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Volume 24, Number 5September/October 1973

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Bridge Across The Bosporus

One of the world’s longest suspension spans now links the Middle East to Europe.

Written and photographed by Robert Arndt

Within sight of the spot where King Darius built his bridge of boats in 512 B.C., leading 700,000 men west to war, another bridge, fulfillment of an age-old dream, now reaches across the Bosporus' waters to unify in peace a city, a country and two continents. It is no emperor, in this century, who is the builder of the first permanent link between Europe and Asia. It is a consortium of European firms in partnership with the Turkish Government: not war but commerce pencils this graceful 4900-foot arc across Istanbul's landscape. But nonetheless, the Bosporus bridge, Europe's longest single span, probably carries a load of emotion even weightier than the eight million vehicles expected to cross it every year.

The project is hardly a new one. After Darius, every emperor, every sultan, every miscellaneous local potentate probably thought of a bridge across Istanbul's "garland of waters." Leonardo da Vinci, who designed a bridge to cross the Golden Horn in 1502, must have looked at the far greater challenge of the Bosporus with longing and, ultimately, despair. French engineers proposed a masonry-arch bridge during the 18th century, and a different sort of longing from Leonardo's motivated a German bridge proposal in 1905. In the 1930's a European technical congress made plans to try as did, in 1953, the government of Turkey, but like the earlier plans they were not realized.

By 1963, however, two factors had changed. One was that Istanbul, on both banks of the Bosporus, had grown in 10 years as much as the 1953 planners had expected it to grow in the next quarter-century. On the two bridges across the Golden Horn, traffic had reached the saturation point; to cross the city by car could take two hours, or up to six if a ferry crossing of the Bosporus was involved. The other change was that the technology of bridge building had advanced unbelievably; all five large suspension bridges built between 1953 and 1963 had broken some record or inaugurated some advance in the precarious spidery art of hanging mid-air roadways on spun steel strands. Thus, when London's Freeman Fox and Partners, one of the most experienced and innovative firms in the field, was given the job of designing anew the Bosporus bridge, the dream could at last become a reality.

This time the bridge was to be the center section of a ring highway around Istanbul that ran from behind the old city, just outside the Theodosian land walls, almost to the sea of Marmara's Asian shore at Moda. The site chosen for the leap across the strait was not the narrowest crossing, the nearly 2,300 feet from Kandilli to Rumeli Hisar, but lay just short of a mile further south, at a point between Ortakoy on the European side and Beylerbeyi, in Asia, that would preserve the architectural and historic integrity of both parts of Istanbul. There, too, the hills that line the Bosporus on both sides leave a narrow shelf on the water's edge that can accommodate the bridge's main supporting towers, and the shape of the hills themselves makes a unique and economical bridge design possible.

The basic geometry of each of the world's eight great suspension bridges is the same, despite progressive improvement in design and method of construction since NewYork's George Washington Bridge was completed in 1931. Two high double towers frame the central suspended span; heavy steel main cables run across the river from one concrete and steel anchorage, embedded in the bedrock of the bank, to the peaks of the nearest tower, down and up in a swooping catenary curve to the far tower tops, and down again to the firm support of the opposite anchorage. All along the main cables, which are generally over a foot in diameter, slender hanger cables reach down at short intervals to the roadway, supporting it in mid-air across the distance to be spanned. Thus the downward weight of the roadway, and later of the traffic on it, is transferred by the vertical (or near-vertical) hangers to the main cables. There the pull becomes a horizontal one, tensing the main cables as they stretch from anchor age to anchorage, and tending to tug the towers into the stream from their tops. As long as the anchorages hold the cable ends firm, though, the horizontal tension 'tries' to pull the arching cables straight, driving the towers into the earth the way a bowstring drives an arrow. Thus the force becomes vertical again, and is easily borne by the towers on their concrete footings.

Forces other than the dead weight of roadway and traffic, however, have to be taken into account in the design of a suspension bridge. Wind forces blow the hanging roadway out of place, and since as much as 70 percent of that lateral force is transferred to the towers, they must be able to resist both indirect and direct wind pressure. Special circumstances—an earthquake or a uniquely heavy vehicle—must be allowed for, their effects calculated, and the structure designed to withstand those effects.

The Bosporus bridge fits this general description, of course, but it also has some design features unique in the small worldwide family of these spans. Since the main towers are on land, Freeman Fox was able to support the approach spans—the part of the roadway that leads from the anchorages up to the towers—on slim steel piers rather than by hanging them from the main cables. The total load on the cables was thus reduced by almost a third, and important savings in strength—and thus cost—could be made in the design of the towers and the anchorages. It is this feature, in fact, that brought the bridge so many puzzled second looks as its outlines were growing clear: the main cable backstays are innocent of the lacework of hanger lines that show faintly against the sky in the main span, and they hang in a different, tauter curve, carrying only their own weight.

