The Design of Modern Steel Bridges - TEDI

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58 The Design of Modern Steel Bridges all bridges on public roads are designed for the same loading, whereas in the other countries, the design loading depends on the type of route. Most codes prescribe the simultaneous action of a single concentrated or truck loading, and a uniformly distributed load. Some codes consider the effect of a heavier axle or wheel load. The impact effect is already included in the design loads in some codes, but in the other codes the design loading has to be increased by a factor which generally decreases with the length of the member. The intensity of loading decreases with the increase in the loaded length in most of the codes. Most codes also allow a reduction when several traffic lanes have to be loaded. This study also included a valuable numerical exercise of calculating the total bending moments caused by the live loads of the various codes on a simply supported bridge. Separate calculations were made for the bridge carrying two, three and four traffic lanes and spanning 10–100 m. The total load on the whole bridge was considered for this comparison, as if the bridge was supported by one single beam. The impact factor and the reduction due to multiple land loading were taken into account; any difference between the various codes on the allowable stress levels was also allowed for by multiplying the bending moment by a ratio: yield stress of steel specified in the national material specification allowable stress of steel in the bridge design code The bending moment M thus obtained was converted into an equivalent uniformly distributed load qeq in kN/m, given by 8M/L 2 . These qeq values indicate the structural strength of bridges built according to the loading specifications of the various countries. Figures 3.4(a) and (b) show these qeq values for bridges with two and four lanes in the carriageway, respectively. Very wide differences between different countries are evident, the AASHTO loading being by far the lightest for spans over 25 m. 3.4 Recent developments in bridge loading In recent years it has been found in several countries that the standard loading does not satisfactorily reflect the effect of a long queue of vehicles in a traffic jam situation, particularly for long loaded lengths of, say, over 40 m. In the USA, proposals[8] were made for a new loading standard which will consist of a uniformly distributed load U and a concentrated load P for each lane, both of which depend on the length of the bridge to be loaded for the worst effect. U depends also on the percentage of heavy goods vehicles in the traffic. Table 3.6 gives typical values. No allowance for impact needs to be added, as the loading represents a static jam situation. In multiple lanes, a second lane shall have 70% of the basic lane load and all other lanes shall each have 40%.
Loads on Bridges 59 In Germany, a new draft bridge loading standard has been proposed in which, in addition to the heavy vehicle of 600 kN on the principal lane a heavy vehicle of 300 kN is also taken on a second lane, and the class 12 loading for lightly used roads has been abandoned. In Britain, observations on traffic Figure 3.4(a) Comparison of National Bridge Loadings for two-lane bridges. (Courtesy OECD.)
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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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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: a coefficient that depends on the 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 and 104: Chapter 5 Rolled Beam and Plate Gir
Page 105 and 106: as possible. But deep and thin webs
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
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The strain energy of the elastic re
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Figure 5.6 Buckling of plates in co
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Figure 5.8 Buckling of plate under
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techniques. It may be noted that th
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Rolled Beam and Plate Girder Design
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5.4.3 Effect of residual stresses R
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Aw ¼ 40 mm 2 , allowing for a smal
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Basler and Thurlimann were the firs
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Ostapenko/Chern gave the following
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where m ¼ Mp b 2 twsyw ty ¼ syw p
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the case of equal flanges, the tota
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emains flat until an elastic critic
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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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206 The Design of Modern Steel Brid
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