A metal bridge, by definition, is one whose structure is made of metal, mainly steel, although from the end of the century until the end of the century, first cast iron and then padded iron were used.[1] On the other hand, there are structural solutions that combine steel with concrete, called mixed bridges when both elements have an important load-bearing function.[2] Some of the typologies considered here include concrete decks, supports or towers, so the border between both materials It is not always very clear, although metal structures are usually considered to be all those that include visible beams, lattices or suspension or cable-stayed cables made of steel or, in the past, iron.
History
The first documented use of iron to build bridges dates back to the beginning of the Christian era in China and the Himalayan region, where wooden walkways suspended from iron chains have since been built.[3].
But the generalization of metal bridges only became possible thanks to the enormous development of the steel industry during the beginning of the Industrial Revolution, which involved the availability in large quantities first of cast iron, then of wrought iron and finally of steel, which allowed the construction of increasingly larger structures, initially linked to the expansion of the railway.[4] The rapid development of metal bridges in the century was due both to their advantageous mechanical characteristics and to the increasing knowledge of their behavior. resistant.[5].
Cast iron bridges have their origin in the Iron Bridge, built in 1779 by Abraham Darby III over the River Severn "River Severn (United Kingdom)"), in Coalbrookdale, which is still preserved. It would be 17 years until the second cast iron structure was built, the missing Wearmouth Bridge "Wearmouth Bridge (1796)") over the River Wear, which had a surprising 72 m span arch designed in 1796 by Thomas Paine (mainly known for being one of the founding fathers of the United States). These first structures inherited the typologies of stone and wooden bridges.[4].
Among the most notable achievements of the time are structures by notable engineers such as Thomas Telford (with numerous cast iron arches to his credit) and John Rennie (who designed the old Southwark Bridge over the River Thames, with three arches spanning 64+73+64 metres; the largest cast iron bridge ever built).[6] Initially they were made by joining pieces (often large) with bolts, although later smaller segments linked with pins, as in the case of the Alexander III Bridge in Paris, a 107 m span arch over the Seine River completed in 1900.[6] Other notable examples of cast iron arches built in France are the Pont des Arts in Paris,[4] designed in 1803 by Louis-Alexandre de Cessart and Jacques Dillon; the original Austerlitz Bridge designed by Becquey de Beaupré also over the River Seine; and the Carrousel bridge built in Paris in 1834 by Antoine-Rémy Polonceau, which marked the heyday of cast iron bridges.
Construction of Metal Bridges
Introduction
A metal bridge, by definition, is one whose structure is made of metal, mainly steel, although from the end of the century until the end of the century, first cast iron and then padded iron were used.[1] On the other hand, there are structural solutions that combine steel with concrete, called mixed bridges when both elements have an important load-bearing function.[2] Some of the typologies considered here include concrete decks, supports or towers, so the border between both materials It is not always very clear, although metal structures are usually considered to be all those that include visible beams, lattices or suspension or cable-stayed cables made of steel or, in the past, iron.
History
The first documented use of iron to build bridges dates back to the beginning of the Christian era in China and the Himalayan region, where wooden walkways suspended from iron chains have since been built.[3].
But the generalization of metal bridges only became possible thanks to the enormous development of the steel industry during the beginning of the Industrial Revolution, which involved the availability in large quantities first of cast iron, then of wrought iron and finally of steel, which allowed the construction of increasingly larger structures, initially linked to the expansion of the railway.[4] The rapid development of metal bridges in the century was due both to their advantageous mechanical characteristics and to the increasing knowledge of their behavior. resistant.[5].
Cast iron bridges have their origin in the Iron Bridge, built in 1779 by Abraham Darby III over the River Severn "River Severn (United Kingdom)"), in Coalbrookdale, which is still preserved. It would be 17 years until the second cast iron structure was built, the missing Wearmouth Bridge "Wearmouth Bridge (1796)") over the River Wear, which had a surprising 72 m span arch designed in 1796 by Thomas Paine (mainly known for being one of the founding fathers of the United States). These first structures inherited the typologies of stone and wooden bridges.[4].
Among the most notable achievements of the time are structures by notable engineers such as Thomas Telford (with numerous cast iron arches to his credit) and John Rennie (who designed the old Southwark Bridge over the River Thames, with three arches spanning 64+73+64 metres; the largest cast iron bridge ever built).[6] Initially they were made by joining pieces (often large) with bolts, although later smaller segments linked with pins, as in the case of the Alexander III Bridge in Paris, a 107 m span arch over the Seine River completed in 1900.[6] Other notable examples of cast iron arches built in France are the Pont des Arts in Paris,[4] designed in 1803 by Louis-Alexandre de Cessart and Jacques Dillon; the original Austerlitz Bridge designed by Becquey de Beaupré also over the River Seine; and the Carrousel bridge built in Paris in 1834 by Antoine-Rémy Polonceau, which marked the heyday of cast iron bridges.
wrought iron bridges, a material capable of much better resistance to traction and bending without cracking, replaced cast iron bridges, overcoming the problems caused by the fragility of this material. The collapse of several structures in the United Kingdom, as in the case of the Tay Bridge disaster in 1879, led to the avoidance of the use of cast iron in railway bridges, which would be replaced by wrought iron. The first large structures built with this material were the Conwy railway bridge (1849; span 125 m) and the Britannia Bridge (1850; spans 70+2x142+70 m), two box girders designed by Robert Stephenson that cross the Menai Straits. Its novel configuration is considered one of the most innovative designs of all times, becoming the first large girder and box girder bridges in history, and the first structures to systematically use wrought iron.[6].
The transition from cast iron to wrought iron represented a radical change in the way metal bridges had been conceived until then. Casting pieces were produced by molding, so that their sections were usually solid, more suitable for working in compression (like the ashlars of a stone bridge) than in flexion. However, wrought iron, to acquire its mechanical properties, needed to be rolled or extruded and repeatedly hammered, for which the simplest thing was to manufacture it by giving it the shape of relatively thin plates, which could be bent longitudinally to produce bars with simple profiles, suitable for withstanding traction, compression and bending efforts with equal effectiveness. In this way, the availability of iron sheets led to the appearance of the box girder typology, while the use of profiles in the shape of long bars was at the origin of lattice arches and triangulated beams, the latter typology that became omnipresent during the second half of the century and the first decades of the century due to its simplicity of calculation, ease of construction and reduced cost compared to other solutions.[7].
