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Design of Looping Cable Anchorage System for New SanFranciscoOakland Bay Bridge Main Suspension SpanJohn Sun, P.E.1; Rafael Manzanarez, P.E.2; and Marwan Nader, P.E.3Abstract: Located at the rocky edge of the Yerba Buena Island, the west anchorage of the San FranciscoOakland Bay Bridgesuspension span serves as the anchor for this single tower self-anchored suspension bridge. With extensive comparative studies onnumerous alternatives, the new looping cable anchorage system is recommended for the final design of the west anchorage of theself-anchored suspension span. The looping cable anchorage system essentially consists of a prestressed concrete portal frame, a loopinganchorage cable, deviation saddles, a jacking saddle, independent tie-down systems, and gravity reinforced-concrete foundations. Thisanchorage system is chosen for its structural efficiency and dimensional compactness. This paper describes major design issues, designphilosophy, concept development, and key structural elements and details of this innovative suspension cable anchorage system.DOI: 10.1061/ASCE!1084-07022002!7:6315!CE Database keywords: California; Bridges, suspension; Design; Bridges, cable-stayed.IntroductionIn May of 1998, the Bay Area Metropolitan Transportation Com-mission selected the self-anchored, single tower suspension alter-native as the signature span of the San FranciscoOakland BayBridge east span seismic safety project T. Y. Lin 2001!. Therendering of the bridge is shown in Fig. 1.While the single tower asymmetric suspension bridge satisfiesthe aesthetic preference of the bridge type selection committee,the concept of self-anchored is dictated by the geotechnicalcondition, shown in Fig. 2. At the east anchorage pier, the com-bined depth of young bay mud, old bay mud, and sand layersreaches more than 100 m above the Franciscan rock formation.Such soil conditions make construction of the conventional earthanchorage undesirable in both technical and economic terms. Thedeck anchorage system becomes the natural choice for the eastanchorage, and consequently for the west anchorage.The general elevation and plan of the east bay bridge mainspan are shown in Figs. 3 and 4. With a 385 m main span and a180 m side span, the asymmetrical suspension bridge has a mainspan length to side span length ratio of 4.3, when compared to aconventional symmetrical suspension span. This spans asymme-try is recommended primarily to accommodate the main spannavigation clearance and to highlight the grace of the catenarycurve of the main cable.The 70-m-wide deck structural system consists of twin ortho-tropic box girders transversely connected with cross beams. Theseemingly single tower is, in fact, a four closely spaced steel shaftframe connected by intermittent shear and tension links along theheight of the tower. The deck is monolithically connected to westanchorage pier W2, and is supported on sliding bearings for ser-vice load conditions and effectively pinned for safety evaluationearthquake SEE! loads at east pier E2. The architectural require-ment of a gateway effect created by two planes of hangers meansthat the main cable must be anchored to the outer sides of the boxgirders at both east and west ends, and converge on the tower topsaddle, as shown in Fig. 4.Among all of the key structural components in this suspensionstructural system, the west anchorage is one of the most criticalelements for several considerations.The west anchorage piers take up to 70% of the total baseshear in the critical longitudinal direction under critical seis-mic loads. This requirement results from the limited shear ca-pacities of a very flexible tower and the east anchorage pierbeing founded on a flexible pile foundation system in the baymud.1Senior Bridge Engineer, T. Y. Lin International, 825 Battery St., SanFrancisco, CA 94111.2Vice President, T. Y. Lin, International, 825 Battery St., San Fran-cisco, CA 94111.3Associate, T. Y. Lin International, 825 Battery St., San Francisco,CA94111.Note. Discussion open until April 1, 2003. Separate discussions mustbe submitted for individual papers. To extend the closing date by onemonth, a written request must be filed with the ASCE Managing Editor.The manuscript for this paper was submitted for review and possiblepublication on June 19, 2001; approved on January 30, 2002. This paperis part of the Journal of Bridge Engineering, Vol. 7, No. 6, November 1,2002. ASCE, ISSN 1084-0702/2002/6-315324/$8.001$.50 per page.Fig. 1. Rendering of east bay suspension spanJOURNAL OF BRIDGE ENGINEERING / NOVEMBER/DECEMBER 