Special Report Cable Stayed Bridges With Prestressed Concrete Fritz Leonhardt Prof. Dr. Ing. Consulting Engineer Stuttgart West Germany he number of cable stayed bridges stand the basic principles of cable stay T with concrete or steel has increased bridge design and construction. The dramatically during the last decade. Ref. purpose of this article is to present the 1 presents a survey of some 200 cable latest state of the art on cable stayed stayed bridges that have been designed bridges while elaborating upon the or built throughout the world, fundamentals and possibilities pertain- Although most of these bridges are ing to such structures. The text will made of steel, many of them could have cover highway, pedestrian and railroad been designed with prestressed con- bridges using prestressed concrete. crete. In fact, by using this material, the design, structural detailing and construc- tion method can be simplified, thus pro- 1. Range of Feasibility of ducing a bridge that is economically and Cable Stayed Bridges aesthetically superior. There are some misconceptions re- Unfortunately, there are many bridge garding the range of feasibility of cable engineers today that do not fully under- stayed bridges. For example, the state- ment is often made that cable stayed Note: This paper is based upon the Keynote Lee- bridges are suitable only for spans from hire presented by the author at the FTP Congress in 100 to 400 in (about 300 to 1300 ft). This New Delhi, India, February 16, 1986, and a similar lecture, including steel bridges, at the Symposium statement is wrong. Pedestrian bridges on Strait Crossings in Stavanger, Norway, in Oc- with only about 40 m (130 ft) span can be tober 1986. built to be structurally efficient and cost 52 Based on his more than 50 years of experience as a consulting engineer on specialized structures, the author presents a state of the art report on prestressed concrete cable stayed bridges with authoritative advice on the design-construction intricacies involved with such structures. effective. Employing prestressed con- For the crossing of the Messina Straits crete, the superstructure can be made in Italy (see Fig.1), our firm designed an very slender using a few cable stays and all-steel cable stayed bridge with a main a deck with a depth of only 25 to 30 cm span of 1800 m (5900 ft) for six lanes of (10 to 12 in.). road traffic and two railway tracks with- Highway bridges can be built of pre- out encountering any structural or con- stressed concrete with spans up to 700 in struction difficulties. (2295 ft) and railroad bridges up to about Many bridge engineers believe that 400 m (1311 ft). If composite action be- for spans above 400 m (1311 i), a sus- tween the steel girders and a concrete pension bridge is preferable. This as- deck slab is utilized, then spans of about sumption is false. In fact, a cable stayed 1000 and 600 in (3279 and 1967 ft) for bridge is much more economical, stiffer highway and railroad bridges, respec- and aerodynamically safer than a com- tively, can be attained safely. parative suspension bridge. The author Fig. 1. Messina Straits Bridge, linking Sicily and Italian peninsula; design by Gruppo Lambertini (1982). PCI JOURNALISeptember-October 1987 53 h Suspension Bridge Cable stayed Bridge " h t. t 5 weight t 50000 width of bridge 38m 46 000 t 1.0 Gip 1 46 000 = 240 30 000 f9 200 0 12300.2.16 yQ 20? 000 5 70o L,^ e ^ \ r°Q 19 2DD t 1z 30o apt°` 1;^ ocio i 5, e 5 700 0 500 1000 1500 1800m span Fig. 2. Comparison of quantity of cable steel for suspension bridges and cable stayed bridges. showed this was true as early as 1972) stiffness which must be about ten times In designing a cable stayed bridge larger than that of the cable stayed there is first the problem of finding the bridge. The suspension bridge needs required quantity of high strength steel additional heavy anchor blocks which for the cables. For a bridge with a 1800 can be extremely costly if the navigation m (5902 ft) main span and 38 m (125 ft) clearance is high and foundation condi- width, a suspension bridge needs 46000 tions are poor. The total cost of such a t (50700 tons) of steel whereas a cable suspension bridge can easily be 20 to 30 stayed bridge only needs 19200 t (21170 percent above the costs of a cable stayed tons) of steel (see Fig. 2). In the latter bridge. case this represents only about 40 per- Currently, the second Nagoya Har- cent the amount of steel. bour Bridge (in Japan), at 600 m (1967 Furthermore, a suspension bridge ft), has the longest span of a cable stayed needs a stiffening girder with a bending bridge in steel. This