Wrought iron allowed the development of large triangulated structures, with notable examples such as the Royal Albert Bridge (1859; two arches of 135 m span) by Isambard Kingdom Brunel over the estuary of the Tamar River,[8] and the numerous works designed by the Frenchman Gustave Eiffel, designer of the Garabit Viaduct (1886; a lattice arch of 165 m span) and the Don Luis I Bridge (1886; a lattice arch spanning 174 m) in Porto together with his partner Théophile Seyrig. Another unique work of wrought iron, although designed for lighter loads than those of the railway, was the Clifton suspension bridge (1864; central span of 214 m span) in Bristol, a chain-suspended bridge designed based on a proposal by Brunel.[9].
However, the idea of triangulated beams originated in the United States. Based on the designs of wooden bridges, an abundant and affordable material widely used in North America, different wrought iron lattice systems suitable for the passage of railways were patented (Town, 1820; Howe, 1840; Pratt, 1844; or Whipple, 1847)[10] that competed with each other to obtain the greatest possible lightness and resistance. These first designs, which still combined steel and wooden bars, were quickly assimilated in Europe, where the scarcity of wood and the availability of iron meant that from the beginning the lattices became completely metallic. Europe would precisely be the origin of the Warren beam, the culmination of the process that tended to simplify the geometry of the different types of triangulated beams.[11].
Steel bridges represented an important new technological leap, brought about by the development of metallurgical converters (Bessemer and Thomas), and especially the Martin-Siemens furnace, which allowed steel of increasingly better quality and in large quantities to be available from the last two decades of the century. Although it had previously been used in isolation (as in the Vienna Chain Bridge, designed in 1828 by Ignaz von Mitis),[12] almost half a century had to wait until in 1874 the St. Louis Bridge in Missouri over the Mississippi River was built using steel on a widespread basis. Designed by the American engineer James Buchanan Eads, it consists of three lattice arches (152+157+152 m span), and is considered the first large steel bridge, in which numerous novel construction procedures were also introduced, such as the execution of the arches by successive cantilevers without the need for falsework (provisionally supporting them from piers and abutments).[12] Curiously, more than a decade later, Eiffel is still there. He built the Garabit viaduct in 1886 with padded iron, preferring a material that he knew and mastered rather than risk relying on steel. However, the introduction of steel as a substitute for wrought iron was already unstoppable, and the two largest bridges at the end of the century were built with this material: the Brooklyn Bridge (1883; 1825 m and main span of 486 m) over the East River in New York, an iconic model of suspension bridge designed by John Augustus Roebling; and the Forth Bridge in Scotland, (1890; 210+521+521+210 m span) attributed to John Fowler among others, a huge corbel bridge with an unusual figure that meant doubling (reaching 500 m) the maximum span of this type of structures (until then, belonging to the Sukkur Bridge over an arm of the Indus River in Pakistan, with 250 m span). light).[13].
The emergence of high-performance steel manufacturing techniques meant that metal truss bridges multiplied in parallel with the development of the railway network. The golden age of this type of structures lasted until the First World War, despite the appearance in France in 1898 of a formidable competing material, reinforced concrete (later prestressed), which a few decades later would manage to almost completely replace steel in small span bridges, thanks to its versatility and lower maintenance requirements. However, in long-span bridges, metal structures maintained their primacy throughout the century, which they retained in the first decades of the century. With steel as the protagonist, large lattice arches were built (such as the Hell Gate Bridge (1916; 298 m span) in New York, or the Sydney Harbor Bridge (1932; 503 m span)), and especially a series of spectacular suspension bridges of increasingly larger spans that followed the path initiated by the Brooklyn Bridge, among which we can mention the Golden Gate Bridge in San Francisco (California) "San Francisco (California)") (1937; span 1280 m); the Verrazano-Narrows Bridge in New York (1964; span 1,298 m); the Humber Bridge between Wales and England (1981; span 1,410 m); or the one with the greatest span so far, the Great Akashi Kaikyō Bridge inaugurated in Japan in 1998, which reaches 1991 m span.[14].
The cable-stayed bridges deserve special mention, the first modern example of which is the Strömsund Bridge), built in Sweden in 1955 with a maximum span of 183 m. This typology, which can be considered halfway between metallic and mixed structures due to its considerable use of concrete, has developed extraordinarily in the second half of the century, with achievements such as the Normandy Bridge (1995; 856 m maximum span), the Great Tatara Bridge (1999; 890 m maximum span), the Río-Antirio bridge (2004; 560 m maximum span and 2883 m total length) or the spectacular Millau bridge. Cable-stayed bridges have demonstrated great versatility, finding applications in spans between 100 and 1000 m in length, making them competitive with other types of structures thanks to the slenderness of their decks. whose rigidity contributes to enhancing its structural efficiency.[15].
Materials
Contenido
A lo largo del desarrollo de los puentes metálicos, se han empleado fundamentalmente tres tipos de materiales siderúrgicos en tres períodos sucesivos. Así mismo, de forma excepcional se han utilizado otros metales, como el aluminio:.
Iron foundry
Cast iron is an alloy of iron and carbon (in percentages greater than 2%), which normally contains other impurities, such as phosphorus, silicon and sulfur (which tend to make the metal more brittle), as well as inclusions of slag "Glut (metallurgy)") (which make it brittle). This material began to be produced in large quantities thanks to the development at the beginning of the century by Abraham Darby I of blast furnaces fueled with hard coal transformed into coke. Due to its high carbon content, cast iron is melted at temperatures that are not too high (around 750 °C), a circumstance that makes it relatively easy to liquefy in low ovens to be poured into molds, with which large solid pieces with the desired geometry could be manufactured. This resulted in the construction of the first cast iron bridge at the end of the century, the famous Coalbrookdale Iron Bridge, in which some of its cast pieces measure more than 15 meters.[6].
