Challenging Design Aspects of a 3-Tower Cable-Stayed Bridge

Transcription

1 Challenging Design Aspects of a 3-Tower Cable-Stayed Bridge John BRESTIN Vice President Buckland & Taylor Seattle, WA, USA jnbn@b-t.com John Brestin, born 1968, received his Bachelors of Science in Civil Engineering from the University of Nebraska and his Masters of Science in Civil Engineering from Purdue University. He worked for Kiewit Construction and HNTB before joining Buckland & Taylor to direct major bridge projects. Tian-Jian (Steve) ZHU Executive Engineer Buckland & Taylor Vancouver, BC, CANADA tjz@b-t.com Tian-Jian (Steve) Zhu, born 1960, received his Bachelors of Engineering from Zhejiang University in China and his Masters of Engineering and Doctor of Philosophy from McMaster University in Canada. He is an executive engineer with Buckland & Taylor. John FINKE Department Manager Jacobs St. Louis, MO, USA John.Finke@jacobs.com John Finke, born 1964, received his Bachelors of Science in Civil Engineering from the University of Missouri at Rolla and his Masters of Science in Structural Engineering from Washington University in St. Louis. He is the manager of the Structures Department in St. Louis, MO. Summary Keywords: Cable-Stayed Bridge, Three Towers, Dynamic Analysis, Foundations, Durability, Accelerated Schedule. 1. Introduction A three tower cable-stayed bridge brings with it a unique set of challenges for the design team. This presentation focuses on those challenges by looking in depth at the Downtown Louisville crossing over the Ohio River. The final design of this structure presented unique geotechnical conditions, site specific seismic design spectrum developed, aggressive scour conditions, erection method, wind engineering analysis all completed on an extremely aggressive design and construction schedule will be discussed. We will also explain the inherent flexibility of a three tower cable-stayed bridge with no anchor cables to stiffen the center tower and foundations consisting of a single row of shafts at each tower and anchor pier. Probabilistic service life design to attain 100 years of life is employed on this bridge to assure a proper level of durability, service life design is in its infancy in North America and therefore afforded an additional level of complexity to the design. A collaborative effort well underway between the Kentucky Transportation Cabinet, Walsh Construction, Jacobs and Buckland & Taylor has led to a landmark project well on its way towards an expected opening in Fig. 1: Night Bridge Rendering

2 2. Bridge Description The downtown crossing of the Ohio River Bridge in Louisville, Kentucky is a three tower cable-stayed bridge founded on a rock profile with significant slope. The 2106 ft. (642 m) long bridge has two main spans of 750 ft. each (229 m), and end spans of 303 ft. each (92 m) and carries 6 lanes of traffic. The 750 ft. (299 m) spans are supported by 15 cables spaced at roughly 45 ft. (15m) along the Edge Girders. The Edge Girder Anchorages are outboard of the centerline of the girder and the superstructure is made up of traditional composite steel section. The towers are constructed on drilled shafts and pile caps and are primarily concrete with mild steel reinforcing. The upper section of the tower contains no cross strut between the pylons, and the individual pylons are 5 sided pentagons. The tower anchor box is steel construction made composite with the reinforced concrete pylons. There is primary navigation channel with a clearance envelope for shipping of 71 ft. (22m) high, by 680 ft. (207m) wide, and an auxiliary navigation channel width 465 ft. (142m) in width. Wind fairings that attach to the edge of the deck along 50 percent of the length of spans 3 and 4 provide stability during wind events. Fig. 1: Bridge General Arrangement Fig. 2: Bridge Deck Cross Section 3. Design Challenges 3.1 Site Condition The bedrock profile at the bridge site slopes upwards significantly from the Kentucky to the Indiana shore resulting in uneven foundation stiffness along the bridge. The overburden soils above the bedrock are relatively thick at Anchor Pier 2R on the Kentucky shore and diminish towards the Indiana side with no overburden soils at Tower 5R and Anchor Pier 6R.

