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By : Prof.Dr .\Nabil Mahmoud

Steel Bridges. By : Prof.Dr .Nabil Mahmoud. Content. 1-Introduction 2-Parts of steel Bridge 3-Loads on bridge 4-Allowable stresses 5-Fatigue 6-Plate girder 7-Length of flange plate girder 8-Connection of flange plate with web

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By : Prof.Dr .\Nabil Mahmoud

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  1. Steel Bridges By : Prof.Dr.\Nabil Mahmoud

  2. Content 1-Introduction 2-Parts of steel Bridge 3-Loads on bridge 4-Allowable stresses 5-Fatigue 6-Plate girder 7-Length of flange plate girder 8-Connection of flange plate with web 9-Web stiffnner 10-Web splice 11-Splice of flange plate 12-Maximum defleciton in Bridge 13-Bearing 14-Road way bridge

  3. 1-Introdution • 1.1 Introduction • 1.2 Railway Bridge • 1.3 Highway Bridge • 1.4 Types of Bridges

  4. 1.1 Introduction • Most bridges are built for the transportation of highway or railway traffic across natural or artificial obstacles. A deck bridge supports the roadway or railway on its top chords (truss bridge) or flanges (plate girder bridge), while a through bridge supports the floor system at or near the lower chords or flanges, so that traffic passes through the supporting structure (main girder).

  5. The rolled-beam bridge supports its roadway directly on the top flanges of a series of rolled beams placed parallel to the direction of traffic and extending from abutment to abutment. It is simple and economical. It may also be used for multiple spans where piers or intermediate bents can be built economically. Beam bridges may be economical for spans up to 15 m. a typical beam bridge for highway traffic is illustrated in Fig. (1.1).

  6. For crossings greater than those which can be spanned economically by a rolled-beam bridge, (deck or through) plate-girder bridges may be used. In this simplest form, a plate girder consists of three plates (two flanges and web) welded together in the form of an I. Ties and rails for railway bridges may rest directly on the top flanges of the deck plate-girder bridge. When clearance below the structure is limited, a through plate girder bridge is used. The floor system may consist of a single line of stringers under each rail, supported by floor beams (cross girders) forming into the main girders just above their lower flanges.

  7. If an open floor is objectionable, ballast may be laid on concrete or steel-plate decking supported by closely spaced floor beams (cross girders) without stringers. Knee braces (U frames) are used to support the top flanges of through bridges, as illustrated in Fig. (1.2) .Highway plate-girder bridges are usually of the deck type. The floor slab is usually supported directly on the main girder, as in the beam bridge Fig. (1.1). In orthotropic steel-deck plate construction the floor consists of a steel deck plate stiffened in two mutually perpendicular directions by a system of longitudinal and transverse ribs welded to it (Fig. (1.3)).The deck structure functions as the top flange of the main girder and floor beams. This system makes efficient and economical use of materials, particularly for long-span construction.

  8. When the crossing is too long to be spanned economically by plate girders, a through or deck truss bridge may be used. Deck bridges are more economical than through bridges because the trusses can be placed closer together, so that the span of the floor beam is shortened. For multiple spans there is also a saving in the height of the piers. back

  9. 1.2 Railway Bridges • The stringers, cross girders and the main girders are the main load carrying members. The design of various elements is done in the sequence in which the load is transmitted. In railway bridge, there will be either an open timber floor or a ballasted floor. • STRINGERS • CROSS-GIRDERS • MAIN GIRDERS

  10. 1.3 Highway Bridges The floor is a part of bridge which carries the load directly. The floor system in case of Highway Bridges generally consists of reinforced concrete slab or steel deck plate and wearing surface. In case of deck type plate girder Highway Bridges, the slab is supported directly by the plate girders. In case of through type Highway Bridges, the reinforced concrete slab is supported on stringers, and cross-girders, or by the cross-girders alone. Many times, the reinforced concrete slab provides its own traffic surface. In addition to this, the bituminous, asphalt or carpet surface is also furnished. This acts as a wearing surface. The design of reinforced concrete slab has not been discussed in the text. • STRINGERS • CROSS-GIRDERS • MAIN GIRDERS

