Chapter 2
The first Bridgess were made by nature itself – every bit simple as a log fallen across a watercourse. The first Bridgess made by worlds were likely spans of wooden logs or boards and finally rocks, utilizing a simple support and trave agreement. Some early Americans used trees or bamboo poles to traverse little caverns or Wellss to acquire from one topographic point to another.
Figure 2.1 the The three Span Concrete claghorn span over blacklick brook
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Some of the earliest surviving Bridgess day of the month from around the 2nd Century BC. Typically, they were stone arches, a signifier that dominated bridge building until the reaching of wrought Fe and steel in the early eighteenth Century and, 150 old ages subsequently, concrete. Most Bridgess were built by the church and two Renaissance rock Bridgess can still be seen in Paris – the Pont Notre Dame ( 1305 ) and the Pont Neuf ( 1606 ) . It was in the eighteenth Century that bridge design began to develop into a scientific discipline, led by an technology school founded in Paris. Its manager, Jean Perronet, perfected the masonry arch, with its low sweeping curve and slender wharfs. Soon afterwards, attending switched to England where the innovation of the steam engine called for stronger Bridgess. In 1794, Fe was foremost used for the concatenation overseas telegrams of a suspension span over the River Tees and 1779 saw the first all-iron span over the Severn at Coalbrookdale. This arch span, crossing 100ft, is still in service.
Merely when the masonry arch span was making its extremum around the beginning of the twentieth Century, reinforced concrete arrived on the scene. Since so, it has become the major building stuff for Bridgess as it has for most structural and civil technology applications, with its intrinsic versatility, design flexibleness and, above all, natural lastingness. Although several British applied scientists had been utilizing concrete early in the nineteenth Century, its usage in British Bridgess did non develop until the latter half of the twentieth Century. It is estimated that at least 75 % of the Highways Agency concrete span stock has been built since 1960.
The earliest known illustration of a mass concrete span in the UK, utilizing lime concrete, was on the District Line, near Cromwell Road, West London, designed by Thomas Marr Johnson for Sir John Fowler and built c.1865. Other British applied scientists began to utilize apparent concrete for span superstructures, notably Philip Brannan, who erected a three-span concrete arch, including a 50ft in-between span, at Seaton in Devon in 1877. The usage of strengthened concrete likely started with the Homersfield Bridge over the River Waveney on the Norfolk/Suffolk boundary line in 1870, when Fe was embedded in concrete, but it was non until the first decennary of the twentieth Century that support, as we know it today, was introduced. This was due about wholly to L.G. Mouchel, the UK agents for the Hennebique system. The first undertaking in the UK was an 18ft span span at Chewton Glen in Hampshire in 1902, followed two old ages subsequently by a 40ft span beam and slab span at Sutton Drain in Hull.
The three Bridgess circa 1928, the Royal Tweed Bridge is about ready for opening ( center )
By 1930 there were about 2000 reinforced concrete Bridgess in UK and noteworthy interior decorators such as Sir Owen Williams emerged between the two World Wars e.g. Montrose ( 1930 ) . Other major Bridgess of this period were the Royal Tweed Bridge, Berwick ( Mouchel ) ; Chiswick and Twickenham River Thames Bridges ( Considere ) ; King George V Bridge, Glasgow and, perchance the best of the period, Waterloo Bridge, London. The outstanding characteristic of concrete Bridgess both during and after the Second World War was the coming of prestressed concrete, used to reconstruct the many Bridgess that had been destroyed, particularly on the Continent.
By 1950, Bridgess by Freyssinet and Magnel had been built, utilizing precast sections joined by concrete or howitzer ; Finsterwalder had constructed the first unmoved box girder span utilizing cantilever building and in the 1960s, the first incrementally launched span was built in Germany. Reinforced concrete was still being used in the 1950s for larger Bridgess, particularly arches, notably Lune Bridge transporting the M6, but by the terminal of the sixtiess, prestressed concrete had mostly superseded reinforced concrete with box girders being the dominant structural signifier.
Expansion of the expressway web demanded big Numberss of concrete Bridgess, a functional and cost-efficient solution to society ‘s demands. The chief accent on span design became economic system and lastingness instead than manner. This divine any figure of developments and building techniques, chiefly affecting precast segmental. The lastingness of concrete has besides been recognised by its important usage on major estuarine crossings. Design and building techniques continue to germinate to fulfill the increasing demands of the UK conveyance web e.g. built-in Bridgess.
Hangzhou Bay Bridge ( China ) : World ‘s Longest Trans-Oceanic Bridge
2.2 Tonss on Bridgess
2.3 Type of Bridgess
There are six chief types of Bridgess: beam Bridgess, cantilever Bridgess, arch Bridgess, suspension Bridgess, cable-stayed Bridgess and truss Bridgess.