2002 / 315The postyield behavior of the west anchorage pier columns hasa significant effect on the residual displacement, or perma-nent set of the main suspension span.The 17,400 5.4-mm-diameter wire cables are capable of devel-oping up to 700 MN or 70,000 tons, cable-breaking capacity!of cable force with about 280 MN or 28,000 tons! of uplift atthe deck level.The west anchorage structural system must resist longitudinalcompression thrusts from the orthotropic deck without signifi-cant local effects, such as bending or shear lag.The anchorage must also satisfy all geometry requirements,such as roadway clearance, minimum saddle radius, the singleplane requirement of the anchorage cable layout, and the road-way elevation differential.The west anchorage shall be compact and clean so that it is anatural part of the grace of the bridge.West Anchorage Structural Concept DevelopmentThe deck anchorage at the east pier E2 requires that compressionthrust delivered to the deck box girders be countered by the westanchorage at the elevation of the main box girder centroid. There-fore, the main cables should be anchored in the deck at the westpier or deviated by a saddle bearing against the deck. This meansthat the traditional earth-anchorage system cannot be employed.A number of anchorage systems may be used to deliver therequirements. All of the deck-anchorage systems can be catego-rized into the following two types: 1! a deck anchorage system;and 2! a hybrid anchorage system with reference to deck an-chorage and earth anchorage!.Deck Anchorage SystemsIn theory, there are several viable deck anchorage systems for thewest anchorage. Brief descriptions of deck anchorage alternativesinvestigated are presented below.Box-side Cable Splay Anchorage System IFigs. 5 and 6 show a plan and an elevation of a box-side splayanchorage system I layout. In this system, the steel box girdercontinues over the west anchorage pier. The main cable will bedeviated at the splay saddle at the west anchorage pier on the sideFig. 2. East bay bridge soil profileFig. 3. New east bay bridge main span structural layoutplanFig. 4. New east bay bridge main span structural layoutelevation316 / JOURNAL OF BRIDGE ENGINEERING / NOVEMBER/DECEMBER 2002of the box. The strands of the main cable are anchored to a stiff-ened grillage welded to the interior faces of the box girderflanges. The longitudinal compression thrust from the east an-chorage will be balanced by the opposite cable force transmittedto the deck through the cable anchorage stiffening grillage at thewest pier.It is important that the splayed saddle be positioned in thespecified service loading cable plane to avoid excessive out-of-plane bending action on the splay saddle for various service loads.To spread the strands of the main cable to anchor locations, thelength of the splayed strands should be about 35 m behind thesplay saddle. This requirement results in the anchorage box sec-tion of 1418 m in depth, as shown in Fig. 6.In addition to the aesthetic impact due to the size of the an-chorage box, the large shear and bending on the anchorage deckcaused by large uplift action and local effects, such as shear lag,are causes of concern.Box-side Splay Anchorage System IIThe box-side splay anchorage system I described in the previoussection can be further refined by introducing the closely spacedhangers in front of the splay saddle, as shown in Figs. 7 and 8, inplan and elevation, respectively. Tie-down forces can be appliedto the hangers so that the main cable is deviated to reduce thecables approaching angle, and consequently the uplift action tothe anchorage deck and depth of the anchorage box.Due to the large cable size and inclination angle at the westanchorage, up to 24 rope-hanger sets, spaced across about 25 m ofcable length, are required to achieve the deviation. This is a clearaesthetic concern. Also, the first few hangers will be subjected toexcessive forces under seismic loads.Box-side Anchorage with Midair Floating SaddleThe concept of a box-side anchorage with midair floating saddleis illustrated in Fig. 9 in plan and in Fig. 10 in elevation. Thisanchorage system uses a midair floating saddle to splay thestrands of the main cable, then strands are anchored to the side ofthe continuing steel box girder over the west pier. The uplift ac-tion of the main cable is transferred to the stiffened deck at thewest pier, then tied back to rock by prestressing tendons.To keep the lateral splitting action to a reasonable level, thefloating saddle must be located at a minimum of 40 m away fromthe strand anchors. Consequently, a triangle of strands will beformed above the bridge deck, as shown in Fig. 10. This triangledimension mass is even more visible when an enclosing houseFig. 