3 The bridge design considered the following requirements: Maximizing the navigation clearances for the main spans of the three tower arrangement; Minimizing disturbance to river flow and local water level rise at the river foundations; Minimizing debris accumulation at the river foundations; Designing for 100 year scour depths for strength limit states and 50% of 500 year scour depths for extreme event limit states - 30' deep overburden soils at Tower 4R were assumed to be completely scoured away, and the top 30' of the 70' deep overburden soils were assumed to be scoured away at Tower 3R; and Designing for large barge impact loads for the river piers - about 4600 kips for Towers 3R and 4R. The key to address the first three design requirements above was to minimize footprint for the tower foundations in the direction perpendicular to the river flow. 3.2 Foundations The selected foundation system for each tower consists of a single row of four 12' (3.66m) diameter drilled shafts aligned with the river flow, as shown in Figure 4. The two tower legs are supported by two separate waterline pile caps with each founded on two drilled shafts, and the two pile caps are connected by a cross beam in between. This foundation system minimizes the dimension perpendicular to the river flow. As a result, it maximizes the main span navigation clearances and minimizes disturbance to river flow and the risk of debris accumulation. Due to the design's result of significantly reducing the number of shafts required per tower, the selected foundation system shortens the installation time and accelerates the overall construction schedule. Fig. 4: 3D Tower Foundation Rendering All drilled shafts consists of an 11.5' (3.51m) diameter rock socket ranging from 18 to 32 feet (6-10m) in length and a 12' (3.66m) inner diameter and 1" (25mm) wall thickness steel casing above the bedrock. The steel casing is seated into the bedrock by about 1' to achieve proper sealing. At Tower 4R, the steel casings are vibrated down to bedrock. At Tower 3R and Anchor Pier 2R, the steel casings are oscillated down to bedrock to provide intimate contact between the steel casing and the overburden soils. Vertical geotechnical capacity of the shafts was determined considering the rock socket only. Lateral resistance to the shafts by overburden soils above bedrock was taken into account at Tower 3R and Anchor Pier 2R only after proper scour depths were considered. In the transverse direction of each tower foundation, the pile caps plus the cross beam provide framing action for the four shafts. However, the foundation system relies on cantilever action of the shafts to resist overturning moments resulting from longitudinal loads instead of pile axil loads in a conventional pile group foundation. As a result, the foundation system of this cable-stayed bridge is much more flexible in the longitudinal direction as compared to a typical cable-stayed bridge founded on a pile group foundation system with piles spaced in both directions. This longitudinal flexibility created unique challenges for both design and erection of this bridge.

4 3.3 Three-Tower Arrangement The three tower arrangement as shown in Figure 2 is required by the owner. The Request for Proposal (RFP) requires the center tower being taller than the side towers within a specified range. This results in greater flexibility in the center tower. Unlike a conventional two-tower arrangement where top of the towers is properly restrained by back stays attached to the anchor piers, the center tower in this three-tower arrangement is not properly restrained at the top, resulting in large demands on edge girders near mid-span of the main spans and large longitudinal overturning moment demands on the shafts of the center tower. To address this issue, the following measures were taken: Install two bundled large size back stays spaced at 15' at the top of each side tower. This helps to stiffen the top of the side towers which in turn stiffens the mid-span of the main span decks; Keep the center tower as short as allowed by the RFP; and Increase the edge girder stiffness in the vertical plane by increasing the girder depth (7' deep). 3.4 Articulation (Expansion and Contraction) The three-tower arrangement, the single row of shafts foundation and the sloped bedrock profile created unique challenges to develop proper articulation for the bridge structure. Different options of articulation for the continuous superstructure were considered in the pre-bid design phase including: Fixed connections between edge girders and tower legs at all three towers; Complete longitudinal floating of the superstructure at all three towers; and Different combinations of longitudinal connections of the superstructure with the three towers using links or bumpers. The optimum solution was to provide longitudinal restraint of the deck at Tower 5R only using bumpers where the bedrock is the highest and the free-standing length of the shafts is the shortest, as shown in Figure 5. Horizontally oriented laminated elastomeric bearings are used for the longitudinal bumpers. The continuous superstructure is restrained transversely by deck level bumpers at the thee towers and by transverse shear keys connected to the end floor beams at the two anchor piers as shown in Figure 5. Horizontally oriented laminated elastomeric bearings are used for the transverse bumpers, and horizontally oriented disc bearings are used at the transverse shear keys. The edge girders are vertically supported by disc bearings at the three towers. Vertical tie-downs (pin-pin articulation) connect the two ends of the edge girders to the two anchor piers as shown in Figure Flexible System Fig. 5: Tower 5R Deck Bumper Single row of shafts foundations, three-tower arrangement and longitudinal superstructure connection at Tower 5R only result in a flexible structural system in the longitudinal direction.

