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Articles > Cement Shoes 1

Cement Shoes
Part One - Angers 1850

Currently in the world of art and literature (also on stage and in films) they use the word trope when referring to any simplistic image or device generally understood by the average audience where our comprehension doesn’t come directly from the literal meaning of what we see or read. We use tropes for electronic communication. It is impressive to see how rapidly younger people have adopted tropes.

By Don Sayenga

Published in Rope News & Sling Technology June 2015

If you have any doubts about this, consider all those Three Letter Acronyms, or take a look at the terms in current use on the internet, such as meme and/or teme. I’ve chosen to call this kind of phenomenon a catchy tune. If we all know what I mean by catchy tune there is no need for me to create any TLA to explain myself. Do you get the picture? …LOL…or maybe I should insert the typewriter smiley : - )

The trope in my title links to the world of gangster movies. Actors such as Robinson, Cagney, Bogart, and Brando portrayed thugs who disposed of their enemies using cinder blocks on their feet sealed with cement. The bodies were dropped into water whereupon they sank to the bottom.

Today many people believe this was done frequently despite the fact very few dead bodies have been found in that condition. Even if it is only an urban legend, the concept of cement shoes provides a very good metaphor to illuminate a major scientific error associated with wire suspension bridge cables and the techniques for anchoring them.

For decades, cement shoes were attached to the wire cables of suspension bridges due to a mistaken belief in the permanent nature of the fixture. A few engineers (including some brilliant engineers) continued to recommend the practice of using cement shoes on cables despite a very serious catastrophe in 1850. The structural failure at Angers, France was the worst suspension bridge accident of all time.

Angers bridge before

PARDON MY INTERRUPTION
The History of Technology is the weakest category of human history for two reasons. Professional historians often do not understand changes in technology, while at the same time the innovators who change technology often do a mediocre job when documenting their efforts. The combination of the two produces a weird collection of garbled tales, myths, and urban legends, such as Al Gore inventing the internet. These tend to be viewed in a humorous manner by the general public. When an unexpected event interrupts a sequence of technological change, some times we don’t get the whole story of how it happened.

As of 1850, France had been the world leader in the technology of wire cable suspension bridges for almost three decades. French engineers had constructed so many wire bridges, no one yet has counted all of them. The website created by David Denenberg, www.bridgemeister. com, displays images and descriptions of several hundred French bridges built in that period. It was the transformation predicted by the engineering expert Navier in his famous 1823 report: “It is likely that the use of suspension bridges will soon become widespread; by this means we will create communication links in places where it seems at present impossible”.

Prof. Tom Peters explains in his book Transitions in Engineering (1987) how the academic posture of the French engineering establishment cleared a path for sudden interruptions within bridge technology: “Not only were the French engineers the best informed of any, they were also the best trained. The academically educated French engineer played an important role in the development of modern technological method, an essential component in the evolution of the wire cable suspension bridge”.

WHAT HAPPENED IN 1850 AT ANGERS, FRANCE?

The capital of the Maine-et-Loire province in France is Angers, formerly the stronghold of the counts of Anjou. We have a tendency to characterize the seriousness of a bridge catastrophe on the basis of how many people were killed. For example, when the Silver Bridge over the Ohio River fell in 1967, 46 commuters lost their lives. At Angers, the wire cable suspension bridge known as the Pont de Basse Cha"ne collapsed into the Maine River on April 16, 1850. The disaster, which occurred in the middle of the day, was witnessed by hundreds of people who were watching a military parade near the medieval fortress

Nobody is absolutely sure how many people were killed when the Basse Cha"ne bridge went down. The best estimates are more than 200. A plaque at the site says 223. One of the two main cables broke, pulling over one of the four metal towers, breaking the other main cable, and thereby dropping the bridge deck into the water. At the moment of failure, the bridge was crowded with spectators, soldiers, horses, and a marching band. It was the worst suspension bridge failure on record up until then and it hasn’t been surpassed since. The only encouraging thing we can say about it is that it was seen by so many bystanders. They were motivated to rush down to the river to pull men out of the water, preserving many of them from drowning.