Beam Bridgess are horizontal beams supported at each terminal by wharfs. The earliest beam Bridgess were simple logs that sat across watercourses and similar simple constructions. In modern times, beam Bridgess are big box steel girder Bridgess. Weight on top of the beam pushes directly down on the wharfs at either terminal of the span
Cantilever Bridgess are built utilizing cantilevers-horizontal beams that are supported on merely one terminal. Most cantilever Bridgess use two cantilever weaponries widening from opposite sides of the obstruction to be crossed, meeting at the Centre
Arch Bridgess are arch-shaped and have abutments at each terminal. The earliest known arch Bridgess were built by the Greeks and include the Arkadiko Bridge. The weight of the span is thrust into the abutments at either side
Suspension Bridgess are suspended from overseas telegrams. The earliest suspension Bridgess were made of ropes or vines covered with pieces of bamboo. In modern Bridgess, the overseas telegrams hang from towers that are attached to coffers or caissons. The coffers or caissons are deep-rooted deep into the floor of a lake or river
Like suspension Bridgess, cable-stayed Bridgess are held up by overseas telegrams. However, in a cable-stayed span, less overseas telegram is required and the towers keeping the overseas telegrams are proportionally shorter
Truss Bridgess are composed of affiliated elements. They have a solid deck and a lattice of pin-jointed or gusset-joined girders for the sides. Early truss Bridgess were made of wood, and subsequently of wood with Fe tensile rods, but modern truss Bridgess are made wholly of metals such as shaped Fe and steel or sometimes of strengthened concrete.
2.4 General description of a span type
The four chief factors are used in depicting a span. By uniting these footings one may give a general description of most span types.
span ( simple, uninterrupted, cantilever ) ,
arrangement of the travel surface in relation to the construction ( deck, pony, through ) ,
Material ( rock, concrete, metal, etc. ) ,
Form ( beam, arch, truss, etc. ) .
2.4.1 Span
The three basic types of spans are shown below. Any of these spans may be constructed utilizing beams, girders or trusses. Arch Bridgess are either simple or uninterrupted ( hinged ) . A cantilever span may besides include a suspended span.
Bridge rudimentss
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2.4.2 Placement
In a Deck constellation, traffic travels on top of the chief construction ; in a Pony constellation, traffic travels between analogue superstructures which are non cross-braced at the top ; in a Through constellation, traffic travels through the superstructure ( normally a truss ) which is cross-braced above and below the traffic.
2.4.3 Material
2.4.3.1 Reinforced and Prestressed Concrete
Concrete is an all-around building stuff. Almost every edifice contains some concrete. Concrete is poured into signifiers as a stiff but feasible mix, and it can be given any form ; this is an advantage and a danger. The building of good lasting concrete requires particular know-how – which the span applied scientist is assumed to hold. Good concrete attains high compressive strength and opposition against most natural onslaughts though non against de-icing seawater, or CO2 and SO2 in contaminated air.
The steel bars merely truly come into drama after the concrete clefts under tensile emphasiss. If the reinforcing bars are right designed and placed, so these clefts remain as all right “ hair clefts ” and are harmless. A 2nd method of defying tensile forces in concrete constructions is by prestressing. Prestressed concrete revolutionized the design and building of Bridgess in the 1950ss. With prestressed concrete, beams could be made more slender and span well greater distances than with strengthened concrete. Prestressed concrete – if right designed – besides has a high weariness strength under the heaviest traffic tonss. Prestressed concrete Bridgess shortly became much cheaper than steel Bridgess, and they need about no care – once more presuming that they are good designed and constructed and non exposed to de-icing salt. So as from the 1950ss prestressed concrete came good to the bow in the design of Bridgess.
All types of constructions can be built with strengthened and prestressed concrete: In span edifice, concrete beams and arches predominate. The defining of concrete is normally governed by the want to utilize formwork which is simple to do. Plain surfaces, parallel borders and changeless thickness are preferred. This gives a stiff visual aspect to concrete Bridgess, and avoiding this is one undertaking of good aesthetic design.
There is one great disadvantage to concrete as it emerges from the signifiers: the inexpressive, dull grey coloring material of the cement tegument. The surfaces often show discolorations, irregular runs from puting the concrete in changing beds, and pores or even pits from lacking compression, which ire so patched more or less successfully. These lacks have lead to a widespread antipathy to concrete, As good as to attempts for betterment. Some of the methods used to accomplish a good concrete coating in edifices, like profiles and forms on the formwork, ribs or accentuated lumber venas etc are non by and large suited for Bridgess.
The best consequence is obtained by bush pound as was usual between 1934 and 1945 for the Bridgess of the German autobahn system. The concrete coating of the rein- forcement is increased by 10 to 15 millimeters, so that a thin bed together with the cement tegument can be taken off by mulct or coarse shrub hammering. The sum is so exposed with its construction and coloring material.
