The Design of Modern Steel Bridges - TEDI

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200 The Design of Modern Steel Bridges the stiffening girders, the changes in the cable tensions and the changes in the bridge profile due to the removal of the superimposed dead loads. For this exercise, the cable stiffnesses should be based on their tangent modulus given by equation (7.4), corresponding to the assumed tension in the cables for the completed bridge. After obtaining the reduction in the cable tensions, the process should be reiterated with the cable stiffness based on their secant modulus given by equation (7.3), appropriate to their initial tension and final tension obtained in the previous iteration. This process should be repeated a few times until satisfactory convergence is obtained. At the end of this process, the correct profile and bending moments of the stiffening girders and the correct tensions in all the cable-stays occurring before the application of the superimposed dead loads will be obtained. (c) In the next stage, imagine removing the dead weight of the closing section of the stiffening girder at the middle of the main span. From the structural model of the bridge (which should have two pinned joints at the two ends of the closing section), obtain the changes in the bending moments and the profile of the stiffening girder and the changes in the cable tensions, by following a procedure similar to that described in (b). These are the values correct at the stage before the insertion of the closing section. (d) From now on in this exercise, the structural model will consist of one tower, one approach span and half the central span. In the next step, imagine removing the last cable-stay at the tip of the cantilever near the middle of the central span. The effect of losing the tension in this stay, and its self-weight, on the stiffening girder and on the other cable-stays shall be determined from the structural model by an iterative process similar to the ones described already. (e) Next, an appropriate length of the stiffening girder cantilevering beyond the current last cable-stay shall be removed and its effects determined. (f) This process of successive removal of cable-stays and stiffening girder segments shall be continued in the reverse sequence of the adopted erection sequence, involving cable-stays and stiffening girder segments in both the main span and the side span. (g) The removal of the anchor cable, i.e. the one between the tower top and the pier at the end of the side span, shall conform to the stage at which it is proposed to be installed. (h) Thus the profile of the stiffening girder and the tensions in the cablestays at all stages of erection can be obtained and used for controlling the actual erection process. (i) For the installation of any cable-stay, its unstressed length can be determined from equation (7.2), using the tension that it is designed to carry when just installed. Jacking it up to its correct length between the stiffening girder profile and the tower should automatically ensure the correct tension in the stay.
Reference Cable-stayed Bridges 201 (j) The structural behaviour under live loading should be determined from the full structural model of the bridge. Initially the cable axial stiffnesses should be based on their tangent modulus appropriate to the tension in them under full dead load. In the next iteration the secant modulus appropriate to the dead load tension and the live load tension calculated from the previous step should be used and the process repeated until satisfactory convergence is obtained. 1. Troitsky M. S. (1977) Cable-stayed Bridges: Theory and Design. Crosby Lockwood Staples.
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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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Table 2.1 Properties of some commer
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Chapter 3 Loads on Bridges 3.1 Dead
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3.3 Design live loads in different
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interstate highways are designed fo
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a coefficient that depends on the n
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Loads on Bridges 59 In Germany, a n
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Table 3.6 Typical values of U and P
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3.5 Longitudinal forces on bridges
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wind loading on the traffic. An upl
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where r is the air density ¼ 1.226
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Loads on Bridges 69 minimum and max
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3.8 Other loads on bridges There ar
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References Loads on Bridges 73 wher
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Chapter 4 Aims of Design 4.1 Limit
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critical components of the structur
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But this is an attempt to achieve u
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If the basic variables R and S are
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educed variables defined by oi ¼ x
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will have a probability of failure
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Aims of Design 87 g m ¼ g m1 g m2
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Table 4.2 Failure probabilities of
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Chapter 5 Rolled Beam and Plate Gir
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as possible. But deep and thin webs
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eam, with an effective area equal t
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and stresses caused by lateral defl
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Figure 5.3 Beam cross-section. In t
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Rolled Beam and Plate Girder Design
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Table 5.2 Effective length factors
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stress is equal to p 2 E/(Le/r) 2 )
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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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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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Page 169 and 170: must be at least i.e. 1 IsxbB 4 2 a
Page 171 and 172: Chapter 6 Stiffened Compression Fla
Page 173 and 174: Stiffened Compression Flanges of Bo
Page 175 and 176: cross-section in which the secant s
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Page 179 and 180: webs of main girders is denoted by
Page 183 and 184: wherefrom Ms ¼ 2 3 PE P The net be
Page 195 and 196: Chapter 7 Cable-stayed Bridges 7.1
Page 197 and 198: Cable-stayed Bridges 185 cables con
Page 199 and 200: The following is a list of cable-st
Page 201 and 202: (a) Fan system (b) Harp system (c)
Page 203 and 204: 7.3.2 Ropes causing lateral compres
Page 205 and 206: The spiral winding of wires around
Page 207 and 208: polyethylene or even steel tube. Or
Page 209 and 210: See Fig. 7.7. Imagine a cable-stay
Page 211: It may be noted that: Et is higher
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