In the spring of 1922, a man was working inside a caisson on the bed of the Delaware River in Philadelphia, Pennsylvania, in compressed air at roughly thirty pounds per square inch. That was above the normal atmospheric pressure within a steel chamber submerged eighty feet down, through river sediment in water the color of silt, at the bottom of a river that had been carrying the drainage of the Delaware Valley for ten thousand years. He had no wish to be interrupted. The air pressure was essential; without it, the river would have flowed through the permeable sediment, flooded the chamber, and drowned the men inside.
With it, the men could dig the riverbed by hand, removing material so that concrete could be placed to form the foundation of a tower where the sediment had been. Working inside a caisson is work done under conditions the human body was not designed to endure. Nitrogen in the compressed air dissolves into the bloodstream at a rate proportional to the pressure. A man who rose too quickly from the pressurized chamber after his shift, returning to normal atmospheric pressure faster than his blood could release the dissolved nitrogen, risked bubbles forming in his joints and bloodstream.
The condition was called caisson disease in 1922. Today it is called decompression sickness. In 1922 it was managed with lock chambers that were supposed to control the rate of pressure change, and with medical chambers that were supposed to treat the men for whom the locks had not worked. The men who worked in the caissons on the Delaware River during the construction of the bridge understood the risks the way workers in dangerous construction understand them: they knew the dangers existed, and they knew they would do the work anyway, because that was what they had come for.
The bridge they were building had no name yet. It would be called the Delaware River Bridge when it opened, then renamed the Benjamin Franklin Bridge in 1956, thirty years after its opening, when the city of Philadelphia decided that a structure of this size deserved the name of a man of that stature. In 1922 it was called the Delaware River Bridge, which described it accurately. The men who worked in the caissons at the riverbed called it whatever working men call a job they are in the middle of, which is usually not its official name.
To understand why Philadelphia had spent most of the previous two decades trying to build a bridge across the Delaware River, and why this attempt consumed the engineering attention of two states, a federal government, a bond market, and a workforce of several thousand men, you have to understand what the Delaware River was, and what crossing it by any means other than a bridge cost people on both sides. The Delaware River at Philadelphia is about two thousand feet wide at the crossing site the engineers eventually chose. It carries commercial shipping traffic from the port of Philadelphia downstream to Delaware Bay and the open Atlantic. The minimum navigational clearance for the river’s shipping traffic—the oceangoing freighters, tankers, and passenger ships that the port of Philadelphia handled in the first decades of the twentieth century—was 135 feet of vertical clearance above the high-water mark.
A bridge that did not provide 135 feet of vertical clearance at the navigation channel was not a bridge that shipping could pass under on the river. A bridge that provided 135 feet of vertical clearance at the channel had to begin rising to that height well before the edge of the channel. That meant that a bridge of any conventional design would require approach ramps of enormous length on both shores, eating into the urban fabric of a city that was already built to the water’s edge on the Pennsylvania side and substantially developed on the New Jersey side at Camden. The ferry service between Philadelphia and Camden had been operating for more than a century before the bridge was built.
The ferries crossed the river in about fifteen minutes. They ran on a schedule. They carried vehicles and pedestrians. They were inadequate for the traffic volumes that the automobile economy of the 1920s was generating.
The number of vehicles crossing the Delaware River between Philadelphia and Camden by ferry in 1920 was large enough that ferry operators were turning vehicles away during peak hours, and the waiting line on the Philadelphia side stretched back into the city’s street grid. A city the size of Philadelphia and a port the size of the port of Philadelphia could not operate efficiently when crossing a two-thousand-foot river was managed by boats running on a fixed schedule and fixed vehicle capacity, with a queue growing longer every year as car ownership increased. The bridge was not a luxury. It was what the commerce of two states required.
The first serious proposals for a bridge across the Delaware at Philadelphia appeared in the early years of the twentieth century. The proposals immediately ran into the judicial reality that the Delaware River is a boundary between two states. Pennsylvania owns the crossing from its shore to the middle of the river. New Jersey owns the crossing from the middle of the river to its shore.
Building a bridge across the river required both states to agree on the same structure, the same financing, the same operating authority, and the same distribution of construction costs. The Delaware River Joint Commission was created by an interstate compact in 1919 to do what neither state could do alone: act as a single authority capable of designing, financing, building, and operating a crossing that neither state fully controlled. The commission engaged Ralph Modjeski, a civil engineer of Polish origin who had been practicing bridge engineering in the United States since the 1890s and had designed and built bridges over the Missouri, the Mississippi, the Ohio, and the St. Lawrence Rivers.
