I. General Introduction For thousands of years humans have been trying to span rivers, valleys, chasms, and even parts of the ocean. Bridges are used to accomplish this task and range from the incredibly simple beam bridge, made of only a flat deck, to complex cable bridges. One successful type of bridge is called a truss bridge. Truss bridges use triangles as supports above or below the deck to increase the amount of weight the bridge can hold .
TRUSS BRIDGE Teacher : MS Nguyen Tuan Bình Name : Nguyễn Hữu Thành Code : 1321171 Class : British Bridge and Road K54 TABLE OF CONTENTS Contents I General Introduction _ 1.1 Definition of truss bridge 1.2 Historical of truss bridge 1.3 Some of famous truss bridge II Components and Features 2.1 The deck and floor system _ 2.2 Truss members _ 2.3 Truss Bracing _ 11 2.4 Truss joints 13 I GENERAL INTRODUCTION I General Introduction For thousands of years humans have been trying to span rivers, valleys, chasms, and even parts of the ocean Bridges are used to accomplish this task and range from the incredibly simple beam bridge, made of only a flat deck, to complex cable bridges One successful type of bridge is called a truss bridge Truss bridges use triangles as supports above or below the deck to increase the amount of weight the bridge can hold 1.1 Definition of truss bridge We can understand in a simple way, the truss bridge is the bridge supported by trusses According to Wikipedia: “A truss bridge is a bridge whose load-bearing superstructure is composed of a truss, a structure of connected elements usually forming triangular units” Truss bridge is a type of bridge whose main element is a truss which is a structure of connected elements that form triangular units Truss is used because it is a very rigid structure and it transfers the load from a single point to a much wider area Truss bridges appeared very early in the history of modern bridges and are economic to construct because they use materials efficiently 1.2 Historical of truss bridge The first bridge of a truss design was built in the 1840s, by Alfred Neville in France However, this bridge used an isosceles triangle framework The Warren truss bridge design was patented in Britain by James Warren and Willoughby Theobald soon after in 1848, Page I GENERAL INTRODUCTION with an equilateral triangle framework This design was used widely to build bridges all across Britain and India Traditionally, most of the railway infrastructure used this design These designs were borrowed from Europe, and became a common sight throughout the US in the 20th century From the first truss bridge, engineers experimented with different forms of truss bridges trying to find better shape and the one that will suit them for the particular problems Because of that we have today many forms of truss bridges 1.3 Some of famous truss bridge Minato Bridge (Japan) Minato Bridge is a double-deck overpass in Osaka; Japan It was opened for traffic in 1974 and is longest overpass after Quebec Bridge and Forth Bridge Its approximate length is 510 m (1673 ft) Page I GENERAL INTRODUCTION Foorth Bridge Next is Forth Bridge which is a beam overpass beyond the Firth of Forth, situated in the eastern side of Scotland, and about miles in the western side of Edinburgh City Centre Forth overpass was opened for traffic in March, 1890 Its approximate length is 8196 feet It is occasionally referred as Forth Rail overpass Francis Scott Key Bridge (USA) The Francis Scott Key Bridge is a Steel Arch-Shaped Continuous Through Truss Bridge spanning the Patapsco River in Baltimore, Maryland, USA The main span of 1,200 feet (366 m) is the third longest span of any continuous truss in the world The bridge was opened in March 1977 and is named for the author of the Star Spangled Banner, Francis Scott Key The bridge is the outermost of three toll crossings of Baltimore’s Harbor The span is 1,200 feet (366 m) long and carries an estimated 11.5 million vehicles annually Page II COMPONENTS AND FEATURES II Components and Features COMPONENTS AND FEATURES OF TRUSS BRIDGE Typical truss bridge components Page II COMPONENTS AND FEATURES 2.1 The deck and floor system Deck and floor system include: deck, stringers and floor beams 2.1.1 Bridge deck The deck is the structural element providing direct support for vehicular loads Carrying traffic load and tranfer these load to floor beam and then from floor beam to the two trusses Nowaday, they can create many type of bridge deck, but one of the most common type is reinforced concrete deck They can composite with the stringers or not This is done by using 'shear connectors' fixed to the steel beams and then embedded in the concrete Shear connectors can be welded on, perhaps using a 'stud welder', or better still on export work, by fixing nuts and bolts Page II COMPONENTS AND FEATURES 2.1.2 Stringer Stringers are longitudinal beams, set parallel to the direction of traffic They are used to transmit the deck loads to the floorbeams If stringers are not used, the deck must be designed to transmit vehicular loads to the floorbeams Stringers have to resist flexural and shear stresses, and to limit deflection The crosssection of stringer often is I shape, which is suitable to bear flexural moment 2.1.3 Floor beam Floor beams which are important tranverse elements in the support of vehicular loading, are either vertical or set normal to roadway grade These beams span between the two longitudinal trusses to provide the primary support for live loads The floorbeams are located at the panel points so we can determine the floorbeams spacing through length of span The main function of floorbeams is transmiting the deck loads to the trusses Connection between stringer and floor beam There are two kind of connection between stringer and floor beam: Overlap connection and intergrated Overlap connection: The characteristic of Overlap connection: - Where depth does not control the design, the method is applied; - The stringer is continuous spans supported at floorbeams; - Overlap connection is made by bolts or bearings (steel plates); Floor connection: - Where depth control the design, the method is applied; - Floor connection is made by bolt splices; - This method is practically applied in modern bridges; Floor systems are made rigidty