The Brooklyn Bridge Experiment
by Don Sayenga
In honor of the Brooklyn Bridge’s 100th anniversary we are re-publishing this article from a 2018 issue of Wire Rope News & Sling.
The four main cables of the Brooklyn Bridge were completed in October 1878. It would be another five years before the bridge went into service, but since its opening day on May 24, 1883, each of the four main cables has been carrying a load in excess of a thousand tons. Although the burden of the fabricated metal bridge structure they are supporting has been reconfigured several times, the cables themselves remain essentially as they were 140 years ago. They are unique. No other bridge ever has been built with the same kind of main cables. In this essay, we’ll analyze the various considerations we should have in mind if we try to look at the Brooklyn Bridge as the world’s oldest and largest experiment in metal fatigue.
The phenomenon called “metal fatigue” is a weakened condition induced into metallic structures by repeated stresses or loadings. It causes metals to fail at much weaker stress than originally predicted. According to online Encyclopedia Britannica, it involves a number of physical changes including atomic slip, crack initiation, and crack propagation. When it was first recognized by European scientists, they adopted a very simplistic engineering approach originally called the “Safety Factor”. Today it is known as the Design Factor. For ordinary mechanical and civil engineering it is Six-to-One.
In reality, the Brooklyn Bridge was built by contractors and subcontractors. The general superintendent was William Kingsley who was the primary promoter for the whole enterprise. A wheeler-dealer in Brooklyn, he had all kinds of political connections plus he controlled the city’s leading newspaper. He also gave himself the contract to build the eastern anchorage. Brooklyn was the 3rd largest city in the USA at the time (after New York City and Philadelphia) which made it very important to Kingsley that all of the Brooklyn Bridge main cable wire ought to be made right there in Brooklyn. It was the largest single steel wire buy in world history as of that moment. Washington Roebling was the bridge’s chief engineer. When he was writing the specification for the main cables, had no official standards to apply because no one had ever done anything like this before. He designed the cables six times stronger than needed, adhering to the accepted design factor. He did not have a good relationship with Kingsley; he said he felt “an intolerable aversion” to him. There is no indication the two men ever discussed the spec. Kingsley did not have any kind of engineering background, making it seem unlikely he would have understood much even if they had talked. Steel wire had been used for many decades to make small items such as needles, but this main cable application was much larger than anything ever tried before.
At the time new metallurgical methods of making steel, such as the Bessemer-Kelly process and the Siemens Martin process were being introduced. Roebling asked one of the assistant engineers, William Paine, to run tests on all the steel wire products readily available in the marketplace. When in 1876 the megalithic stone towers were at last completed and cable making was about to begin Roebling wrote: “Tests have been going on at intervals for the past six years, and constantly during the last two years … Samples were obtained from most of the principal foreign as well as domestic makers of steel wire, comprising all of the various grades of steel, but especially cast steel…The principle was to obtain a certain amount of strength for the least amount of money.”
The focus on verifiable wire strength and the preference for cast steel (both dictated by Roebling himself) marked the beginning of the big experiment. As of 1872, cast steel prices were often as high as 20 to 25 cents per pound. The crucible casting process limited a batch of steel to the weight that could be lifted by two men working together; about 100 pounds. Almost seven million pounds of wire would be purchased. No order that large had ever been placed before. The Board of Directors was aware Roebling’s family business, John A. Roebling’s Sons Co., might want to become the wire supplier – they passed a motion blocking this which forced Washington Roebling to sell his stock in his own family company. David McCullough’s book The Great Bridge describes the chicanery involved. Nine wiremakers, both foreign and domestic, responded to the call for bids. When the proposals were opened, the contract went to a wire mill located on what is now Pioneer Street in the Red Hook section of Brooklyn.
