PAPER SP 23-39
DESIGN OF LONG-SPAN CONCRETE BRIDGES WITH SPECIAL REFERENCE TO PRESTRESSING, PRECASTING, ERECTION, STRUCTURAL BEHAVIOR, AND ECONOMICS By T. Y. LIN and BEN C. GERWICK, JR. The design of long-span concrete bridges, considering conditions in the u.s.A., is discussed with reference to recent advances in materials and construction techniques. The use of prestressing, precasting, and erection methods are presented as applied to different types of bridges such as cantilever, continuous, rigid-frame, truss, arch, and suspension structures, Design approaches emphasizing strength and behavior requirements are mentioned, together with the use of computer programs for optimization. Material, labor, and erection costs are compared as indices of total economy in bridge design. Keywords: bridges (structures); concrete construction; connections; costs; cranes; girders; long span; post-tensioning; precast concrete; prestressed concrete; prestressing; prestressing steel; reinforced concrete; reinforcing steel; structural design; superstructures.
0 In many countries concrete is the accepted material for achieving long-span bridge construction up to 1000 ft (300 m) and plans are made for even longer spans. It has proven to be technically suitable, economically sound, and aesthetically attractive. In the u.s.A. concrete bridges of span greater than 200 ft (60 m) are a rarity, and steel is arbitrarily adopted for practically all cases, often because of an erroneous assumption of economy while actually because of unfamiliarity with the design and construction potentials of concrete. It is therefore pertinent to summarize recent developments in materials and techniques as they affect the economical design and layout of long-span concrete bridges. ADVANCES IN MATERIALS The days of 3000-psi (210 kg/cm2) concrete for bridge construction are gone, and 5000 psi (350 kg/cm2) is now the accepted strength for field-placed concrete. For precast concrete, strength of 6000 to 8000 psi (420 to 560 kg/cm2) strength is frequently achieved with a cement factor of 8 sacks per cu yd (450 kg/m3). Using selected aggregates, with maximum size of coarse aggregate of 1/2 in. (13 em), rather coarse sand, 10 sacks of cement per cu yd (560 kg/m3), minimum water, and a waterreducing admixture, concrete compressive strengths of 10,000 psi (705 kg/cm2) can be consistently and practically obtained for plant-produced elements. Achieving this high strength consistently does require trained personnel, special vibrating equipment, and strong forms. High-quality structural lightweight concrete is now available and utilized throughout the United States for major building construction. It is used in only a few mediumspan bridges, including the suspended span of two bridges in Florida, With care in selection of aggregates, mix design and control, and special placing and curing techniques, field concrete of 5000 psi (350 kg/cm2) compressive strength and plant693
694
CONCRETE BRIDGE DESIGN
T. Y. LIN, a Director of ACI, is Professor of Civil Engineering at the University of California in Berkeley, California. For biographical sketch, see Paper SP 23-27. BEN c. GERWICK JR., ACI member, received his BSCE degree from the University of cailforma in 1940. Following five years of service in the u.s. Navy during World War II, he returned to Ben c. Gerwick, Inc., and, since 1952, has served as President of the firm, which engages primarily in bridge, marine, and foundation construction. Mr. Gerwick is a Fellow of ASCE, past president of PCI, United States representative to the International Federation of Prestressing, a former Director of ACI, and currently a member of ACI Committees 349, Criteria for Nuclear Containment Vessels, and 543, Concrete Piles, and ACI-ASCE Committee 423, Prestressed Concrete. He has published articles on prestressed bridge construction.