to subjected by deck loads Page II COMPONENTS AND FEATURES There are three kinds of connection between stringers and floorbeams: Page II COMPONENTS AND FEATURES 2.2 Connection between floorbeam and truss planes Truss members 2.2.1 Basic dimension of truss planes Basic dimensions of truss planes Distance between two truss planes: B With truss bridge located on the vehicle lanes, they ussually choosing the distance between two truss planes greater than the width of vehicle lanes is 1,0 – 1,5m for distributing the space of truss member Page II COMPONENTS AND FEATURES With truss bridge located under the vehicle lanes, they ussually choosing the distance between two truss planes is from 5-7 m Height of Truss:H The height of truss plane is the distance from the centerline of top chord to the centerline of bottom chord To be easy, we can imagine the height of truss plane like the height of girder in girder bridge For simple-span trusses, experience has shown that a depth-span ratio H/L of 1:5 to 1:10 will ensure a economical design For continuous trusses with reasonable balance of spans, a depth-span ratio of 1:12 to 1:10 should be satisfactory Length of a panel (distance from node to node): d The length d is choosed to maintain the suitable angle α of diagonals: - The appropriate angle of diagonal: α = 30 – 60 degree Optimum angle α: α = 60o in case we don’t have vertical and hanger; α = 45o in case truss include both vertical and hanger; In the case α > 60o: this cause the self weight of truss member be larger In the case α < 30o: it hard to construct the joint They ussually choosing the popular dimension of d is from 3m to 6m 2.2.2 Components and function of truss planes member Bottom chord: generally, subjected to axial compression force, however in continuous truss, it can be compressed or tensile depending on moment diagram; Top chord: it is similar to bottom chord but instead of comprssion, top chord subjected to axial tensile force In continuous truss, it also can be compressed or tensile; Diagonal: diagonals bear both compression and tension, depending on location of live load; Vertical member: it only be compressed when live load appear; Hanger: it does not subject any load It only have to reduce the effective length of chords Page II COMPONENTS AND FEATURES 2.2.3 Type of cross-section The following shapes for truss members are typically used in almost truss bridge: H sections, made with two side segments with solid web Modern bridges almost exclusively use H sections made of three plates welded together H section have very simple composition, easy to connect with other member as well as connet to nodes But the disadvantage of H – section is that the radius of inertial in direction of H – section is not the same, and water can not drain when H – section is used for top or bottom chord To handle this problem, small holes (D = 40 – 50 mm ) are made on the web for water to drain Channel sections, made with two angle segments, with solid web These are seldom used on modern bridges Box sections, modern bridges use primarily four-plate welded box members The cover plates are usually solid, except for access holes for bolting joints Box shapes generally offer greater resistance to vibration due to wind, to buckling in compression, and to torsion, but require greater care in selection of welding details It requires expensive edge preparation for weldding Figure 1: Proposal section for truss member Type 1,2,3: Box section; Type 4: H-section We can use box section for all member in the truss planes, but generaly, for member only subjected to tension, H-section should be used Page 10 II COMPONENTS AND FEATURES 2.3 Truss Bracing There are three common kinds of bracing in truss bridge.They are lateral bracing, sway bracing and portal bracing 2.3.1 Lateral bracing Placed between the top chords and bottom chords of a pair of trusses Lateral bracing in truss bridges is provided for transmitting the longitudinal live loads and lateral loads to the bearings and also to prevent the compression chords from buckling This is done by providing stringer bracing, braking girders and chord lateral bracing In case of highway truss bridges, concrete deck, if provided, also acts as lateral bracing support system Type of lateral bracing 2.3.2 Sway bracing (cross bracing) Sway bracing is provided between the trusses in the plane of either verticals or diagonals, and its primary purpose is: - Minimizing the relative vertical deflections between the trusses - It is provided for distributing the transverse loads to the lateral system - It is also for providing torsional rigidity to the truss frame Sway bracing contains vertical, horizontal and diagonal member to form a stiff frame Some kind of bracing in truss bridge: Page 11 II COMPONENTS AND FEATURES Sway bracing in the truss bridge 2.3.3 Portal bracing Portal bracing is sway bracing placed in the plane of the end posts The main purpose of portal bracing is providing end supports to the top lateral bracing system Portal bracing and sway bracing Page 12 II COMPONENTS AND FEATURES 2.4 Truss joints Truss joint is the most important element in truss bridge, located at the nodes of a truss where the web members are connected to the chords, there is a change in load in the chord which necessitates a change in its cross-section area Structural principle of joint The centerline of each member in a joint must to intersect at one point; Center of gravity of group of bolts must be in the centerline of member; The load bearing capacity of joint must be larger than load bearing capacity of each member in the joint to ensure that if failure occur, the damage will happen in members first Page 13 ... Bridges are used to accomplish this task and range from the incredibly simple beam bridge, made of only a flat deck, to complex cable bridges One successful type of bridge is called a truss bridge. .. truss bridges 1.3 Some of famous truss bridge Minato Bridge (Japan) Minato Bridge is a double-deck overpass in Osaka; Japan It was opened for traffic in 1974 and is longest overpass after Quebec Bridge. .. area Truss bridges appeared very early in the history of modern bridges and are economic to construct because they use materials efficiently 1.2 Historical of truss bridge The first bridge of