  11. Figure (1.4)shows the common types of simple-span bridge trusses. By varying the depth of a truss throughout its length (Fig. 1.4c)forces in the chord members can be more nearly equalized and the forces in the web reduced. Trusses of economical proportions usually result if the angle between diagonals and verticals ranges from 45 to 60. However, if long-span trusses are made deep enough for adequate rigidity as well as for economy

  12. , a suitable slope of the diagonals may produce panels too long for an economical floor system. Using the subdivided panels (Figure 1.4(f and g))solve this problem. Certain objections to subdivided panels overcome with the invention of the K truss (Fig. 1.4h). Cantilever bridges(Fig. 2.2) , continuous bridges (Fig. 2.3), arch bridges (Fig. 2.4), suspension bridges (Fig. 2.5) and three Chord Bridge (Fig. 2.6) are common types of structures suitable for long spans.

  13. A cantilever bridge consists of two shore, or anchor, spans flanked by cantilever arms supporting a suspended simple span. Positive bending moments are decreased because of the shorter simple beam, while the cantilever and anchor arms subjected to negative bending moments. Positive bending moments in continuous bridges are reduced because of the negative moments at the piers. Arch bridges may be fixed, single-hinged, two-hinged, or three-hinged. The principal supporting elements of the suspension-bridge superstructure are the cables which pass over the towers to be anchored in foundations at each end. Back

  14. Figure 1-1 Figure 1-2 Back Figure 1-3

  15. Back Figure 1-4

  16. 1.4 Types of Bridges Introduction In designing the different parts of a bridge we must investigate carefully how the loads are transmitted from one member to the next. We must follow the loads from the point of application up to the abutment. All members and all connections should have the same factor of safety. The strength of the whole structure depends on the weak part. The design of the details is just as important as the design of the main members; failure is generally caused by a weak or wrong detail. For the computation and design of the different parts of a steel bridge, the same method is used as for the corresponding members of steel buildings

  17. But on account of the bigger spans and greater loads, much bigger cross section is required. Bridges can be classified according to many factors like purpose of the bridge, system of main girders, considering the position of the bridge floor, square or skew bridge, and fixed or movable bridges. In the following section we can see these different classifications.

  18. 1.5 Classification of bridges 1.5.1 Classification according to purpose of the bridge a.   Railway Bridges. b.    Roadway Bridges. c.     Foot bridges. d.     Combined bridges as Embaba Bridge.

  19. 1.5.2 Classification according to system of Main Girder a-Simple Bridges. The main girders are resting on two supports only. They may be: - beams, plate girders or trusses. One of the supports is hinged while the other is movable and thus these bridges are externally statically determinate. But internally they may be either determinate or indeterminate. b-Continuous Bridges. The main girders are continuous trusses or plate girders on three or more supports. One bearing only of each girder is hinged, while all the other must be movable to avoid

  20. temperature stresses. Vertical loads acting on a continuous girder give also vertical reactions, but the bridge is statically indeterminate. A settlement of one of the piers produces additional stresses; therefore continuous bridges should be built in places where we have firm soils. c- Cantilever Bridges. The main girders extended over several spans but they have many intermediate hinges that the reactions are statically determine. For n supports we have to odd n-2 hinges. In a cantilever bridge the settlement of support does not affect the stresses. When foundation is not firm enough, either simple bridges or cantilever bridges should be used.

  21. D- Arch Bridges. An arch is a structure which under vertical loads produces inclined reactions at both supports. We have 3-hinged, 2-hinged and fixed arches. 1-Three-hinged arches are statically determinate; hence, horizontal displacement of the abutments does not produce any additional stresses on the structural system. 2- Two-hinged arches and the fixed arches are statically indeterminate; hence, displacement of the abutments produces additional stresses in the structural system. Furthermore, foundations of such arches should be on rock or on very solid gravel.