Modjeski understood long-span bridge design by actually building them, not through theory alone. He understood the problem of crossing the Delaware River the same way he understood the problems of every crossing he had undertaken before: the river defines the problem, and the engineer’s task is to determine the solution the river requires. The solution the Delaware required was a suspension bridge. Navigational requirements ruled out any type of bridge that required piers in the navigation channel.
The width of the crossing ruled out any conventional design that could span two thousand feet without intermediate support. A suspension bridge with towers on each bank and a main span that cleared the navigation channel without intermediate support was the only structural solution that met all constraints at once. The main span of the bridge Modjeski designed was 1,750 feet between tower centers. It was, at the moment of its completion in 1926, the longest suspension bridge span in the world.
It was more than 150 feet longer than the previous record holder. The record it broke belonged to the Ambassador Bridge in Detroit, which was under construction at the same time, and whose completion date was a matter of competition between the two projects; although the competition was not publicly acknowledged the way commercial rivalry is, because both projects were civil infrastructure, not private ventures. The Delaware River Bridge opened first. The record went to Philadelphia.
The towers that carry the main suspension cables rise 380 feet above the high-water level of the river. These towers stand on foundations built by the caisson crews of 1922 on the bed of the Delaware River through eighty feet of sediment and decomposed organic material before reaching the bedrock that engineering borings had shown to be there. The Pennsylvania tower foundations rest on bedrock at 94 feet below the river’s surface. The New Jersey tower foundations rest on bedrock at 74 feet below the river’s surface.
The difference in depth reflects the nature of the riverbed geology, which is not uniform across the two-thousand-foot width of the crossing. On the Pennsylvania side, the sediment column is deeper. On the New Jersey side, bedrock appears somewhat sooner. So the diver on the Pennsylvania side had to go deeper and stay under higher pressure for longer.
The caisson is a working chamber lowered through the sediment under its own weight. The walls of the chamber act as a cutting edge that sinks into the material while excavation inside removes the support underneath. An air lock at the top of the caisson maintains the pressure difference that keeps water out. Compressed air is pumped in.
Excavated material is removed through locks designed for that purpose. Concrete is pumped down to fill the void behind the cutting edge. As the chamber descends, the men inside work in compressed air, breaking up sediment with picks, shovels, and pneumatic tools, loading the broken material into containers that the lock carries to the surface, and descending with the chamber as the work removes material from beneath them. The caisson crews on the Delaware River Bridge were drawn from the marine construction workforce along the northeastern seaboard.
Men experienced in compressed-air work from tunnels and foundations on industrial waterfronts. They were not a homogeneous workforce. Caisson work on major public infrastructure projects in the northeastern United States in the early 1920s was done by a workforce that included recent immigrants from southern and eastern Europe—Italians, Poles, Slovaks, Greeks—who had arrived in the decade before the war and entered the construction labor market at its lowest levels, and Black men from the American South who had come north in the Great Migration (accelerated by wartime labor shortages) and found in the construction industry one of the few sectors of northern urban economies that employed them, at wages below those of the white workers beside them. The industry’s hierarchy assigned them the jobs closest to the hardest and most dangerous work.
Exposure to compressed air in the caisson environment does not vary by race. Caisson disease does not choose its victims based on national origin. Nitrogen dissolves into every man’s blood at the same rate, and bubbles form in every man’s joints according to the same physiology. The men who came up too fast from the caisson on the Pennsylvania side of the Delaware River did so because the decompression protocols of 1922 were based on an incomplete understanding of the physiology involved, and the economic pressure to move men through the locks faster than strict safety procedures would allow was present on every caisson project of that era.
They faced the disease regardless of what they were paid to be there. The documentation of disease cases on the Delaware River Bridge project is incomplete. Medical records that would allow a comprehensive accounting of the injuries that occurred and the men who suffered them do not exist in any publicly accessible archive in a form widely cited by researchers. What the construction records do show is that the caisson work was completed on the schedule the contract required, that the foundations reached the bedrock the borings had identified as bearing material, and that the concrete poured into the caissons formed the base upon which the towers were built.
What the construction records do not show, because the records of 1922 were not required to show it, is the full account of what the compressed-air work cost the men who did it. The towers themselves were built by ironworkers. A different workforce with a different skill set and a different set of risks. The steel structure of a suspension bridge tower rises from the foundations through 380 feet of open air.
It rises from the bottom up. That means the men connecting steel at the top of the tower are working 380 feet above the river on structural members not yet fully connected to the members below them, on connections that are not yet the final connections the design specifies. They work on temporary platforms regardless of whatever weather the Delaware River makes, which in winter is wind coming off the open coastal plain of New Jersey, at temperatures that make bare metal too cold to grip with bare hands except briefly, and that briefness is the essence of the work. You cannot complete a structural steel connection wearing gloves.
