermittent additional heavy forces of loads. This constant fluctuating tension tends to pull the material apart.

The collapse of the I-35 bridge over the Mississippi in Minneapolis, Minnesota, was due to fatigued steel failing, though it has been said that the fact that the bridge was excessively loaded with construction equipment at the time probably hastened the collapse.

A bridge in Cowley County, Kansas, said to be similar in design to the ill-fated I-35 bridge in Minneapolis. Note how the trusses are arch-shaped. Notwithstanding, they are not fully in compression, so are flexed over time. The Achilles heel in these bridges is often the relatively thin metal plates that are used to attach all the truss pieces together. These plates are subjected to heavy wear and flexing under loads, and are crucial to binding all the individual truss pieces into one unit. Over time, the constantly changing tensile forces work to tear these plates apart.

In modern bridges, to prevent such catastrophic failures, extra structural members are added such that if one fails another structural member is able to prevent an immediate collapse, hopefully allowing the failure to be found during an inspection and rectified before the bridge goes out completely. A well-built arch bridge does not have these kinds of problems as the arch, if always operating in compression, does not flex up and down the same way a steel bridge does.

Arches vs. Slabs

Slab bridges by their very nature are prone to being “worked” over time by loads forcing them down and up as they cross the bridges. If the weight on the slab was fairly steady, this wouldn’t be a problem. Yet a slab bridge, with its variable traffic loads, is prone to constant flexing. Truss bridges suffer from the same effect; a truss bridge does feature elements in compression, but has other elements in tension, and is subjected to constant wear.

The beauty of a true arch operating under compression is that loads push the structure together. Thus, rather than being flexed as traffic crosses an arch bridge, the arch simply gets pressed tighter together into its ideal, compressive operating state. As long as the arch is never so overloaded as to cause material crushing nor allowed to operate in any form of tension, an arch, being always operated in compression, becomes tighter and thus stronger with use.

So Why Slabs and Trusses?

The primary advantages of slabs and trusses is that they are easily built (an arch usually requires some kind of massive falsework), are easily calculated (how forces travel through an arch is still something of a mystery, at least as far as precise calculations are concerned), are usually (though not always) cheaper up front, and require less specialized skill to build. Then, too, an arch has a nonlinear curved shape that sometimes makes implementation difficult. Perhaps replacing the slab and truss bridges every 50 to 120 years or so may be considered acceptable; it is, at least, expected for these bridges to have relatively short lifespans. And, of course, as an extra benefit, some bridges last longer than their design life. When it comes down to it, trusses and slabs are (broadly speaking) easier to build than arches.