5 The RFP does not permit transverse struts between tower legs above the deck. The DBT elected to eliminate any transverse struts below deck to reduce cost and improve construction schedule. As a result, the system is also flexible in the transverse direction due to lack of struts between the tower legs above the foundation. The flexibility of the system created the following challenges for design of the permanent structure: For the towers with taller free-standing shafts (Towers 3R and 4R), the foundation design was governed by erection loads. Therefore, analyses of the completed structure and critical erection stages had to be performed concurrently at an early stage of the design; Even with added reinforcement in the shafts to deal with erection loads, relatively severe restrictions are still required for the erection sequence at the center tower (Tower 4R). Therefore, earlier discussions with the Contractor were needed to ensure that such restrictions would not significantly impact their field operations and can be properly incorporated into their erection plan; Proper determination of wind loading Fig. 6. Anchor Pier Tie-Down through wind tunnel testing became critical for this flexible system for both the completed structure and the critical erection stages. At the time of foundation design, wind tunnel testing had not started. Wind loading was initially evaluated based on buffeting analysis using reasonably conservative force coefficients from prior experience, and wind loads were then progressively refined as results from section model testing and as 3D aeroelastic model testing became available; System flexibility also created some challenges in developing appropriate measures to mitigate vortex-shedding induced instability. Wind tunnel testing was performed on fairings of different shapes and sizes, and the option of using tuned mass dampers was also considered. The final solution was to use a two-sloped shape wind fairing after considerations of effectiveness, reliability, initial cost, future maintenance cost and other factors for the different options. Due to flexibility of this bridge, the two-sloped wind fairing has dimensions somewhat larger than those typically used on other cable-stayed bridges, as shown in Figure 3. Wind fairing is provided over the center half span for each of the two main spans; Longitudinal bending demands on the drilled shafts are sensitive to installation tolerances of the shafts (both out of plan location and plumb tolerances). Therefore, it was important to establish practical and achievable tolerance ranges after discussions with the Contractor based on installation methods (e.g. oscillation vs. vibration at different locations), local soil conditions, and previous installation experience. Upon reaching an agreement on an installation tolerance it was clear to the contactor that a 2-stage template would be needed during casing installation to meet the relatively tight tolerances. Early discussions on shaft tolerance were a must to the success of this unique foundation design. The agreed upon tolerance ranges were incorporated in computer modelling of both the completed structural system and the critical erection stages. The contractor has installed more than half of the drilled shafts by early 2014 and has been able to meet the required location and plumbness tolerances to date

6 Because of the system flexibility, it was important to properly capture the secondary P-delta and slenderness effects. Geometric nonlinearity was properly modelled in the global model of the completed bridge structure and the models of the critical erection stages. Installation tolerances for the shafts were also modelled. To capture soil-structure interaction properly, p-y springs for refined soil and rock layers were distributed along the length of drilled shafts. The nonlinear behaviour of the p-y springs was modelled depending on the limit states considered; The shaft foundations were initially designed based on the geotechnical capacities and p-y spring stiffness properties evaluated based on the extensive borehole data along the bridge length. O-Cell and stat-namic tests were then performed on an 8' diameter technique shaft. The test results were used to calibrate the design parameters and optimize the shaft design. This calibration enabled some reduction of the rock socket length (ranging from 2' to 6') resulting in sizable cost saving for the Contractor. A world record was set for the magnitude of the applied load in the O-Cell testing of the technique shaft; and The flexibility of the center tower leads to complex behaviour of the edge girders under combined axial, bending and shear effects for different load combinations at different limit states. The current AASHTO design standards do not adequately address such cases. The DBT worked together with the owner's Engineer and resolved this issue by applying the appropriate provisions of the Eurocode for edge girder design providing more uniform and consistent safety margins consistent with the overall project design criteria. 3.6 Service Life Design The Ohio River Bridge Downtown Crossing in one of the first major bridges in North America where a rational durability study was explicitly performed and incorporated in the design process. The RFP required non-replaceable elements of the bridge to have a 100 year service life. We therefore developed a corrosion protection plan that provided the bridge a 90 percent Fig. 6. & 7- Tower and Anchor Pier Elevations showing Exposure Conditions