Despite being much more resistant to compression than stone ashlars or wooden pillars (which made cast iron a suitable material for building arches similar to those made of stone, where practically no traction is generated), the cast pieces could present internal defects (such as bubbles or large inclusions of slag), capable of significantly reducing their load-bearing capacity. In addition, they were difficult to join together (molded lugs were used at the same time as the pieces and pins to connect them), and their fragile behavior made their use problematic in the large beams required by railway bridges, so they tended to be reinforced with pieces of wrought iron, a much more ductile material that would eventually completely replace cast iron in the middle of the century.
wrought iron
Wrought iron differs from cast iron fundamentally in its chemical composition and mechanical properties, given that it is a material with a very low carbon content, much more ductile, and capable of withstanding bending and deformation before breaking. For the pig iron to acquire these properties, it must be refined in a reverberatory furnace to eliminate its carbon content and most of the embrittlement elements such as phosphorus, sulfur and silicon, and subject the resulting material to a subsequent mechanical process of rolling and hammering, with which it was possible to reduce the proportion of slag inclusions and at the same time give them a fibrillar arrangement, which was used to increase the flexibility of the profiles in the form of bar. The industrial process to obtain wrought iron was called pudding. Fully developed in the middle of the century, it consisted of treating pig iron in a reverberatory furnace, vigorously manually stirring the molten iron to ensure that the oxygen in the air burned most of the carbon and impurities dissolved in the metal. When the carbon content was reduced sufficiently (to reach 0.01%),[16] the melting point of the material increased locally and solid iron lenses were formed in the bath of molten metal and slag, which were removed from the furnace to be subjected first to cingling to remove most of the slag and then to a rolling and forging process.
This material became the most used in metal bridges during the second half of the century, beginning with the Britannia Bridge in 1849 and culminating with the great achievements of Gustave Eiffel (such as the Garabit Viaduct of 1886), when it definitively gave way to steel from the beginning of the century. The laminating process and the standardization of laminated products and profiles led to the appearance of triangulated typologies, which would continue to be widely used until the middle of the century.[17].
The method used to join wrought iron pieces was locking, a system that provided highly resistant rigid joints.
Steels
The steels currently used for constructions are iron-carbon alloys with a low proportion of carbon (between 0.2% and 2%), with other metals added to improve certain mechanical characteristics.[18] They have their origin in the development of converters (especially Bessemer and Martin-Siemens) during the second half of the century, and the constant improvement of refining techniques during the first half of the century, which have allowed the commercialization of steels with characteristics increasingly better mechanics.
For aesthetic or safety reasons, other types of steel can sometimes be found, such as Corten steel or stainless steel.
In general, the steels used in the construction of bridges have the following characteristics:[19].
For safety reasons, the steel in the structures is calculated to work well below its elastic limit. Fatigue phenomena limit the allowable stresses to approximately half of this limit, i.e. 120 MPa "Pascal (unit)") for ordinary mild steel and 180 MPa for high-strength steel. It is this fatigue limit that is used as a reference in the calculation of structures.
Other characteristics may also be involved: temperature, resistance to corrosion due to mechanical stress") or resistance in a saline environment.
The most common system for joining steel pieces today is electrogenic welding.
Aluminum
Some mobile aluminum bridges have been designed, in which the lightness of the material allows reducing the cost of the machinery that must operate the opening of the bridge. A well-known example is the Hendon Dock Bridge in Sunderland (England). With a span of 27 meters, it was built in 1948 and was in service for 28 years. It is a solution that has not become widespread, due to the maintenance problems of this type of structures.[20].
Metal parts joining systems
Los procesos de ensamblaje de piezas de acero son la unión mediante pernos, el remachado, el atornillado y la soldadura. Hoy en día todos los puentes metálicos son soldados, y los remaches dejaron de utilizarse hace mucho tiempo. En cuanto a los tornillos, todavía se utilizan para puentes provisionales, que traídos en partes separadas, se pueden montar mediante atornillado muy rápidamente.
Union by bolts
Two types of joints using smooth bars began to be used in castings: articulated joints by inserting a pin perpendicular to the axes of the pieces in a pair of lugs (one per piece); or butt joints of two pieces, sewing several pairs of lugs with pins parallel to the axis. This system continued to be used in America until the beginning of the century, but the collapse of the first Quebec bridge in 1907 surely caused it to stop being used.[13] Today it is still used in some types of articulated joints.
bolted joint
The bolts act by riveting one of their ends hot in order to lock a pair of flat elements between their two heads. Furthermore, when applied hot, when they cool the resulting compression strengthens the bond. It was the most used system until the 1920s, when they were completely replaced by electric welding systems. For example, on the Forth Bridge, all the joints are bolted.[21].
Riveted joints were the most common type of connection before welding became widespread. The straight truss bridges of the century and the beginning of the century were riveted. The photo to the side shows a detail of the classic assembly of these lattices using slats. The vertical and horizontal uprights are made up of angular profiles and flat plates riveted together. The inclined uprights are made from U-shaped bars. At the junction of each set, plates, called gussets and joint covers, are added to give rigidity to the nodes of the structure.
Threaded bolts are cold set. Initially they were mainly used to make temporary assemblies or to replace rivets in certain special cases in which they would malfunction. They have a threaded head "Thread (mechanical)") into which a nut fits that is screwed using torque wrenches that allow the applied tightening torque to be adjusted. High-strength prestressed screws join the pieces thanks to the introduced compression and induced friction, while in bolts and bolts the tensile and shear resistance of the tenon is relied upon.[22].
welded joint
Welding ensures assembly at the joints or contact planes of the elements to be assembled, generating steel cords that provide mechanical continuity to the linked elements. For this purpose, coated steel rods called electrodes are used, which melt under the extremely high temperature of an electric arc.
The photo on the right shows the typical modern assembly between metal parts. These are two welded full web beams. The transverse beam, called the "bridge piece" or "spacer", is welded to the longitudinal piece, called the side member (whose web can be seen flat at the bottom of the photo). Also note the presence of a vertical stiffener, which ensures the non-deformability of the assembly. This stiffener is terminated at its base by a gusset.
A key aspect in the weldability of steel is its chemical composition, so that the presence of impurities such as phosphorus above certain minimum thresholds makes effective joining between pieces extremely difficult. This circumstance means that many old metal structures (before the 1950s) cannot be repaired by welding, and screwed joints must be used.
