BRIDGES

BRIDGES . All over the world, in different religions and cultures, there are vivid descriptions of a perilous way that the dreamer, the ecstatic visionary, or the deceased has to follow on his journey to the otherworld. One of the perils may be a bridge leading across a chasm, a rapacious stream, or the void. Success in crossing the bridge may depend on the traveler's own behavior during life or on the sacrifices he or his surviving relatives have performed. Ethical qualifications are not always needed.

Parallel with these eschatological ideas, actual bridge-building on earth has been connected with sacrifices and with religious, folkloric, and magical conceptions. At times, the construction of a tangible bridge—whether for day-to-day use or for ritual use only—is related to the soul's passage to the afterlife. Finally, the bridge in itself has often been a very useful symbol to signify the transcendence of the border between two realms or the ascension from a lower to a higher dominion.

History

One of the striking characteristics of the bridge as symbol is its universality among traditions from all over the world.

Indo-Iranian religions

In the Hindu religion, from the Ṛgveda (9.41.2) onward, the bridge occurs as a link between earth and heaven, the world of illusion (māyā ) and reality. The sacrifice terminology and the lofty speculations of the Upanisads use it in a figurative rather than a literal sense, though the popular imagination might suppose the latter to be the case. Just as the gods enter the heavenly world by means of the "southern fires" (dakṣiṇā s) in the Agnicayana ritual, using them as steps and ladders, so the sacrificer "crosses a bridge and enters into the world of heaven" (Yajurveda, Kāṭhakam 28.4). It is necessary to make a ladder or a bridge with sacrificial gifts in order to ascend into that heavenly realm.

On the other hand, this luminous brahman world is attained by recognition of ātman, the spiritual reality. To the extent that this universal self is conceived of as a person, God himself is called "the highest bridge to immortality" (Svetāśvatara Upaniṣad 6.19). That bridge, God, in itself is not ethical, but all evil is excluded from the brahman world: "Therefore, if blind people go over the bridge they receive their sight, if wounded, they are cured; therefore, if the night crosses the bridge, it is turned to day" (Chāndogya Upaniṣad 8.4.1).

According to Herman Lommel (1930, pp. 264ff.), the greatest significance of the ancient Indian bridge is that it holds two worlds, heaven and earth, apart. But, speaking of the Nāciketas fire—"the bridge of the sacrificers to that eternal highest brahman "—Lommel does not mention some later lines where the road is described as the "sharp edge of a razor, difficult to pass over" (Kāṭhaka Upaniṣad 3.14). The bridge, however, is not explicitly mentioned in this connection.

A comparison of the Indian and Iranian sources reveals common as well as differing conceptions of the bridge and of the whole structure of ideas to which it belongs. The paradisiacal delights of the virtuous are pictured in a very concrete way in Kauṣītaki Upaniṣad 1.4: "Five hundred nymphs [Apsara s] go to meet him, one hundred with fruit in their hands, one hundred with ointment in their hands, one hundred with wreaths, one hundred with raiment, one hundred with fragrant powder in their hands" (Lommel, p. 270).

Space does not allow a thorough account here of the Iranian sources with their various chronologies, varying content, and difficulties of philological interpretation (cf. Lommel, pp. 263ff., with Nyberg, 1966, pp. 180ff.). Some general ideas may be summarized, however, with the help of a present-day specialist.

In ancient Iran, the ceremonies of the first three days after death were regarded as very important for the soul of the dead person. It had to be protected against evil powers and had to be strengthened before the dangerous journey to the otherworld. Originally, at least, princes, warriors, and priests might hope to come to a luminous paradise with all its delights. The "crossing of the Separator" (Av., Chinvatō Peretu) was imagined as passing over a bridge that began on top of Mount Harā and ended on the road to Heaven. Only worthy souls, perhaps those who had given rich offerings, could reach the heavenly way; the others fell down into the subterranean Hell (Boyce, 1979, pp. 13ff.).

Zarathushtra taught that everyone had the possibility of gaining Paradise and that the successful passing of the bridge depended on the moral qualities of the departed, not on social rank or costly sacrifices. Three godly judges weigh the soul's good and bad thoughts, words, and deeds. If the good are heavier, the bridge is made broader, and the soul can pass, accompanied by a beautiful maiden, Daēnā, its own heavenly double or good conscience. Otherwise, the bridge gets as narrow as a blade edge, and the soul is propelled into the place of torment by an ugly hag (Boyce, 1979, p. 27). The classical sources of this short abstract are the ritual law contained in the Younger Avesta, Vendidad 19.28–32, combined with the fragment of the Hadhōkht Nask belonging to the same canon, composed in pre-Christian times and supplemented with later Pahlavi literature, from the ninth century ce (Mēnōg i Khrad 2; Bundahishn 30), as quoted in detail by Nyberg (pp. 180ff.; cf. Duchesne-Guillemin, 1973, pp. 333ff.).

Judaism and Islam

Probably through Iranian influence, a similar conception appears in Judaism. It has been mistakenly cited as existing in the Jewish apocalypse of Ezra (2 Esdras 7:8ff., end of first century ce). There, according to Silverstein (1952, pp. 95f.), the bridge, whose width accommodates only a single person's feet, becomes broader when the righteous cross it and narrower for the sinners. But this passage actually refers to two different ways (not bridges), one belonging to this earthly world, the other to the heavenly one. The ordeal of crossing the bridge over the Valley of Jehoshaphat, which according to Jewish eschatology occurs in the Last Judgment, is of a later date.

The corresponding eschatological ideas in Islam also seem to be dependent on Iranian tradition, perhaps with a Jewish intermediary. The Islamic name of this bridge is Ṣirāṭ, which simply means "way." Thus it has been possible to discover the bridge in Qurʾanic passages concerning the afterlife that refer only to a way (36:66, 37:23f.). But the later tradition (ḥadīth ) describes a real bridge, "thinner than a hair and sharper than the edge of a sword," which leads the dead over Hell's uppermost part, Jahannam. Out of compassion or in recognition of the good deeds of the person concerned, God makes it possible for believers to pass over the bridge. The just and those who have received forgiveness come over without mishap, while sinful Muslims plunge down into Hell and remain there for a limited time in a sort of purgatory. Unbelievers, however, remain in those portions that function as places of punishment. Gabriel stands before the bridge and Michael upon it; they question those who pass over about the lives they have led (Gardet, 1968, p. 87).

According to a tradition that goes back to Ibn Masʿūd, everyone must cross the bridge. In accordance with their works, they do it more swiftly or more slowly: as the wind, as a bird, as a fine horse or a camel, as a running person, or as a person walking only on the big toes, who is immediately shaken off into the fire by the sharp, slippery bridge bristling with barbs. In his passage, the walker is also attacked by angels with fiery pitchforks (Jeffery, 1962, p. 247). The bridge may be arched, ascending for a thousand years, running level for a thousand, and descending for a thousand years. In this tradition, the gate of Paradise is opened only if the deceased gives the right answers. Then he is accompanied over the now soft and level bridge by an angel (Coomaraswamy, 1944/1945, p. 203, with references).

Christianity

In the literature of classical antiquity and in the Bible, no soul-bridge is known. But the classical writer of the Syrian church, Ephraem (fourth century), speaks of the cross of Christ as a bridge leading over the terrible abyss with its menacing fire. This river or sea of fire as an obstacle on the journey of the soul can be transcended in other ways, too; the righteous may even pass through it without being damaged. Because of their vividly striking descriptions, it is sometimes difficult to say whether these passages are to be taken literally or figuratively (Edsman, 1940, pp. 121ff.; cf. pp. 52ff.).

In medieval Russian spiritual songs, which represent popular religion, the language is very realistic (Edsman, 1959, pp. 106ff.). In medieval times, the world of folk imagery knew well the perilous bridge. It also has a place in the literature of Christian visionaries, with its roots in the ancient church's rich outpouring of apocalyptic descriptions of the hereafter. To a great extent, these descriptions are found in the extracanonical apocrypha; these, in turn, are descended from Judaism and Iran.

However, the classic Christian image of a bridge, which has been very influential throughout history, is contained in the Dialogues of Gregory I (c. 540–604). The framework is the same as in the famous vision of Er in the last book of Plato's Republic: one who seems to be dead revives and tells those around him what he has experienced. In Gregory's story, it is a certain soldier who tells of a bridge under which a dark and stinking river runs, but that leads to the heavenly green meadows and shining mansions inhabited by men in white clothes. Any wicked person who tries to go over the bridge tumbles down into the river, whereas the righteous pass safely across. The soldier also saw a fight between angels and demons over a person who had slipped on the bridge so that half his body was hanging over the edge (Dialogues 4.36). As Howard Rollin Patch (1950, pp. 95f.) points out, Gregory quotes Matthew 7:14 as part of his interpretation: "For very narrow is the path which leads to life." This quotation, in turn, demonstrates the technique of combining biblical and extrabiblical source material.

