1、 中文 1590字 between towers increase if the number of cables increase and the angle of inclination of the cables remains the same. B.READING MATERIAL FUTURE OF CABLE-STAYED BRIDGES I would like to begin with a view back on the development of cable-stayed bridges during the last 25 years.It started with
2、 Dischingers publication shortly after the end of Word War II. He pointed mainly to the necessity of gonging to high steel stresses in the stays to produce stiffness in the system.The first bridge following Dischingers recommendations,was built in Sweden,designed by Demag,a German steel construction
3、 firm,consulted by Dischinger. Then in 1953-54 the three Duesseldorf bridges were designed,all of them,with parallel stay cables but different tower arrangements,in order to have a family of similarly appearing bridges. The fundamental concept of these early designs was retained for over almost 20 y
4、ears,which it took to built them. We learned by detailing and erecting these bridges. In these bridges,only a few stay cables were chosen；some engineers designed their bridges with even only one stay cable. This resulted in large cable forces causing difficulties to anchor the cables in the beam str
5、ucture. Heavy cross beams were necessary, the ropes had to be formed. To gain sufficient space for the anchors, adjustment of cable lengths becomes difficult. In addition, the large distance between the stay cables complicate the erection requiring heavy equipment, auxiliary trusses,even auxiliary p
6、iers were necessary to build the Maracaibo bridge and the Kniebridge. Auxiliary stayes were needed for cantilevering the beam plate girder to the next stay cable. In addition, long spans between supports provided by stay cables,cause large bending moments in the continuous beam and hereby a consider
7、able depth of the girders is needed. Form all of this experience, we concluded for our later and future designs ,that a) a large number of stay cables should be chosen in a way b) that one anchorage socket can be used to simplify the placing of the cable, c) by short spacing of the cables, bridge gi
8、rder bending moments are low so that a depth of 1 to 2 m is sufficient, just providing a deflection line curvature satisfying traffic requirements and providing safety against buckling in the deflected stage. d) The spacing of the cables should be such, that no heavy erection equipment is needed to
9、cantilever out for placing the next following stay cable. e) Feasible spacings may be between 6 and 12 m for concrete girders and between 8 and 16 m for steel girders. In order to satisfy these rules, my office developed a new type of cable anchorage in cooperation with BBR Switzerland, which allows
10、 ultimate cable forces up to almost 2,000 tons, using parallel wires or strands of very high strength, inside a polyethylene tube for perfect corrosion protection. The anchorage was developed to get high fatique strength, therefore called High Amplitude(HiAm) cable. These cables can be prefabricated
11、 and shipped on reels and allow a simple and inexpensive erection. Several bridges have been designed lately using these principles:The Pasco bridge, bridges in Parana, Argentina, and others. As we designed these bridges, I knew already the favorable effect of system damping in multi-stay cable brid
12、ges by experience which I had gained from the behavior of a pedestrian stay cable bridge in Stuttgart, but we had to prove the dynamic safety for these larges. A dynamic model test was made at the Ismes Institute of Profssor Oberti in Italy, 18 m long designed for full dynamic similitude. Short and
13、long trains or just locomotives could run on the rails with different speeds-no adverse oscillations could be detected. Then the test engineers excited artificially oscillations going through all possible modes and frequencies and at many points of the bridge. Whenever they succeed in building up a
14、small amplitude, it broke quickly again down to small amplitudes. It was impossible to find a mode of oscillation which would build up large amplitudes by resonance. Any mode of oscillation broke down as soon as the amplitudes starts to grow, because each of the cables has a different natural freque
15、ncy and disturbs the oscillation of deck structure by interference so strongly, that large amplitudes cannot develop. We get a very effective system damping which does not allow resonance oscillation with dangerous amplitudes. Of course, this effect is only obtained with stiff and highly stressed ca
16、bles and with a sufficient number of cables in close spacing. We must recognize that the dynamic behavior of the suspension bridge is perfectly different from that of a multi stay cabled bridge. In s suspension bridge without stiffening girder, there is full freedom for the dangerous first antimetri
17、c mode of oscillation, combining torsional and bending movement. Small force can excite this mode of oscillation and build up large amplitudes by resonance. These oscillations can be restrained by stiffening trusses with large bending and torsional rigidity and resonance can mainly be avoided by a l
18、arge difference between the natural frequencies of bending and torsional oscillations. Sectional mode tests in wind tunnels and theories have been well developed for these suspension bridge problems. The multi stay cable bridge-on the other side-cannot oscillate in low order modes, it especially can
19、not move into combined torsional and flexural oscillations. With stays along the edges of the bridge, torsional oscillations are almost impossible and flexural oscillations assume quickly high order modes with only small amplitudes. The important fact is,that resonance is impossible for the reasons
20、described. As a consequence, we must learn that the theories which were developed to check aerodynamic safety of suspension bridges are not valid for multi stay cabled bridges. Wind tunnel tests with sectional models must be made with realistic restraint by system damping which, however, is difficult to imitate for a sectional model. * * * Fritz Leonhardt, Dr. Ing. Stuttgart, Germany