The tolerance on bolted connections in high-strength steel is too tight for gloves to be a practical tool for the men doing that work. The men who built the towers of the Delaware River Bridge worked bare-handed on steel in winter at heights where any error in balance or any lapse of concentration was fatal. The main cables of a suspension bridge are not manufactured in a single process. They are built in place on the structure through a process called aerial spinning, in which individual steel wires are carried back and forth across the gap by a spinning wheel traveling on a temporary cableway, laying one wire at a time.
The cable is built up from individual wires until the full count specified by the design is complete, then the bundle is compacted and bound into the circular cross-section the design requires. The main cables of the Delaware River Bridge contain 18,666 individual steel wires, each measuring 0. 196 inches in diameter, each galvanized to resist the atmospheric corrosion the cable would face over the life of the structure. The spinning operation laid those 18,666 wires in succession across the 1,750-foot main span, with additional passes over the side spans from the towers to the anchorages on each shore.
The spinning crews worked on the aerial cableway for the spinning wheels and on temporary footbridges suspended below the cable line, from which the spinning operation was watched, corrected, and maintained. The footbridges were temporary structures adequate to support the men working on them, and were not designed to provide any protection against the weather the Delaware River makes at that height. The wind in the main span of a suspension bridge under construction is not the wind at street level. At two hundred feet above the river, and over two thousand feet of open water, the winds from the southwest, which dominate Philadelphia’s weather in the summer months, arrive uninterrupted by buildings and terrain as they are at street level.
The men worked on the footbridges during cable spinning in those winds, with the equipment the work required and the protection that era provided, which was a canvas work jacket if they had one, knowing that if the wire-spinning wheel jumped its track it could knock a man off the footbridge without warning. The number of men who died during the construction of the Delaware River Bridge is not prominently documented in any publicly available source. Construction death records in the early 1920s were subject to state labor laws that required reporting to the appropriate labor department, but did not require publication in any form readily accessible to the public or to historians. The bridge-building industry of that era understood worker fatalities as a tax on the work, recording them where the law required, reporting them to authorities when needed, and continuing the work.
The men who fell from the towers or the footbridges or the cable-spinning equipment were entered into whatever record-keeping system the state of Pennsylvania or the state of New Jersey maintained for such events. Those records, to the extent they have survived the past century, are not in circulation in any account of the Delaware River Bridge project that has been widely read. The anchorages that hold the main cables against the tension of the loaded span are concrete blocks large enough to be considered engineering projects in their own right. The Pennsylvania anchorage contains approximately 85,000 tons of concrete.
The New Jersey anchorage contains approximately 55,000 tons. The anchor blocks were poured in place within excavations that reached the bearing material the engineers had identified, and the steel cable eyes that receive the cable strands at the anchorage end are embedded in those concrete blocks, so that the tension of the cable is transferred to the anchor block and from it to the ground, rather than allowing the cable to pull the tower back over its base. The concrete in these anchor blocks has been in continuous service since 1926. It has not moved.
The suspension cables carry the dead weight of the bridge deck and the live traffic load through suspenders—vertical steel rods—down to the stiffening truss that runs along both sides of the roadway for the length of the main and side spans. The stiffening truss is not a feature of older suspension bridge designs. It was incorporated into suspension bridge design after the experience of earlier long-span bridges showed that a deck suspended from cables without a stiffening element is prone to oscillation under dynamic loads. Wind loads, traffic loads, the kind of loads constantly generated by a major urban crossing carrying mixed vehicle and transit traffic.
The engineers who designed the Delaware River Bridge understood that a 1,750-foot main span carrying the traffic volumes expected at a crossing of this size required a stiffening truss deep enough to resist the dynamic loads produced by traffic and wind without excessive deflection or oscillation. The stiffening truss of the Delaware River Bridge is 25 feet deep. It is clearly visible from the river. It is the visual feature that distinguishes the bridge’s silhouette from the more slender appearance of later suspension bridges that incorporated more elegant solutions to the same structural problem.
The transit tracks incorporated into the original design of the bridge reflect the transportation planning assumptions of 1920. The engineers and transportation planners of Philadelphia and Camden understood in 1920 that an urban crossing of this size would carry both automobile traffic and transit trains. Because in 1920, the rail transit systems in both cities were carrying passenger numbers that the automobile had not yet displaced, and was not expected to displace. The bridge was designed with two transit tracks on the lower level of the stiffening truss, providing clearance for the electric railway vehicles operated by the transit systems of Philadelphia and Camden.