But the Brooklyn wire mill didn’t keep up its side of the deal. They committed fraud when furnishing the wire. McCullough’s history has several chapters explaining what they did. Also, there was a simplistic explanation published by the New York Times on December 30, 1879. An unknown amount of wire which failed the acceptance testing already had been inserted into the cables before the fraud was discovered. Because the diameter of the wire had been specified “Number 8” (referring to the gauge system commonly used in the wire business) Paine realized minor variations in size might be encountered. The tools for checking the size were not codified in scientific terms. He concentrated on strength, ductility, stretch, and other physical attributes. Every one of the short lengths of wire was examined in the wire mill by an assistant engineer named Arthur Abbott.
Erica Wagner’s eloquent book Chief Engineer explains what happened next. Paine and Roebling tried to estimate how much deficient wire had been put into the main cables. There was no way to remove it. Roebling reported a make shift solution to the trustees. Fourteen tons of extra wire was to be added to the four main cables, and the wire mill would furnish it without charge. Roebling tried to explain this sort of unanticipated problem was exactly the reason why civil engineers employed the Six-to-One design factor. He wrote: “There is still left a margin of safety of at least five times, which I consider to be perfectly safe, provided nothing further takes place”. Most of the trustees had very little technical training and may not have understood what happened. The public was told nothing. Cablemaking began mid-year 1877 and was completed in October the following year. With the extra wire added, each of the main cables was slightly larger than fifteen inches in diameter when its outer wrapping was in place. The steel bands affixed to the cables for attachment of the suspender ropes were squeezed on without requiring any changes.
All of the deficient wire is still there today inside the cables. No one has a clue which cable contains a portion of it. The continuous wire wrapping prevents observation of the load-bearing wires everywhere except inside the anchorages where they pass around the first pins of the anchor chains. If any of the wire has failed internally inside a main cable while service since 1883, we have absolutely no way to know about it.
New York City’s East River is actually a tidal strait on the Atlantic seacoast. Ordinary carbon steel wire corrodes rapidly in a seacoast atmosphere unless the surface is protected. Corrosion at the surface leads to pitting which reduces the cross-section of the wire, making it weaker at that spot. Starting in 1837, a French scientist named Stanislaus T. M. Sorel developed a series of patents for coating ferrous metal by dipping articles in a bath of molten zinc. As a result of his success (which he called “galvanizing”) this method of protecting wire was adopted very quickly in Europe and America for protecting wire from rust.
Hot dip galvanizing is called a sacrificial process. The corrosion caused by the atmosphere (which cannot be stopped) attacks the zinc coating first. The surface of the steel is protected by the preferential attack on the coating until it has been eaten away. Common sense makes it seem obvious the thickness of the coating is a consideration we must take into account when estimating how long the zinc protection will last. When the cable wire was purchased there was no engineering standard for galvanizing by the hot dip method. Obviously, no thickness was specified. If any of the internal wires has failed because the coating is gone and pitting has begun, we have no way of knowing about that, either.
The Brady Splices
The Brooklyn Bridge is the only bridge ever built with Brady splices. Those little couplings are the most experimental factor in this giant metal fatigue experiment. Never used before or since, there is no track record for comparison. At each splice there are circular circumferential V-grooves cut into the wire by the coupling. In the fundamental principles of mechanical engineering being taught today, such grooves are well known as stress-raisers. When the main cables were being made, the Brady splices were an un-tried solution devised by Washington Roebling and his engineering staff as a way to save time. There was no debate with anyone about their decision in 1877. Today most people are completely unaware of the numerous little splices everywhere in the cables.
