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The Pont du Gard Aqueduct near Nimes, France, remains an engineering marvel 2,000 years after the Romans built it.
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How would the Romans build Glass City Bridge?

How would the Romans build Glass City Bridge?

What if you had to build a 130-foot-high bridge across the Maumee River without the help of the 900-ton cranes, the internal combustion engine, and electricity?

How about without steel?

The ancient Romans came awfully close to performing such a feat on other rivers, building bridges that span nearly as far and just as high as the $220 million Veterans Glass City Bridge.

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So effective were their construction techniques, a few of those bridges are still standing, and carrying traffic.

George Houston, a retired classics professor at the University of North Carolina at Chapel Hill, will talk about how the Romans would have tackled their own Glass City span at 7:30 p.m. Friday in a free public lecture in the Little Theater at the Toledo Museum of Art.

Romans were prolific bridge builders.

I think the earliest bridge that survives of any size dates to around 170 B.C., Mr. Houston said. There are roughly 270 bridges that survive ... and of those, probably several dozen are still in use, mostly as pedestrian bridges.

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To an empire with some 50,000 miles of road, bridge building was an essential skill.

There was no great leap in knowledge about engineering or construction that turned Romans into successful bridge engineers, Mr. Houston said. Instead, other skills underlay their construction success.

Greeks built bridges. Probably Egyptians did some bridge building as well. But the Romans were good at organizational matters. Bringing to bear their intelligence of design and organization, they were able to build much more spectacular bridges than before.

The basic unit in Roman bridge building was the arch, Mr. Houston said, which is far stronger and more durable than simply extending beams across a river or gorge.

The shape of the arch is its strength. The weight of heavy objects crossing the top of an arch transfers toward the sides. If the arch is set against a rocky cliff, the cliff walls push back, keeping the arch stable.

But Romans did more than stretch a single arch from cliff to cliff. In their biggest bridges, they used multiple arches.

The Pont Du Gard, a Roman-built bridge in France, carried water across the Gardon River Valley more than 150 feet below. That 900-foot-long aqueduct is actually three decks of arches. At the base are six large arches. These are topped by about twice as many smaller arches. And this second deck is topped by yet another row of 35 even smaller arches.

That is a good example of the Roman accomplishment of being able to build very high bridges, Mr. Houston said.

But a look at this bridge, with its many piers sunk into the river, makes it clear why Roman construction methods wouldn t have worked for the Glass City span: A modern ship would never fit under the arches or between the bridge piers.

Bridge piers created problems for the Romans as well, Mr. Houston said.

For instance, the Romans built their longest bridge in Spain in a town called Merida. But they probably didn t intend it to be their longest.

Every time you build one of these piers, they act like little dams, Mr. Houston said. The piers supporting the arches of the Merida bridge were so wide it dammed up the water. The water moved and eventually outflanked the bridge.

As a consequence, another long bridge was attached to the first one in order to cross the newly created river channel. Now more than 60 arches support the structure.

But how did the Romans manage to build these piers on the river bottom?

Mr. Houston said Romans created cofferdams not too dissimilar to the ones used for the Glass City structure.

The Romans would drive long timbers into a great circle into the riverbed, Mr. Houston explained. This ring of timbers would be encircled by another ring. Then the space between the concentric rings would be packed with clay. Finally, the water would be pumped from the center section, creating a dry space to lower huge building stones.

Just transporting and lifting stones of some 500 to 800 pounds presented a whole different series of challenges for the Romans.

To begin with, stones were never brought from very far away, certainly not more than 20 miles. To lift stones, Romans used simple cranes that suspended pulley arrangements from timbers.

They also had a circular contraption that looks like a gerbil running wheel. Men would get into that and scramble, and their weight would turn the capstan, and lift stones. But Mr. Houston said this type of crane probably wouldn t have been used for bridge construction.

None of the stones in Roman bridges were secured with mortar. Rather, force created by the arch shape held rock in place. To create that shape, Romans probably built wooden forms.

We re pretty sure they would put a couple of stone piers up, then have false work wooden scaffolding in the shape of the arch, extending from one pier to the next. Over that you would lay the stones that formed the arch, and once you got all the arch stones in place, you then remove the form work.

Evidence for the use of such scaffolding is apparent in bridges that remain today. Some contain ledges for scaffolding, or have sockets in the piers for the placement of scaffolding, Mr. Houston said.

While there are apparently no records of bridge accidents, Mr. Houston said it is likely that Roman bridge construction may have included deaths, just as five men have been killed during construction of the Veterans Glass City Skyway.

With the nature of the materials, and the heights involved, I strongly suspect there were occasional fatalities, he said.

Mr. Houston, an expert in Latin literature and Roman history, taught courses in Roman technology at UNC. The classes would cover everything from Roman food and clothing to Roman construction. Today he studies Roman libraries.

Contact Jenni Laidman at jenni@theblade.com or 419-724-6507.

First Published April 30, 2007, 10:04 a.m.

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