The bridge motif occurs in many different categories of medieval literature and belongs to Celtic and Germanic mythology also (Patch, 1950; Dinzelbacher, 1973). It can have greater or lesser importance in the Christian representations of the general topography of the otherworld, where the account of the soul's union with the Savior on the other shore may outweigh the description of the horrors on the way (Dinzelbacher, 1978). Moreover, the eschatological ideas were combined with the practical construction of a bridge; this was considered a pious work that was also helpful for the future fate of the builder. Frequently, a bridge had its own chapel for prayers and often its own hospital. Papal and episcopal indulgences encouraged such construction. Consequently, legacies became common, and bridge-building brotherhoods were founded beginning at the end of the twelfth century (Knight, 1911, p. 856; Boyer, 1967, p. 798).

Buddhism

That the soul-bridge also turns up in Buddhism is hardly surprising, since Buddhism is a daughter religion of Hinduism. As conceptions of an afterlife influenced by Islam can be found in the great stretches of Central Asia into which the Muslim religion has penetrated (Paulson, 1964, 152f.; cf. Eliade, 1964, pp. 482ff.), so a corresponding eschatology has followed Buddhism into East Asia. But here a new feature is observed. Even though people have tried in various ways in the different religions to affect the fate of deceased persons (for example, during the Christian Middle Ages, by accomplishing the actual construction of a bridge as a spirit-gift for the deceased), it is only in "northern" Buddhism that there is found a comprehensive symbolic bridge-building ceremony combined with the funeral. One of the classical Sinologists, J. J. M. de Groot, has written in careful detail about the Buddhist rites for the dead in Amoy, which lies in Fukien opposite Taiwan (1885, pp. 97ff.). According to de Groot, the bridge ceremony that takes place in connection with these rites is based on a quotation from a relatively late description of Hell. According to the latter, no less than six bridges of different materials lead from the underworld to the world of rebirth. There, the souls are sorted out, and their impending fates in the six different forms of existence are decided in detail.

The rites for the dead are intended to help the souls over some of these bridges. If a deceased person has not completely atoned for his crimes by enduring the torments of Hell or has not, through the actions of clergy, been freed from his remaining punishment, he has to plunge down into the pit, which is filled with snakes and writhing monsters. Therefore, in the room where the rites for the dead are being carried out, the priests build a temporary bridge out of boards that are laid upon chairs or out of a long bench without a back. The soul-bridge is also provided with railings of bamboo and cloth or paper, sometimes even with an overhead canopy. As soon as the ceremony, which is called "the beating of Hell," is completed, they undertake "the crossing of the bridge." The happy completion of this is reported to the powers of the underworld, so that they will not be able to hinder the soul in its progress.

Just as the variations in such rituals are numerous, the afterlife concepts that lie behind them also change. Thus, Nai-ho Bridge ("the bridge without return") is also found in the popular color illustrations that the Jesuit father Henri Doré reproduced in his instructive and comprehensive work on what he calls "superstitions" in China (1914, pp. 194f., fig. 52). The text quoted by Doré describes the picture exactly: souls receive the wine of forgetfulness from the ten underworld judges before they continue to what is also called the Bridge of Pain, which goes over a red foaming stream in a hilly region. When the souls have read a text on the conditions of existence, they are seized by two devils, Short Life and Quick Death, who hurl them into the stream. They are swept out through the stream into new existences to live as men, four-footed animals, birds, fish, insects, or worms. As late as after World War II such ceremonies were carried out both in mainland China (Hsu, 1948, p. 165) and on Taiwan.

Ritual Sacrifices

The Latin word pontifex is composed of the noun pons and the verb facere, and it signifies "he who makes or builds bridges." One cannot prove historically that the incumbents of this ancient Roman office literally had such a function. However, the etymology, which existed as early as the Roman librarian and scholar Varro (116–27 bce), is disputed both in antiquity and among modern researchers. Evidently an old Indo-European meaning is hidden in pons, giving it the sense of a "path" or "way," not necessarily over a river (Szemler, 1978, cols. 334ff.).

Discussing the various interpretations of the term pontifices and rejecting that of "bridge builders," the Greek writer Plutarch (b. 46?–d. around 119 ce) gives us the arguments of those who defend that theory: the name refers to the sacrifices performed at the very ancient and sacred bridge Pons Sublicius over the Tiber, which were necessary to prevent a sacrilegious demolition of the entirely wooden construction (Numa 9.2). The Roman poet Ovid (43 bce–17 ce) makes reference to this ceremony, at which the Roman high priest, pontifex maximus, officiated together with the first Vestal: "Then, too, the Virgin is wont to throw the rush-made effigies of ancient men from the oaken bridge" (Fasti 5. 621f.). This ancient festival in the middle of May is also mentioned by another historian, Dionys of Halikarnassos (fl. 20 bce). Dionys already understands that the puppets are a substitute offering for men (1.38.3).

The purpose of this and corresponding sacrifices is interpreted in different ways by both ancient and modern authors. According to James G. Frazer, the river god must be propitiated when humans intrude into his domain and transcend a border. Or, as Eliade (1957) explains, any building, to withstand its hardships, ought to have a life and soul that are transferred to it by a bloody sacrifice.

A Greek folk song, "The Bridge at Arta," which speaks of this matter, has become famous. The song describes how people keep on building the bridge for three years, but the last span is never finished because what is built by day collapses by night. When the builders begin to complain, the demon or genius loci, perhaps originally the river god, lets his voice be heard: he tells the people that unless they sacrifice a human life, no wall is securely founded. They are not, however, to give an orphan child, a traveler, or a foreigner but instead the construction foreman's beautiful wife. From one of the Ionian islands, Zante (Gr., Zákinthos), there is the tale that, as late as the second half of the nineteenth century, the people had wanted to sacrifice a Muslim or a Jew at the building of the more important bridges. There is also a legend that a black person was walled up in an aqueduct near Lebadea in Boeotia (Lawson, 1910, pp. 265f., 276f.; Armistead and Silverman, 1963). In 1890, China's department of public works paid the price of ten pounds for a human bridge sacrifice, if one is to believe a highly respected English reference work (Knight, 1911, p. 850). In Western countries sacrificial ceremonies at the building of bridges have survived as only partly understood reminiscences; they take the form of children's games (Knight, 1911, p. 852; Edsman, 1959).

Symbolism

In Christian metaphorical language, life is likened to a pilgrimage. One is not to become so captivated with the joy of traveling, whether by wagon or ship, that one forgets the destination. It is a matter of using the world, not of enjoying it. Augustine conveys this theme in On Christian Doctrine (1.4), while in his discourses on the Gospel of John (40.10) he speaks of the world as a lodging where one has temporarily stopped over during one's journey.

This metaphor can easily be reformulated using the bridge symbol. One can then consider a saying of Jesus that is lacking from the New Testament, a so-called agraphon, that has survived in Islamic tradition. It is best known through the inscription that, in 1601, Emperor Akbar caused to be affixed at the chief entrance to the great mosque in Fathepur Sikri in North India: "Jesus, peace be upon him, has said: 'The world is a bridge, walk over it, but do not sit down on it.'" The saying can be traced through Islam as far back as the seventh century (Jeremias, 1955).

Among the extremes of modern psychoanalytical interpretations of the bridge is the Freudian-inspired theory that the bridge constitutes a phallic symbol, with all that suggests about sexual fantasies, castration complexes, and incest (Friedman, 1968). A different interpretation is found in the writings of Hedwig von Beit, who was inspired by C. G. Jung to apply his categories to research into fairy tales. The bridge, which divides two land areas, would thus reflect a psychic situation in which a gap in consciousness occurs or where a transition is occurring to another area. It is at just such a point that the "demons" of the unconscious are free to make an appearance (Reimbold, 1972, pp. 66f., pp. 71f.).

Mircea Eliade, who has also been influenced by Jung, gives a phenomenological interpretation of the bridge symbol in the initiation of shamans among the Mongolian Buriats of southern Siberia. A climbing ceremony is involved in which the candidate climbs nine birches that are tied together with a rope and called a "bridge." Eliade interprets the red and blue ribbons, which further bind this arrangement with the yurt, as a symbol of the rainbow. This would lend support to the interpretation that Eliade gives to the whole ceremony: it is a visualization of the shaman's heavenly journey, his rite of ascension. Therefore, the initiation of the Mongolian shaman can be connected with the crossing of the Chinvat Bridge in ancient Iranian eschatology, which also constitutes a test or an initiation. But both pertain to an even larger framework: the reinstitution of the paradisiacal antiquity when humans and gods could converse with each other without difficulty, thanks to the bridge that then connected them (Eliade, 1964, pp. 116ff.; cf. Berner, 1982). Eliade has treated this theme in his fiction, in a tale entitled "Bridge" (1963) included in his collection Phantastische Geschichten (1978), as Berner has pointed out. Eliade's critics, in turn, consider a hermeneutic of this kind fantastic; other specialists (e.g., Blacker, 1975) have found his interpretation confirmed by their own material.

See Also

Chinvat Bridge; Pontifex.

Bibliography

Armistead, Samuel G., and Joseph H. Silverman. "A Judeo-Spanish Derivate of the Ballad of the Bridge of Arta." Journal of American Folklore 76 (January-March 1963): 16–20. Contains a rich bibliography.

Berner, Ulrich. "Erforschung und Anwendung religiöser Symbole in Doppelwerk Mircea Eliades." Symbolon 6 (1982): 27–35.

