OTEC 2002

October 22, 2002 Columbus, Ohio

Elements in the Design and Construction of Cable-Stayed Bridges

by David Goodyear, PE SE Senior Vice President, TY Lin International

Introduction The cable-stayed bridge has come into great favor as a medium or long span solution in the United States, due to both the economy and aesthetics of the unique structural form. While it is ideal to have the favor of good aesthetics with optimal economics, this condition is far from guaranteed with cable-stayed bridges at all sites. The modern cable-stayed bridge form hails from Western Europe in the post-WWII era. Faced with a considerable number of bridges to replace, and a limited amount of capital with which to do so, Europe embraced the efficiency of the cable-stayed form for crossing the moderate spans of Continental rivers. In the first applications, only a few cables were used, principally as replacements for piers in the channel. However, with the advent of digital computers, the efficiency of the modern form became apparent.

Stomsund – Dischinger’s First in 1955 Tartara – The Newest Record, 1998 The efficiency of the cable-stayed bridge form is derived from the cable in pure tension. The three basic cable arrangements – harp, fan and semi-fan – do less to distinguish the engineering behavior of these bridges than whether the deck is the rigid (generally box girder) type or the flexible (generally plate girder) type of superstructure.

Harp – Dames Point Fan – Pasco-Kennewick Semi-Fan – East Huntington

The efficiency of the cable stayed bridge for medium to long spans has been demonstrated by competitive designs and competitive bidding around the world. Once departing the economical range of cantilever box girder and truss solutions (in the 550 to 750 foot range), the cable stayed bridge stands alone for most conventional sites up to the emergence of suspension spans in the 2500 foot range. The premium for cable-stayed bridges is the height and associated cost of the pylon, both on the short and long sides of its competitive range. The efficiency is in the web of taught cables, each readily optimized for their part in the overall structural system. Cable stayed bridge structures are often major crossings of waterways. This allows waterborne equipment to deliver and erect prefabricated bridge segments, taking further advantage of the modular nature of the cable-deck units that make up a modern cable-stayed bridge.

Contrast of Cross-Sections

Design Concepts Rigid and flexible deck designs behave differently, both during construction and in service. Referring first to the beam on elastic foundation analogy (a very useful preliminary design tool), the more rigid box girder has a broader influence line, and spreads concentrated loads more broadly across the array of cables. The rigid box is, in effect, the next generation of solution after the original long span girders with pier replacements by cable-stays. In addition, a closed box girder section provides torsional stiffness, allowing a single cable plane option. A space truss yields the same behavior. Where a rigid girder floats on an elastic foundation moments remain relatively small. However, at boundary conditions of hard support, such as at pylon or pier supports, the box girder develops high moments due to the greater rigidity. While there are a number of space truss systems in service, the preferred section for this type of design in the US is the concrete box girder. Steel box girders were common for the early solutions in Europe, but they proved to be uncompetitive as the cable stayed system evolved in the US, costing almost 30% more in the case of East Huntington.

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Skyway – Concrete Box System East Huntington – Edge Girder System

The flexible cable-stayed deck design is based on a different design philosophy than the deeper, more rigid box. The flexibility of the edge girder system is designed to maximize the utility of the stay cable. It results in a more concentrated influence line, and relies on the cable for both local and global stiffness. The objective with the flexible girder system is to keep structural weight to a minimum. Consequently, steel edge girder systems are preferred. The combination of concrete deck and steel edge girders results in a composite system where the steel edge girder resists the local bending, while the concrete deck resists much of the compression associated with the stay cables. Surprisingly, there is not as great a difference in quantities between rigid and flexible solutions as one might think. Walther1 compared the equivalent concrete thickness of various concrete cable-stayed bridge designs over a range of designs, and found that the variance between designers was more pronounced than the variation between rigid and flexible systems (most have an equivalent thickness close to 0.5m). What is different, of course, is the mix of material between the steel, steel composite and concrete solutions. Philosophy of Design Effects Approach to Construction Just as behavior of the two designs differs, so too does the approach to construction. The structural theory for each option is the same, and so too is the basic approach to analysis. However, the behavior of the two bridge types (rigid and flexible) is so different that the practical issues with construction are not the same at all.

