The Design of Modern Steel Bridges - TEDI

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92 The Design of Modern Steel Bridges Figure 5.1 Different types of plate girders. Sometimes inclined plates are attached between the flange and the web, as shown in Fig. 5.1(e); they support the flange tip, as well as acting as a longitudinal stiffener to the web, and more importantly, they provide some torsional rigidity to the girder. The material in the web is not as efficient as that in the flange in resisting bending moment. But the shear resistance of an I-beam is related only to the web area, and thus determines the minimum web area. The most efficient design of a plate girder cross-section is thus to make the web as thin and deep
as possible. But deep and thin webs are liable to buckling and may require stiffening. Vertical stiffeners are provided for the webs to improve their buckling resistance, and in the case of very deep and thin webs horizontal stiffeners may also be necessary. Increased depth of beams also adds to the cost and length of the embankments in the bridge approaches. The concrete deck on top of the top flange may be made integral with a plate girder by means of shear connectors. Plate girders are used in continuous spans by splicing adjacent lengths by either bolting or welding. Where the bridge consists of several spans, a decision has to be made whether to provide simply supported girders over each span or to make the girders continuous over several spans. The advantages of simply supported spans are: (1) Relative settlement of any support will not change the dead load stresses. (2) If each span length is within the limits of transportation, then site splicing of girders is avoided. (3) Expansion joints at the ends of the spans have to cope with the expansion of a single span length only. The advantages of continuous spans are: Rolled Beam and Plate Girder Design 93 (1) The number of expansion joints can be reduced, often to only one; however, the joint will have to cope with the expansion of several span lengths. Expansion joints tend to deteriorate with traffic and cause bumpiness of riding and are potential sources of trouble. (2) The forces due to braking and acceleration of vehicles (see Chapter 3) can be resisted at one bridge support or shared between several supports; in the case of simple spans these same forces are to be resisted fully at several supports and cannot be shared. (3) The number of bearings will be reduced on each pier, which can therefore be narrowed in elevation. (4) The structure will be generally more rigid, with reduced deflections and vibrations. (5) There is scope and engineering justification for increasing the girder depths over supports, thus improving the appearance of the bridge. (6) Finally, materials in the girder can be reduced and longer spans can be built with fewer piers, thus achieving overall economy. There is another arrangement of suspended and cantilever spans, as shown in Fig. 1.1. This arrangement has the advantages (5) and (6) of the continuous spans, the number of expansion joints being less than for simple spans but more than for continuous spans; each support takes half the longitudinal forces and carries only one bearing, and like simple spans is unaffected by any settlement of supports. This arrangement is thus particularly suitable for bridge sites vulnerable to foundation settlements, e.g. in mining areas. The AASHTO specification[1] requires a minimum depth of steel beams as 1/25 of simple spans, or of the distance between points of contraflexure under
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The Design of Modern Steel Bridges
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The Design of Modern Steel Bridges
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Contents Preface vii Acknowledgemen
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Preface Bridges are great symbols o
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Acknowledgements The figures in Cha
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2 The Design of Modern Steel Bridge
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10 The Design of Modern Steel Bridg
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Page 54 and 55: 42 The Design of Modern Steel Bridg
Page 58 and 59: Table 2.1 Properties of some commer
Page 63 and 64: Chapter 3 Loads on Bridges 3.1 Dead
Page 65 and 66: 3.3 Design live loads in different
Page 67 and 68: interstate highways are designed fo
Page 69 and 70: a coefficient that depends on the n
Page 71 and 72: Loads on Bridges 59 In Germany, a n
Page 73 and 74: Table 3.6 Typical values of U and P
Page 75 and 76: 3.5 Longitudinal forces on bridges
Page 77 and 78: wind loading on the traffic. An upl
Page 79 and 80: where r is the air density ¼ 1.226
Page 81 and 82: Loads on Bridges 69 minimum and max
Page 83 and 84: 3.8 Other loads on bridges There ar
Page 85 and 86: References Loads on Bridges 73 wher
Page 87 and 88: Chapter 4 Aims of Design 4.1 Limit
Page 89 and 90: critical components of the structur
Page 91 and 92: But this is an attempt to achieve u
Page 93 and 94: If the basic variables R and S are
Page 95 and 96: educed variables defined by oi ¼ x
Page 97 and 98: will have a probability of failure
Page 99 and 100: Aims of Design 87 g m ¼ g m1 g m2
Page 101 and 102: Table 4.2 Failure probabilities of
Page 103: Chapter 5 Rolled Beam and Plate Gir
Page 107 and 108: eam, with an effective area equal t
Page 109 and 110: and stresses caused by lateral defl
Page 111 and 112: Figure 5.3 Beam cross-section. In t
Page 113 and 114: Rolled Beam and Plate Girder Design
Page 115 and 116: Table 5.2 Effective length factors
Page 117 and 118: stress is equal to p 2 E/(Le/r) 2 )
Page 119 and 120: equation (5.16) has been adopted fo
Page 121 and 122: The strain energy of the elastic re
Page 123 and 124: Figure 5.6 Buckling of plates in co
Page 125 and 126: Figure 5.8 Buckling of plate under
Page 127 and 128: techniques. It may be noted that th
Page 133 and 134: 5.4.3 Effect of residual stresses R
Page 135 and 136: Aw ¼ 40 mm 2 , allowing for a smal
Page 141 and 142: Basler and Thurlimann were the firs
Page 143 and 144: Ostapenko/Chern gave the following
Page 145 and 146: where m ¼ Mp b 2 twsyw ty ¼ syw p
Page 147 and 148: the case of equal flanges, the tota
Page 149 and 150: emains flat until an elastic critic
Page 153 and 154: longitudinal compression is given b
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where Peq ¼ s1tB PE Ps Pcro Peq1
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Figure 5.27 Tension-field forces in
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5.5.6 Design of longitudinal web st
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when Le is the effective length for
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pffiffiffiffiffiffiffiffiffiffiffif
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een derived as a condition for trea
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For interaction of various stress c
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must be at least i.e. 1 IsxbB 4 2 a
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Chapter 6 Stiffened Compression Fla
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Stiffened Compression Flanges of Bo
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cross-section in which the secant s
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longitudinal compression of the lon
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webs of main girders is denoted by
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wherefrom Ms ¼ 2 3 PE P The net be
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Chapter 7 Cable-stayed Bridges 7.1
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Cable-stayed Bridges 185 cables con
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The following is a list of cable-st
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(a) Fan system (b) Harp system (c)
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7.3.2 Ropes causing lateral compres
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The spiral winding of wires around
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polyethylene or even steel tube. Or
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See Fig. 7.7. Imagine a cable-stay
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It may be noted that: Et is higher
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Reference Cable-stayed Bridges 201
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204 The Design of Modern Steel Brid
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