precast concrete of over 6000-psi (420 kg/cm2) are now obtainable whose shrinkage and creep characteristics are similar to those of normal-weight concrete. It is conceded that the modulus of elasticity is lower than that of normal-weight concrete, but the dead-load strains of lightweight concrete can be controlled by prestressing or prestraining during various stages of construction, and the live-load deflections and vibrations are usually not a controlling factor in long-span bridges. The low tensile strength and hence the low resistance to shear can be essentially balanced by prestressing. Since dead weight, and in certain regions seismic loading, is the important factor affecting the economic design, lightweight concrete will play a major role in long-span bridges, The development of high-strength steel, particularly in conjunction with prestressing, has revolutionalized concrete bridge construction. Steel strands of 270,000 psi (19,000 kg/cm2) tensile strength, wires of 250,000 psi (17,500 kg/cm2) and bars of 160,000 psi (11,000 kg/cm2) are now standard materials. Engineers in the United States are accustomed to these steels for short-span pretensioned girders as produced by precasting plants. The application of high-strength steel pretensioning to long spans promises higher economies yet to be explored.
PRESTRESSING The techniques of pretensioning and post-tensioning have been widely applied to buildings in the United States. Although the United States has more pretensioning plants than any other country, the use of their products for bridges has been limited by transportation capabilities to spans not much over 100 ft (30m). However, it is envisaged that by combining precasting with post-tensioning, such as segmental construction, a new fertile and economical field can be opened up for long-span bridge construction. External tendons have been extensively employed in European-engineered bridges, and were utilized on aportionoftheSan Francisco-Oakland Bay Bridge reconstruction. They do not require large ducts in the webs and thus they permit thinner webs, The inverted dome roof of the 400-ft (120m) Oakland Coliseum has precast ribs with webs 7 ft (2.1 m) high but only 2-1/2 in. (6,4 em) thick. It is an example of what constructors can do if given the opportunity and the challenge. Naturally, care must be taken to insure against corrosion. One idea, utilized by Dr. Leonhardt of Germany, is to have the external tendons in a metal sheath which is encased in concrete and then injected with mortar, for double protection. Alternatively, epoxy or bitumastic coating
LONG.SPAN BRIDGES
695
of the encasement concrete can be used to prevent moisture penetration; this technique has been employed in Australia. Another development in prestressing is to use large tendons providing much higher forces. Single tendons of 300-ton (272 t) effective force and higher are available commercially. There seems to be no limit to the possible concentration through the technique of dividing the tendons near the anchorage into three or more groups, thus permitting anchors of reasonable size, as at the nuclear reactor in Wales. A combination of prestressing and conventional reinforcing promises to be another technique for obtaining economy, While prestressing will carry the main forces in the bridge, reinforcing for local tensile stresses (known also as partial prestressing) may help to economize the sections and thus reduce the dead weight. Where compression becomes excessive, compressive reinforcement may increase the strength and reduce the strain. Spiral or tubular reinforcement may further increase the compressive strength. Some laboratory tests at the University of California using expanding cement with spiral reinforcement showed compressive strength up to 20,000 psi (1,410 kg/cm2), For long-span bridges with deep webs, web prestressing with either vertical or inclined tendons can be applied to neutralize practically all diagonal tension. When the webs are spaced farther apart than about 12 ft (3,6 m), transverse prestressing of the road slabs or the floor beams will reduce the dead weight appreciably. Thus a long-span bridge can well be prestressed in three dimensions, a system successfully utilized on the Oosterschelde Bridge in the Netherlands, Europe's largest bridge. Stage prestressing, already employed for long-span roof construction (Oklahoma and Arizona State Fair Coliseum) and waterfront structures (LaGuardia Airport, New York) in the u.s.A., is often a necessary practice for long-span bridge construction. It involves a carefully controlled sequence of erection, concreting, and stressing. Certain tendons are left unstressed or partly stressed for a period of time and they must be properly protected and sealed against moisture entry. Prestraining of long indeterminate members to compensate for shrinkage and elastic and creep shortening, previously applied to long-span arches, is now also adopted for prestressed rigid-frame bridges.
