What is a Bridge? Chicago Edition II

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.

Other posts in this series:

What is a Bridge? Pittsburgh Edition I (July 18, 2012)

What is a Bridge? Pittsburgh Edition II (July 27, 2012)

What is a Bridge? Chicago Edition I (April 18, 2013)

What is a Bridge? Bethlehem Edition I (July 1, 2020)

What is a Bridge? Bethlehem Edition II (July 15, 2020)

Layers of the City: Chicago Edition

The first place to show me how a city can be stacked like a layer cake, Chicago provided ample opportunities to explore all levels of the city. The experience of noticing the expansion joints in roads that I assumed were on solid ground opened my eyes to the possibilities of stacking uses.

Underground Life

A vacant lot two stories below street level solved the mystery of the expansion joints, by exposing the inner guts.  Two more roads sit below street level to segregate trash pick-up and deliveries from the flow of traffic.  These lover levels also provide some opportunities for parking without monopolizing valuable real estate above.  Retail shops connected by pedestrian passageways are also interspersed in these layers.

River Life

At the same elevation as the “underground life,” the Chicago River flows through the heart of the city.  On and along the river are a variety of activities.  Pedestrian paths, cafes, housing, parks, industrial uses, and homeless encampments line the shores.  Meanwhile, the river abounds with ducks, boat tours, water taxis, construction staging, and marinas.

Street Level

Back up on the street level, life buzzes.  Vehicular and pedestrian traffic rush passed, occasionally pealing off to visit the numerous shops, offices, museums, restaurants, cafes, parks, and trails.

Pie in the Sky

Yet, more life looms above.  Several of the skyscrapers have penthouse, or nearly penthouse, restaurants.  Others have rooftop observation platforms.  Between these and the street are many other opportunities for enjoying life including a religious sanctuary, the “L”, gardens, art, pedestrian bridges, and of course, offices, apartments, hotel rooms, and shops.

Unlike Pittsburgh, in Chicago, the public is welcome in some form on every level to gain a full experience of the city.

What is a Bridge? Chicago Edition I

Stetson Ave, Chicago, from the Hyatt

In July 2012, a few months into the height of my Pittsburgh bridge-walking, I pondered the definition of a bridge in What is a Bridge? Pittsburgh Edition I and What is a Bridge? Pittsburgh Edition II.  Asking what a bridge is may seem a little odd.  After all, it’s one of those things that you know it when you see it, right?  Yet, in walking bridges, I’ve discovered it really isn’t that simple.  In everyday life, the actual semantics of what a bridge is, is not important–you either cross it or you don’t and move on with your life.  But if you’re trying to walk as many Pittsburgh bridges as you can (as I am) or to count the number of bridges a city has to see which has the most of any city in the world (as others have), spelling out a clear definition of a bridge becomes important.  By the end of my previous posts on the definition of a bridge, it seemed like I had covered almost all the difficulties: is a bridge still a bridge if it no longer bridges anything? what’s the difference between a bridge and a ramp? how many is one bridge?….but then I went to Chicago and faced a new facet to this problem.

The issue Chicago brings up is best illustrated by the image above.  Is there a bridge in that picture?….No, right?….Try again.  If you walk past the buildings on the right, you get a new perspective on the seemingly solid ground you stand on (pictured below).

Discovering Ground Level on Stetson Ave
Looking Back Toward Hyatt on Stetson

Walking around on this street and the blocks around it, you may notice metal joints like those normally found only in bridges running across the roads and sidewalks.  You may also feel the ground bounce like a bridge as a large truck drives by.

Metal Joints across road and sidewalk on Michigan Ave
Metal joint across road on Columbus Drive

Are these roads, then, really bridges?  If no, what are they?  If yes, where does one bridge end and the other begin?  For example, is the Michigan Avenue Bridge across the Chicago River, visible in the back left of the image above-left, just a continuation of the Michigan Avenue road-bridge or is it a separate bridge?

Other posts in this series:

What is a Bridge? Pittsburgh Edition I (July 18, 2012)

What is a Bridge? Pittsburgh Edition II (July 27, 2012)

What is a Bridge? Chicago Edition II (February 1, 2020)

What is a Bridge? Bethlehem Edition I (July 1, 2020)

What is a Bridge? Bethlehem Edition II (July 15, 2020)