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One of the fundamental principles in erection is that the stress and force conditions within a structure are established by the change in geometry from the unstressed to the stressed state. This means that in theory, once the cambered shape of the girder and the fabrication lengths of cables are established, the entire stress state in the bridge can be determined only by knowing the geometry of the completed bridge. This theory applies only to complete prefabrication, and is compromised by cast in place construction and inelasticity (creep and shrinkage), but it still is a very important tool in understanding and controlling cable-stayed bridge erection. Erection of cable-stayed bridges is dominated by the cable installation process. The range of control during erection includes cable force measurement via pressure gages on jacks, cable length and shortening in the case of pre-fabricated cables, and deck elevation/tower sway as a measure of geometry. All measurements have tolerances, but in order of reliability, elevation is the most direct measurement, cable shortening the next most direct, and forces the least direct measurement. Cable shortening itself is generally the most accurate, but that accuracy must be combined with general superstructure geometry (location of anchorages) in every case, so that the accuracy of the controls by cable length and bridge geometry are similar. Cable force measurement is the least reliable measurement owing to gage tolerances, friction and both mechanical and human factors in reading and seating. Stiff Girder Systems: The rigidity that gives the large box its broad influence line precludes local deformation and flexibility during erection. The shape of fabrication - be it precast segmental, cast in place, or fabricated steel - is the shape constrained during erection. Local adjustments of stay cables have little effect on local girder shape. Therefore, erection of stay cables on stiff systems is dominated by the constraint induced by the shape of the prefabricated girder. The rigidity of the girder allows cables to be pulled within the general tolerance of jacking systems without an appreciable effect on girder profile. By the same token, corrections to deck geometry that might be necessary due to fabrication/casting errors or variances in weight or behavior (e.g. shear lag) generally can not be achieved through cable installation. In the case of precast segmental box shapes, shimming is normally required to correct geometry.

Clark – Flexible Edge Girder Skyway – Stiff Box Girder Huntington – Flexible Concrete

Flexible Girder Systems: The flexibility of the slender composite deck designs is even more pronounced during erection than during service. The bare steel frame that exists prior to addition of the concrete deck is extremely flexible. Controlling such a section with a cable force is somewhat akin to pulling a free weight up with a string. The geometry changes with the length of the string, but the force remains the same. The tolerance on cable force during the jacking operation can move a flexible bridge frame well outside the geometric tolerances for deck erection. This movement is less pronounced once the steel frame is stiffened with addition of the concrete deck, but relative to the box girder type superstructure, the shallow edge girder/composite system is still quite flexible. This flexibility allows corrections to geometry that might be needed due to variations in weight, errors in camber, or geometric deviations from bolt fit-up to be achieved in the field through cable length adjustments.

