While poking around the map collection at Chicago’s Public Library, I discovered that the structures carrying roads over the railroad tracks in Grant Park are considered viaducts. The word viaduct brings to my mind a Roman structure soaring over a valley floor and conveying a flat road on top of towering stone arches. The Chicago “viaducts” do not fit that image. To me, they look like bridges with no striking difference between them and Chicago’s river bridges other than color and style of ornamentation. Both groups of structures have fairly flat roadbeds supported by piers. I am puzzled why one set of structures is called bridges and the other viaducts.
The main difference seems to be that the bridges are crossing a body of water while the viaducts are crossing an obstacle on land. This doesn’t feel like the answer, though. I’ve walked over 50 bridges in Pittsburgh, the majority of which are over land. Not one of these structures is called a viaduct. To help find clues to solve this puzzle, I looked up the definitions of bridge and viaduct.
Merriam-Webster’s definitions are not much help:
Viaduct: a long elevated roadway usually consisting of a series of short spans supported by arches, piers, or columns
Bridge: a structure carrying a pathway or roadway over a depression or obstacle
The Oxford English Dictionary has similar definitions:
Viaduct: an elevated structure consisting of a series of arches or spans, by means of which a railroad or road is carried over a valley, road, river, or marshy low-lying ground
Bridge: a structure forming or carrying a road, path, or (in later use) a railroad, etc., which spans a body of water, a roadway, a valley, or some other obstacle or gap, and allows a person or vehicle to pass unimpeded over or across it
Based on these definitions, the features that seem to distinguish a viaduct from a bridge are elevation and short spans. This still doesn’t help solve the question of why the Grant Park structures are viaducts and the Chicago River structures are bridges.
Next, I turned to “How to Read Bridges” by Edward Denison and Ian Stewart. The glossary defines viaduct as “a type of bridge over land formed by a series of small (usually arched) spans.” This definition seems to work best for the viaducts in Chicago. They are over land and have a series of spans, though the spans are not arched.
To further develop my understanding, I paged through the nine examples of viaducts in “How to Read Bridges.” All nine examples are elevated (such as the 330-foot high Goltzsch Viaduct and the 407-foot high Garabit Viaduct) and have multiple arches or piers (such as the 21-arch Glenfinnan Viaduct, the 8-arch Wharncliffe Viaduct, and the 5-pier Busseau Sur Grusse Viaduct). All, but one, have no support system above the road deck. The exception is the Millau Viaduct, which uses a cable-stayed deck on top of seven piers that range from 253 feet to 800 feet tall. All, but one, are over land. The Garabit Viaduct spans the Truyère Valley and one-third of its length crosses the Truyère River. In another example, the approaches to the Cubzac-les-Ponts over the River Dordogne are considered viaducts, but it is a bridge that crosses the river.
I find myself again at a loss. While the definition in this book seems to work for the Chicago viaducts, the examples do not look like the Chicago structures. To help the Chicago viaducts fit in, I propose a new definition built from the three definitions and nine examples above:
Viaduct: an elevated roadway, supported by many arches or piers over land.
Yet, there are exceptions to every rule. The nine examples in “How to Read Bridges” include exceptions to the support system of the viaduct and to the obstacle spanned by a viaduct. Expanding upon this, the Chicago viaducts, which are supported by multiple piers over land, become the exception to the elevated part of the rule or definition.
My biggest take away is that it is no easier to define a viaduct than it is to define a bridge. In most cases, as with bridges, a viaduct is something you know when you see it, even if you cannot define it. However, sometimes it takes a label or a sign, such as the maps of Chicago’s viaducts and, prior to its restoration, the maps of Heth’s Run Bridge, to know what you are seeing.
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