  22. e- Suspension Bridges. Cables of suspension bridges are made from very high tensile steel. The allowable deflection is about 10 cm. The floor is hung by vertical suspenders from cables. These cables are carried by vertical steel towers A-Q, B-V over which it posses and are anchored at P and V. A saddle top of each tower is provided to relieve the tower from B.M. The reaction at top of tower is nearly vertical. Stiffening, girders must be used to reduce the deflection and vibration of the bridge due to the moving loads. Suspension bridges are of good appearance but they are economical only for long spans (> 300 m). F- Cable stayed bridges

  23. G- Three Chord System Bridges. The arch trusses with a tie bow-string are simply supported. They are externally statically determinate, and once internally statically indeterminate. They are good appearance but rather expansive than trusses with two chords. 1.5.3 Classification according to position of bridge floor Fig(2-7) a- Deck Bridges. In which the floor is or near the top chord or flange of the main girders.

  24. b- Through Bridges. In which the floor is or near the bottom chord or flange of the main girders. The distance (h) is called the height of construction, it is the height between the top of rails or road way and the lowest line of the bracing. If there is a sufficient height of construction a deck bridge should always be arranged as it is more economical stiffer, and of better appearance than through bridge. In a railway deck bridge the distance between the two main girders can be made less than in a through bridge therefore the weight of the cross-girder and wind bracing would be less.In Roadway bridge, the reinforced concrete floor may rest on several main girders.

  25. 1.5.4 Classification according to the layout of the bridges (square or skew bridges) The centerline of the square bridge is perpendicular to centerline of the canal, while in skew bridge they are at oblique angle. Fig(2-8)

  26. 1.5.5 Classification according to Fixed bridges and Movable bridges. Movable spans are required in bridges crossing navigable streams if the height below the bridge is not sufficient for the passage of ships. Three major types of movable bridges are in common use:- a- The vertical lift bridges. b- The bascule bridges. c-The swing bridges.

  27. A End bracket Scale=(1:50) SEC (A-A) X.G L.W.B M.G M.G X.G Stringer bracing X.G A Lower wind bracing End bracket Back Fig 2-1

  28. Systems of Main Girder End Stiff. Web Plate Upper Flange Lower Flange L1 = (0.6 - 0.75) L2 L2 L1 Cantilever Plate Girder Bridge C1 C1 L1 L2 L1 Structural System End Stiff. Upper Flange Web Plate Lower Flange C1 C1 L1 L2 L1 Cantilever Plate Girder Bridge C1 C1 L1 L2 L1 Structural System Back Fig 2-2

  29. Systems of Main Girder Transverse Web system Bracing Upper Chord Lower Chord L1 = 0.8 L2 L2 L1 ContinuousTruss Bridge End Stiff. Upper Flange Web Plate Lower Flange L1 = (0.6 - 0.75) L2 L2 L1 Continuous Plate Girder Bridge Structural System Back Fig 2-3

  30. Systems of Main Girder 3-Hinged Arch Bridge L 2-Hinged Arch Bridge Fixed Arch Bridge L L Back Figure 2.4

  31. Systems of Main Girder Saddle Saddle Cable Steel Tower Stiffening Girder L Suspension Bridge Figure 2.5 Back

  32. Systems of Main Girder L Three Chord Truss Girder Bridge L Bow-String Truss Girder Bridge L Bow-String Plate Girder Bridge Fig 2-6 Back

  33. Upper Chord h = L/ 10 1.5 m Sleeper Lower Chord B = Bridge Width Back

  34. h = L/ 10 B = Bridge Width Railway Through Bridge h = L/ 10 B = Bridge Width Railway Deck Bridge Back Figure 2.7

  35. B = Bridge Width M.G. M.G. Square Bridge B = Bridge Width M.G. M.G. SkewBridge Fig 2-8 Back

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