Pasco-Kennewick - Concrete

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There is no clear line of demarcation between the stiff and flexible systems. The difference between concrete edge girder systems like East Huntington, edge girder boxes like Pasco-Kennewick and box deck systems like Skyway is a matter of scale. Over the long main span, all are flexible relative to the stiffness of stay cables. In the short span from cable to cable, the steel frame system gains significant stiffness once the composite deck is erected, and becomes similar to the concrete edge girder system of East Huntington or Dames Point. Ultimately it is the ratio of girder to cable stiffness that defines behavior, considering deck stiffness, cable spacing and cable angles. Construction Engineering The engineering text books teach us that structural efficiency is to be measured in terms of the weight of material necessary to safely carry loads. Yet to the financial sponsor of the project - most often the tax paying public - efficiency is measured in terms of cost. The question of aesthetics notwithstanding, the first and most publicized cost is that on bid day. Construction cost often does not equate with structural efficiency. Construction costs are certainly affected by the quantity of materials in a design, but it is more greatly affected by the amount of time and labor needed to assemble those materials, by the access to fronts of construction, and by the temporary works needed to build the permanent structure. A clear example is the condition of the tower in a cable stayed bridge. In its final form, a cable stayed bridge tower can be optimally pinned at the base. Yet such a tower requires considerable temporary works not only during erection of the superstructure, but during erection of the tower itself. As a general rule, structures that can self-erect (that is, erect without any temporary works) are more cost effective than those that require temporary works. This is one of the economic advantages of the cable-stayed solution constructed in free cantilever. However, self erection for the typical cable-stayed bridge requires a tower section far more robust than needed for service. Nevertheless, this solution has proven to be more cost effective, and towers needing temporary guyed support are increasingly rare today. The greatest demand on many of the bridge components in a cable-stayed bridge is during construction. The segmental erection of a cable-stayed bridge results in a new statical system with each field section and cable. Each stage of erection must be engineered in order to have a completed bridge that conforms to the desired profile and array of forces within the structure at completion. In the case of a flexible bridge, the process is made more interesting due to the nonlinearity of flexible girders, tall towers, and slackened cables that exist at various intervals throughout erection. In addition, the aerodynamic stability of a free standing tower with cantilevered deck is generally more vulnerable than the final configuration with the deck closed against approach piers. Just as there are two basic approaches to design, there are two basic methods to erection engineering of cable-stayed bridge construction. The two approaches are theoretically the same, but practically quite different. In the first method, the bridge is “deconstructed” from the final profile, member and cable force regime. This is an intuitive approach based on the concept of linear superposition. If one begins with the final profile and cable forces, it is assumed that by reversing the construction process, the installation requirements for each field section and cable can be defined. In the second method, the bridge is analyzed just as it is erected, from the start. The starting point for analysis is based on computation of unstressed cable lengths for the geometry and cable forces of the final bridge. This method does not require application of superposition, since forward analysis can easily be nonlinear (recall that superposition is only valid for linear systems).

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Clark Bridge Erection to Grade

In both methods, the direct solution (de-constructing in one case, and unstressed geometry in the other case) breaks down once intermediate cable or camber adjustments are needed in order to control stresses or geometry. These intermediate adjustments are unknown at the outset, and need to be determined based on stress or displacement constraints that evolve during erection analysis. The constraints may be top steel flange limitations (the top flange is composite in the final case, but quite slender in the non-composite case); concrete stresses behind heavy equipment on the work front; accommodation of long term effects; or global geometry adjustments for time-dependent movements, line up for main span closure, etc. The approach that my office uses is the forward analysis approach. While the choice is a matter of preference, we find that this approach offers several advantages. First is the direct method of accounting for nonlinearity in the analysis. Second is the ability to “iron out” pre and post closure cable operations, minimizing the number of post closure adjustments – a practice that is only possible when establishing intermediate targets independent of the final plan solution. This method also avoids associating erection analysis to unknown design assumptions, since targets are only local strength and deflection constraints during erection. Quite often we find that we can adjust cable sizes or move strands around to improve the solution from a de-construction solution.

Intermediate Adjustment for Stresses – Clark Bridge

Nonlinearity in Design and Construction I have mentioned a few times now the nonlinearity in a cable stayed bridge. The degree of nonlinearity in cable stayed bridge analysis and design varies with each particular design – obviously the nonlinearity of a heavy box girder will be different than for a slender edge girder. Leaving time dependent behavior aside for the moment, we have two general sources of nonlinearity in structures. The first is material nonlinearity – that which comes from a deviation from Hook’s law. The second is geometric nonlinearity, which stems from the change in stiffness as loads are applied on the deflected shape of a structure. For practical conditions within the working load range of cable-stayed bridges, we can disregard material nonlinearities (other than creep/shrinkage), both during construction and in service. (The safety philosophy for LRFD of nonlinear structures is another subject entirely, and beyond this present discussion.) However, you will find countless references to geometric nonlinearity in cable-stayed structures. The most obvious aspect of nonlinearity is cable response. Podolny 2 shows a chart of changing cable stiffness vs. tension and geometry of a cable. The Ernst equation is used to describe the chord stiffness of a cable, while catenary equations are used to describe the free length of cable more precisely. The most interesting result in Podolny’s presentation is the tangent stiffness of cables at sag ratios less than OTEC 2002 pg 6