Blacker, Carmen. "Other World Journeys in Japan." In The Journey to the Other World, edited by Hilda R. Ellis Davidson, pp. 42–72. Totowa, N.J., 1975.

Boyce, Mary. Zoroastrians: Their Religious Beliefs and Practices. Boston, 1979.

Boyer, Marjorie Nice. "Bridgebuilding." In New Catholic Encyclopedia, vol. 2, p. 798. Washington, D.C., 1967.

Coomaraswamy, Luisa. "The Perilous Bridge of Welfare." Harvard Journal of Asiatic Studies 8 (1944/1945): 196–213.

Dinzelbacher, Peter. Die Jenseitsbrücke im Mittelalter. Vienna, 1973.

Dinzelbacher, Peter. "Ida von Nijvels Brückenvision." Ons Geestelijk Erf 52 (June 1978): 179–194.

Doré, Henri. Recherches sur les superstitions en Chine, vol. 2. Shanghai, 1914.

Duchesne-Guillemin, Jacques. La religion de l'Iran ancien. Paris, 1962. Translated as Religion of Ancient Iran (Bombay, 1973).

Edsman, Carl-Martin. Le baptême de feu. Uppsala, 1940.

Edsman, Carl-Martin. "Själarnas bro och dödens älv." Annales Academiae Regiae Scientiarum Upsaliensis 3 (1959): 91–109.

Eliade, Mircea. "Bauopfer." Die Religion in Geschichte und Gegenwart, 3d ed., vol. 1, p. 935. Tübingen, 1957.

Eliade, Mircea. Shamanism: Archaic Techniques of Ecstasy (1951). Rev. & enl. ed. New York, 1964.

Friedman, Paul. "On the Universality of Symbols." In Religions in Antiquity: Essays in Memory of Erwin Ramsdell Goodenough, edited by Jacob Neusner, pp. 609–618. Leiden, 1968.

Gardet, Louis. Islam (1967). Cologne, 1968.

Groot, J. J. M. de. "Buddhist Masses for the Dead." In Actes du Sixième Congrès International des Orientalistes tenu en 1883 à Leide, vol. 4, pp. 1–120. Leiden, 1885.

Hsu, Francis L. K. Under the Ancestors' Shadow: Chinese Culture and Personality (1948). Enl. ed. Stanford, 1971.

Jeffery, Arthur, ed. A Reader on Islam: Passages from Standard Arabic Writings Illustrative of the Beliefs and Practices of Muslims. The Hague, 1962.

Jeremias, Joachim. "Zur Überlieferungsgeschichte des Agraphon: Die Welt ist eine Brücke; Zugleich ein Beitrag zu den Anfängen des Christentums in Indien." Nachrichten der Akademie der Wissenschaften in Göttingen 4 (1955): 95–103.

Knight, G. A. Frank. "Bridge." In Encyclopaedia of Religion and Ethics, edited by James Hastings, vol. 2. Edinburgh, 1911.

Lawson, John Cuthbert. Modern Greek Folklore and Ancient Greek Religion: A Study in Survivals (1910). Reprint, New York, 1964.

Lommel, Herman. "Some Corresponding Conceptions in Old India and Iran." In Dr. Modi Memorial Volume, pp. 260–272. Bombay, 1930.

Nyberg, H. S. Die Religionen des alten Iran. 2d ed. Osnabrück, 1966.

Patch, Howard Rollin. The Other World According to Descriptions in Medieval Literature. New York, 1950. See the index, s. v. Bridge.

Paulson, Ivar. "Jenseitsglaube der finnischen Völker: In der wolgafinnischen und permischen Volksreligion." Arv 20 (1964): 125–164.

Reimbold, Ernst T. "Die Brücke als Symbol." Symbolon 1 (1972): 55–78.

Silverstein, Theodore. "Dante and the Legend of the Miʿraj: The Problem of Islamic Influence on the Christian Literature of the Otherworld." Journal of Near Eastern Studies 11 (1952): 89–110, 187–197.

Szemler, G. J. "Pontifex." In Real-encyclopädie der klassischen Altertumswissenschaft, supp. vol. 15, cols. 331–396. Munich, 1978.

Carl-Martin Edsman (1987)

Translated from Swedish by David Mel Paul and Margareta Paul

Bridges

Forces acting on a bridge

Dynamic loads

Model testing

Types of bridges

Cantilever bridges

Trusses

Arch bridges

Suspension bridges

Pontoon bridges

Movable bridges

Resources

Bridges are structures that join two otherwise inaccessible points of land, such as the two shores of a river or lake, or the two sides of a canyon or deep gully. Bridges are designed to carry railroad cars, motor vehicle traffic, or foot travel by pedestrians

and/or animals, or to support pipes, troughs, or other conduits used for the movement of goods and materials, such as an oil pipeline or a wateraqueduct.

Humans have been constructing bridges since ancient times. The earliest bridges were probably nothing more than felled trees used to cross rivers or ditches. As civilization advanced, artisans discovered ways to use stone, rock, mortar, and other naturally occurring materials in the construction of longer and stronger bridges. Finally, as physicists and engineers began to develop the principles underlying bridge construction, they incorporated other materials such as iron, steel, and aluminum into the bridges they built.

Bridges can be classified in a number of different ways, according to their intended use (railroad bridge or pedestrian walkway, for example), according to the material of which they are made (steel, wood, or concrete), or according to whether they are fixed or moveable. Moveable bridges are used when the height of ships traveling on a waterway will be greater than the bridge floor. In such cases, the bridge is built so that the roadway can be raised or pivoted to allow marine traffic to pass under or through it. Probably the most useful way to classify bridges for technical purposes, however, is according to their structural form. There are three major types of bridges: arch, cantilever, and suspension.

Forces acting on a bridge

Three kinds of forces operate on any bridge: the dead load, the live load, and the dynamic load. The first of these terms refers to the weight of the bridge itself. Like any other structure, a bridge has a tendency to collapse simply because of the gravitational forces acting on the materials of which the bridge is constructed (i.e., the wood, concrete, steel, or aluminum). The second term refers to traffic that moves across the bridge as well as normal environmental factors such as changes in temperature, precipitation, and winds. The third factor refers to environmental factors that go beyond normal weather conditions, factors such as sudden gusts of wind and earthquakes. All three factors must be taken into consideration in the design of a bridge.

For example, suppose that it is necessary to build a bridge across a span that is 325 feet (100 m) wide. It would not be possible to build a beam bridge, one that consists of a single slab of steel of that length. The weight of the material used to construct the bridge plus the weight of the traffic on the bridge would be too great for the bridge to remain standing. An engineer would have to design some other kind of bridge—an arch or suspension bridge, for example—that would be able to hold up that amount of weight.

Dynamic loads

Dynamic loads can present special problems for the designer. A bridge must be able to withstand not only the forces of normal everyday traffic, but also unusual forces of unexpected magnitude. In California, as an example, bridges require special kinds of reinforcement to withstand possible earthquakes. The fact that engineers have not completely solved the problems presented by dynamic loads is reconfirmed from time to time. For example, during the 1989 earthquake in the San Francisco Bay area, a section of the San Francisco–Oakland Bay Bridge collapsed, leaving a gaping hole. A freeway overpass in Oakland also failed during the earthquake, taking the lives of about two dozen motorists.

Wind gusts have been responsible for a number of bridge failures in the past. Even if wind speeds are relatively low, dynamic loads may become too great for a bridge to withstand. One reason for this phenomenon is that the bridge may begin to vibrate so violently that it actually shakes itself apart. Such was the case, for example, with the Tacoma Narrows Bridge in 1941. On November 7 of that year, with wind speeds registering only about 40 mph (25 km/h), the bridge vibrated so badly that it collapsed. The actual force experienced by the bridge was considerably less than the dead and live forces for which it had been designed. But the oscillations produced by wind gusts on the day in question were sufficient to shake the bridge apart.

As a result of failures such as those in the Bay Bridge and the Tacoma Narrows Bridge, engineers have developed methods for making bridges more aerodynamically sound. For example, lighter materials arranged in geometric structures that are more aerodynamically stable are now used in bridges where earthquakes, wind gusts, or other unusually severe environmental problems can be expected.

Model testing

At one time, the only test of a bridge design was actual use. Engineers could incorporate all the scientific knowledge and technological craftsmanship they had to produce a sound design. But how well the bridge would stand up under actual use and dynamic loads could only be discovered in the real world.

Today, engineers have two powerful tools with which to test their ideas: wind tunnels and computer-aided design (CAD). Wind tunnels have long been used by aeronautical engineers to test aircraft designs. Now they are routinely used also for bridge designs. A wind tunnel is an enclosed space in which rapidly moving air from giant fans passes over the model of a bridge. Possible structural and design problems can be detected by photographing and studying patterns of air movement over the model.

As in so many other fields, bridge design has benefited greatly from the growth and development of computer programs. Such programs can incorporate huge amounts of information about various ways in which bridges and the materials of which they are made will react to various kinds of stresses. CAD can be used to draw and test bridge designs on screen without even making the models needed for wind tunnel testing.