The roadway itself is of a design used only once before in a suspension bridge. All large suspension bridges, up to and including Freeman Fox's Firth of Forth Bridge (completed in 1964), featured a roadway stiffened beneath by a jackstraw tangle of horizontal, vertical and diagonal girders called a truss—a method of achieving the necessary rigidity in a structure that has hardly changed since the Eiffel Tower. By contrast, the roadway of the Bosporus bridge is a unitary construction called a box girder. In this design, traffic will run on the top surface of a large hollow steel box, stiffened internally by lateral and longitudinal steel membranes. The box is roughly rectangular in cross-section, with cantilevered 'wings' on both sides to provide space for pedestrian walks, and the construction as a whole is fully 20 percent lighter than a truss-stiffened design. Here again, weight savings in the roadway have a multiplier effect that makes possible major savings in the strength and cost of other components of the bridge: the hangers, main cables, towers and anchorages. The smooth surface of the box girder also reduces the effect of wind on the structure—thus further savings—and makes maintenance a simple annual repaint of some 430,000 square feet of clean surface, rather than the unending job that it is in the wind-trap tangle of a truss-stiffened roadway.

Wind effects cannot be eliminated entirely, of course, especially here in the steep-sided Bosporus, a natural north-south wind funnel. Both the Russian north wind and the hot southern 'lodos' reach 60-mile-anhour force several times a year, measured at ground level; and the tops of the bridge towers, reaching above the shelter of the surrounding hills at their 540-foot height, are exposed to constant blasts. Though the roadway's cross section is aerodynamically stable, no shape 107 feet wide, 10 high and 3,521 feet long, hung in mid-air, can help being affected by winds: what must be avoided at all costs is the possibility of self-reinforcing oscillations that could build till they shake the bridge apart—the 'Galloping Gertie' effect that makes bridge designers wake up sweating. Wind tunnel tests of the roadway shape have eliminated that possibility for the Bosporus bridge, and Freeman Fox designers then added a further aerodynamic damping factor by drawing the normally vertical hanger cables at distinct angles to the vertical, the better to transmit horizontal forces to the main cables. The zigzag of fine diagonal lines at varying angles between roadway and main cables was another striking visual distinction of the Bosporus bridge as it took its final form.

Indeed, the bridge has been attracting attention since work was begun in April 1970. At that time the German tunnel and foundation experts Hochtief AG began to dig toward bedrock on the Bosporus shores. When they found it—55 to 78 feet down on the western (Ortakoy) side and 16 to 32 feet deep at Beylerbeyi—steel was placed and concrete poured to support the tower legs. At the same time, similar work proceeded at the anchorage sites. Each huge anchorage transfers a total of 30,800 tons of pull to the ground, and consists of a pair of chambers, in which the strands of the main cables splay apart, and a concrete block in which the ends of the strand are fixed. Each of the 19 strands in each main cable (plus four smaller strands that reach only from anchorage to tower top) is looped around a semicircular steel 'cable shoe.' Two huge bolts hold each shoe to a separate steel slab a yard high and almost a foot thick, and that in turn is held by 12 further bolts sunk 40 feet into the concrete block. Each chamber, with its anchor block—two chamber/block units make up each anchorage—is set in a separate trench in the hillside bedrock; the entire anchorage is unified by a concrete back wall. The Ortakoy anchorage, complete with the section of roadway that roofs it, contains 60,000 tons of concrete, and its analogue in Beylerbeyi, only somewhat less.

As the steel framework for the anchor blocks was taking shape at the anchorages, the legs of the bridge towers began to rise. Twenty-three by 17 feet at the base, each leg was assembled from prefabricated steel slabs made in Italy, now set on end and edge-welded together. As the finished height exceeded the reach of ground-level cranes, lifting cranes were attached to the legs themselves, and with each 60-foot increase in height the crane 'inchwormed' itself higher on the leg it was building. These remote-controlled cranes were themselves as remarkable as the bridge they were helping to construct. Each capable of lifting a 45-ton load and lowering it by fractions of a millimeter to precise position, they were so reliable that workers were not afraid to place their unprotected hands in the narrowing gap between steel slabs.

As the tower legs reached a height of 140 feet above their foundations (150 feet above the ground), a crosspiece, called a portal, was hoisted up to span the 92-foot gap between them and, by joining the legs into one unit, strengthen the entire construction. It is on the top surface of this 32-foot-high crossbeam that the roadway itself now rests, supported on huge expansion bearings. Another, lighter portal joined the legs of each tower at 330 feet and a third at 500, just below the saddles that capped the tower legs and held the main cables as the nock of an arrow holds the bowstring. When the legs had reached half height, single steel cables were strung from the legs to the anchorages over 750 feet away and tensioned to keep the legs from lashing under stress. At full height, another set of cables was added, and both sets were tightened enough actually to pull the huge steel towers off the vertical by some 14 inches at the top. Then the saddles were placed, not squarely on the 23- by 10-foot tower tops, but offset some 25 inches toward the anchorages to bring the total deflection to more than three feet.