1%, which is about the level in service. At this stage of stress, there is little effect of nonlinearity in the cables, and cable nonlinearity is not a major factor in the analysis of cable-stayed bridges for service loads. It is the construction stage where geometric nonlinearity is most pronounced. In the first instance stay cables are often initially installed to less than 10% of Fpu for composite girder systems, and stays can be highly nonlinear as installed both due to tension stiffness and the deflection of the cable axis from the chord. In the second instance, the girder itself is quite flexible before the concrete deck is added and main span closure is made, compounding the nonlinearity of the bridge as it is being erected. In the case of the more rigid concrete box girder deck structures, the deck nonlinearity is not significant, and owing to the heavy weight of the deck, cables are generally installed closer to final force levels. However either flexible or rigid deck systems may be erected in free cantilever about a tower that is quite flexible before the backspan is constructed to the anchor pier. Stay cables aid the stability of the pylon, even in the free cantilever state. However, the moment magnification on these towers is often significant. Economy and Competition During the formative stages of cable-stayed technology in the US, the FHWA had a policy of bidding alternative designs. These were almost exclusively competitions between steel and concrete alternatives, and the process forged the economical solutions that we have today. The earliest competition was on the East Huntington Bridge, connecting Procterville, Ohio and Huntington, West Virginia. The main piers for this crossing had already been constructed for a steel box girder cable-stayed bridge when FHWA requested an alternative design in concrete. The resulting concrete edge-girder design bested the steel alternative by almost 30%. This bid was the end of the orthotropic box for cable-stayed designs in the US. About the same time as the hybrid design of concrete edge girders and steel floorbeams at the East Huntington project was entering construction, both Dames Point and Sunshine Skyway were developing designs that extended edge girder systems. Dames Point was an all concrete system, while the steel alternative of Sunshine was a steel frame / concrete composite precast panel deck system. Although not the successful alternative (the steel alternative had more total concrete than the concrete alternative, emphasizing the importance of foundation design), this alternative proved to be the template for contemporary composite systems of today. Later competitions resulted in both the concrete edge girder and the delta-frame box system being refined to compete with the steel frame/precast panel deck system. As we look at the predominate systems in steel (steel composite) and concrete (box or edge girder), there are a number of factors to consider in terms of efficiency and cost. These factors differ for bridges over land when compared with bridges over water. Since most long span cable-stayed bridges in the US cross water, I will deal with only the latter.

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Factor Commentary Foundation Requirements When it comes to large water crossings, “the

glamour is in the air, but the money is in the ground.” The quantity of foundation material and the type of foundation can overwhelm the cost differential for superstructure. The decision factor is whether this element of design is driven by superstructure weight, or other factors that are weight neutral.

Material quantity and cost While least weight is often not least cost, the quantity of material for a given structural system and construction concept is a direct scale factor on cost. Excess weight tracks through the entire system, more so for a long span bridge than for a typical structure. The decision factor is whether additional weight secures greater economy in other cost factors to more than compensate for the weight.

Special Equipment Specialized equipment invariably becomes a sunk cost in construction. Planning and promotion notwithstanding, most specialized equipment is written off on the project, and not discounted for later use. As a general rule, lighter equipment is more readily available, and therefore more economical. The decision factor is whether the need for specialized equipment secures a greater cost advantage for labor or time for completion when compared to alternatives.

Cables and related hardware Cable cost is a large percentage of construction cost – in the range of 10% or more. In addition, cable installation is a major critical path activity, compounding the cost issue. There is not much latitude with cable weight once other design options are chosen. However, the type of anchorage and blend of girder, tower and cable designs are all decisions that affect cost, reliability and speed of construction.

Prefabrication and Construction Schedule The type of design and implied construction methods need to be compatible with the global logistics of the site and the flow of work during construction. Deck erection is always on the critical path, and so the decision factor is whether the costs and time of prefabrication, quicker delivery, and faster erection are offset by any economies of in-situ construction. In general, when there is lead time for major foundation construction, then there is an advantage to steel and precast concrete, all else being equal.

Labor Risk Big bridge construction is fraught with risk, and labor risk is high on the list of exposure. Designs

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that minimize field labor risk, risk of rework, and risk of delays can often carry an additional burden of material cost and still be more economical. This is an aspect of cost that can only be defined with competition, and is an issue that drives the debate over alternative designs.