Types of bridges

The simplest type of bridge corresponds to the felled tree mentioned above: a single piece of material that stretches from one side of the gap to be bridged to the other. That piece of material—the beam, or girder—rests directly on the ground on each side or is supported on heavy foundations known as piers. The length of a beam bridge of this kind is limited by the weight of the beam itself plus the weight of the traffic it has to bear. Longer beam bridges can be constructed by joining a number of beams to each other in parallel sections.

The concept of a beam bridge can be extended to make a stronger product, the continuous bridge. A continuous bridge differs from a beam bridge in that the latter has at least one additional point of support beyond the two found in a beam bridge. The longest existing continuous bridges now in use are the Astoria Bridge that crosses the Columbia River near Astoria, Oregon, and the Oshima Bridge that connects Oshima Island to the mainland in Japan.

Cantilever bridges

A cantilever bridge is a variation of the simple beam bridge. A cantilever is a long arm that is anchored at one end and is free to move at the opposite end. A diving board is an example of a cantilever. When anchored firmly, a cantilever is a very strong structure. Imagine a 200-pound (91 kg) man standing on the end of diving board. The board bends only slightly, showing that it can hold a relatively large weight (the man).

A cantilever bridge consists of three parts: the outer beams, the cantilevers, and the central beam. Each of the outer beams of the bridge is somewhat similar to a short-beam bridge. The on-shore edge of the bridge is attached to the ground itself or to a pier that is sunk into the ground. The opposite edge of the outer beam is attached to a second pier, sunk into the ground at some distance from the shore.

Bridge piers are vertical columns, usually made of reinforced concrete or some other strong material. In many cases, they are sunk into massive supporting structures known as abutments. Abutments are constructed in large holes in the ground, in contact with bedrock if possible, to withstand the forces created by the dead and live loads created by bridges and the traffic they carry.

Also attached to the off-shore pier is one end of a cantilever. The free end of the cantilever extends outward into the middle of the gap between the shores. An incomplete cantilever bridge consists, therefore, of two halves—one anchored to each side of the gap to be bridged and consisting of a cantilever facing toward the middle of the gap. The space between the two cantilevers, finally, is bridged by another beam, similar to that of a short beam bridge joining the two cantilevers to each other.

The distribution of forces in a cantilever bridge is fairly straightforward. The dead and live loads are borne by the two sets of piers that hold up the bridge, the outermost piers that hold up the outer edges of the bridge, and the inner piers that anchor the fixed end of the cantilever.

The two longest cantilever bridges in the world are the Forth Railway Bridge in Scotland, completed in 1890, and the Quebec Bridge in Canada, built in 1917. The former is 1,700 feet (520 m) in length and the latter, 1,800 feet (550 m) long.

Trusses

The strength of a cantilever bridge can be increased by the use of trusses. A truss is structure that consists of a number of triangles joined to each other. The triangle is an important component of many kinds of structures because it is the only geometric figure that cannot be pulled or pushed out of shape without actually changing the length of one of its sides. By combining a number of triangles into a single unit, the unit is given a great deal of strength.

The cantilever beam, end beams, and joining beams in a cantilever bridge are often strengthened by adding trusses to them. The trusses act somewhat like an extra panel of iron or steel, adding strength to the bridge with relatively little additional weight. The open structure of a truss also allows the wind to blow through them, preventing additional stress on the bridge from this factor.

Trusses are used not only in cantilever bridges, but in all other kinds of bridges also. In fact, you have probably noticed the complex pattern of intersecting triangles on bridges over which you have passed. These truss patterns are one of the most efficient ways of adding strength to any type of bridge an engineer designs.

Arch bridges

As its name suggests, the main supporting structure in an arch bridge is one or more curved elements. The dead and live forces that act on the arch bridge are transmitted along the curved line of the arch into abutments at either end. These abutments are sunk deep into the earth, into bedrock if at all possible.

They are, therefore, essentially immovable and able to withstand very large forces exerted on the bridge itself. This structure is so stable that piers are generally unnecessary in an arch bridge.

The deck of an arch bridge can be placed anywhere with relationship to the arch: on top of it, beneath it, or somewhere within the arch. The deck is attached to the arch by vertical posts (ribs and columns) if the deck is above the arch, by ropes or cables (suspenders) if the deck is below the arch, and by some combination of the two if the deck is somewhere within the arch.

Most arch bridges today are made either of steel or of reinforced concrete. The longest existing steel arch bridge is the New River Gorge Bridge in Fayetteville, West Virginia, built in 1977. It is 1,700 feet (518 m) long. The longest reinforced concrete bridge is the Jesse H. Jones Memorial Bridge at the Houston Ship Channel, Texas, with a length of 1,500 feet (455 m).

Suspension bridges

The longest bridges in the world are suspension bridges. Some examples are the Humber Bridge in Hull, England, with a length of 4,626 feet (1,410 m), the Verrazano Narrows Bridge in lower New York Bay (4,620 feet [1,298 m]); the Golden Gate Bridge over the entrance to San Francisco Bay (4,200 feet [1,280 m]); and the Mackinac Straits Bridge connecting the Upper and Lower Peninsulas of Michigan (3,800 feet [1,158 m]).

In a suspension bridge, the dead and live loads are carried by thick wire cables that run across the top of at least two towers and are anchored to the shorelines within heavy abutments. In some cases, the bridge deck is supported directly by suspenders from the cables, while in other cases, the suspenders are attached to a truss, on top of which the deck is laid. In either case, the dead and light load of the bridge is transmitted to the cables which, in turn, exert stress on the abutments. That stress is counteracted by attaching the abutments to bedrock.

The towers in a suspension bridge typically rest on massive foundations sunk deep into the river or sea bed beneath the bridge itself. The wire cables that carry the weight of the bridge and its traffic are made of parallel strands of steel wire woven together to make a single cable. Such cables typically range in diameter from about 15 inches (38 cm) to as much as 36 inches (91 cm). Smaller cables can be ordered from a factory, while thicker cables may have to be assembled on the construction site itself.

An interesting hybrid of the cantilever and suspension bridge is known as the cable-stayed bridge. The 1,200-foot (366-m) Sunshine Skyway across the entrance to Tampa Bay in Florida is one of the most beautiful examples of the cable-stayed bridge. In a cable-stayed bridge, the deck is cantilevered outward in both directions from a central tower. The deck is then attached to the tower by a series of cables, similar to those in a suspension bridge. Often, a cable-stayed bridge will make use of two towers. In that case, the cantilevered sections extending towards each other in the middle of the bridge can be joined together, producing an unusually long central span. The advantage of the cable-stayed bridge is that support for dead and live loads come from three distinct places: the towers, the cables, and the abutments to which the bridge is attached at each end.

Pontoon bridges

Pontoon bridges float on water. They find use primarily in two situations: First, they find application during wars when engineers need to construct a simple bridge quickly and easily. In such instances, they can be assembled from inflatable rubber or plastic and put into place in a matter of hours. Second, they can be used in rivers and lakes where the river bottom makes it very difficult or impossible to install piers. Lake Washington, in the state of Washington, for example, once had three floating bridges. All were made of large hollow concrete blocks tied to each other.

Movable bridges

Traditionally, three kinds of movable bridges have been constructed. In one, the swing bridge, the deck is rotated around a central span, a large, heavy pier sunk into the river bottom. The swing bridge has one serious disadvantage: The central pier, on which the bridge rotates, is usually located in the deepest part of the waterway. Ships with significant drafts may, therefore, have difficulty passing through such bridges. The swing bridge also has one important advantage: Since it never moves upward, it will not interfere with air traffic that might be present in the area.

In the second type of movable bridge, the bascule bridge, the deck is raised, either at one end or at two ends. The bascule bridge acts, therefore, something like a cantilever in which the free end is raised to permit passage of seagoing vessels.

In the third type of movable bridge, the vertical-lift bridge, the whole central portion is raised straight up by means of steel ropes. One disadvantage of the vertical-lift bridge, of course, is that it can not open

KEY TERMS

Abutment— Heavy supporting structures usually attached to bedrock and supporting bridge piers.

Cable-stayed bridge— A type of bridge that is a mix of cantilever and suspension bridge, in which the deck is supported both by one or more central towers and cables suspended from the tower(s).

Dead load— The force exerted by a bridge as a result of its own weight.

Dynamic load— The force exerted on a bridge as a result of unusual environmental factors, such as earthquakes or strong gusts of wind.

Live load— The force exerted on a bridge as a result of the traffic moving across the bridge. Piers—Vertical columns, usually made of reinforced concrete or some other strong material, on which bridges rest.

Suspenders— Ropes or steel wires from which the deck of a bridge is suspended.

Truss— A very light, yet extremely strong structural form consisting of triangular elements, usually made of iron, steel, or wood.

entirely above the waterway, but can only be raised to a given maximum height.

Resources

BOOKS

Brash, Sarah, Matthew Cope, Charles Foran, Do´nal Kevin Gordon, and Peter Pocock. How Things Work: Structures. Alexandria, VA: Time-Life Books, 1991.

“Bridge,” McGraw-Hill Encyclopedia of Science & Technology. 7th ed. New York: McGraw-Hill Book Company, 1992, volume 2, pp. 49-58.

Corbett, Scott. Bridges. New York: Four Winds Press, 1978. DeLony, Eric. Landmark American Bridges. Boston: Little, Brown and Company, 1993.