One eye-witness has provided us with a frightening description. Corporal Charles Duban, a company supply clerk from the 3rd Battalion of the 11th Infantry Regiment, had gone into the city to purchase bread for the troops. As a result, he wasn’t in the parade. His regiment was on its way to Marseilles to be transported overseas. The front ranks of his unit already had crossed the bridge. Their commanding officer wanted them to look spiffy. He had given the order to fix bayonets on their weapons.

A thunderstorm blew into the city. The heavy wind caused torsional movements of the bridge deck. Although the troops were instructed to break step when crossing a bridge, the twisting motions of the deck forced them involuntarily to get into step again when trying to keep their balance. This intensified the twisting. Duban and many other onlookers were stood on the structure, a terrible crash was heard: the end of the deck from city’s side of the river began to break up! …the enormous weight of men and horses jostled … and plunged into the river… They were thrown pell-mell, spilling into each other”. They fell 36 feet down to the water. The river was 20 feet deep under the bridge. Many got stabbed by the bayonets as they went down. After the deck went into the river, collapsing iron and stone columns landed on some of them. “I’ve seen in my career, many war events, terrible catastrophes… but never, no, never have I seen a picture as horrible, so sad!”

Agers bridge after

HYDRAULIC CEMENT

Before we take another step toward studying how and why the bridge accident happened at Angers, we need to explore a better understanding of our English word cement. In the USA there is a tendency to use the words cement and concrete as if they mean the same thing. Cement is the brownish or grayish compound which begins as a powder, becomes a slurry when water is added, and lastly hardens into artificial stone. Concrete is a cement mixture containing other natural substances such as pebbles and sand to achieve better physical properties.

According to a description provided by Thomas Hahn and Emory Kemp: “ The use of calcium compounds as the basis for mortar, concrete, stucco, and other applications requiring a cementitious material dates from ancient times… For thousands of years builders have used cements of various kinds to bind and bed materials in their domestic structures, monumental buildings and public works”. When very old structures are being repaired these days, our most common modern cement (i.e. Portland Cement) is avoided because it didn’t exist prior to the 19th Century.

A French engineer named Jean C. Petot published a table in 1833 describing the various types of cement ranked in order based upon the amount of clay in the mixture. One of the most common products on his list is still in daily in use all over the world. It is known as hydraulic cement. It has a long service life and can be installed under water where it hardens and maintains its strength even when submerged. It is very useful in vertical applications because when the powder is mixed with water, the slurry can be poured into place like a liquid. It expands as it hardens. The key ingredients are lime mortar with the addition of clay, either naturally or artificially.

A BRIDGE WITH TROUBLED WATERS

The engineers who designed and built the Pont de Basse Cha"ne in 1838 were Theodore Bourdillon and Joseph Chaley. Ironically, Chaley (1795-1861) was one of the foremost French bridge engineers. Prior to the disaster, he’d already created the world’s record wire bridge at Fribourg in Switzerland with a span of 273 metres, more than 2-1/2 times the span at Angers. Furthermore, his big bridge at Fribourg had been inaugurated by a grand military parade augmented with troops and cannon. 2000 people crossed all at once in that celebration.

At that time it was one of the favored practices in France to seal all suspension bridge anchorages with “chaux” builders believed this ancient technique would prevent water from rusting the iron cables. It was thought the lime mortar somehow was able to coat itself on to the surface of iron in the same way that protective metals such as tin and zinc formed impervious coatings on iron. In reality the lime mortar had the opposite effect but they had no way of knowing that. The flaw in their reasoning is more obvious to us now.

In those days wrought iron and cast iron were plentiful, cheap, and stronger than wood or stone. Iron was being adopted everywhere for massive structural applications. Iron, however, is elastic. Lime mortar (after hardening) is inflexible. In the case of iron wire suspension bridge cables, Chaley used cement shoes to create his anchorage by inserting the cable ends into a vertical chamber, wedging them with masonry, and finally pouring in the sealing slurry of expanding hydraulic cement.