Access to the work front In real estate the axiom is “location, location, location.” Contractors have told me that in terms of construction costs, the parallel is “access, access, access.” When the bridge is entirely over water, the method of delivery is simple. It is when the backspans are over land that the issue becomes more complex. Changing delivery methods for large components is costly and time consuming. It often involves changes to erection methods as well as delivery. Section designs that are modular and offer some flexibility between heavy and light lifting methods are better suited to dual delivery schemes.

The Cable The cable element is the most unique structural element in the cable-stayed bridge. The early, more primitive cable-stayed bridges had few cables, and were based on wire rope and strand technologies associated with suspension bridges. But once the modern cable-stayed bridge arrived in the US, post-tensioning technology became the preferred solution. Led first by the parallel wire cable, design of these new cable systems focused on fatigue strength and anchorage design. The parallel wire cable with “Hi-Am” anchor developed in the 60’s by Leonhardt and BBR reined supreme in terms of fatigue resistance, and was the cable specified for the first modern cable-stayed bridge in the US – the Pasco-Kennewick Bridge in the State of Washington. The Hi-Am parallel wire cable had to be prefabricated, and its use in stay cables in the US came at a time when the parallel wire prestressing tendon was on the wane. Shortly before the third major Hi-Am cable bridge was to be constructed in the US – the East Huntington Cable Stayed Bridge between Ohio and West Virginia – the last wire production line in the US was shut down, and the Hi-Am cables came from Japan. At the same time, designers such as Jean Muller were working with post-tensioners to apply special stay cable anchorages with prestressing strands, with the first major application taken from Jean Muller’s Brotonne Bridge in France. This development allowed field assembled stays which were more versatile and less expensive than Hi-Am stays.

Hi-Am Anchorage Strand Anchorage (see www.dywidag-systems.com)

1 - Ring Nut 2 - Sealing / Spacer 3 - Strands 4 - elastomeric Bearing 5 - Anchor Block 6 - Bearing Plate 7 – HDPE Tube

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Development of strand stay cable systems continues to this day. Hi-Am/parallel wire cables are still used in the Far East, but in a competitive environ are limited to applications such as railway bridges, where their superior fatigue resistance is utilized. The leading guideline for stay cables is the PTI Recommendations for the Fabrication, Design and Installation of Stay Cables. Currently in the 4th Edition, development of these provisions is a chronicle of the industry’s priorities for producing high quality stays cables, and how those priorities have changed with developing technology. Discussion of this history helps to keep development moving forward without retracing too many steps.

“The disadvantage of men not knowing the past is that they do not know the present.” (G. K. Chesterton, 1933)