MacGregor, Anne and Scott. Bridges: A Project Book. New York: Lothrop, Lee & Shephard Books, 1980.

Trefil, James. Encyclopedia of Science and Technology. The Reference Works, Inc., 2001.

OTHER

Brantacan. “Bridges” <http://www.brantacan.co.uk/bridges.htm> (accessed November 6, 2006).

Public Broadcasting Service (PBS). “Building Big: Bridges” <http://www.pbs.org/wgbh/buildingbig/bridge/index.html> (accessed November 6, 2006).

David E. Newton

BRIDGES

BRIDGES. Bridges have been essential to America's growth, and countless types were devised to carry highways, railroads, and even canals. Location, materials, cost, traffic, and the ingenuity and creativity of bridge engineers all have influenced the evolution of American bridge technology.

The Colonies and the Early Republic

Large-span bridge building in North America began with the Charles River Bridge at Cambridge, Massachusetts, in 1662. Its pile and beam construction was not unlike that used for centuries in Europe. The design placed heavy timber beams across piles that were hand driven into the riverbed. Side members were then tied together by cross beams, and wood decking was attached to stringers running parallel to the sides. Spans built this way were limited by the length of available timber and the depth of the water.

A more versatile bridge form, the truss, first came into use in the United States during the late eighteenth century. Trusses composed of a series of triangles were assembled from short lengths of timber that, depending upon their location, resisted the forces of either compression or tension. Because of the way in which it was assembled, the truss bridge could be lengthened to span distances far greater than the simple pile and beam bridge. Structural integrity in these bridges came from a balance of the opposing forces inherent in their construction. Because these spans were not self-supporting during construction,

however, they were built on false work or framing that was later removed. As with all bridges regardless of type, they had to be designed to support their own weight or dead load, as well as the moving weight or live load that passed over them.

During the first two decades of the nineteenth century, Theodore Burr of Connecticut was one of the best-known American bridge builders. His 1817 patent for a combination arch and truss design was widely used in covered bridges. He had erected some forty-five highway spans in New York, New Jersey, and Pennsylvania by the time of his death in 1822. The wooden walls and roofs that were typical of covered bridges were necessary to protect the truss's countless wooden joints from the ravages of the weather.

By far the most lasting bridges built in the eighteenth and early nineteenth centuries were those made of masonry, but masonry construction was expensive and a shortage of qualified masons in the early Republic limited the number that were constructed. Those bridges were most often built in the form of an arch and assembly proceeded on timber falsework, for these bridges were self-supporting only after the last stone was put in place. When America's first railroads began laying out their routes in the 1820s and 1830s, their bridges were apt to be of stone, since once erected such bridges required little attention and were highly durable. As the pace of railroad expansion quickened toward the mid-nineteenth century, however, the need for bridges burgeoned and permanency was abandoned for expediency. The prevailing attitude was that quickly assembled and even temporary timber bridges and trestles could be replaced at some later date with more lasting structures once a rail line was producing revenue. The railroads' need for quickly constructed spans spurred the development of the truss bridge, constructed primarily of wood, throughout the first half of the nineteenth century.

The Expanding Nation

Individual types of truss bridges can be identified by the way their members were assembled. A large number of designs were patented during the nineteenth century. Some trusses were of no practical value, while others were over-engineered or too expensive to build. Those popular during the nineteenth century did not necessarily find similar acceptance in the twentieth century. In time, the broad range of trusses was gradually reduced to a few basic types that proved to be the strongest and most economical to build. By the early twentieth century, the Pratt and Warren were the most commonly used trusses and into the 1920s the truss was the most common bridge type in America.

As the railroads used ever heavier and faster rolling stock, it was necessary to replace wooden bridges with heavier and sturdier construction. Beginning in the 1840s, cast and wrought iron were being substituted for wood members in some bridges and during the 1850s railroads began turning to bridges made entirely of iron. During the 1870s, steel production increased greatly and the price fell to levels that made it reasonable for use in bridges. By 1930, the expansion of American railroads was over and their influence on the structural development of bridges was at an end.

Two significant advances in bridge technology took place in the late 1860s and early 1870s with the construction of the long-span metal arch bridge across the Mississippi River at St. Louis. First, not only was this bridge the first major spanning of North America's largest river, but engineer James B. Eads specified the use of steel in the bridge's arch members. The three tubular arches rested on masonry piers built on wooden caissons sunk in the riverbed. Second, this was the initial use in the United States of the technique in which excavation work inside a caisson took place in an atmosphere of compressed air. Prior to that workers had labored under water or in areas where water was diverted in some way. Air pressure within the caisson equaled the force exerted by the river water outside and the shell did not flood as excavation work inside progressed down toward bedrock. This technology was crucial in the successful execution of all subsequent subaqueous foundation work.

Limitations imposed by location have forced bridge builders to be innovative. It would be impractical, if not impossible, to erect a bridge across wide, deep ravines if the bridge required the support of extensive false work during construction. As a result, a method evolved that avoided the use of staging. In October 1876, engineer Charles Shaler Smith embarked on the construction of the first modern cantilever railroad span to bridge the 1,200-foot-wide and 275-foot-deep valley of the Kentucky River. Smith refined a bridge-building technique little used outside of ancient China. Cantilever construction employed counter balancing forces so that completed segments supported ongoing work as it progressed inward toward the span's midpoint.

Although the suspension bridge was not new in 1842, its future form was forecast when Charles Ellet's Philadelphia wire suspension bridge was opened to highway traffic that year. Suspension bridges in which the roadway or deck was suspended from heavy wrought iron chains had been built for years. For the first time in a major American span, the deck was suspended from relatively lightweight wire cables. Ellet used a European cable-making technique in which cables were composed of a number of small-diameter parallel wires. The shape of each cable was maintained and its interior protected when the bundle's exterior was wrapped with additional wire. The scale of the bridges that followed increased tremendously, but the basic technology for cable making remained the same.

Civil engineer John A. Roebling was the preeminent suspension bridge designer of the nineteenth century. His career began with a suspension canal aqueduct at Pittsburgh in the 1840s, and each of his following projects reflected a growing skill and daring. His combined railroad and highway-carrying suspension bridge across the Niagara River gorge was completed in 1855. In it a suspended double-deck wooden truss carried the two roadbeds. Although other types of bridges would be built to carry highway and urban rail systems, this was the lone example of a suspension bridge constructed to carry both. The overall design and appearance of his Ohio River suspension bridge at Cincinnati in 1867 foretold of his plans for New York City's even larger and monumental Brooklyn Bridge of 1883. The extensive use of steel throughout the bridge, and especially for its cables, was a watershed in bridge technology.

The Early Twentieth Century

A number of large suspension bridges were built during the first half of the twentieth century. They were ideal for spanning the broad waterways that seemed to stand in the way of the growth of modern America. Neither their construction nor their final form posed an impediment to the nation's busy waterways. During the first four decades of the century, New York City was the focus of much of that construction. The city's boroughs were joined by the Williamsburg (1903), Manhattan (1909), Triborough (1936), and Bronx-Whitestone (1939) suspension bridges. However, none compared in size to the magnificent George Washington Bridge, completed in 1931. It was the first bridge linking Manhattan and New Jersey, and represented a remarkable leap forward in scale. Its 3,500-foot-longsuspended span was double the length of the next largest. The bridge's four massive suspension cables passed over towers soaring more than 600 feet high and its roadway was suspended 250 feet above the Hudson River. It was never truly completed, as the masonry facing called for on each of its steel towers was omitted because of the Great Depression.

The federal government responded to the depression by funding many massive public works projects. Partially as a form of unemployment relief, San Francisco undertook the construction of two great suspension bridges. They spanned greater distances than any previously built bridges. Strong Pacific Ocean currents and the depth of the water in San Francisco Bay made the construction of both the San Francisco–Oakland Bridge (1936) and the Golden Gate Bridge (1937) particularly challenging, and no part of the project required more technical expertise than building the subaqueous tower piers.

Triumphs in bridge building have been tempered by failures and perhaps no span has received more notoriety because of its collapse than did the Tacoma Narrows Suspension Bridge across Puget Sound in Washington State. Beginning in the late 1930s, its construction progressed uneventfully until the bridge was completed in July 1940. The valley in which the bridge was built was subject to strong winds and gusts that set the bridge in motion even while it was under construction. These wind-induced undulations increased in frequency as the bridge neared completion. So noticeable were the span's movements that they earned the bridge the sobriquet Galloping Gertie. Several months after its opening, the bridge was subjected to a period of intense high winds, during which it literally tore itself apart. The rising, falling, and twisting of the deck was so violent that it broke loose from its suspender cables and crashed into the sound. It was later determined that the failure resulted from the bridge being too flexible. The narrow deck and the shallow profile of the steel girders supporting the deck provided little resistance to aerodynamic action. The bridge's collapse prompted a reevaluation of suspension bridge design and resulted in a move away from flexible designs toward much stiffer and wind-resistant construction. A redesigned span across Puget Sound was completed in 1950.