The technology of welded joints, which had already been studied since the 1920s, saw its first realizations in the 1930s, but would not be completely mastered until the Second World War in Germany, where it reached its maturity in the postwar period, when it was systematically used in the reconstruction of the country's infrastructure.[22].
Sheets and profiles
Depending on their dimensions, steel sheets receive the following commercial names:[23].
metal beams
Las vigas metálicas tienen generalmente el perfil de una (también conocida como doble T, aunque también se fabrican con perfiles en U, en L o incluso en perfiles rectangulares cerrados.
Se distingue entre vigas de alma llena y vigas de celosía.
Full web beams
Full web beams are essentially composed of one or more vertical parts that form the web&action=edit&redlink=1 "Web (beam) (not yet drafted)") and horizontal parts called flanges, arranged on either side of the web. They can be formed hot by rolling (with profiles of standard dimensions), or produced cold by assembling flat welded plates (or in the past, by riveted angular profiles). The wings constitute what are called the chords of the beam (the upper chord and the lower chord).
Lattices
Lattice beams, also called triangulated beams, are made up of chords connected by bars (vertical or inclined, instead of a full web), with which they form a triangulated whole. These bars are arranged differently depending on the triangulation system adopted.
The commonly used systems are:
Types of metal bridges
Straight girder bridges
The beams with the smallest span usually have a "T" section, they change to the "double T" type (widening the lower flange) as the span grows, and for much longer spans either the web is lightened (which gives rise to lattice beams) or the resistant efficiency of the section is increased to the maximum (giving rise to full web beams or box beams).[24] Regardless of their typology, structurally they can be continuous or edge-to-edge. variable, or be continuous beams with several intermediate supports or be simply supported.
After the Second World War, the development of welding and the widespread use of laminated sheet metal led to the replacement of triangulated beams with box beams. In addition, orthotropic slabs were introduced, capable of supporting concentrated loads at any point on the deck as required by automobile traffic.[14] Depending on the structure of the beam, a distinction is made between:.
Portal bridges
The portal bridge is an intermediate structural form between the beam and the arch. In practice, an arc with a simplified guideline can be considered a polygonal arrangement. They can be two-mounted, two-jointed or three-jointed. An outstanding example of a metal gantry bridge is the Sfalassa Bridge in Italy. Inaugurated in 1973, it has a span of 360 meters.[26].
Lenticular bridges
Structurally halfway between a beam and an arch, lenticular bridges are a rare solution, which consists of adopting a triangulated arrangement whose upper chord is a segmental arch and its lower chord is symmetrical to the upper chord with respect to a horizontal plane. Famous examples of this typology are Brunel's Royal Albert Bridge or the New Bridge over the Elbe").[27].
Arches
The first iron bridges were arches, such as the famous Coalbrookdale Iron Bridge, similar to stone bridges. However, it did not take long for all the typologies used until today to be developed, using lattices or full-web beams.[28] From a functional or simply aesthetic point of view, arches can be classified depending on the position of the board with respect to the arch (inferior, intermediate, coinciding with the keystone or even above the keystone), but from a structural point of view there are two fundamental types of arches: those in which the lateral thrust of the arch is compensated by the abutments or bridge piers; and those in which this thrust is compensated by the deck, which works in tension like the lower chord of a lattice beam. In fact, this second configuration, known in English as "bowstring", behaves like a simply supported straight beam, and does not transmit horizontal thrusts in the abutments.
Two notable examples of this type of structure are the Bayonne Bridge at Kill Van Kull (1931; 510 m span), and the New River Gorge Bridge (1976; 518 m span), both in the United States.
The metal arches with large spans built from triangulated elements until the first half of the century have given way to smaller metal arches with a closed section (with spans generally between 100 and 250 m) in which very varied typologies have been developed, in which the configuration of the starts (simple or open in an inverted "V"), the number of arches (a single arch between two panels or a deck between two arches), the arrangement of the arches (vertical, or inclined to approximate the keystone), or the way of joining boards and arches (vertical or crossed cables, hangers, lattices...). The city of Zaragoza in Spain brings together a set of metal arch bridges over the Ebro River of different typologies and periods that illustrate this circumstance.
inverted arches
A metal inverted arch bridge includes a curved chord whose keystone is located below its legs, as if a conventional arch had been "flipped" in which the keystone is located above the legs. Structurally, the curved chord works in tension (unlike what happens in a conventional arch, where it works in compression), so it has points in common with suspension bridges on the one hand, and with lattice beams on the other, and can be considered a limit solution between both types. (engineer).) Guadalquivir.[29].
Cable-stayed bridges
The idea of using cables to reinforce truss girders (such as the Whipple truss) or the Howe truss)[30] or suspension bridges (as in the case of the Brooklyn Bridge, where you can see stays stretched from the towers that supplement the suspension hangers), had already been used in the 19th century, but did not acquire its own development until the middle of the 19th century.
Structurally, a cable-stayed bridge combines a series of inclined cables stretched from one or several towers to a deck,[15] in which the inclination of the stays converts part of the weight of the deck into a compression effect. This compression effect functionally differentiates them from suspension bridges, in which the deck is suspended vertically from the main cable. The use of concrete in the towers and especially in the decks, sometimes means that they are considered mixed structures.
As in the case of arches, it is an extraordinarily versatile typology from an aesthetic point of view, and in fact numerous configurations have been developed that play with the number of cables (individual braces or in bundles), with their arrangement (on each side of the board or only in its center), with the way of distributing them (in a harp when they are parallel; in a fan when they converge at a point on the tower; or in a star when they converge at a point on the board), and especially with the configuration of the towers (in "H", in "delta", monolithic, vertical, inclined...).
The cross section of the deck is conditioned by the position of the stays,[31] so that in bridges with stays in the axis a two-cell box with lateral cantilevers is usually provided, while with the stays arranged laterally a single box reinforced internally is usually used.
Among the numerous creations of this type of structures, two archetypal examples built in Spain can be cited: the Ingeniero Carlos Fernández Casado Bridge (1983; 440 m maximum span) over the Barrios de Luna reservoir, designed by Carlos Fernández Casado; and the Puente del Alamillo (1992; 250 m span); designed by Santiago Calatrava.