The first edition of the Recommendations focused on establishing a framework for specifying cable stay systems within the context of the AASHTO Standard Specification. Prompted by industry concern over the divergent fatigue criteria in the Dames Point and East Huntington specifications, Cliff Freyermuth, then the Director of PTI, assembled a small group of industry and design leaders in the cable-stayed bridge field, and set out an agenda for a concise standard covering the basics of stay-cable supply. The preponderance of effort was devoted to establishing fatigue criteria. Led by Professor Breen of UT at Austin, the fatigue subcommittee established the basic safety philosophy that remains in the Recommendations to this day. The Recommendations set levels of fatigue resistance for different cable systems based on testing, and also established the relationship between strengths of individual cable elements vs. the cable bundle as a whole. The research work performed by the fatigue subcommittee also uncovered a disparity between lead patented and air patented prestressing steel, the latter of which had become more the standard in the US and Japan, vis a vis Europe. This difference in manufacturing process was one of the items that came to explain a decline in basic fatigue strength and toughness from newer strand suppliers when compared with historical data from older European supplies. Fundamental to this new fatigue safety philosophy was the quality control testing imposed on full scale anchor prototypes designed and supplied by a vendor. The stay anchorage is not subject to conventional design rules, but is instead subject to performance qualification testing. The basic requirements are defined to assure that the quality of design and manufacture of vendor’s anchorage designs do not depreciate tendon strength more than a nominal amount. In earlier specifications for Hi-Am cables, this depreciation was specified as zero against GUTS. However, as strand and bar cables became more prevalent, the more common post-tensioning rule of 5% depreciation was adopted as recognition that boundary and anchorage effects would control actual fatigue strength. In order to address material over-strength, the targets for residual strength were established against the actual strength of the wire or strand material used for the tests. One of the fatigue issues was the proper load spectrum for fatigue design. Some early projects included safety factors on fatigue load cases, and most included patterned uniform loading as a fatigue load case. While this patterned loading made sense for railway bridges, the typical highway bridge was indicating significant fatigue only when this patch uniform load was included. A study (Goodyear4) was conducted to investigate fatigue loads based on realistic truck traffic. This study resulted in the use of single trucks as fatigue vehicles, eliminating pattern lane loading from the fatigue case. This first edition of the PTI Recommendations was a pioneering document that gained instant recognition as a world-wide standard, even though the standard was written to complement the AASHTO Code. However, like most new standards for developing technologies, there was a need to test and update for field experience. One of the issues that emerged was the issue of corrosion. The typical corrosion protections schemes were those for conventional post-tensioning – a duct with grout. While not fully resolved in the Second Edition, the issue was acknowledged by expanding the recommendations for grouting procedures, stay cable duct and pipe, and PVF (Tedlar) tape covering over the pipe; and by introducing greased/sheathed and epoxy coated strand. While other strategies were noted in commentary form, the basic corrosion protection medium continued to be the conventional PT approach of grouted ducts.

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Fatigue continued to be a major focus of the PTI Committee. While there was a good deal of discussion about results of recent fatigue tests, there were few changes in the fatigue provisions in the Second Edition. One element that was added was a requirement to de-bond steel pipe ducts from the fatigue specimens. This recommendation was adopted in lieu of having the pipes and pipe connections themselves qualified for fatigue, which would have been necessary if their strength was to credit the main stay material. At the time nearing the end of the schedule for the second edition there were further issues arising regarding fatigue tests and acceptance criteria. A major component of the discussion at this time was the quality control / design qualification aspect of the stay fatigue tests, and the significance of the “white gloved” technicians assembling the test rigs, vs. the sledge hammer toting ironworkers assembling the stays in the field. Certain tests had failed because of tolerances or refinements in fitting out the test specimens, and the difference in tolerances between lab testing and field testing were vigorously debated. It was at this time that the issues of design quality and proof testing of a proprietary design came to be recognized as one of the more important functions of the fatigue test. This was the first aspect of performance specifications to be recognized by the Committee (for to date, most of the Recommendations were patterned after methods specifications). By the Second Edition (1989-1990) PTI had also published the new Concrete Segmental Guide Specification, then known as NRHRP 20-7/32. Experiences on a number of cable-stayed projects in the US had raised questions about appropriate erection controls for cable-stayed bridges, as well as cable installation methods. So perhaps the most significant revision to the Second Edition was the addition of “Installation” to the title, and the addition of recommendations for erection control of stay cables. These provisions were written to address such items as cold unreeling of PE pipe and stays, handling of prefabricated stay anchors, and the first requirement for an engineering cable erection program by the contractor. The Third Edition of the Recommendations was being outlined as the Second Edition was in to print. There was a major expansion of the Recommendations going into the Third Edition. There were a growing number of bridges under design, and a growing interest with any number of specification details that come up when design practice expands to those without experience in development of a new technology. There was a decided emphasis on corrosion protection, albeit still centered on contemporary post-tensioning methods and technology. Grout was still a mainstay in the systems prescribed, but now there were far more extensive recommendations for epoxy coated strand, greased and sheathed strand, galvanized strand (although not available in the US) and the material specifications and testing requirements for these components. A significant volume of material was added for inspection, shipping, and storage of stay materials, supplanting or reinforcing many of the state standard specifications or contract special provisions that had been used for these items in the past. The design criteria for loadings and stay removal conditions were expanded, as were the provisions for cable installation. It was in this Third Edition that the practice of erection to grade was introduced (prior to this, the Recommendations were established for erection control using either cable forces or unstressed cable lengths) in the Installation section. The issue of fatigue was never off the agenda for any meeting. As noted above, there were some outstanding issues on fatigue testing left over from the Second Edition. The recommendations on fatigue were expanded to include more detail on the techniques and criteria for detection of wire breaks, and added further definition of what was acceptable. There was a slight relaxation of the standards for fatigue acceptance (that would later be retracted) that was promoted, in part, as a practical simplification in evaluating results. The post-fatigue strength of the specimen was changed from 95% of actual tensile strength to 95% of guaranteed tensile strength. This change allowed additional testing margin, for up to 5% over-strength in actual strand tensile could be used without penalty. The effect was to lower the quality control on anchorage design and performance – a measure that was later rebutted by the suppliers who initially proposed it. While each of the first 3 editions came in rapid succession (about every 3 years) during a period of intense design activity, the Fourth Edition was about 7 years in the making. As the Fourth edition was being planned, AASHTO began moving (or so we thought) to the LRFD specification, following the format