PRECASTING A fertile area of long-span bridge construction is the combination of precasting with post-tensioning, sometimes known as the segmental method. This method is widely employed in Europe and Australia but has had only limited use so far in the United States. The precast element may be a wide but short segment or it may be a long and narrow one, as determined by the economics of erection, Precasting puts labor in the plant and thus cuts down the cost and ensures higher quality control, so vital to long-span bridges. Shrinkage and creep are minimized, Dead-load deflections during construction can be controlled and compensated, When the precast elements are wide and short, joining them has been achieved in at least three ways: (1) Cast-in-place concrete joints with width about 24 in (60 em) and overlapping reinforcing steel, the joint concrete obtaining accelerated strength through steam curing or infra-red heating, (2) Cast concrete joints about 3 in. (8 em) thick, using high-strength concrete with 3/8-in, (1 em) maximum aggregate. (3) Dry joints, using direct face-to-face contact, without any gasket material or need for grinding, and with or without an epoxy "glue," Such joints were recently tested for the San Francisco Bay Area Rapid Transit District,
696
CONCRETE BRIDGE DESIGN
Techniques include precasting against thick milled plates, with careful temperature, mix, and curing-cycle control, Tolerances achieved were of the order of 1/64 in. (0,4 mm), plus or minus. On the San Francisco-Oakland Bay Bridge reconstruction, precast units were precast against their match-marked mates, and then were match-marked, They were subsequently prestressed directly against each other with dry joints, obtaining a face-to-face contact, This procedure was also used by French engineer-contractors on the bridges at Oleron and Choisy-le-Roi. For steel cantilever or suspension bridges, considerable economy can be achieved through the use of precast concrete stringers spanning between floor beams (which may be cast-in-place or of composite construction), enabling long spans to be built with a minimum amount of falsework. In contrast to steel stringers of 20 to 30ft (6 to 9 m) length, these prestressed concrete stringers should range from a minimum of 50 ft (15m) to about 100 ft (30m) when transported by land, and to 300 ft (90 m) or more when barged on water. ERECTION METHODS The method of erection is probably the most important single item in the planning and design of a long-span concrete bridge, The structural design cannot be separated from its erection procedure. There are at least six categories of erection methods: 1. Crane Erection- Cranes are required in almost any type of bridge erection, whether by themselves or in conjunction with other methods. Three graphs concerning crane costs plotted from actual data are shown: Fig. 39-1 shows the erection cost per ft of girder in relation to the length of the girder. Note that the cost of erection increases from approximately $1,50/ft for 60-ft (18m) spans to $15/ft for 250-ft (75 m) spans. (The point shown for a 250-ft girder is an estimated price,) Fig. 39-2 relates the erection cost per ton to the weight of girder in tons. Again notice the sharp rise in cost per ton with the weight of the girder. (For the 160-ton point on the graph, the data has been obtained from large precast concrete elements for bridge piers.) Fig. 39-3 shows the daily cost for crane or derrick with various effective capacities. Note that, while the daily cost per crane or derrick does increase with its capacity, the cost per ton per day actually decreases with the size of the crane. A study of the three graphs reveals that the high cost of crane operation for heavy lifting is actually due to the longer time required for handling heavier pieces and also due to the small number of pieces handled for the project, thus increasing the percentage of fixed cost charged to each girder. When these large cranes are used for lifting of multiple pieces, it is believed that the cost per ton can be lowered. In San Francisco, a steel bridge is currently being erected by a 500-ton (450 t) floating derrick. Two-hundred-ton and 250-ton (180-t, 225-t} derricks are commonly available, Eight-hundred-ton (725-t) shear legs have been used in offshore work, Two 800-ton shear legs were recently employed to erect precast sections of the Volga River Bridge (Fig, 39-4}. So this ability for over-water bridges should be exploited by utilizing large units, Large units also can be floated into place without necessarily being lifted into position by a crane or derrick. The double-cantilever section shown in Fig, 39-5 was assembled on shore of precast elements, post-tensioned, and floated into place over a pier, 2. Jacking or Counterweighting - For pieces weighing over a few hundred tons, jacking or counterweighting has been employed for lifting steel suspended spans