Lore Croghan of the Brooklyn Daily Eagle newspaper says: “Many of the people who swarm to the iconic span’s promenade to stroll and snap selfies ignore the painted white lines that separate its bike and pedestrian lanes. Because signs posted on the bridge about the walking and biking paths are written solely in English, they’re unreadable for some visitors… The pedestrians don’t pay attention. They treat the bike lane as their own.” Brooklyn Eagle photo by Lore Croghan Roebling’s father had his own method for splicing wires end-to-end. Two wire ends were tapered with a hammer and scratched to a rough surface with a file. The two roughened surfaces were overlapped and the connection was secured with a wrapping of fine wire. At a rope factory, it was a process that could be done all the time without any interruptions but it was utterly unsuitable for a huge project like the big bridge where the cost prediction of the project had been grossly underestimated by John A. Roebling the original designer of the bridge. The splices were created by a tiny Brooklyn machine shop, Brady Manufacturing Company. We are fortunate that the owner, James Brady, provided the story in a piece he wrote for American Machinist magazine March 16, 1905. He had been called into court to testify in a patent infringement lawsuit. The technique he used for couplings was known as a “vanishing thread on a cone”. He began his explanation of the contract by emphasizing how the use of crucible cast steel wire severely limited the wire lengths: “I secured a contract for furnishing the couplings (about 200,000) required in connecting the wires for the bridge cables. The state if the art at that date limited the lengths of the wire used to about 600 feet…these wires were made practically continuous by the use of the couplings.”
(When I’m reading Brady’s explanation, I find it difficult to keep in mind he is talking about cutting two internal threads in thousands of little chunks of metal smaller than the USB flash drive I’m using to record this essay.)
“The threads were of course right and left and the abutting ends of the wires were cut obliquely so that they interlocked as shown when drawn together, the object of this interlocking being to prevent uncoupling by unscrewing while being laid into the cables… the form of thread used, technically known as a vanishing thread…was of the V form, or practically an equilateral triangle in section, being at the small end of the cone the full size of the standard pitch used, and tapering its entire length to a vanishing point at the other end…It seemed to puzzle those who saw these…how the thread could be so perfectly made…we made two conical ends alike…we also made a sleeve having a taper hole to perfectly fit the entire length of the cone” By simply twisting the coupling onto the ends of the wire coils (which were called “rings”) any of the unskilled workmen could easily complete a firm connection, which greatly accelerated the work of making all those splices. The long-term consequences of V grooves cut into both ends of every length of wire in the bridge were not recognized at the time. In his explanation Brady made some interesting observations about the wire: “Perhaps it may be of interest to some of your readers to know that the wire used in the bridge cables was spring temper, and by the use of the vanishing thread above described the tensile tests showed that none of the couplings failed to give at least 95 percent of the full strength of the wire and, as a whole, they averaged higher; also that the cutting of the threads on this spring tempered wire called for a special construction of dies, the standard make proving an absolute failure”. Due to the continuous outer wrapping wire around the main cables, and the many layers of protective painting applied since the bridge opened for business, it is nearly impossible to visually locate one of the Brady couplings on the exterior of the main cables to evaluate its condition as of today. During the last two decades of the 1800s, the sweeping adoption of large-volume processes for making steel stimulated wire mills to draw longer and longer lengths of wire for suspension bridges. During the same time period new ways to weld lengths of wire end-to-end eliminated the need for couplings like Brady’s. They were never used again in a bridge.
What Happens Next?
The wisdom of the design factor becomes obvious when all the risky aspects are added together. Although it was contrived to be used as a Rule-of-Thumb, it was exactly changing conditions such as these that caused the bridge to be built five times stronger than needed. In the case of the Brooklyn Bridge we can add one other very important consideration which is rarely the case with other wire bridges. When it was built it was intended to carry horse-drawn cart and carriage traffic with a cable-car system in the middle to transport pedestrians. The walkway at the highest level was described as a “promenade” because it was intended for leisurely strolling. For a while, the routing of heavy mass-transit trains overloaded the original capacity. Removing the heavy railroad traffic and later the rerouting of heavy semi-trailers has reduced the burden significantly. The next most obvious adjustment for the future would be a change preventing automobile traffic from using the bridge during rush hour, and, at a later time, stopping all vehicle traffic on it except for motorcycles. Recent publications such as several art books by Richard Haw, and Erica Wagner’s Chief Engineer have stimulated pedestrian tourism to a degree beyond the wildest imagination of the 1870s. It seems almost incredible to realize the first major design change made by Washington Roebling when he revised his father’s initial ideas was to eliminate the sidewalks!