Suspension bridges
A suspension bridge is characterized in that the deck only transmits vertical loads (through a series of hangers) to the supporting elements, which historically began as ropes, then chains, later articulated bars, and finally steel cables. The main virtue of suspension bridges is at the same time their main problem: their lightness.[32] The deck is associated with the rigidity beam, whose structure is generally a metal lattice, although suspension bridges are also designed whose deck is a metal box girder.
In addition to the deck, an element of great importance are the towers that support the cables, which in the first small bridges used to be made of wood. However, the increase in spans and the loads they entail meant that they had to be replaced first by masonry or ashlar structures (such as the Egyptian-inspired pylons of the Clifton Suspension Bridge in Bristol, or the pointed arches of granite blocks of the Brooklyn Bridge) and then by metal frames, although in some cases the latter were covered with stone or brick to give them a more architectural appearance. However, this trend was abandoned throughout the century (the George Washington Bridge in New York is a good example of this), and all subsequent large suspension bridges have left the structure of their steel or sometimes concrete towers exposed.
Taking into account their general configuration, practically all large modern suspension bridges have the same configuration: two support cables that span the central span and the two accompanying spans, which pass through the crown of the towers and usually end in two lateral anchoring masses. However, from the middle of the century and at the beginning of the century some suspension bridges with several spans were built (such as the Robinet bridge over the Rhône River in 1847)[32] and in 1990 the Konohana bridge was inaugurated), an exceptional suspension bridge with a main span of 300 m provided with a single supporting cable.[32]
Movable bridges
Mobile bridges are special cases of some of the previous typologies. In general, they tend to be beams of considerable rigidity to allow them to withstand the forces generated by the rotation, tilting or vertical displacement movements to which they are subjected. It is for this reason that they are usually steel bridges, although some have been built with aluminum such as the aforementioned Hendon Dock Bridge") in Sunderland.[20].
Find more "Construction of Metal Bridges" in the following countries:
[6] ↑ a b c d Fernández Troyano, 2004, p. 203 T-I.
[7] ↑ Fernández Troyano, 2004, p. 204 T-I.
[8] ↑ Serna García-Conde, 2006, p. 63.
[9] ↑ Beaumont, Martin (2015). Sir John Hawkshaw 1811–1891. The Lancashire & Yorkshire Railway Society www.lyrs.org.uk. pp. 108-111. ISBN 978-0-9559467-7-6.
wrought iron bridges, a material capable of much better resistance to traction and bending without cracking, replaced cast iron bridges, overcoming the problems caused by the fragility of this material. The collapse of several structures in the United Kingdom, as in the case of the Tay Bridge disaster in 1879, led to the avoidance of the use of cast iron in railway bridges, which would be replaced by wrought iron. The first large structures built with this material were the Conwy railway bridge (1849; span 125 m) and the Britannia Bridge (1850; spans 70+2x142+70 m), two box girders designed by Robert Stephenson that cross the Menai Straits. Its novel configuration is considered one of the most innovative designs of all times, becoming the first large girder and box girder bridges in history, and the first structures to systematically use wrought iron.[6].
The transition from cast iron to wrought iron represented a radical change in the way metal bridges had been conceived until then. Casting pieces were produced by molding, so that their sections were usually solid, more suitable for working in compression (like the ashlars of a stone bridge) than in flexion. However, wrought iron, to acquire its mechanical properties, needed to be rolled or extruded and repeatedly hammered, for which the simplest thing was to manufacture it by giving it the shape of relatively thin plates, which could be bent longitudinally to produce bars with simple profiles, suitable for withstanding traction, compression and bending efforts with equal effectiveness. In this way, the availability of iron sheets led to the appearance of the box girder typology, while the use of profiles in the shape of long bars was at the origin of lattice arches and triangulated beams, the latter typology that became omnipresent during the second half of the century and the first decades of the century due to its simplicity of calculation, ease of construction and reduced cost compared to other solutions.[7].
Wrought iron allowed the development of large triangulated structures, with notable examples such as the Royal Albert Bridge (1859; two arches of 135 m span) by Isambard Kingdom Brunel over the estuary of the Tamar River,[8] and the numerous works designed by the Frenchman Gustave Eiffel, designer of the Garabit Viaduct (1886; a lattice arch of 165 m span) and the Don Luis I Bridge (1886; a lattice arch spanning 174 m) in Porto together with his partner Théophile Seyrig. Another unique work of wrought iron, although designed for lighter loads than those of the railway, was the Clifton suspension bridge (1864; central span of 214 m span) in Bristol, a chain-suspended bridge designed based on a proposal by Brunel.[9].
However, the idea of triangulated beams originated in the United States. Based on the designs of wooden bridges, an abundant and affordable material widely used in North America, different wrought iron lattice systems suitable for the passage of railways were patented (Town, 1820; Howe, 1840; Pratt, 1844; or Whipple, 1847)[10] that competed with each other to obtain the greatest possible lightness and resistance. These first designs, which still combined steel and wooden bars, were quickly assimilated in Europe, where the scarcity of wood and the availability of iron meant that from the beginning the lattices became completely metallic. Europe would precisely be the origin of the Warren beam, the culmination of the process that tended to simplify the geometry of the different types of triangulated beams.[11].
Steel bridges represented an important new technological leap, brought about by the development of metallurgical converters (Bessemer and Thomas), and especially the Martin-Siemens furnace, which allowed steel of increasingly better quality and in large quantities to be available from the last two decades of the century. Although it had previously been used in isolation (as in the Vienna Chain Bridge, designed in 1828 by Ignaz von Mitis),[12] almost half a century had to wait until in 1874 the St. Louis Bridge in Missouri over the Mississippi River was built using steel on a widespread basis. Designed by the American engineer James Buchanan Eads, it consists of three lattice arches (152+157+152 m span), and is considered the first large steel bridge, in which numerous novel construction procedures were also introduced, such as the execution of the arches by successive cantilevers without the need for falsework (provisionally supporting them from piers and abutments).[12] Curiously, more than a decade later, Eiffel is still there. He built the Garabit viaduct in 1886 with padded iron, preferring a material that he knew and mastered rather than risk relying on steel. However, the introduction of steel as a substitute for wrought iron was already unstoppable, and the two largest bridges at the end of the century were built with this material: the Brooklyn Bridge (1883; 1825 m and main span of 486 m) over the East River in New York, an iconic model of suspension bridge designed by John Augustus Roebling; and the Forth Bridge in Scotland, (1890; 210+521+521+210 m span) attributed to John Fowler among others, a huge corbel bridge with an unusual figure that meant doubling (reaching 500 m) the maximum span of this type of structures (until then, belonging to the Sukkur Bridge over an arm of the Indus River in Pakistan, with 250 m span). light).[13].