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(such as the Carquinez and Quebec bridges). The heaviest pieces ever lifted for prestressed concrete were for the main span of the Moscow River Bridge, whose two halves, each weighing 6,600 tons (6000 t), were assembled along the shore and then barged into position. 3. Cantilever Erection - This method is frequently used for extending over deep waters. With anchor spans, cantilevers are extended in one direction. From intermediate piers, double cantilevering in both directions is often done. Temporary piers may be added where required. A moving scaffold for castin-place cantilevering or segmental method of precast construction can be used. This system of erection is very widespread in almost all parts of the world except the United States. It has been used for major long-span bridges in Germany, England, France (Fig. 39-6}, Sweden (Fig. 39-7}, Portugal, Greece, Austria, Japan and India. 4. Metal Erection Trusses - These can be economically employed when there is a repetition of spans. The erection truss, which is built of aluminum or steel, may be used to launch precast girders or precast segments to be posttensioned to~&~iher (Fig. 39-6). Erection trusses can be moved from span to span, panel to panel, or transversely sidewise on the supporting piers to accommodate various girders. 5. Falsework Piers- For arch or suspension bridges, falsework piers may prove to be economical provided the water is not too deep. In a long-span suspension bridge, temporary piers can support the deck until prestressing of the suspenders lifts the girders off the falsework. 6. Cable Erection - The Mount Hope Bridge was erected in a manner similar to that for a steel suspension bridge, with post-tensioned concrete gir~ers. Doctor Finsterwalder* has advocated ribbon bridges erected by a similar method. A 500-ft (150-m) span suspension type using precast segments was designed for the Colorado River in Costa Rica, where the erection is controlled by stage post-tensioning and the segments are precast and slid along the cables into position and grouted. Undoubtedly there will be other methods or a combination of methods. E:~ct~on rocedures must be geared to each particular site. But it is only the lack o~ imtiative ~at prevents the economical construction of long-span concrete brid.ges, smce there are clearly many methods of erection to be chosen by the engineer-butlde-r • u
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DESIGN APPROACHES The design of long-span bridges cannot always be based on conventional allowable stresses which were set up for bridges of short and medium span. The predominance of dead load and the employment of stronger materials may indicate higher allowable stresses. On the other hand, the shrinkage, creep, and temperature movements which are insignificant in shorter spans may become major items in the design of long spans. Strength and behavior requirements are the real major considerations. Because of the relatively high dead load, reserve strength for live load is very high and there is little trouble in meeting the usual live-load-factor requirements. On the other hand, behavior requirements may dominate the design. Because of the weight of concrete, vibration under either live load or wind is generally not a problem, but vibration may become controlling for extremely slender members. For compression members, the problem of buckling of members or lateral expansion of steel bars under sustained force may need attention. For tension zones, cracks should be controlled either by prestressing or by the addition of nonprestressed reinforcement. Large deflections and horizontal movements, when they can be predicted and provided for, can be adequately taken care of by existing techniques. Because of the importance of saving materials by proper proportioning and design, it is advisable to resort to computer programs for optimization. Many designs can be evaluated by a computer program to determine the economical ratio of side span to main span for a three-span continuous bridge, for example. Clearly, many variables may be chosen and many designs made to arrive at optimum values without excessive effort.
