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怎么折小星星Evolution of Cable Supported Bridges
The principle of carrying loads by suspending a rope,chain or cable across an obstacle has been known since ancient times.However,it was not until 1823that the first permanent bridge supported by cables compod of drawn iron wires was built in Geneva by the Frenchman Marc Seguin,one of five brothers who,in the following two decades,built hundreds of suspension bridges around Europe.All of the bridges were of modest size but they marked an important step on the way to the more impressive structures that followed.
The application of thin wires in the main load-carrying elements gave ri to a number of problems especially in relation to durability,as an efficient method for corrosion protection had not been found at that time.Therefore,some of the leading engineers preferred to construct suspension bridges with the main load-carrying elements,the catenaries,compod of pin-connected eye-bars forming huge chains.
This principle was applied by the British engineer Thomas Telford in the world’s first bridge to cross a strait ud by ocean-going vesls,the Menai Bridge between the British mainland and the Isle of An
gley (Figure 1.1).Opened to traffic in 1826,this bridge had its 176m long main span supported by chains asmbled from wrought iron eye-bars,each with a length of 2.9m.
Cable Supported Bridges:Concept and Design ,Third Edition.Niels J.Gimsing and Christos T.Georgakis.Ó2012John Wiley &Sons,Ltd.Published 2012by John Wiley &Sons,
Ltd.
Figure 1.1The suspension bridge across the Menai Strait (UK)征文题目
The chain support was generally preferred by the British engineers of the nineteenth century and a number of notable bridges were built,among the the famous Clifton Suspension Bridge,initially designed by Isambard Kingdom Brunel, but not actually constructed until after his death.The bridge was opened to traffic in1864and it comprid a main span of 214m–an impressive span,considering that the strength-to-density ratio of the wrought steel in the chains was less than one-fifth of the ratio of modern cable steel(Figure1.2).
幼儿园转学证明To erect the eye-bar chains,a temporary footway had to be established between the supporting points on the pylon tops and at the anchor blocks.In the ca of the Clifton Suspension Bridge,this temporary footway was supported by wire ropes, so the principle of cable support was actually applied,although only in the construction pha.
A most unusual bridge bad on application of eye-bar chains is the Albert Bridge across the Thames in London (Figure1.3).The bridge was built from1871to1873and it is characterized by combining the cable stayed and the suspension system.A part of the deck load is transferred to the strong top chain through hangers and the rest is carried by a number of straight chains radiating from the pylon tops.The system is statically indeterminate to such a degree that it was impossible with the available tools to calculate forces and moments to get even clo to the exact values.Nevert
heless the bridge with its122m-long main span is still in rvice although there are restrictions on the traffic allowed to pass over it(Figure1.4).
Chain support was also applied in a number of bridges on the European continent,but here it was to a larger extent in competition with cable supported suspension bridges.Thus the longest free span in Europe was for veral decades found in the wire supported Grand Pont Suspendu across the Sarine Valley at Fribourg in Switzerland.The bridge was completed in1834and it had a main span of273m.In the Grand Pont Suspendu,each of the four main cables was compod of over1000wires,grouped in20strands,each asmbled on the ground and lifted individually into position.The bridge was in rvice for almost a century until it wasfinally demolished in1923.
On a global level,the Swiss span record was beaten in1849by the completion of the Wheeling Suspension Bridge across the Ohio River in the USA.This bridge had a main span of308m,carried by a total of12parallel-wire cables,six on either side of the roadway.
The Wheeling Bridge is still in existence,although not in its original version.Five years after its completion,in1854,a violent gale blew the bridge down.Subquently it was reconstructed and later,in1872,further strengthened by a fan-shaped system of stays.The principle of strengthening the
suspension system with stays was originally introduced during the construction of the suspension bridge across the Niagara Gorge.This bridge was designed by the famous
bridge designer John A.Roebling,who was born in Germany but emigrated to the United States of America at
the
Figure1.2Clifton Suspension Bridge(UK) 8Cable Supported Bridges:Concept and Design
鲢鱼怎么做好吃age of 25.The Niagara Bridge was constructed in the period from 1851to 1855and it was the first major suspension bridge to have air-spun wire cables,a system invented by Roebling.