structure, which 54 Fig. 3. Nagoya Harbour Bridge, Japan. was designed by Dr. Ing_ Kunio one cable at each tower (see Fig.4), Hoshino (a former student of the au- leaving long spans requiring a beam thor), is now under construction. When with large bending capacity. Soon three completed, it will look similar to the first to five cables were used, decreasing the Nagoya Harbour Bridge (see Fig.3). The bending moments of the beam and the author is confident that in the near fu- individual cable forces to be anchored. ture similar such spans will be built However, structural detailing and con- using prestressed concrete. struction procedures were still difficult. Range of span is only one important The desired simplification was ob- parameter in designing cable stayed tained by spacing the cables so closely bridges. Special situations such as that single cables with single anchor curved spans and skew crossings can be heads were sufficient which could eas- easily solved using stays. ily be placed and anchored. The spacing The engineer can choose between a became only 4 to 12 in (13 to 39 ft), al- large variety of configurations: sym- lowing free cantilever erection without metrical spans with two towers, un- auxiliary cables. Simultaneously, the symmetrical spans suspended from one bending moments became very small, if tower, or multi-spans with several towers an extremely small depth of the longi- in between. This is an ideal field for tudinal edge beams and very stiff cables creativeness in design to satisfy the were chosen. functional requirements of the project This multi-stay cable system can no and to obtain an aesthetically pleasing longer be defined as a beam girder. structure which complements the envir- Rather, it is a large triangular truss with onment. the deck structure acting as the com- pressive chord member. The depth of the longitudinal beams in the deck 2. The Main Girder System structure is almost independent of the main span, but it must be stiff enough to 2.1 Development of multi-stay cable limit local deformations under concen- system trated live load and to prevent buckling Some of the first cable stayed bridges due to the large compressive forces (like the Maracaibo Bridge) had only created by the inclined cables. PCI JOURNALSeptember-October 1987 55 Fig. 4. Development of structure to multi -stay cable bridge. Note that additional cables allow beam of superstructure to be progressively more slender. The stiffness of this new system de- length of the tower head and get a pends mainly on the angle of inclination semi-fan arrangement (as shown in and on the stress level of the cables. Fig. Fig.8), which also improves the appear- 5 shows the enormous influence of the ance of the bridge. stress in such cables on the effective In the authors early bridges across modulus of elasticity. Low stresses give the Rhine River in Dusseldorf, the ar- a large sag and low stiffness. The stress chitect wanted to have all the cables in the cables should be at least 500 to parallel and the anchorages equally dis- 600 N/mm2 (72500 to 87000 psi). For tributed over the height of the tower. long spans, stiffening ropes (as shown in This is called the harp-shaped arrange- Fig. 6) are needed to prevent an exces- ment (see Fig. 9). The system requires sive change of sag. The simplification of more steel for the cables, gives more the structural design and of the erection compression in the deck and produces process gained by this multi-stay cable bending moments in the tower. How- system should be used whenever suit- ever, the structure is aesthetically able. pleasing, especially when looking at the bridge under a skew angle. Dr. Ulrich 2.2 Arrangement of stay cables Finsterwalder, the world renowned If all the cables are anchored at the bridge engineer, prefers this harp ar- top of the tower, the structural system is rangement with many closely spaced called a fan-shaped configuration (see cables. Such a scheme was chosen for Fig.7). Unfortunately, the concentration the Dame Point Bridge in Florida (see of the anchorages causes structural diffi- Fig.10), with a main span of 396 m (1298 culties when the number of cables is f3), which is now under construction. large. Therefore, it is preferable to dis- In these multi-stay bridges, the deck tribute the anchorages over a certain is hung up with cables near the tower 56 Y Z f2 E, eff - o 12 G3 Eeff N1mm2 205 000 700 180 000 ^; SO 600 steel stress Sao 140 000 ^p0 100 000 oti T 60 000 sag T {C 0 100 200 300 400 fc Fig. 5. Effective modulus of elasticity depends primarily on steel stress in cable which in turn affects the cable sag.