STRUCTURAL TYPES For long-span concrete bridges, the choice of structural type is closely related to the method of erection. Economy of materials is not the only consideration; how the materials are made into desired shapes and erected into position becomes the major concern. Structural types for long spans and their potentials are discussed below. Long simple spans of concrete bridges are economical only when they are precast and can be erected with reasonable cost. Generally, pieces with length over 100ft (30 m) or width over 12 ft (3. 7 m) cannot be easily transported on land. Over water, however, much larger and heavier pieces can be transported, up to 300ft (90 m) and more. For spans larger than about 150ft (45 m), continuous layouts are more economical than simple-span layouts so far as the quantity of materials is concerned. Other problems, such as the cost of anchorages and ease of erection, enter into the picture so that continuous spans may be cheaper for shorter spans while simple spans may be suitable for longer ones. For example, continuous spans can be erected by the cantilever method, while simple spans can be economically erected if there is enough repetition. For long bridges requiring expansion joints, cantilever spans are often convenient. The cantilever method of construction is often used to eliminate the falsework of the main span. As previously mentioned, prestress is applied to each completed section before proceeding to the next. Suspended spans can also be lifted or pushed into position to bridge the gap between the cantilevers. Prestressed rigid frames can be economically used for long-span concrete bridges. Not only are the main girders prestressed, but also the piers can be posttensioned for stress control. The problem of expansion and contraction requires attention; temporary hinges can be provided and compensating strains introduced to minimize the secondary stresses. A 300-ft (90-m) span designed for the City of Oakland utilizes the frame principle.
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CONCRETE BRIDGE DESIGN
The possibilities of trussed concrete bridges have been only occasionally explored. The Zaza River Bridge, Las Villa, Cuba, has a span of 300 ft (90 m), and the total amount of concrete for the superstructure was only 1,6 cu ft per sq ft (0,49 m3jm2) of roadway, with prestressing steel at 5,1 psf (25 kg/m2) and reinforcing bars at 17,8 psf (86 kg/m2), When there is sufficient repetition of spans, it may be worthwhile to build special floating and lifting equipment. Erection trusses may also be used for assembling or launching truss spans. Concrete suspension bridges make use of the force in the cables and anchor them into the girders, thus prestressing the girders. These may be visualized as selfanchored suspension bridges using concrete girders which are thus automatically prestressed for flexural resistance. A different type of suspension design for precast concrete girders (Mt. Hope) was discussed previously.
COST DATA The amounts of materials, labor, and equipment naturally vary with each bridge depending on its location, span length, structural type, live loading, and other requirements, However, the economy of design depends not on the materials alone. This relationship is shown in Fig. 39-8, which gives average costs of materials, labor, form work, and erection cost for various span lengths, up to 500 ft (150m). For spans over 500 ft, special designs should be made, so that the quantity of materials will not be excessive. At the same time, the erection cost will be the major consideration. First, a computer design was made for a three-span continuous box-type bridge, with the side-span lengths equal to 0.6 and o. 7 of the center-span length. These results are plotted in Figs. 39-9, 39-10, and 39-11; designs using the cantilever T-section for the suspended span and box section for the cantilever arms and anchor spans show considerable saving in concrete and prestressing steel.
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CONCLUDING REMARKS The great pioneering steel bridges of the United States were built by an open or covert alliance between designers and constructors. The turnkey approach of designerconstructor has developed and built our chemical plants, refineries, steel plants, and nuclear power plants. It is time to ask, seriously, whether we may not have adopted a restrictive approach by divorcing engineering and construction in the field of bridge construction. If a contractor-engineer, by some stroke of genius, were to present to design engineers today a wonderful new scheme for long-span prestressed concrete bridges that made them far cheaper, he would have to make these ideas available to all other contractors, even limiting or watering them down so as to "get a group of truly competitive bidders." The engineer would have to make sure that he found other contractors to bid against the ingenious innovator.
If an engineer should, by a similar stroke of genius, hit on such a unique and brilliant scheme, he would have to worry, wondering if the low bidder would be one who had any concept of what he was trying to accomplish or was in any way qualified for high-class technical work.
Bridge engineers in the United States are constrained to work within the pattern of separation of designer and builder. Can a pattern be evolved which will open the doors for large-scale use of long-span prestressed concrete bridges in the u.s.A.? It is a challenge which we have a professional responsibility to meet.