The span of the Niagara Bridge was not quite as long as for the largest suspension bridges of that time but,due to the fact that the bridge carried both a railroad track and a roadway,its span of 250m was still a very impressive achievement.As a most unusual feature the truss of the Niagara Bridge had the railroad track on the upper deck and the roadway on the lower,inside the two truss.
Another unusual feature of the Niagara Bridge was the u of wood in the truss.This might today em to be an awkward combination of structural materials but it must be remembered that in the early days of railroad building in North America,wood was the preferred material for bridges across rivers and gorges.For the Niagara Bridge,the application of a wooden truss resulted in a relatively short lifespan as the bridge had to be replaced in 1897after 42years of
征文比赛评分标准rvice.
Figure 1.3Albert Bridge across the Thames
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(UK)
Figure 1.4Traffic restrictions on the Albert Bridge (UK)强要
Evolution of Cable Supported Bridges 9
The largest of Roebling’s bridges completed during his lifetime,the Cincinnati–Covington Bridge across the Ohio River,was completed in 1866with a record-breaking span of 322m (Figure 1.5).In this bridge he tested many advanced features before they were adopted in his most sublime achievement:the design for the Brooklyn Bridge across the East River in New York.
Brooklyn Bridge
The Brooklyn Bridge across the East River between Manhattan and Long Island (Figure 1.6)is justifiably regarded as the ancestor of all modem suspension bridges and it was to a large degree detailed by Roebling before his death in 1869shortly after the start of construction of this,the greatest bridge of his career.Opened to traffic in 1883,the Brooklyn Bridge had a centre span of almost 500m (486m)and side spans of a total cable supported length of 1058
m.
Figure 1.5The suspension bridge across the Ohio River between Cincinnati and Covington
(USA)
Figure 1.6Brooklyn Bridge across the East River in New York (USA)
10Cable Supported Bridges:Concept and Design
Bad on his experience during design and construction of veral suspension bridges,and through his investigations into accidents such as the collap of the Wheeling Bridge in 1854,Roebling had acquired a profound understanding of the aerodynamic problem.This is clearly indicated in his own description of the Brooklyn Bridge concept:
But my system of construction differs radically from that formerly practid,and I have planned the East River Bridge [as the Brooklyn Bridge was initially called]with a special view to fully meet the destructive forces of a vere gale.It is the same reason that,in my calculation of the requisite supporting strength so large a proportion has been assigned to the stays in place of cables.
This description proves that Roebling knew very well that a cable stayed system is stiffer than the suspension system,and the fact that the stays of the Brooklyn Bridge carry a considerable part of the load can be detected by the configuration of the main cable having a smaller curvature in the regions where the stays carry a part of the permanent load than in the central region,where all load is carried exclusively by the main cable.
The efficiency of the stay cables (Figure 1.7)is clearly demonstrated by the following remark by Roeb
ling:‘The supporting power of the stays alone will be 15000tons;ample to hold up the floor.If the cables were removed,the bridge would sink in the center but would not fall.’
Roebling had started his engineering career at a time when the design of bridges was still more of an art,requiring intuition and vision,than a science.Therefore,he had to acquire a profound understanding of the structural behaviour of cable supported bridges through obrvations and by experience.He gradually learned how to design structures of great complexity,as he could combine his intuitive understanding with relatively simple calculations,giving adequate dimensions for all structural elements.
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In the ca of the Brooklyn Bridge,the system adopted is one of high indeterminateness as every stay is potentially a redundant.A strict calculation bad on the elastic theory with compatibility established between all elements would involve numerical work of an absolutely prohibitive magnitude,but by stipulating reasonable distributions of forces between elements and always ensuring that overall equilibrium was achieved,the required safety against failure could be attained.After Roebling,the next generation of engineers was educated to concentrate their efforts on the calculations,which required a stricter mathematical modelling.As systems of high statical indeterminateness would involve an insuperable amount of numerical work if treated mathematically
stringently,the layout of the structures had to be chon with due respect to the calculation capacity,and this was in many respects a step backwards.Conquently,a cable system such as that ud in the Brooklyn Bridge had to be replaced by much simpler systems.
The theories available for the calculation of suspension bridges in the cond half of the nineteenth century were all linear elastic theories,such as the theory by Rankine from 1858,dealing with suspension bridges where the deck
comprid Figure 1.7The cable system of the Brooklyn Bridge at the pylon (USA)
Evolution of Cable Supported Bridges 11