This deflection from the vertical was the first of many necessary preliminaries to the most difficult and time-consuming part of the construction: the spinning of the main cables. Each of the huge cables would weigh 2,700 tons, and the pull of that weight on the towers, plus the gradual adjustment of the placement of the saddles, would reestablish the vertical stance the towers must have to be able to carry the roadway and its traffic without toppling. The morning of January 11, 1972, saw the Bosporus closed to shipping as a tiny bright-yellow barge towed an inch-thick steel cable across the strait from tower to tower; three other cables followed that afternoon and the next day, and the first connection between the European and the Asian towers was made. "City Hall's been trying to join the two halves of Istanbul for years," cracked one Turkish engineer. "We've done it in two days!"

After these cables were fastened at the tower tops and the anchorages, they were used as supports to carry further cables across aerially, until finally a dozen spiral steel strands spanned the Bosporus. Sheets of steel mesh were laid on the cables, slid out and fastened, and a pair of catwalks—steel-mesh troughs hanging in loose curves between the two upstream and the two downstream tower legs—took shape. For the first time since Darius, men could walk across the Bosporus.

More cables were then stretched from the base of each tower leg up to the catwalks' lowest point and across to the opposite legs. When these cables, the 'storm system,' were tightened, they formed a mirror image of the catwalks' curve, which they pulled lower and tighter. The gain in safety for the workers was considerable—previously, the midpoint of the catwalks had been lashing in 30-foot arcs as the wind blew—but the main purpose of the storm system was to tighten the catwalks into approximately the catenary curve that the much heavier cables would assume when spinning was completed. (Without this measure, as the cables grew and sagged under their own weight they would rest on and finally rip loose the catwalks.) Then with the most painstakingly precise measurement assuring accuracy, the first two galvanized steel wires of the main cables were stretched from anchorage to tower to tower to the opposite anchorage. Marked with black paint at intervals along their length, they became the respective guide wires for the two sets of nineteen 550-wire strands to be spun. Each five-millimeter wire in each strand of the main cables would lie in a precise, unchanging relationship to each other wire and to the guide wire for that cable, so that each strand of parallel, untwisted wires—and ultimately the whole main cable—would have the minimum of bulk and the maximum of strength.

The actual spinning of the more than 11,000 wires of each main cable took over three months to complete, a job that was simple in its essentials but—at least at first—agonizing in the details of its execution. Properly speaking, the word 'spinning' is a misnomer, since none of the wires or strands is twisted in any way. Rather, the wires are laid down in successive passes from one anchorage to the order of a large steel wheel which which travels like a cable car on an overhead system of moving wires. As a strand was begun, the end of five millimeter wire was temporarily fixed to the cable shoe at the back of the Ortakoy anchorage chamber, and a bight of the wire was laid around the rim of the wheel. As the wheel traveled from the Ortakoy anchorage toward Beylerbeyi, it drew wire from huge drums through counterweight towers that maintained the proper tension. As it moved, the wheel laid down a 'still' wire from its lower edge, which workers along the line carefully laid and levered into the proper position in the strand, reaching high to grasp the wire as the wheel rolled by overhead. The 'live' moving wire, feeding out from the anchorage over the top edge of the wheel, was laid into pulleys alongside the strand and allowed to run out freely to the traveling wheel. When the wheel arrived at the Beylerbeyi anchorage, the semicircle of wire on its rim was removed and looped around the proper cable shoe there. The empty wheel returned to the European side while its twin, working on another strand, ran loaded in the opposite direction; as it returned, workers lifted the now-static 'live' wire from its pulleys and eased it into place in the strand next to the 'still' wire laid down in the same pass. Arriving back at the Ortakoy anchorage, the empty wheel was reloaded with another bight of the same length of wire, and traveled out again. The end of the wire, after the last pass, was joined by a hydraulically clamped steel sleeve to the end left free at the cable shoe when the strand was begun, and the entire strand thus became a continuous loop; all 550 wires in it—actually one wire crossing back and forth 550 times— carefully clamped fast and adjusted for tension and position at each passage of the wheel over a tower top.