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that by then had been adopted by most of the civilized world. We were also chartered on the road to metric, and so the Committee elected to convert the Recommendations to both metric and LRFD. The move to LRFD was calibrated against the working stress design criteria of the Third Edition for Strength I versus Group I loads. This calibration was not direct, since live load as a percentage of total load varies among concrete and steel designs, as well as between longer and shorter spans. The basic fatigue design was patterned after LRFD, but the foundation of criteria remained the original basis established in the first edition, and tuned for results from fatigue testing since the original work. There were several major additions to the Fourth Edition that expanded the content and coverage of the Recommendations. The first dealt with an aged orphan of the past editions – the cable saddle. Saddle cables had been used in the US since Sunshine Skyway – itself a replica of Brotonne in France. Yet past Recommendations did not contain any specific design rules for saddles, but instead attempted to treat saddles as anchorages. This position was due, in part, to FHWA’s position against using saddles for technical reasons, the principle one being the harmful effect that transverse pressure has upon fatigue strength of strand. This situation all came to the fore when a saddle bridge in Delaware could not pass cable tests for the saddles. This became a complicated contractual matter, for saddles were designed by the Owner’s engineer, yet required to be tested by the supplier. Assigning responsibility for the fatigue strength of the saddle to the supplier was questionable indeed (much the same as trying to assign responsibility to a contractor using a methods specification). The testing failures served to expose the limitation of the Recommendations in this regard, and the development of saddle provisions was placed on the agenda for the Fourth Edition. The approach to saddles was first to recognize that such designs are integral to the bridge design, and not a supplier designed item. Therefore, responsibility for design and testing was placed on the designer, rather than on a supplier. Diagnosis of past tests, along with testing on post-tensioning conducted at UT at Austin served as the basis for design guidelines for strands within the saddle, considering the effects of lateral pressures, bending radius, and strand cushioning on the static and fatigue strength. Cushioning and strand isolation was introduced as design factor in the new criteria, based in part on performance of past fatigue tests with epoxy coated and sheathed strands. The result is a provision that addresses the designer’s responsibilities for saddle design and performance testing, removing this feature from the requirements placed on the stay supplier. A second major change to the Fourth Edition was a new approach to corrosion protection. Through the first three editions, corrosion protection had been addressed through methods specifications for grouting, stay pipe, and strand coatings. This was a traditional approach based on post-tensioning technology, but always an uncomfortable approach to the unique conditions of stay cables, and one that seemed to result in doubts on every project. The committee decided to change the approach to one of performance specifications. The challenge was considerable, and the committee went through a wide range of alternatives for both establishing and testing performance. In the end, the committee agreed on a series of tests for leak testing of both inner and outer barriers (there being two required for any cable), leaving the design of those barriers to the suppliers. The acceptance criteria were relatively simple – the barriers can not leak to the point where any cable material will corrode.

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The third major addition to the Recommendations was in the form of wind driven vibrations of stay cables. A number of major bridges around the world had experienced significant vibrations during construction, and several had also experienced rain-wind vibrations during service. Some vibrations were so severe as to fail secondary components of the bridge. The current state of the art in diagnosing and designing for cable vibrations was included in the Recommendations, and guidance was offered for dealing with special cases during construction.