Vibratory forces caused by transitory loading cause the deck of a suspension bridge (no matter how rigidly it has been built) to transmit those forces into the flexible cables. Loading waves pass from end to end of the cables. French wire bridges built in the 1830s and 1840s had decks deliberately designed to be flexible enough to reduce the stresses of that motion against the joints of the deck. Over time, minor vibration waves in the cables caused the cement shoes of the anchorages to break free of the wires leaving tiny open spaces where water could collect. Because most of the bridges were built at damp places near water, wire cable anchorages became vulnerable to corrosion. The damage to the iron wire was occurring at inaccessible locations where the lime mortar seal made inspection impossible.

JUST THE FACTS, MA’AM

A commission of inquiry made a thorough analysis of the Angers bridge failure. They found the deck of the bridge had ample strength to support the weight loaded on it at the time. The engineering design was not implicated because all of the specifications had been met. The 1850 examination report clearly stated it was not simply a matter of strength vs. loading. A study of the wreckage had revealed a combination of three other causes which were difficult to remedy from a design standpoint.

First conclusion: The winds during the thunderstorm caused some twisting of the slightly flexible bridge deck due to aerodynamic effects which, at that time, were poorly understood. The twists resonated from one end to the other on the bridge. Even though the commander of the troops had ordered the soldiers to move in quick-time, he was blamed for an error to have attempted parading across the bridge during a storm when it was obvious the deck was slightly twisting.

Second conclusion: An amplification of the twisting resonance was created by the steps of the soldiers. Although they were not marching in cadence, each man was forced to keep his own balance against the pitch and roll of the deck. This caused them to remain in step anyway although they’d been ordered not to do it.

Third conclusion: The movement of the deck transmitted major oscillations into the main cables. The waving motion passed back and forth down into the anchorages to the ends of the cables where it was dampened by the cement shoes. The ends of the main cables were rusted inside the anchorages. One of the main cables failed at one end of the bridge due to the corrosion of the wires, unbalancing the entire bridge.

OOPS

Because the report concluded without suggesting a good remedy, the French government declared a halt to the construction of wire bridges, pending an assessment of the other wire bridges scattered all over their nation. At first, the French moratorium on construction of wire suspension bridges was more or less ignored elsewhere. In the USA there was no equivalent to the French civil engineers’ organization. Most of the men who were advertising themselves as CE didn’t have a college degree. The American Society of Civil Engineers wasn’t organized until 1852.

As of 1850, the science of ferrous metallurgy was still in its infancy. The reality of the Angers situation was complicated by another urban legend in general circulation. Many intelligent people believed a notion that under certain conditions iron would suddenly “crystalize” and shatter like glass. The conversion of iron into the form known as steel was not well understood although it had been practiced for centuries.

Charles Ellet, Jr. had introduced the French wire bridge concept into the USA in 1839. Ellet was opposed to the practice of sealing the anchorages with cement shoes. When he created America’s first wire cable bridge at Philadelphia he specifically fixed the loops at the ends of his cables to iron bars inside an open inspection chamber. His 1843 report asserts:

“These bars are attached to the cables in open archways formed for their protection, and the connexion is such that the cables can be approached and e x - amined throughout their entire length”.

Unfortunately, Ellet’s primary rival, John A. Roebling, felt otherwise. Roebling was one of the most brilliant American engineers but he mistakenly believed cement laced with lime mortar formed an impervious coating on the surface of iron. In his November 1845 treatise written for the Franklin Institute he referred to “the known quality of calcareous cements to prevent oxidation”. Roebling used cement shoes protecting the cable attachments in his major structures. Years after his death, his son Washington Roebling wrote: “Mr. Roebling was a great believer in the preserving quality of cement on iron imbedded in it.”

Further Reading

  • Cement Shoes Part 2 Niagara Follies
  • The best source for information about the tragedy at Angers is the website posted by the Musée du Génie, www. musee-du-genie-angers.fr
  • See the article “Angers – La Catastrophe du Pont de Basse-Cha"ne”
  • The book by Prof. Tom Peters Transitions In Engineering, (Birkhäuser 1987) provides an excellent technical analysis of all circumstances leading to the tragedy at Angers.
  • Thomas Hahn and Emory Kemp give a very complete explanation of various cements in their IHTIA monograph Cement Mills Along The Potomac River (Morgantown WV 1994)
  • Charles Ellet’s 1843 treatise A Popular Notice Of Suspension Bridges is now on-line.
 
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