5. Box-side splay anchorage system IplanFig. 6. Box-side splay anchorage system IelevationFig. 7. Box-side splay anchorage system IIplanFig. 8. Box-side splay anchorage system IIelevationFig. 9. Box-side anchorage with midair floating saddleplanFig. 10. Box-side anchorage with midair floating saddleelevationJOURNAL OF BRIDGE ENGINEERING / NOVEMBER/DECEMBER 2002 / 317is installed to protect splayed strands from aggressive weather andpotential vandalism. Another concern for this cable anchoragesystem is that the most outer strands are subjected to excessivestress under seismic loads.Loop Cable AnchorageFigs. 11, 12, and 13 show a plan, a front elevation, and a sideelevation of the loop cable anchorage concept. The anchoragesystem consists of several key structural elementsnamely, theprestressed concrete portal frame, multicolumn piers, the loopanchorage cable and anchorage saddles, and independent tie-down systems.In this anchorage system, the main cable is not splayed at thewest piers; instead, it is continuously looped around under theprestressed concrete cap beam. The cable looping is achieved byusing a pair of deviation saddles at the outer edges of both road-ways, and a jacking saddle located at the centerline of the capbeam. The massive tension force in the main cable is balanced bythe distributed compression stress behind the deviation saddles.The three-dimensional compression thrust behind the deviationsaddle bearing plate is resisted by bridge box girders in thebridges longitudinal direction, the cap beam in the transversedirection, and the independent tie-down cables in combinationwith the cap beam gravity in the vertical direction.The loop cable anchorage has an overall depth of 7.1 m, thesmallest anchorage system among all of the feasible anchoragealternatives. The direct transfer of cable tension into concretecompression in the cap beam joint delivers the structural effi-ciency. The prestressed concrete cap beam functions as a com-pression member to distribute the three-dimensional compressionthrust in both longitudinal and transverse directions, and as acounter weight for cable uplifts at service loads. The independenttie-down cable resists additional tension uplift! induced by seis-mic loads and provides a redundant vertical load path for bridgeglobal stability. The multicolumn reinforced-concrete RC! piersprovide strength, flexibility, and ductility. This anchorage systemis also one of the most cost-competitive alternatives investigated.Much discussion about this system has also been concentrated onwhether or not the deviation saddles should be fixed. Pros andFig. 11. Loop cable anchorageplanFig. 12. Loop cable anchoragefront elevation318 / JOURNAL OF BRIDGE ENGINEERING / NOVEMBER/DECEMBER 2002cons are studied for both fixed and sliding saddle solutions for thefull spectrum of loads, including load conditions during erection,service, and seismic events. Based on an overall evaluation, it isconcluded that a fixed deviation saddle solution, combined with ajacking saddle, best suits the service performance and reducesexcessive requirements to the tower under seismic loads.Another issue about this anchorage system is the constructa-bility of the main cable. The design philosophy with respect tothis issue is that structural safety and efficiency must be ensuredfirst, and the system can be erected without major difficulties.After extensive studies, it is concluded that the loop cable anchor-age allows the cable to be erected by both the traditional aero-spun method and the recently developed parallel wire strandmethod.Hybrid Anchorage SystemsGiant Block Anchorage SystemA hybrid cable anchorage system is an anchorage that combines atraditional earth-anchorage form with the essential characteristicsof the deck anchorage or anchoring the strands of the cable anddelivering the resisting mechanism against the compression thrustin the deck!. A typical giant block anchorage, shown in Fig. 14, isa typical example of this type of anchorage. The main structuralsystem is a concrete wall box that will support the deviationsaddle and provide a bearing wall for the deck box girder. Thesplayed saddle is generally a rocking saddle, and strands of maincable will be splayed through the saddle and anchored to the basefloor blockout by high strength anchor bars connected to strandshoes or strand sockets. The anchorage base floor blockout can betied back to the rock if needed.The advantage of this system is that it is