PTI Acceptance Criteria (from PCI)

In the Fourth Edition, as in all others, the subject of fatigue was revisited. The acceptance criterion for post-fatigue strength was changed to recognize both the conditions of GUTS and actual tensile strength by requiring fatigue specimens to develop the greater of 92% of actual or 95% of GUTS. In addition, the fatigue testing process was tied to the corrosion provisions, first by using one of the fatigue test specimens for corrosion testing, and second by inspecting all fatigue specimens for corrosion during the post testing dissection. Maintenance and Performance of Cable-Stayed Bridges The economic advantage of a cable-stayed bridge is derived from the structural efficiency of a cable in pure tension. Yet this advantage in first cost becomes an obligation for future maintenance. Cable supported bridges of all types are generally the highest cost to maintain of all fixed bridges. The cable that is so efficient is naturally more vulnerable to deterioration. The great exposure and large surface area of stay cables adds to the challenge of inspection and cost of maintenance on these bridges. While the issues of corrosion and fatigue have been addressed in the PTI Recommendations, this does not in any way lessen the need for bridge owners to maintain these delicate structures over the course of time. Older cable-stayed bridges do not generally satisfy the newer anti-corrosion design criteria, and inspection for signs of stay corrosion is even more urgent for some of these structures. PTI has focused on fatigue design criteria that emphasize quality control, for actual field assembly and service can be expected to be far more demanding than the pristine conditions of the laboratory. Construction tolerances that may result in strand fretting, pinching, and local stay bending or life-long attack from road chemicals all create demands for inspection and maintenance for cable-stayed bridges. Construction handling may result in damage to protective barriers on stay cables; damage that is too often ignored during the trials of erection. These become focal points for ongoing inspection and maintenance operations. Anchorages are the most vulnerable elements of the cable, and provisions for inspecting anchorages are generally part of contemporary design. Deck durability is perhaps more important for cable-stayed bridges than for conventional structures, since the deck carries compression as well as local bending. That same compression generally improves durability in concrete decks, but the founding quality is established during construction, so inspections should be focused based on construction records. Global deflections can be harbingers of distress, and a comprehensive array of geometry control hubs should be established at the end of construction for this purpose. (Too often this is ignored in the relief of completing the rigors of construction, leaving a void in knowledge about bridge behavior). Geometry should be assessed as part of routine inspection, and changes in geometry compared to design

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predictions. The most taxed elements of the deck girder are at the rigid boundaries – the anchor piers and the main pylons – and these areas should be scrutinized during regular inspections. All of these requirements, and all of the baseline data that results from design and construction, should be organized into a maintenance manual for the Owner. This spares Operations from digging through design and construction archives, and focuses attention on the needs for maintenance during the design and construction process. Closing Cable-stayed bridge design has matured over the last score of years. Computer capabilities have broadened due to the general advance of technology and have extended access to cable-stayed bridge engineering. In this environ it is worth keeping in mind the thoughts of Henry Petroski in his book To Engineer is Human, “…answers are approximations…and come from a feel for the problem and do not come automatically from machines…” Design remains an art, and construction a talent. Understanding both structural behavior and proper erection technique is vital to the design and construction of efficient, artful, economical and durable cable-stayed bridges.

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References

1. Walther, Houriet, Isler, Moia; Cable Stayed Bridges; Thomas Telford, London; 1988 (English Ver) 2. Podolny, Walter, Jr.; Scalzi, John; Construction and Design of Cable-Stayed Bridges, 2nd Ed;

Wiley; 1986 3. PTI, “Recommendations for Stay Cable Design, Testing and Installation”; 4th Edition; Post

Tensioning Institute; Febr, 2001 4. Goodyear, D; “Stay Cable Fatigue Design Loading”; PCI Jnl; Vol 32, No 3; May/June 1987 5. Lenohardt, F; Zellner, W.: “Cable-Stayed Bridges”; IABSE Periodica, 2/1980; Vol S-13/80

Alamillo East Huntington

Brotonne Pasco-Kennewick

Normandie Tartara

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