The emergence of high-performance steel manufacturing techniques meant that metal truss bridges multiplied in parallel with the development of the railway network. The golden age of this type of structures lasted until the First World War, despite the appearance in France in 1898 of a formidable competing material, reinforced concrete (later prestressed), which a few decades later would manage to almost completely replace steel in small span bridges, thanks to its versatility and lower maintenance requirements. However, in long-span bridges, metal structures maintained their primacy throughout the century, which they retained in the first decades of the century. With steel as the protagonist, large lattice arches were built (such as the Hell Gate Bridge (1916; 298 m span) in New York, or the Sydney Harbor Bridge (1932; 503 m span)), and especially a series of spectacular suspension bridges of increasingly larger spans that followed the path initiated by the Brooklyn Bridge, among which we can mention the Golden Gate Bridge in San Francisco (California) "San Francisco (California)") (1937; span 1280 m); the Verrazano-Narrows Bridge in New York (1964; span 1,298 m); the Humber Bridge between Wales and England (1981; span 1,410 m); or the one with the greatest span so far, the Great Akashi Kaikyō Bridge inaugurated in Japan in 1998, which reaches 1991 m span.[14].
The cable-stayed bridges deserve special mention, the first modern example of which is the Strömsund Bridge), built in Sweden in 1955 with a maximum span of 183 m. This typology, which can be considered halfway between metallic and mixed structures due to its considerable use of concrete, has developed extraordinarily in the second half of the century, with achievements such as the Normandy Bridge (1995; 856 m maximum span), the Great Tatara Bridge (1999; 890 m maximum span), the Río-Antirio bridge (2004; 560 m maximum span and 2883 m total length) or the spectacular Millau bridge. Cable-stayed bridges have demonstrated great versatility, finding applications in spans between 100 and 1000 m in length, making them competitive with other types of structures thanks to the slenderness of their decks. whose rigidity contributes to enhancing its structural efficiency.[15].
Materials
Contenido
A lo largo del desarrollo de los puentes metálicos, se han empleado fundamentalmente tres tipos de materiales siderúrgicos en tres períodos sucesivos. Así mismo, de forma excepcional se han utilizado otros metales, como el aluminio:.
Iron foundry
Cast iron is an alloy of iron and carbon (in percentages greater than 2%), which normally contains other impurities, such as phosphorus, silicon and sulfur (which tend to make the metal more brittle), as well as inclusions of slag "Glut (metallurgy)") (which make it brittle). This material began to be produced in large quantities thanks to the development at the beginning of the century by Abraham Darby I of blast furnaces fueled with hard coal transformed into coke. Due to its high carbon content, cast iron is melted at temperatures that are not too high (around 750 °C), a circumstance that makes it relatively easy to liquefy in low ovens to be poured into molds, with which large solid pieces with the desired geometry could be manufactured. This resulted in the construction of the first cast iron bridge at the end of the century, the famous Coalbrookdale Iron Bridge, in which some of its cast pieces measure more than 15 meters.[6].
Despite being much more resistant to compression than stone ashlars or wooden pillars (which made cast iron a suitable material for building arches similar to those made of stone, where practically no traction is generated), the cast pieces could present internal defects (such as bubbles or large inclusions of slag), capable of significantly reducing their load-bearing capacity. In addition, they were difficult to join together (molded lugs were used at the same time as the pieces and pins to connect them), and their fragile behavior made their use problematic in the large beams required by railway bridges, so they tended to be reinforced with pieces of wrought iron, a much more ductile material that would eventually completely replace cast iron in the middle of the century.
wrought iron
Wrought iron differs from cast iron fundamentally in its chemical composition and mechanical properties, given that it is a material with a very low carbon content, much more ductile, and capable of withstanding bending and deformation before breaking. For the pig iron to acquire these properties, it must be refined in a reverberatory furnace to eliminate its carbon content and most of the embrittlement elements such as phosphorus, sulfur and silicon, and subject the resulting material to a subsequent mechanical process of rolling and hammering, with which it was possible to reduce the proportion of slag inclusions and at the same time give them a fibrillar arrangement, which was used to increase the flexibility of the profiles in the form of bar. The industrial process to obtain wrought iron was called pudding. Fully developed in the middle of the century, it consisted of treating pig iron in a reverberatory furnace, vigorously manually stirring the molten iron to ensure that the oxygen in the air burned most of the carbon and impurities dissolved in the metal. When the carbon content was reduced sufficiently (to reach 0.01%),[16] the melting point of the material increased locally and solid iron lenses were formed in the bath of molten metal and slag, which were removed from the furnace to be subjected first to cingling to remove most of the slag and then to a rolling and forging process.
This material became the most used in metal bridges during the second half of the century, beginning with the Britannia Bridge in 1849 and culminating with the great achievements of Gustave Eiffel (such as the Garabit Viaduct of 1886), when it definitively gave way to steel from the beginning of the century. The laminating process and the standardization of laminated products and profiles led to the appearance of triangulated typologies, which would continue to be widely used until the middle of the century.[17].
The method used to join wrought iron pieces was locking, a system that provided highly resistant rigid joints.
Steels
The steels currently used for constructions are iron-carbon alloys with a low proportion of carbon (between 0.2% and 2%), with other metals added to improve certain mechanical characteristics.[18] They have their origin in the development of converters (especially Bessemer and Martin-Siemens) during the second half of the century, and the constant improvement of refining techniques during the first half of the century, which have allowed the commercialization of steels with characteristics increasingly better mechanics.
For aesthetic or safety reasons, other types of steel can sometimes be found, such as Corten steel or stainless steel.