conventional. It pro-vides adequate spaces for the splay saddle and for splayed strandsbehind the saddle. Some of the main concerns in using this sys-tem for this bridge include the aesthetic impact due to the size ofthe anchorage and the negative effect on the global seismic re-sponse of the bridge system. The extreme structural stiffness ofthis type of anchorage shortens the fundamental period of thebridge under lateral load, and raises the seismic rock motion tothe deck level, which results in unbearable demands on the bridgedeck, tower, and east piers. Based on these considerations, theconventional earth anchorage system was not adopted for the finaldesign.Integrated Pier AnchorageThe integrated pier anchorage is another hybrid system that de-viates the main cable downward with a side-splay saddle bearingagainst the deck box girder, and then anchors the cable strands toa curved strong wall inside the pier shaft. The front and sideelevations of this system are illustrated in Figs. 15 and 16, respec-tively. In fact, this system is an extension of the giant block an-chorage when the anchor chamber is reduced to a pier.The most critical issue for these anchorage systems is the seis-mic performance. First, with cable strands being anchored in thepier shaft, the pier shaft dimension is in the range of 10 3 12 m.This factor coupled with a large axial compression force goesagainst the pier displacement capacity. Second, the large stiff piershafts reduce the structural fundamental period significantly,which in turn tends to increase the seismic demands. Third, thevertical load carrying capacity of this system is interdependent onthe lateral load resisting system. The large lateral deformationunder seismic loads will lead to damage or failure of cable strandsand, consequently, the overall structural system.Loop Cable Anchorage SystemBased on comparative studies among all of the cable anchoragealternatives, the looping cable anchorage is chosen for the finaldesign for its structural safety, efficiency, compactness, and cost.Anchoring 70,000 tons of cable force at the deck level is onlyone of the main requirements for the west anchorage. Other re-quirements, such as the design of the west anchorage piers forglobal response, the joint between the main girder and the westanchorage cap beam, and the west anchorage to west approachspan transition details, are no less important. In the followingsections, the global design concept of the loop cable anchorage,design issues, and critical details of the main components of theanchorage structural system are briefly described.Loop Cable Anchorage Geometry LayoutDesign of the loop cable anchorage starts with the anchoragecable geometry layout. The following factors must be accountedfor in developing a rational anchorage cable geometry:Fig. 13. Loop cable anchorage side elevationFig. 14. Giant block anchorage systemJOURNAL OF BRIDGE ENGINEERING / NOVEMBER/DECEMBER 2002 / 319The overall loop anchorage cable must be placed on a singleplane to keep the out-of-plane bending on cable saddles to aminimum;The longitudinal compression resultant thrust developedwithin the west anchorage cap beam joint should be in thesame elevation as the centroid of each main box girder toavoid locked-in bending of the deck under dead load;The minimum possible saddle radius should be used to keepthe anchorage dimension compact; andOther factors affecting the loop cable geometry are the road-way layout, overhead clearance for the most outer lanes, andcable geometry at dead and service loads near the west anchor-age.The determination of the loop cable geometry is the result of aninteractive procedure.Fig. 17 shows a loop anchorage cable layout in its true plane.This generic loop cable geometry can be determined by twoworking points, WPW! and WPE!; radii of the deviation saddlesand the jacking saddle; and the inclining angle of the cable ap-proaching in side elevation.Working PointsThe working points shown in Fig. 17 are the centers of two circlesdefining two deviated cable segments at the corners of anchoragecables. These working points are the theoretical converging pointsof the bearing stress induced by compression on the saddle baseplates supporting the main suspension cable. In the side elevation,the working points are located on the same elevations of the boxgirder centroid lines. They are also on the centerline of the westanchorage frame cap beam in the transverse direction, so thecable force component on one end in that direction can be effi-ciently balanced by the opposing cable force component on theother end of the cable anchorage. The loop cable