Of the few suspension spans built in the United States after the middle of the twentieth century, one of the more remarkable was the Verrazano-Narrows Bridge, which opened in November 1964. The bridge, situated across the entrance to New York Harbor, connected Staten Island to Brooklyn. It was designed by engineer Othmar H. Ammann, who during his career designed a number of New York City's bridges, including the George Washington Bridge. While no new techniques were introduced in its construction, the bridge is remembered for it huge overall dimensions and the unprecedented size of its individual parts as well as the speed—five years—with which it was erected.

The Late Twentieth Century

Although the first cable stay bridges appeared in seventeenth-century Europe, this type of bridge emerged in a rationalized form only during the 1950s. Their con-figuration may vary in appearance and in the complexity of the tower or towers as well as in the symmetry and placement of the cables. The most recognizable spans are characterized by a single tower or mast and multiple diagonal cables that, if arranged in a single vertical plane, pass over the tower and are affixed at opposite points along the center line of the deck. Decks can be assembled in cantilever fashion from sections of pre-cast, pre-stressed concrete. They offer many of the advantages of suspension bridges, yet require neither the lengthy and costly process of cable spinning nor large cable anchorages. Their overall load-bearing capacity is less than the more complex suspension bridge. One of the most notable American examples is the Sunshine Skyway Bridge completed across Tampa Bay, Florida, in 1987.

With the expansion of American railroads nearing its end, highways became a major factor in bridge design and construction during the 1920s. As a result, the majority of spans constructed during the remainder of the twentieth century were relatively light, reinforced, and prestressed concrete highway bridges. Reinforced concrete bridge construction, in which steel imbedded in the concrete controls the forces of tension, was introduced in the United States in the late nineteenth century. Eventually, a variety of reinforcing systems were patented. The interstate highway system's rapid growth during the 1950s and 1960s fostered the widespread use of pre-stressed concrete beam bridges. Beams fabricated in this way were strengthened by built-in compressive forces. These bridges became the most common type of span in late-twentieth-century America.

BIBLIOGRAPHY

Condit, Carl W. American Building Art: The Nineteenth Century. New York: Oxford University Press, 1960.

———. American Building Art: The Twentieth Century. New York: Oxford University Press, 1961.

Jackson, Donald C. Great American Bridges and Dams. Washington, D.C.: Preservation Press, 1988.

McCullough, David. The Great Bridge. New York: Simon and Schuster, 1972.

Petroski, Henry. Engineers of Dreams: Great Bridge Builders and the Spanning of America. New York: Knopf, 1995.

Scott, Quinta, and Howard S. Miller. The Eads Bridge. Columbia: University of Missouri Press, 1979.

Van der Zee, John. The Gate: The True Story of the Design and Construction of the Golden Gate Bridge. New York: Simon and Schuster, 1986.

Wittfoht, Hans. Building Bridges: History, Technology, Construction. Dusseldorf, Germany: Beton-Verlag, 1984.

William E. Worthington Jr.

See also Brooklyn Bridge ; Eads Bridge ; George Washington Bridge ; Golden Gate Bridge ; Toll Bridges and Roads ; Verrazano-Narrows Bridge .

Bridges

Bridges are structures that join two otherwise inaccessible points of land, such as the two shores of a river or lake , or the two sides of a canyon or deep gully. Bridges are designed to carry railroad cars, motor vehicle traffic, or foot travel by pedestrians and/or animals, or to support pipes, troughs, or other conduits used for the movement of goods and materials, such as an oil pipeline or a water aqueduct.

Humans have been constructing bridges since ancient times. The earliest bridges were probably nothing more than felled trees used to cross rivers or ditches. As civilization advanced, artisans discovered ways to use stone, rock, mortar, and other naturally occurring materials in the construction of longer and stronger bridges. Finally, as physicists and engineers began to develop the principles underlying bridge construction, they incorporated other materials such asiron, steel , and aluminum into the bridges they built.

Bridges can be classified in a number of different ways, according to their intended use (railroad bridge or pedestrian walkway, for example), according to the material of which they are made (steel, wood , or concrete for example), or according to whether they are fixed or moveable. Moveable bridges are used when the height of ships traveling on a waterway will be greater than the floor of the bridge. In such cases, the bridge is built so that the roadway can be raised or pivoted to allow marine traffic to pass under or through it. Probably the most useful way to classify bridges for technical purposes, however, is according to their structural form. There are three major types of bridges: arch, cantilever , and suspension.

Forces acting on a bridge

Three kinds of forces operate on any bridge: the dead load, the live load, and the dynamic load. The first of these terms refers to the weight of the bridge itself. Like any other structure, a bridge has a tendency to collapse simply because of the gravitational forces acting on the materials of which the bridge is constructed (i.e., the wood, concrete, steel, or aluminum). The second term refers to traffic that moves across the bridge as well as normal environmental factors such as changes in temperature , precipitation , and winds. The third factor refers to environmental factors that go beyond normal weather conditions, factors such as sudden gusts of wind and earthquakes. All three factors must be taken into consideration in the design of a bridge.

For example, suppose that it is necessary to build a bridge across a span that is 325 ft (100 m) wide. It would not be possible to build a beam bridge, one that consists of a single slab of steel 325 ft (100 m) long, of that length. The weight of the material used to construct the bridge plus the weight of the traffic on the bridge would be too great for the bridge to remain standing. An engineer would have to design some other kind of bridge-an arch or suspension bridge, for example-that would be able to hold up that amount of weight.

Dynamic loads

Dynamic loads can present special problems for the bridge designer. A bridge has to be able to withstand not only the forces of normal, everyday traffic, but also unusual forces of unexpected magnitude. In California, as an example, bridges require special kinds of reinforcement to withstand possible earthquakes. The fact that engineers have not completely solved the problems presented by dynamic loads is reconfirmed from time to time. For example, during the 1989 earthquake in the San Francisco Bay area, a section of the San Francisco-Oak-land Bay Bridge collapsed, leaving a gaping hole. A freeway overpass in Oakland also failed during the earthquake, taking the lives of about two dozen motorists.

Wind gusts have been responsible for a number of bridge failures in the past. Even if wind speeds are relatively low, dynamic loads may become too great for a bridge to withstand. One reason for this phenomenon is that the bridge may begin to vibrate so violently that it actually shakes itself apart. Such was the case, for example, with the Tacoma Narrows Bridge in 1941. On November 7 of that year, with wind speeds registering only about 40 mph (25 kph), the bridge vibrated so badly that it collapsed. The actual force experienced by the bridge was considerably less than the dead and live forces for which it had been designed. But the oscillations produced by wind gusts on the day in question were sufficient to shake the bridge apart.

As a result of failures such as those in the Bay Bridge and the Tacoma Narrows Bridge, engineers have developed methods for making bridges more aerodynamically sound. For example, lighter materials arranged in geometric structures that are aerodynamically more stable are now used in bridges where earthquakes, wind gusts, or other unusually severe environmental problems can be expected.

Model testing

At one time, the only test of a bridge design was actual use. Engineers could incorporate all the scientific knowledge and technological craftsmanship they had to produce a sound design. But how well the bridge would stand up under actual use and dynamic loads could only be discovered in the real world.

Today, engineers have two powerful tools with which to test their ideas: wind tunnels and computeraided design (CAD). Wind tunnels have long been used by aeronautical engineers for the testing of aircraft designs. Now they are routinely used also for the testing of bridge designs. A wind tunnel is an enclosed space in which rapidly moving air from giant fans is made to pass over the model of a bridge. Possible structural and design problems can be detected by photographing and studying patterns of air movement over the model.

As in so many other fields, bridge design has benefitted greatly from the growth and development of computer programs. Such programs can incorporate huge amounts of information about various ways in which bridges and the materials of which they are made will react to various kinds of stresses. CAD can be used to actually draw and test bridge designs on screen without even making the models needed for wind tunnel testing.

Types of bridges

The simplest type of bridge corresponds to the felled tree mentioned above. It consists of a single piece of material that stretches from one side of the gap to be bridged to the other side. That piece of material—the beam, or girder—rests directly on the ground on each side or is supported on heavy foundations known as piers. The length of a beam bridge of this kind is limited by the weight of the beam itself plus the weight of the traffic it has to bear. Longer beam bridges can be constructed by joining a number of beams to each other in parallel sections.

The concept of a beam bridge can be extended to make a stronger product, the continuous bridge. A continuous bridge differs from a beam bridge in that the latter has at least one additional point of support beyond the two found in a beam bridge. The longest existing continuous bridges now in use are the Astoria Bridge that crosses the Columbia River near Astoria, Oregon, and the Oshima Bridge that connects Oshima Island to the mainland in Japan.

Cantilever bridges

A cantilever bridge is a variation of the simple beam bridge. A cantilever is a long arm that is anchored at one end and is free to move at the opposite end. A diving board is an example of a cantilever. When anchored firmly, a cantilever is a very strong structure. Imagine a 200 lb (91 kg) man standing on the end of diving board. The board bends only slightly, showing that it can hold a relatively large weight (the man).

A cantilever bridge consists of three parts: the outer beams, the cantilevers, and the central beam. Each of the outer beams of the bridge is somewhat similar to a short beam bridge. The on-shore edge of the bridge is attached to the ground itself or to a pier that is sunk into the ground. The opposite edge of the outer beam is attached to a second pier, sunk into the ground at some distance from the shore.

