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A Simpler Cable Stayed Bridge

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A Simpler Cable Stayed Bridge A SIMPLER BIOGRAPHY SUMMARY CABLE STAYED R. Shankar Nair, senior vice Cable-stayed designs are the BRIDGE president of exp US Services Inc. dominant bridge type today for in Chicago, has practiced spans of 800 to 2,000 feet; tied structural engineering for more arches are more often the choice than 45 years as a designer, in the 500- to 800-foot range. researcher, author, and lecturer. However, as discussed in this He has a Ph.D. in civil paper, simplification of some of engineering from the University the typical features of steel cable- of Illinois at Urbana-Champaign stayed bridges could make them and is licensed to practice competitive even in the range of engineering in 44 states. spans now dominated by other bridge types. Dr. Nair’s design practice encompasses both buildings and One possible simplification is to bridges. He has designed building make the concrete deck structures of up to 90 stories and independent of the strut, in the 1047 ft in height; his bridge same way that the deck is designs include what was at the independent of the tie in a tied- R. SHANKAR NAIR time the longest tied-arch span arch bridge or the lower chord in a (909 ft) in the world. truss bridge; this would aid construction and also allow easy This work has won numerous deck replacement. Another awards, including four AISC/ simplification is to use exposed, NSBA “Prize Bridge” awards, unsheathed stay cables of standard two ACEC “National Grand galvanized structural strand with Awards” and six “Most standard end fittings, as used in Innovative Structure” awards tied-arch bridge hangers; these from the Structural Engineers cables could be readily inspected Association of Illinois. He has and, if necessary, replaced. also received a Lifetime Achievement Award and a T.R. Yet another simplification is an Higgins Lectureship Award from erection scheme that does not AISC. involve balanced cantilever but retains the benefit of not requiring Dr. Nair is chairman of the AISC shoring in the navigation span. Specifications Committee and a former chairman the AISC Task All of these unusual features are Committee on Stability and the demonstrated in the trial design ASCE Committee on Design of and comparison of alternatives for Steel Building Structures. He is a two-lane river crossing with an also a former chairman of the overall length of 2,400 ft and a Council on Tall Buildings and main navigational span of 600 ft. Urban Habitat. He was elected to membership in the National Academy of Engineering in 2005. A SIMPLER CABLE STAYED BRIDGE Cable-stayed designs are the dominant bridge type balanced by special anchor cables and tie-downs today for spans of 800 to 2,000 feet; tied arches to the foundations at the ends of the outer spans. are more often the choice in the 500- to 800-foot In all span arrangements, there may be either two range. Even in this shorter span range, cable- planes of stay cables, one at each edge of the deck, stayed bridges are competitive in the efficiency of or a single plane of cables along the center with their use of structural materials. However, the deck cantilevered out on each side. The deck difficulties related to some of the common features structure, in all cases, serves as both a of typical cable-stayed bridges have led many compression strut and a stiffening element. public agencies to be wary of selecting this bridge type when reasonable alternatives are available. These are just some of the many possible span, tower and cable arrangements in cable-stayed Typical Cable Stayed Bridges bridges; many other configurations have been used Two of the more common span configurations for successfully over the years. But while the layout cable-stayed bridges are illustrated in Figure 1. In of primary components may vary widely, a few both cases, most of the vertical load on the span is features are shared by most designs: (a) Use of the supported by the vertical components of the entire deck structure to serve as the compression tension in inclined cables. At the upper ends of strut; (b) enclosed and sealed cables, very difficult the cables, towers or pylons absorb the vertical to inspect or replace, intended to last the life of the force components from the cables and transfer structure; and (c) erection by the balanced them to the foundation. At the lower ends of the cantilever method. cables, the horizontal components of cable tension Deck as Compression Strut are absorbed by the deck structure, which acts as a compression strut. Many different deck configurations have been used in cable-stayed bridges; one of the simplest is sketched in Figure 2. It consists of two steel box girders, transverse and longitudinal steel floor framing, and a concrete slab intended to be (a) Multiple Long Spans composite