In general, the steels used in the construction of bridges have the following characteristics:[19].
For safety reasons, the steel in the structures is calculated to work well below its elastic limit. Fatigue phenomena limit the allowable stresses to approximately half of this limit, i.e. 120 MPa "Pascal (unit)") for ordinary mild steel and 180 MPa for high-strength steel. It is this fatigue limit that is used as a reference in the calculation of structures.
Other characteristics may also be involved: temperature, resistance to corrosion due to mechanical stress") or resistance in a saline environment.
The most common system for joining steel pieces today is electrogenic welding.
Aluminum
Some mobile aluminum bridges have been designed, in which the lightness of the material allows reducing the cost of the machinery that must operate the opening of the bridge. A well-known example is the Hendon Dock Bridge in Sunderland (England). With a span of 27 meters, it was built in 1948 and was in service for 28 years. It is a solution that has not become widespread, due to the maintenance problems of this type of structures.[20].
Metal parts joining systems
Los procesos de ensamblaje de piezas de acero son la unión mediante pernos, el remachado, el atornillado y la soldadura. Hoy en día todos los puentes metálicos son soldados, y los remaches dejaron de utilizarse hace mucho tiempo. En cuanto a los tornillos, todavía se utilizan para puentes provisionales, que traídos en partes separadas, se pueden montar mediante atornillado muy rápidamente.
Union by bolts
Two types of joints using smooth bars began to be used in castings: articulated joints by inserting a pin perpendicular to the axes of the pieces in a pair of lugs (one per piece); or butt joints of two pieces, sewing several pairs of lugs with pins parallel to the axis. This system continued to be used in America until the beginning of the century, but the collapse of the first Quebec bridge in 1907 surely caused it to stop being used.[13] Today it is still used in some types of articulated joints.
bolted joint
The bolts act by riveting one of their ends hot in order to lock a pair of flat elements between their two heads. Furthermore, when applied hot, when they cool the resulting compression strengthens the bond. It was the most used system until the 1920s, when they were completely replaced by electric welding systems. For example, on the Forth Bridge, all the joints are bolted.[21].
Riveted joints were the most common type of connection before welding became widespread. The straight truss bridges of the century and the beginning of the century were riveted. The photo to the side shows a detail of the classic assembly of these lattices using slats. The vertical and horizontal uprights are made up of angular profiles and flat plates riveted together. The inclined uprights are made from U-shaped bars. At the junction of each set, plates, called gussets and joint covers, are added to give rigidity to the nodes of the structure.
Threaded bolts are cold set. Initially they were mainly used to make temporary assemblies or to replace rivets in certain special cases in which they would malfunction. They have a threaded head "Thread (mechanical)") into which a nut fits that is screwed using torque wrenches that allow the applied tightening torque to be adjusted. High-strength prestressed screws join the pieces thanks to the introduced compression and induced friction, while in bolts and bolts the tensile and shear resistance of the tenon is relied upon.[22].
welded joint
Welding ensures assembly at the joints or contact planes of the elements to be assembled, generating steel cords that provide mechanical continuity to the linked elements. For this purpose, coated steel rods called electrodes are used, which melt under the extremely high temperature of an electric arc.
The photo on the right shows the typical modern assembly between metal parts. These are two welded full web beams. The transverse beam, called the "bridge piece" or "spacer", is welded to the longitudinal piece, called the side member (whose web can be seen flat at the bottom of the photo). Also note the presence of a vertical stiffener, which ensures the non-deformability of the assembly. This stiffener is terminated at its base by a gusset.
A key aspect in the weldability of steel is its chemical composition, so that the presence of impurities such as phosphorus above certain minimum thresholds makes effective joining between pieces extremely difficult. This circumstance means that many old metal structures (before the 1950s) cannot be repaired by welding, and screwed joints must be used.
The technology of welded joints, which had already been studied since the 1920s, saw its first realizations in the 1930s, but would not be completely mastered until the Second World War in Germany, where it reached its maturity in the postwar period, when it was systematically used in the reconstruction of the country's infrastructure.[22].
Sheets and profiles
Depending on their dimensions, steel sheets receive the following commercial names:[23].
metal beams
Las vigas metálicas tienen generalmente el perfil de una (también conocida como doble T, aunque también se fabrican con perfiles en U, en L o incluso en perfiles rectangulares cerrados.
Se distingue entre vigas de alma llena y vigas de celosía.
Full web beams
Full web beams are essentially composed of one or more vertical parts that form the web&action=edit&redlink=1 "Web (beam) (not yet drafted)") and horizontal parts called flanges, arranged on either side of the web. They can be formed hot by rolling (with profiles of standard dimensions), or produced cold by assembling flat welded plates (or in the past, by riveted angular profiles). The wings constitute what are called the chords of the beam (the upper chord and the lower chord).
Lattices
Lattice beams, also called triangulated beams, are made up of chords connected by bars (vertical or inclined, instead of a full web), with which they form a triangulated whole. These bars are arranged differently depending on the triangulation system adopted.
The commonly used systems are:
Types of metal bridges
Straight girder bridges
The beams with the smallest span usually have a "T" section, they change to the "double T" type (widening the lower flange) as the span grows, and for much longer spans either the web is lightened (which gives rise to lattice beams) or the resistant efficiency of the section is increased to the maximum (giving rise to full web beams or box beams).[24] Regardless of their typology, structurally they can be continuous or edge-to-edge. variable, or be continuous beams with several intermediate supports or be simply supported.
After the Second World War, the development of welding and the widespread use of laminated sheet metal led to the replacement of triangulated beams with box beams. In addition, orthotropic slabs were introduced, capable of supporting concentrated loads at any point on the deck as required by automobile traffic.[14] Depending on the structure of the beam, a distinction is made between:.
Portal bridges
The portal bridge is an intermediate structural form between the beam and the arch. In practice, an arc with a simplified guideline can be considered a polygonal arrangement. They can be two-mounted, two-jointed or three-jointed. An outstanding example of a metal gantry bridge is the Sfalassa Bridge in Italy. Inaugurated in 1973, it has a span of 360 meters.[26].