Maintenance of the Brooklyn Bridge and the other three East River bridges is a responsibility of New York City’s Department of Transportation working in collaboration with State and Federal Agencies. Information about work-in progress is posted on-line at several websites. The 2017 condition report published by DoT’s Commissioner Pol ly Trottenberg states:
“The Brooklyn Bridge is one of New York City’s most popular tourist destinations, as well as a major transportation corridor. The existing promenade, shared by pedestrians and cyclists, is narrow and heavily crowded with tourists and mobile vendors vying for space with commuters and recreational users.”
From 2008 to 2015, pedestrian volumes increased 275% on weekends and cyclists increased 104%. The promenade, which was part of the original bridge, narrows to just 10 feet across in places from 17 feet at its widest point. In August 2016, the Agency’s Transportation Planning and Management Division began a seven-month $370,000 engineering study to assess how much weight the bridge can carry and consider options for expansion, such as the structural feasibility of constructing additional space above the roadways on the existing truss system. The goals of the study are to relieve overcrowding, enhance the visitor experience and improve safety, and to greatly reduce conflicts between cyclists, pedestrians, and visitors on the promenade.
Commissioner Trottenberg’s report has alerted motorists to be aware the last days of the Brooklyn Bridge may be notable because it will become the world’s largest pedestrian bridge sooner or later. Before this ever happens, DoT will confront the big unimaginable task of getting a look inside the main cables:
“The study’s analysis showed that while the expanded promenade itself would add additional weight, the greatest increase would come from added pedestrian volume and live loads related to their presence. Results of the study recommended an inspection of the cables before considering a deck expansion. This inspection will take roughly two years and is part of regular bridge upkeep. If the cable analysis shows an expansion to be feasible, and taking into account other bridge needs, the promenade will be widened when entire deck is raised to girder height as part of an upcoming contract.”
Along with the never-ending task of painting the steelwork, New York City’s DoT also contends with some dual maintenance problems arising from two aspects of our modern civilization that didn’t exist in 1883. (Both are actually federal crimes because of the 1964 designation of the bridge’s status as a USA National Historic Landmark.) The aerosol spray can has provided a simple tool for quickly and easily adorning any bridge (and other public structures) with freeform graffiti. Because the defacing is always done secretly, it creates a nuisance for public maintenance employees in New York City and everywhere in America.
The Brooklyn Bridge presents a huge target for those who get gratification out of throwing up lettering etc. with a spray can. By periodically removing it with high-pressure water hoses and strong solvents, the clean-up has a negative impact on the Rosendale cement which was used for the brick and stonework of the bridge. That kind of cement has been displaced by Portland cement in modern times but the bridge (and also the base for the Statue of Liberty) was built using the older form of so called “natural” cement. It decomposes and washes out of the joints.
The bizarre European addiction for attaching padlocks to adorn bridges also has become an added affliction for the promenade of Brooklyn Bridge. New York City DoT attempts to curtail the locks by issuing fines. It is extremely difficult to catch someone in the act of affixing a lock. It is even more difficult to remove the locks. Hopefully, like all other similar juvenile fads, this one will cease to be popular with future generations.
Commissioner Trottenberg’s 2017 minimum estimate of all the expenditures to completely rehabilitate every aspect of the Brooklyn Bridge’s condition is more than a billion dollars = $1,083,600,000! Obviously, such an incredible cost cannot be justified in the long run for a bridge as old as this one when several other ways of getting across the East River are available. The essay you are reading provides comments about the longevity of the steel wire cables. Other examples of anticipated deterioration of the materials used for the structure, such as the Rosendale cement in the masonry, may become more critical than the steel wire in the cables as the bridge ages.
Civil and mechanical engineers, chemists, metallurgists, and civic planners, all will benefit when the world’s largest metal fatigue experiment comes to an end. An analysis of the main cables will allow a final report to be written. Between now and then it is safe to say world history has shown us how scientific changes and political changes can alter our attitudes. When the time comes to read it, we may not care what the report tells us!