The first of the 38 main span strands to be spun took well over a week to complete, thanks to constant 'mis-lays' and backtracking; within a month, though, as the workers' experience and confidence increased, a full strand took only three days from start to finish, working in two shifts. When a strand was completed, spinning shifted to the other cable for three days, and the newly completed strand was 'tuned'—jacked back and forth across the tower-top saddles by infinitesimal amounts to bring the whole strand to exact position within the cable. So sensitive was this work and so precise the measurements that though each wire of the strand had already been separately adjusted when it was laid, the tuning of the strand was done in two separate sessions to allow any tensions to subside in the interim, and done after midnight, when uneven expansion induced by the sun's heat had had time to subside.

Meanwhile, about a half-mile north of the bridge, on a meadow on the Bosporus' Asian shore, roadway assembly had been going on since the early spring of 1972. Like the tower legs, the roadway had been partly prefabricated in Italy; as the pieces were welded together on the Goksu meadow, 60 sections of roadway took shape. Each section was the full final 109-foot width, with space for six lanes of traffic, two pedestrian walks, a center strip and roadside guard fences—but it was only 59 feet long. As successive sections were assembled, they were shifted on rails to loading positions in accordance with their final place in the bridge, and the welds were x-rayed for flaws. One of the lengths of rail on which the road sections were shifted jutted pier-like into the Bosporus, at a height which allowed a specially constructed barge to slip under the rails. The barge, an unadorned rectangle powered by four separate diesel engines driving independently steerable propellers, was to transport the road sections one by one down the Bosporus to the bridge. About 650 feet upstream it would let go an anchor, and, its propellers fighting the current, drop slowly down to exact position under the suspension cables.

While the barge crew made dry run after dry run, in all weathers, the lifting tackle that would hoist the road sections up to the hanger cables was prepared. Each set of tackle consisted of two rigid steel bridges that were, in effect, extensions of the climbing cranes that had built the tower legs, re-rigged to allow them to lift the 148-ton sections. One pair of bridges operated on the Ortakoy side of the midpoint of the cables, the other pair on the Beylerbeyi side; at 8 o'clock in the morning of December 7, 1972, the Ortakoy pair lowered its falls to the roadway section numbered ORT-1. Locking pins were hammered home to attach the falls to lift points welded to the structure of the section, and, extremely slowly, the crane began to take the strain. Ten minutes after lifting began, daylight showed between the road section and the supports it had been resting on, and the barge began to back out from under. Four hours later the roadway section was lifted to its final position just west of the midpoint of the cables and the hangers that would hold it there for the lifetime of the bridge were permanently attached.

To the spectators who, for 14 hours a day during the final stages of construction had camped out on the Ortakoy hillside, it looked as if a serious error had been made: section ORT-1 now hung from the main cables fully 130 feet too high. No such worries affected Freeman Fox engineers or the Turkish workers —largely German-trained—whose muscle and sweat were actually building the bridge. They knew that as further road sections were added, the main cables, huge though they were, would gradually give until, with the full 9000-ton weight of the roadway in place, the predicted 210-foot clearance above the Bosporus current would remain—room with a little to spare to allow passage to the aircraft carrier Enterprise and the liner Queen Elizabeth II, the tallest ships afloat.

In the course of the new year's first months, the roadway sections were hung in sequence from the midpoint toward both towers: the second section in place was BEY-1, the third ORT-2, and so on. Finally, on March 26, 1973, the last section, flag-decked and wet with champagne, was lifted and hung, and attention shifted to the approach viaducts that would carry the highway from its jumping-off place at the anchorages to the towers where the suspended roadway began.

The viaducts were somewhat behind schedule, thanks to a March storm that swept several great steel beams into the Marmara, but after frogmen and cranes fished them out, a second pair of cable-straddling cranes installed the 30-ton spans atop their slender steel piers. When these girders met the steel work of the towers at the end of April 1973, the main structure of the Bosporus bridge was complete.

There was, however, no shortage of work ahead. The roadway deck had yet to be asphalted. The main cables had to be wrapped and painted. French-built, 20-person elevators to carry pedestrians up through the tower legs to walkways 187 feet above the ground were still missing. Approaches had still to be constructed and paved. As a result officials of Turkey's General Highways Administration were this summer, still undecided whether they could officially open the bridge during the October celebrations marking the Turkish Republic's 50th anniversary.

Ceremonies, however, will add little to what has been achieved here in the past three and a half years. Despite inertia, politically motivated threats of violence, harsh weather and the immense difficulty of the task, there looms now over the Bosporus a bridge too high for understanding, too long for perspective; a structure 10 stories high that strides across the skyline; a presence that commands without dominating, that broods over and joins two continents, lightly spanning the ancient strait and the dreams of 30 centuries.

Robert Arndt, who emigrated to America from Turkey as a child, is a photographer and free-lance writer living in Istanbul.

This article appeared on pages 8-17 of the September/October 1973 print edition of Saudi Aramco World.


Check the Public Affairs Digital Image Archive for September/October 1973 images.