geometry is alsoaffected by the radius of the deviation saddles. A deviation saddleradius of 10.5 m in the final design is the result of an optimumcompromise among factors such as the minimum saddle radiusand cap beam overall dimension.Minimum Saddle RadiusMinimum radii of the deviation saddles and the jacking saddle areset to be 10D and 20D, respectively, where D is the diameter ofthe main cable. Since the bending effect in the wires of the cableis more severe when a whole cable is subjected to bending actioninduced by anchorage cable jacking during critical deck erectionstages, a larger minimum radius limit for the jacking saddle 20D!is set. In the case of deviation saddle design, the wire stress in-duced by bending is smaller, because the a plane section remainsa plane hypothesis is only partially valid for the individualstrands during cable erection. Consequently, a smaller saddle ra-dius of 10D is recommended.Fig. 15. Integrated pier anchoragefront elevationFig. 16. Integrated pier anchorageside elevationFig. 17. Loop anchorage cable geometryisotropic view320 / JOURNAL OF BRIDGE ENGINEERING / NOVEMBER/DECEMBER 2002Final Location of Jacking SaddleAnother factor affecting the loop cable geometry layout is theposition of the jacking saddle when it is in its final position. Thefinal jacking distance is determined to be 1.8 m to keep the cableforce differential between the forward cables and the anchor-ing cable during various construction stages and critical serviceloads to the limited level.Inclination Angle in Side ElevationThe average inclination angle of the loop cable anchorage in sideelevation is a big factor affecting the cable anchorage layout. It istaken as the average of the angles of the two approachinglive! cables at the west anchorage under dead load and one-half of the full live lane load. By choosing this side inclinationangle to lay out the anchorage cable, the out-of-plane bending oneither one of the two deviation saddles induced by cable geometryvariation under live load is limited.West Anchorage Portal FrameAs shown in Fig. 18, the west anchorage portal frame consists oftwo multicolumn RC piers, a prestressed concrete cap beammonolithically connected to the piers, independent cable tie-downsystems, and RC foundations supported on RC drilled shafts.Cap BeamThe west anchorage cap beam has several structural functions.First, together with the deviation saddles and the jacking saddle, itanchors the main cable to the cap beam and pier joints of theportal frame; the massive cable force is then redistributed in thetransverse, longitudinal, and vertical directions through the capbeam. Second, as the most important joint of the bridge, itconnects the main cable, the W2 pier, the main span box girders,the west approach span hinges, and the expansion joint. Third, itserves as a counterweight for the main span for design serviceloads.As shown in Figs. 19 and 20, there are three groups of pre-stressing tendons or bars in this cap beamnamely, longitudinaltendons, transverse tendons, and vertical tendons bars!. The lon-gitudinal tendons, parallel to the bridge axis, are the continuitytendons that connect the main girder with the cap beam. Theconnection is designed such that full bending capacities of thebox can be developed. The transverse tendons, parallel to the capbeam axis, are designed to counter the moment induced by thevertical dead load and the in-plane bending induced by the eccen-tricity between the cable force transverse thrust and the centroidof the cap beam sections. Vertical tendons or bars are used pri-marily to resist local tension induced by actions such as burstingand vertical shear of the west approach hinge beam.PiersThe first issue in the pier design is the determination of the pierstiffness or dimension design. Since the east pier is on a flexiblepiled foundation with piles of more than 100 m in length, and avery flexible multishaft tower of 160 m in height, the overallstructural stiffness, especially in the longitudinal direction, is dic-tated by that of the west piers. It should be noted that both lateraland vertical stiffness of the pier could have a significant effect onthe bridge global response.A number of pier cross sections for concrete, steel, andconcrete-steel composite materials are considered at the prelimi-nary design phase. These alternatives include a single hollow rect-angle with four corner columns, a twin column, and a four-column pier section. With extensive comparative studies, the four-RC-column pier is recommended for the final design. ThisFig. 