Bridge piers are vertical columns, usually made of reinforced concrete or some other strong material. In many cases, they are sunk into massive supporting structures known as abutments. Abutments are constructed in large holes in the ground, in contact with bedrock if possible, to withstand the forces created by the dead and live loads created by bridges and the traffic they carry.

Also attached to the off-shore pier is one end of a cantilever. The free end of the cantilever extends outward into the middle of the gap between the shores. An incomplete cantilever bridge consists, therefore, of two halves, one anchored to each side of the gap to be bridged and consisting of a cantilever facing toward the middle of the gap. The space between the two cantilevers, finally, is bridged by another beam, similar to that of a short beam bridge joining the two cantilevers to each other.

The distribution of forces in a cantilever bridge is fairly straightforward. The dead load and live load of the bridge is born by the two sets of piers that hold up the bridge, the outermost piers that hold up the outer edges of the bridge, and the inner piers that anchor the fixed end of the cantilever.

The two longest cantilever bridges in the world are the Forth Railway Bridge in Scotland, completed in 1890, and the Quebec Bridge in Canada, built in 1917. The former is 1,700 ft (520 m) in length and the latter, 1,800 ft (550 m) long.

Trusses

The strength of a cantilever bridge can be increased by the use of trusses. A truss is structure that consists of a number of triangles joined to each other. The triangle is an important component of many kinds of structures because it is the only geometric figure that can not be pulled or pushed out of shape without actually changing the length of one of its sides. By combining a number of triangles into a single unit, the unit is given a great deal of strength.

The cantilever beam, end beams, and joining beams in a cantilever bridge are often strengthened by adding trusses to them. The trusses act somewhat like an extra panel of iron or steel, adding strength to the bridge with relatively little additional weight. The open structure of a truss also allows the wind to blow through them, preventing additional stress on the bridge from this factor.

Trusses are used not only in cantilever bridges, but in all other kinds of bridges also. In fact, you have probably noticed the complex pattern of intersecting triangles on bridges over which you have passed. These truss patterns are one of the most efficient ways of adding strength to any type of bridge an engineer designs.

Arch bridges

As its name suggests, the main supporting structure in an arch bridge is one or more curved elements. The dead and live forces that act on the arch bridge are transmitted along the curved line of the arch into abutments at either end. These abutments are sunk deep into the earth , into bedrock if at all possible. They are, therefore, essentially immovable and able to withstand very large forces exerted on the bridge itself. This structure is so stable that piers are generally unnecessary in an arch bridge.

The deck of an arch bridge can be placed anywhere with relationship to the arch: on top of it, beneath it, or somewhere within the arch. The deck is attached to the arch by vertical posts (ribs and columns) if the deck is above the arch, by ropes or cables (suspendors) if the deck is below the arch, and by some combination of the two if the deck is somewhere within the arch.

Most arch bridges today are made either of steel or of reinforced concrete. The longest existing steel arch bridge is the New River Gorge Bridge in Fayetteville, West Virginia, built in 1977. It is 1,700 ft (518 m) long. The longest reinforced concrete bridge is the Jesse H. Jones Memorial Bridge at the Houston Ship Channel, Texas, with a length of 1,500 ft (455 m).

Suspension bridges

The longest bridges in the world are all suspension bridges. Some examples are the Humber Bridge in Hull, England, with a length of 4,626 ft (1,410 m), the Verrazano-Narrows Bridge in lower New York Bay (4,620 ft [1,298 m]); the Golden Gate Bridge over the entrance to San Francisco Bay (4,200 ft [1,280 m]); and the Mackinac Straits Bridge connecting the Upper and Lower Peninsulas of Michigan (3,800 ft [1,158 m]).

In a suspension bridge, the dead and live loads are carried by thick wire cables that run across the top of at least two towers and are anchored to the shorelines within heavy abutments. In some cases, the bridge deck is supported directly by suspendors from the cables, while in other cases, the suspendors are attached to a truss, on top of which the deck is laid. In either case, the dead and light load of the bridge are transmitted to the cables which, in turn, exert stress on the abutments. That stress is counteracted by attaching the abutments to bedrock.

The towers in a suspension bridge typically rest on massive foundations sunk deep into the river bed or sea bed beneath the bridge itself. The wire cables that carry the weight of the bridge and its traffic are made of parallel strands of steel wire woven together to make a single cable. Such cables typically range in diameter from about 15 in (38 cm) to as much as 36 in (91 cm). Smaller cables can be ordered from a factory, while thicker cables may have to be assembled on the construction site itself.

An interesting hybrid of the cantilever and suspension bridge is known as the cable-stayed bridge. The 1,200 ft (366 m) Sunshine Skyway across the entrance to Tampa Bay in Florida is one of the most beautiful examples of the cable-stayed bridge. In a cable-stayed bridge, the deck is cantilevered outward in both directions from a central tower. The deck is then attached to the tower by a series of cables, similar to those in a suspension bridge. Often, a cable-stayed bridge will make use of two towers. In that case, the cantilevered sections extending towards each other in the middle of the bridge can be joined together, producing an unusually long central span. The advantage of the cable-stayed bridge is that support for dead and live loads come from three distinct places: the towers, the cables, and the abutments to which the bridge is attached at each end.

Pontoon bridges

Pontoon bridges are bridges that float on water. They find use primarily in two situations. First, they find application during wars when engineers need to construct a simple bridge quickly and easily. In such instances, they can be assembled from inflatable rubber or plastic and put into place in a matter of hours. Second, they can be used in rivers and lakes where the river bottom makes it very difficult or impossible to install piers. Lake Washington, in the state if Washington, for example, once had three floating bridges. All were made of large hollow concrete blocks tied to each other.

Movable bridges

Traditionally, three kinds of movable bridges have been constructed. In one, the swing bridge, the deck is rotated around a central span, a large, heavy pier sunk into the river bottom. The swing bridge has one serious disadvantage. The central pier, on which the bridge rotates, is usually located in the deepest part of the waterway. Ships with significant drafts may, therefore, have difficulty passing through such bridges. The swing bridge also has one important advantage. Since it never moves upward in a vertical direction, it will not interfere with air traffic that might be present in the area.

In the second type of movable bridge, the bascule bridge, the deck is raised, either at one end or at two ends. The bascule bridge acts, therefore, something like a cantilever in which the free end is raised to permit passage of seagoing vessels.

In the third type of movable bridge, the vertical-lift bridge, the whole central portion of the bridge is raised straight up by means of steel ropes. One disadvantage of the vertical-lift bridge, of course, is that it can not open entirely above the waterway, but can only be raised to a given maximum height.

Resources

books

Brash, Sarah, Matthew Cope, Charles Foran, Dónal Kevin Gordon, and Peter Pocock. How Things Work: Structures. Alexandria, VA: Time-Life Books, 1991.
"Bridge," McGraw-Hill Encyclopedia of Science & Technology, 7th ed. New York: McGraw-Hill Book Company, 1992, vol. 2.

Corbett, Scott. Bridges. New York: Four Winds Press, 1978.

DeLony, Eric. Landmark American Bridges. Boston: Little, Brown and Company, 1993.

MacGregor, Anne, and Scott MacGregor. Bridges: A ProjectBook. New York: Lothrop, Lee & Shephard Books, 1980.

Trefil, James. Encyclopedia of Science and Technology. The Reference Works, Inc., 2001.

David E. Newton

KEY TERMS

Abutment

—Heavy supporting structures usually attached to bedrock and supporting bridge piers.

Cable-stayed bridge

—A type of bridge that is a mix of cantilever and suspension bridge, in which the deck is supported both by one or more central towers and cables suspended from the tower(s).

Dead load

—The force exerted by a bridge as a result of its own weight.

Dynamic load

—The force exerted on a bridge as a result of unusual environmental factors, such as earthquakes or strong gusts of wind.

Live load

—The force exerted on a bridge as a result of the traffic moving across the bridge.

Piers

—Vertical columns, usually made of reinforced concrete or some other strong material, on which bridges rest.

Suspendors

—Ropes or steel wires from which the deck of a bridge is suspended.

Truss

—A very light, yet extremely strong structural form consisting of triangular elements, usually made of iron, steel, or wood.

BRIDGES

Bridge building as a human activity predates recorded history, and bridges are among the earliest structures described in the historical record. In the fifth century b.c.e. Herodotus reports on a bridge over the Euphrates River made of timber resting on a stone foundation. Roman stone bridges at Segovia (Spain) and Nıˆmes (France) are still standing 2,000 years after their construction. In the Middle Ages, bridge building became the province of specialist monastic orders. Medieval bridges were conceived as places to live, not just as a means of passage from one side of a river to another. London Bridge in 1594 supported 100 houses and shops.

Bridge Engineering

In the nineteenth century, bridge building became a scientific discipline, after a backlash brought about by notorious disasters in which bridges failed to endure mathematically predictable loads. A fascinating 1887 monograph by George L. Vose (1831–1910) reflects the period in which bridge building crystallized into a scientific and mathematical discipline. Vose complained that any charlatan could proclaim himself a bridge builder and find customers, while ignoring the mathematics that made the calculation of safety margins simple. "There is at present in this country absolutely no law, no control, no inspection, which can prevent the building and the use of unsafe bridges" (p. 12). He pointed out that the science of bridge loads was well understood: A dense crowd of people creates a load of up to 140 pounds per square foot, while soldiers walking in step double the strain; snow and ice can create a load of 10 to 20 pounds per square foot, while heavily loaded freight trains can create a strain of 7,000 pounds per square foot.