The bridge continued to carry transit for decades after it opened. It does so today in modified form. The PATCO Speedline uses the lower level of the bridge to carry rapid transit passengers between Philadelphia and Camden on tracks that the engineers of 1920 anticipated without knowing the name of the system that would eventually use them. The bridge opened on July 1, 1926.
The opening ceremony was organized by the Delaware River Joint Commission and the civic authorities of Philadelphia and Camden. President Calvin Coolidge participated remotely by pressing a gold telegraph key in Washington, sending an electrical signal to the bridge’s opening ceremony on the Philadelphia side, which was considered the official declaration of its dedication. The bridge was open. It was the longest suspension bridge in the world in operation.
The newspapers of Philadelphia and Camden and the national wire services covered the opening the way engineering achievements of this scale were covered in 1926: with emphasis on the facts of the structure, its length, its cost, its number of workers, and the kind of civic pride that a project of this investment generates in the cities that commissioned and funded it. The construction cost was approximately $37 million, financed through bonds issued by the Delaware River Joint Commission against the bridge’s anticipated toll revenues. The toll at opening was 25 cents per vehicle. Traffic volumes in the early years of operation exceeded the projections the commission had provided to bond buyers.
Debt service was covered. The bonds were paid off in full. The commission continued to operate the bridge from the toll revenue paid by road users. The men who built the bridge were paid from the construction budget funded by the $37 million.
The caisson workers on the Pennsylvania side, who breathed compressed air at a pressure thirty pounds per square inch above atmospheric in shifts of four to six hours, received the wage that compressed-air workers commanded in the construction market of 1922. The ironworkers who raised the steel towers 380 feet above the Delaware River were paid the ironworker’s wage set by the Philadelphia labor market of 1922. The laborers—the men who moved materials, maintained the site, and performed the tasks that required no recognized trade classification—were paid the common labor rate of 1922, which was below the ironworker’s rate by a margin reflecting their place in the work hierarchy. And the men at the bottom of that hierarchy were mostly those whose position in the larger social order of the 1920s confined them to the bottom of the work hierarchy as well.
Whether those wages were adequate for the work and the risk is a question the payroll records can inform but not answer definitively. The payroll records exist in the archives that hold the documents of the contractors who performed the construction work. Whether those records are accessible, and whether researchers have used them to reconstruct the details of the workforce, is not established in any of the widely circulated historical accounts of the bridge. The names of the commissioners and engineers appear in the administrative history of the Delaware River Joint Commission.
The name of Ralph Modjeski appears in the design records and in the professional literature of bridge engineering. The name of Calvin Coolidge appears in the records of the opening ceremony because the president pressed a telegraph key in Washington on July 1, 1926. The names of the caisson workers, the ironworkers, the cable-spinning crews, and the common laborers exist in the payroll records of the contractors who employed them, in whatever form those records have survived for a century. The bridge still stands.
It carries approximately 100,000 vehicles a day across 1. 8 miles of the Delaware River between Philadelphia and Camden. The 1,750-foot main span was the longest in the world for five years, until the George Washington Bridge opened in 1931 with a span of 3,500 feet, making the Delaware River Bridge’s record a historical footnote rather than a present reality. The Delaware River Bridge is now called the Benjamin Franklin Bridge, renamed in 1956 for a man who did not build it.
The cables are painted blue, and have been blue for most of the bridge’s life because the Delaware River Port Authority chose a color for repainting cycles, and that color became the bridge’s appearance for the people who live beside it, cross it, and watch it from the river. At the midpoint of the bridge, 380 feet above the water, the main cables built by the spinning crews wire by wire in 1924 and 1925 carry the weight of the deck and the traffic below them, and transfer it to the towers raised by the ironworkers in 1922 and 1923. From the towers, the load goes to the foundations that the caisson crews sank into bedrock in compressed chambers at the bottom of the river, in conditions the river did not provide willingly and the men did not enter without knowing the price. The river moves beneath the bridge.
The tide pushes in from Delaware Bay. The water is not the water of 1922. The concrete in the foundations is, and the steel in the cables is. The men who made it are not.
The bridge is open. Traffic continues. The commission operates it. The toll is five dollars for a passenger vehicle heading south.
The crossing takes about four minutes at the speed limit. At the midpoint of the bridge, you are 380 feet above the Delaware River on prestressed concrete slabs laid on structural steel girders suspended from cable suspenders woven from 18,666 individual galvanized wires spun across 1,750 feet of open air by men on temporary footbridges in the wind coming up the Delaware Valley in 1924. The cables are still under tension. The tension they hold is the weight of you and the weight of every vehicle crossing with you, and the weight of every vehicle that has crossed since July 1, 1926.
The cables have not failed. The foundations have not moved. The men who built them exist in the payroll records of the contractors the commission hired.
Their names are not on the bridge.