2.4.3.2 New stuffs
Concrete design specifications have in the yesteryear focused chiefly on the compressive strength. Concrete is easy traveling toward an engineered stuff whose direct public presentation can be altered by the interior decorator. Material belongingss such as permeableness, ductileness, freeze-thaw opposition, lastingness, scratch opposition, responsiveness, and strength will be specified. The HPC ( high public presentation concrete ) enterprise has gone a long manner in advancing these specifications, but much more can be done. Additives, such a fibres or chemicals, can significantly change the basic belongingss of concrete. Other new stuffs, such as fiber-reinforced polymer complexs, nonmetallic support ( glass fiber-reinforced and carbon fiber-reinforced plastic, etc. ) , new metallic supports, or high-strength steel support can besides be used to heighten the public presentation of what is considered to be a traditional stuff. Higher strength support could be peculiarly utile when coupled with high-strength concrete. As our natural resources diminish, alternate sum beginnings ( e.g. , recycled sum ) and farther replacings of cementitious stuffs with recycled merchandises are being examined. Highly reactive cements and reactive sums will be concerns of the past as new stuffs with long-run lastingness become platitude. New stuffs will besides happen increasing demand in fix and retrofitting. As the span stock list continues to acquire older, increasing the useable life of constructions will go critical. Some advanced stuffs, although non economical for complete Bridgess, will happen their niche in retrofit and fix.
( B ) ( degree Celsius )
Figure Wrapping of C fiber sheet ( a ) gluing gum ( B ) wrapping the first bed
( degree Celsius ) bonding on the first bed
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2.4.4 Decks
The most common type of concrete deck, by far, is the Cast-in-place ( CIP ) . Their chief advantage is the ability to supply a smooth siting surface by field-adjustment of the roadway profile during concrete arrangement. In recent old ages mechanization of concrete arrangement and coating has made this system cost-efficient. However, CIP slabs have disadvantages that include inordinate differential shrinking with the back uping beams and slow building. Recent inventions in span decks have focused on betterment to current pattern with CIP decks and development of alternate systems that are cost-competitive, fast to build, and lasting. Focus has been on developing mixes and bring arounding methods that produce public presentation features such as freeze-thaw opposition, high scratch opposition, low stiffness, and low shrinking, instead than high strength. Full-depth precast panels have the advantages of important decrease of shrinking effects and increased building velocity and have been used in provinces with high traffic volumes for deck replacing undertakings.
Several states use stay-in-place ( SIP ) precast prestressed panels combined with CIP exceeding for new constructions every bit good as for deck replacing. This system is cost-competitive with CIP decks. The SIP panels act as signifiers for the topping concrete and besides as portion of the structural deepness of the deck. This system can significantly cut down building clip because field forming is merely needed for the exterior girder overhangs. The SIP panel system suffers from brooding snap, which normally appears over the panel-to-panel articulations. A modified SIP precast panel system has late been developed in NCHRP Project 12-41.
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2.5 subdivision forms
2.5.1 Column subdivision forms
The figure illustrates different possible subdivision form for strengthened concrete columns of span wharfs. The chief pick will be between round and rectangular solid subdivisions.
Figure
Sections A-A and B-B represent the common pick of columns with a round distribution of longitudinal support contained within cross support in the signifier of round basketballs or spirals. These subdivisions are efficient, economical and easy to build. The uninterrupted curve of the cross support consequences in first-class parturiency of the nucleus concrete and besides provides effectual restraint against buckling of the longitudinal flexural support. Section strength and supplanting capacity are independent of way of seismal response.
Section C-C the round longitudinal supports have been supplemented by extra support in the flare part. It is with massive superstructure designs and common to flame up the top of the column to supply better support to the cap beam under bizarre live-load and besides to better aesthetics.
Section D-D has merely peripheral hoop support, which is uneffective in restricting the nucleus concrete and in supplying restraint against longitudinal saloon buckling, and therefore should ne’er be used when malleable response is required of the wharf. Supplying equal parturiency utilizing rectangular basketballs, as is common in edifice columns.
Section E-E is possible for merely relatively little span columns.
Section F-F is common for big rectangular columns.semi- round terminals, or big bevels are used to avoid inordinate screen, with attendant possible spalling jobs and to cut down sensitiveness to diagonal onslaught. The spirals must overlap by a sufficient sum to guarantee that shear strength is non compromised. When longitudinal response of a span with relatively few spans is resisted chiefly by abutments, an extended rectangular wharf subdivision shown in the fig. 26, Section G-G may be adopted. In the cross way, these subdivisions act as structural walls, with high strength and stiffness, but in the longitudinal waies they have low stiffness, therefore pulling small seismal force.
2.5.2 Superstructure Section forms
Solid and voided slabs are appropriate for short span Bridgess, with spans below 15 m. the upside-down T subdivision is besides used for short span Bridgess, with spans below 25 m. Typically an in situ deck is cast on the upside-down T units, utilizing shear connexions between these units and situ slabs. The I beam is a common subdivision for short span Bridgess. The dual T subdivision is used in the lower terminal of the medium-span scope ( 25 to 35 m ) . However, this subdivision is non suited for Bridgess curved in the horizontal plane because of hapless torsional features.
Box girders have the advantage of holding high stiffness and strength for minimal weight, and besides high torsional features. This last characteristic makes the subdivisions suited for Bridgess curved in the horizontal plane.