18. West anchorage portal frameelevation and sectionsJOURNAL OF BRIDGE ENGINEERING / NOVEMBER/DECEMBER 2002 / 321decision is based on the following considerations:The lateral flexibility of the four-column piers increases thefundamental period of the bridge to more than 3.5 s, thussignificantly improving the bridge global response.The multi-RC column has adequate vertical stiffness to limitthe first vertical mode period to less than 0.1 s, which is dif-ferent from the peak vertical response period of 0.15 s.The reinforced-concrete columns have proven displacementcapacity when adequately confined.The concept ensures consistency in the use of RC materialfrom the cap beam to the pier, then to the foundation, conse-quently reducing the difficulties in connection details, result-ing in a more reliable structural system.It maintains the aesthetic harmony between the flanking piersand the tower.It is an economical alternative.Another key design issue regarding the pier design is connec-tivity between the pier and cap beam. Theoretically, there can bethree types of pier-cap beam connections, as follows: 1! bearingconnection; 2! monolithic connection, full moment transfer; and3! monolithic connection, partial moment transfer.The concept of a pinseems attractive, since it eliminates thebending moment transfer to the deck girder from the piers. How-ever, it proves to be difficult to achieve, due to the large verticalforce under combined dead and critical seismic loads. Thepinned bearing is also undesirable for erection loading condi-tions, considering a flexible tower and sliding bearing at the eastpier. Furthermore, the condition of a theoretical pin is question-able within the 150 years of the bridge service life.A fully fixed versus a partially fixed monolithic connec-tion is also studied and compared. The idea of a partially fixedconnection is to design a pier column section so that it functionsas the fixed joint during erection, service, and functional earth-Fig. 19. Cap beam posttensioning layoutplanFig. 20. a! Cap beam posttensioningside elevation; b! Cap beamposttensioningfront elevation322 / JOURNAL OF BRIDGE ENGINEERING / NOVEMBER/DECEMBER 2002quake loads, and functions as a hinge when designed momentcapacities are exceeded under SEEs. It is important to note thatthis connection concept works only when there is a large enoughdifferential between the design moment for erection and the ser-vice load, and that for the safety evaluation seismic loads. Sincethe partially fixed hinge near the cap beam requires that sectionmoment capacity within the hinge is significantly smaller thanthat of adjacent column sections, dimensional narrowing at thehinge sections is required to achieve this design objective. An-other concern about the partially fixed concept is that it will ex-perience more frequent damage and repairs than that of a fullyfixed connection. This may have a negative psychological impacton commuters.With extensive comparative study, a full monolithic connec-tion is recommended. In addition, two performance requirementsguide the design effort of the west anchorage pier seismic design.1.Minimal damage criteria for functional evaluation earth-quakes. Minimal damage implies an essentially elastic re-sponse, and is characterized as minor inelastic response/narrowcrackinginconcrete/noapparentpermanentdeformation. This requirement is represented by limiting themaximum concrete compression strain to less than 0.004,and the maximum tensile strain for the reinforcement to0.001 under functional evaluation earthquake loads.2.Repairable damage criteria for safety evaluation earthquakes.Repairable damage is characterized as the yielding of rein-forcement not necessarily for replacement!/spalling ofconcrete/small permanent deformation. Repairable damageis ensured by limiting the maximum concrete compressionstrain to less than 2/3 of the ultimate concrete strain derivedfrom Manders model, and the maximum tensile strain in thereinforcement to less than 2/3 of the defined ultimate steeltensile strain. In addition, the permanent displacement mustbe less than 300 mm for both the flanking piers and thetower.The reinforcement design of a pier column is shown in Fig. 21.The critical push-over analysis in the bridge longitudinal direc-tion, shown in Fig. 22, clearly demonstrates the adequacy of thepier design.Tie-down System DesignDue to the large main span to side span ratio, large uplift action isintroduced to the west anchorage through cable in tension. Thisaction is largely balanced by the cap beam as a counterweight fordead and service live loads. Uplift under seismic loads, or evenworse, pier failure under unexpected overloads