Lenticular bridges
Structurally halfway between a beam and an arch, lenticular bridges are a rare solution, which consists of adopting a triangulated arrangement whose upper chord is a segmental arch and its lower chord is symmetrical to the upper chord with respect to a horizontal plane. Famous examples of this typology are Brunel's Royal Albert Bridge or the New Bridge over the Elbe").[27].
Arches
The first iron bridges were arches, such as the famous Coalbrookdale Iron Bridge, similar to stone bridges. However, it did not take long for all the typologies used until today to be developed, using lattices or full-web beams.[28] From a functional or simply aesthetic point of view, arches can be classified depending on the position of the board with respect to the arch (inferior, intermediate, coinciding with the keystone or even above the keystone), but from a structural point of view there are two fundamental types of arches: those in which the lateral thrust of the arch is compensated by the abutments or bridge piers; and those in which this thrust is compensated by the deck, which works in tension like the lower chord of a lattice beam. In fact, this second configuration, known in English as "bowstring", behaves like a simply supported straight beam, and does not transmit horizontal thrusts in the abutments.
Two notable examples of this type of structure are the Bayonne Bridge at Kill Van Kull (1931; 510 m span), and the New River Gorge Bridge (1976; 518 m span), both in the United States.
The metal arches with large spans built from triangulated elements until the first half of the century have given way to smaller metal arches with a closed section (with spans generally between 100 and 250 m) in which very varied typologies have been developed, in which the configuration of the starts (simple or open in an inverted "V"), the number of arches (a single arch between two panels or a deck between two arches), the arrangement of the arches (vertical, or inclined to approximate the keystone), or the way of joining boards and arches (vertical or crossed cables, hangers, lattices...). The city of Zaragoza in Spain brings together a set of metal arch bridges over the Ebro River of different typologies and periods that illustrate this circumstance.
inverted arches
A metal inverted arch bridge includes a curved chord whose keystone is located below its legs, as if a conventional arch had been "flipped" in which the keystone is located above the legs. Structurally, the curved chord works in tension (unlike what happens in a conventional arch, where it works in compression), so it has points in common with suspension bridges on the one hand, and with lattice beams on the other, and can be considered a limit solution between both types. (engineer).) Guadalquivir.[29].
Cable-stayed bridges
The idea of using cables to reinforce truss girders (such as the Whipple truss) or the Howe truss)[30] or suspension bridges (as in the case of the Brooklyn Bridge, where you can see stays stretched from the towers that supplement the suspension hangers), had already been used in the 19th century, but did not acquire its own development until the middle of the 19th century.
Structurally, a cable-stayed bridge combines a series of inclined cables stretched from one or several towers to a deck,[15] in which the inclination of the stays converts part of the weight of the deck into a compression effect. This compression effect functionally differentiates them from suspension bridges, in which the deck is suspended vertically from the main cable. The use of concrete in the towers and especially in the decks, sometimes means that they are considered mixed structures.
As in the case of arches, it is an extraordinarily versatile typology from an aesthetic point of view, and in fact numerous configurations have been developed that play with the number of cables (individual braces or in bundles), with their arrangement (on each side of the board or only in its center), with the way of distributing them (in a harp when they are parallel; in a fan when they converge at a point on the tower; or in a star when they converge at a point on the board), and especially with the configuration of the towers (in "H", in "delta", monolithic, vertical, inclined...).
The cross section of the deck is conditioned by the position of the stays,[31] so that in bridges with stays in the axis a two-cell box with lateral cantilevers is usually provided, while with the stays arranged laterally a single box reinforced internally is usually used.
Among the numerous creations of this type of structures, two archetypal examples built in Spain can be cited: the Ingeniero Carlos Fernández Casado Bridge (1983; 440 m maximum span) over the Barrios de Luna reservoir, designed by Carlos Fernández Casado; and the Puente del Alamillo (1992; 250 m span); designed by Santiago Calatrava.
Suspension bridges
A suspension bridge is characterized in that the deck only transmits vertical loads (through a series of hangers) to the supporting elements, which historically began as ropes, then chains, later articulated bars, and finally steel cables. The main virtue of suspension bridges is at the same time their main problem: their lightness.[32] The deck is associated with the rigidity beam, whose structure is generally a metal lattice, although suspension bridges are also designed whose deck is a metal box girder.
In addition to the deck, an element of great importance are the towers that support the cables, which in the first small bridges used to be made of wood. However, the increase in spans and the loads they entail meant that they had to be replaced first by masonry or ashlar structures (such as the Egyptian-inspired pylons of the Clifton Suspension Bridge in Bristol, or the pointed arches of granite blocks of the Brooklyn Bridge) and then by metal frames, although in some cases the latter were covered with stone or brick to give them a more architectural appearance. However, this trend was abandoned throughout the century (the George Washington Bridge in New York is a good example of this), and all subsequent large suspension bridges have left the structure of their steel or sometimes concrete towers exposed.
Taking into account their general configuration, practically all large modern suspension bridges have the same configuration: two support cables that span the central span and the two accompanying spans, which pass through the crown of the towers and usually end in two lateral anchoring masses. However, from the middle of the century and at the beginning of the century some suspension bridges with several spans were built (such as the Robinet bridge over the Rhône River in 1847)[32] and in 1990 the Konohana bridge was inaugurated), an exceptional suspension bridge with a main span of 300 m provided with a single supporting cable.[32]
Movable bridges
Mobile bridges are special cases of some of the previous typologies. In general, they tend to be beams of considerable rigidity to allow them to withstand the forces generated by the rotation, tilting or vertical displacement movements to which they are subjected. It is for this reason that they are usually steel bridges, although some have been built with aluminum such as the aforementioned Hendon Dock Bridge") in Sunderland.[20].
Find more "Construction of Metal Bridges" in the following countries:
[6] ↑ a b c d Fernández Troyano, 2004, p. 203 T-I.
[7] ↑ Fernández Troyano, 2004, p. 204 T-I.
[8] ↑ Serna García-Conde, 2006, p. 63.
[9] ↑ Beaumont, Martin (2015). Sir John Hawkshaw 1811–1891. The Lancashire & Yorkshire Railway Society www.lyrs.org.uk. pp. 108-111. ISBN 978-0-9559467-7-6.