with the steel. Anchor Cable Cables Cables Tie-down Concrete Deck to Pier (composite with girders) (b) Single Long Span; Short Anchor Spans Figure 1. Common Cable-Stayed Span Steel Girder Steel Girder Configurations Figure 2. Cross Section—Deck as Strut When there are multiple long spans (Figure 1a), the cable tension due to span weight will be Other options include steel or concrete or roughly equal on the two sides of each tower and composite steel/concrete box sections extending the horizontal components of the cable tension across the full width of the deck; for very long applied on the tower will be largely in balance; spans, these sections are often shaped for small imbalances can be absorbed by the flexural aerodynamic benefit. strength of the towers. With a main span and short side spans (Figure 1b), the larger load on the main One feature common to essentially all the designs span produces larger horizontal forces on the is that the entire deck structure—longitudinal towers toward the main span; these can be girders, other longitudinal members, deck slab— Page 1 of 7 works as a single composite element in resisting The record of performance of bridge stay cable the compression induced by the horizontal systems is mixed. While some have performed component of the tension in the cables. This is a well, others have required expensive repair and very efficient design in its use of materials, but it retrofit, mainly to correct the effects of failures in carries certain penalties. the corrosion protection system. And even if the most modern cable designs are, in fact, reliable, With the concrete deck slab designed to be part of the difficulty in effectively inspecting and, if the compression strut, the balance of forces necessary, repairing the enclosed cables in service between girders and slab depends on the erection makes some state transportation agencies in the sequence, which can lead to more or less force U.S. reluctant to build cable-stayed bridges when being locked into the slab, and varies with time reasonable alternatives are available. due to creep of the concrete. This leads to a high degree of complexity in design and construction Balanced Cantilever Erection and, most importantly, makes replacement of the concrete deck slab extremely difficult. Erection of cable-stayed bridges is usually done by the balanced cantilever procedure: The tower is Sealed Stay Cables erected first; then segments of deck structure and cables are added on each side of the tower, in Many different stay cable designs have been tried sequence, maintaining balance about the tower. and tested over the years, all with the objective of achieving a cable that would stand up to the Pier or Pier or elements and last the life of the bridge without Center of Center of special maintenance (1, 2, 3). Span Span Casing (filled or empty) 15 13 11 9 7 5 3 1 2 4 6 8 10 12 14 16 Figure 4. Balanced Cantilever Erection Balanced-cantilever erection, illustrated in Figure Single 4, can be very efficient and economical: it does not 7-Wire Strand Bundle of require shoring; crane picks are modest. However, Parallel Strands this method of construction, combined with time- dependent and sequence-dependent effects related Figure 3. Stay Cable Section to the sharing of compression between the strut and the concrete deck, limits flexibility and requires a high level of specialized engineering In typical U.S. practice of recent years, the cable, sophistication on the part of the builder; this can as sketched in Figure 3, consists of many parallel limit competition and increase cost, at least in the strands within a casing. Each strand is made up of moderate span range. seven high-strength steel wires; in most cases the strand is individually greased or waxed and A Simpler Cable Stayed Bridge covered with a high-density polyethylene or high- density poly-propylene extruded sheath. The Cable-stayed bridges are very efficient in their use bundle of strands is sealed in a steel or high- of structural materials. And with a more than 50- density polyethylene casing, which may be either year record of performance, most features of grouted or empty. (The current trend is toward modern cable-stayed bridges have been developed HDPE casings without fill.) to a high degree of refinement. Nonetheless, as discussed, some of these features continue to The end anchorages and jacking systems for the prove troublesome to many bridge owners and cables are usually proprietary items, designed and operators. And as a result, cable-stayed designs do tested by the cable manufacturer-supplier. not dominate the 500- to 800-ft span range as they Page 2 of 7 do longer spans; tied arches and even trusses are For all the designs, the concrete deck is 47 ft wide often preferred for highway bridges with spans including parapets; the loading is AASHTO HL- under 800 ft in the U.S.
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