Vose was a pioneering proponent of safety margins. He argued that bridges should be designed to carry a load four to six times greater than the actual loads they are likely to carry under any foreseeable circumstances. Many existing bridges did not meet these standards; some, in fact, were capable of carrying only the predictable load. Of these, Vose acerbically noted that such a bridge is warranted "to safely bear the load that will break it down" (p. 55). The country, in his estimation, was full of highway bridges "sold by dishonest builders to ignorant officials" and awaiting only "an extra large crowd of people, [or] a company of soldiers" to collapse (p. 16).

According to the structural engineer David P. Billington, however, a second transformation occurred when bridges (along with tall buildings) became uniquely modern works of art by exploiting the properties of new structural materials such as steel and reinforced concrete. In the period after 1880 engineers began "to explore new forms with these materials," the first maturity of which occurred in the period between the two world wars (1983, p. 7). The bridge designs of the Swiss engineer Robert Maillart (1872–1940) are archetypical achievements of this new era.

In the contemporary world the Clifton (Bristol, 1864), Brooklyn (New York, 1883), Golden Gate (San Francisco, 1937), and Tsing Ma (Hong Kong, 1997) Bridges are indeed considered works of art, objects whose function is intertwined with their beauty. For the engineer Henry Petroski "there is no purer form of engineering than bridge building" (1995, p. 14). Whereas houses and buildings are designed for appearance, and then engineered, the process followed in bridge construction is the opposite. A bridge must be designed to perform its function successfully; its beauty emerges from the engineering.

Ethics and Bridges

The ethical issues pertaining to bridges span a range of questions. Is a particular bridge really needed? What impacts do bridges have on the social and natural environments where they are constructed? What levels of safety are appropriate in bridge design?

NEEDS. Insofar as they are major public works projects, the need for bridges has to be obvious and they often have to pass a hurdle of criticism before being constructed. At the same time bridges are sometimes built so that powerful politicians can create jobs and funnel money to their districts, or reward political contributors. According to environmental groups in Alaska, the proposed Gravina Island Bridge is an example. Designed to be 1.6 kilometers long and 24 meters higher than the Brooklyn Bridge, the $200 million structure would link the depressed town of Ketchikan (and its 7,500 residents) to an island that has fifty residents and an airport with six flights a day in the busy season. The island is already well served by ferry, and the bridge would bisect a channel used by shipping and floatplanes.

SOCIAL AND ENVIRONMENTAL IMPACTS. Most people do not want a bridge in their own backyards, with the concomitant loss of views and increases in local traffic, leading to a decrease in property values. Illustrating the NIMBY (not in my backyard) syndrome, even citizens who will benefit prefer that a bridge be sited in someone else's neighborhood. The site originally studied for the George Washington Bridge in New York City was at West 110th Street in Manhattan. Two powerful local institutions, St. Luke's Hospital and Columbia University, strenuously opposed this location. Columbia's president, Nicholas Murray Butler, said that the proposed site was "little short of vandalism" (Petroski 1995, p. 242). The bridge was eventually built (1927–1931) on unused land much further north at West 179th Street.

Robert Moses (1888–1981), the motivating force behind many of New York's best-known bridges and parks, is famous for his ruthless treatment of opponents and of local communities that stood in the way. His beautiful Verrazano-Narrows Bridge, built 1959–1964 with either end in a highly populated neighborhood, caused the seizing and demolition of 800 buildings in Bay Ridge, Brooklyn, displacing 7,000 people. On the Staten Island side, 400 buildings were taken by eminent domain, displacing 3,500 residents.

Moses's determination, and his willingness to counter his opponents in the same visceral language they used to attack him, is evident in a series of monographs issued at his direction. In 1939, when the New York Herald-Tribune opposed his proposed Brooklyn Battery Bridge, Moses had the Triborough Bridge Authority publish a brochure entitled "Is There Any Reason to Suppose They Are Right Now?" It ridiculed the Herald-Tribune, excerpting two decades of editorials opposing previous Moses park and highway projects. Moses painted the newspaper as the voice of millionaires who did not want their neighborhoods tainted by projects that would benefit the common folk.

Another organization opposing the Brooklyn Battery Bridge was the Regional Plan Association, which argued that it was not a natural site for a bridge and would deface the land- and cityscape. In his counterattack, Moses noted that the association had backed a proposal for the construction of a 200-meter obelisk in the Battery, which Moses claimed would obscure the view much more than his proposed bridge. In the end, however, Moses lost the battle, and a tunnel was built in lieu of the bridge. Tunnels are frequently proposed as alternatives to bridge projects; underground, they have the virtue of not being seen, but tend to be more expensive to build and are of necessity narrower, carrying less traffic and freight.

BRIDGE SAFETY. Bridges collapse for one of two reasons. Either their design and construction fail to meet contemporary industry standards, or those standards are inadequate to ensure safety in the face of unexpected circumstances. An example of negligent construction was West Gate Bridge, in Melbourne, Australia, which fell while being erected on October 15, 1970. Thirty-five workers were killed in the collapse. The bridge was being assembled in sections, which were elevated and then bolted to one another. It was discovered that two adjoining sections were not flush with one another as designed; the difference in "camber" was about 3 inches, while the specifications called for a difference of no more than 1 inch. In order to fix the problem, the builders should have lowered the two pieces to the ground again, but this would have caused a delay and a cost overrun, so instead they decided to fix them in place.

They applied a very primitive solution, one of placing 8-ton concrete blocks on the higher span, to push it back into line with the other one. This then caused the steel plates to buckle out of shape by as much as 15 inches. In an ill-fated and foolhardy attempt to eliminate the buckling, the builders decided to remove the bolts holding the steel plates in place. After the first sixteen bolts had been removed, the plates had slipped so much that the remaining bolts were jammed and could not be unscrewed. The workers then tightened each of these until they broke, removing the pieces. Like a man sawing off a tree limb upon which he is sitting, they continued removing bolts, until the entire structure collapsed, killing many of them. A Royal Commission appointed to investigate the disaster concluded that what had happened was "inexcusable" and that the builder's performance "fell far short of ordinary competence" (Royal Commission 1971, p. 97).

An example of a structure that arguably was designed acceptably by contemporary standards, but that fell anyway, was the Tacoma Narrows Bridge (built 1938–1940), popularly known as "Galloping Gertie" because of the alarming way it flailed around under high winds before eventually tearing apart. While most bridge disasters occur when a load crosses the bridge that exceeds its carrying capacity, the Tacoma Narrows Bridge had more than an adequate margin of safety for any traffic load. What the architect had failed to anticipate was that the long and thin bridge had "aerodynamic qualities somewhat like the wing of an aeroplane" (Rastorfer 2000, p. 33). Buffeted by heavy winds on November 7, 1940, the whole span began to twist. Finally, hours after Gertie began its last gallop, the bridge tore itself apart and fell.

Petroski notes that bridge failures follow an approximately thirty-year cycle. A notorious failure leads to the use of a new model, which at first is designed conservatively, but then extended and overextended, until a new failure results, and then a new model emerges. The "high girder" design led to the collapse of the Tay Bridge (Dundee, Scotland, 1879), which resulted in the new cantilevered design, which was responsible for the double collapse (in 1907 and 1916) of the Quebec Bridge, which brought about the suspension model, of which Galloping Gertie was an example. In this sense bridges may illustrate a general dynamic, one society should always take into consideration when attempting to make informed ethical use of science and technology.

JONATHAN WALLACE

SEE ALSO Dams;Water.

BIBLIOGRAPHY

Billington, David P. (1979). Robert Maillart's Bridges. Princeton, NJ: Princeton University Press. Billington has written two other books exploring the work of this famous bridge engineer.

Billington, David P. (1983). The Tower and the Bridge: The New Art of Structural Engineering. New York: Basic.

Kharbanda, O. P., and Jeffrey K. Pinto. (1996). What Made Gertie Gallop? Lessons from Project Failures. New York: Van Nostrand Reinhold.

Mock, Elizabeth B. 1972 (1949). The Architecture of Bridges. New York: Arno Press, published for the Museum of Modern Art.

Petroski, Henry. (1995). Engineers of Dreams: Great Bridge Builders and the Spanning of America. New York: Knopf.

Rastorfer, Darl. (2000). Six Bridges: The Legacy of Othmar H. Ammann. New Haven, CT: Yale University Press.

Royal Commission (E. H. E. Barber, F. B. Bull, and H. Shirley-Smith). (1971). Report of Royal Commission into the Failure of West Gate Bridge. Melbourne, Australia: C. H. Rixon, Government Printer.

Triborough Bridge Authority. (1939). "Is There Any Reason to Suppose They Are Right Now?" New York: Author.

Vose, George L. (1887). Bridge Disasters in America: The Cause and the Remedy. Boston: Lee and Shepard.