7 probability of meeting or exceeding the prescribed service life. Figures 6 & 7 show the exposure conditions assumed for the probabilistic approach the design team used to design the 4. Erection Challenges The bridge superstructure is erected by the balanced cantilever method at each of the three towers. The superstructure is erected in 45' long sections (three floor beam spacing). The flexibility of the structural system, single row of shafts foundations, and three-tower arrangement also created the following unique challenges for bridge erection: More restrictions on placement sequence of superstructure steelwork and precast deck panels are required to minimize the unbalanced erection loads thereby the longitudinal overturning demands on the tower foundations particularly for the center tower (Tower 4R); Measures, such as two stage templates, are required to minimize tolerances in installation of the drilled shafts. Control of the tolerances in the longitudinal direction (both out of plan location and plumb tolerances) becomes critical because the shafts rely on cantilever action to resist longitudinal bending and therefore are very sensitive to longitudinal installation tolerances. The longitudinal deviation of the as-built from the theoretical shaft location needs to be accurately determined from survey and monitored after installation of each shaft, and adjustments to the subsequent shafts may be required depending on the deviations of the previous shafts; A temporary bent is required for the balanced cantilever superstructure erection at each of the three towers, as shown in Figure 8. Because of the system flexibility and the three-tower arrangement, the temporary bent in the secondary navigation channel near the center tower (Tower 4R) has to remain in place for the total duration of superstructure erection (after closure of both main spans); and Because of longitudinal flexibility of the tower foundations, load sharing between the tower and temporary bent becomes more complex prior to closure of each main span. The stiffness of the temporary bent including installation tolerances for the bent shafts needs to be accurately modelled in the erection analysis. The location of the temporary bent needs to be optimized, and more adjustments in the form of vertical jacking and shimming are required at top of the temporary bent. 5. Compressed Schedule Because of system flexibility, demands on the foundations are governed not only by the completed structure but also by the critical erection stages. Therefore, careful analyses of both the completed structure and the critical erection stages capturing the key behavior are required to develop appropriate design loads for the foundations. The construction schedule for this design/build project is very aggressive, and the designer didn't have the luxury of completing all studies and investigations (such as wind tunnel testing and technique shaft load tests) prior to computer model analysis and foundation design. To address the compressed schedule, some initial reasonably conservative assumptions were required based on the available information and experience from previous projects so that foundation design could move ahead. The initial assumptions were then refined and optimized as results from subsequent studies and investigations became available. For example, the design of the first tower (Tower 5R) was somewhat conservative whereas the design of the other two towers were optimized due to refinement of the wind loads based on wind tunnel test results and calibration with the technique shaft load test results after the test results became available.

8 6. Summary KYTC's project requirements and the design/build team's drive to reduce cost and shorten construction schedule led to the development of a three-tower cable-stayed bridge with a flexible foundation system in the longitudinal direction. Working through the site specific challenges and external constraints the design team was able to deliver an affordable solution that could be built in a short period of time. The key measures taken by the design build team to address these unique challenges included the following: The design/build team worked effectively with the owner's engineer to develop project specific design criteria to address the unique behaviour of the flexible system and to provide overall safety margin for the structural system consistent with the project requirements specified in the RFP; At the start of the final design, the designer identified the key system behaviour that needed to be properly captured not only for the completed structure but also for the critical erection stages. Appropriate computer models were developed to ensure such behaviour was properly modelled. Sensitivity of the key behaviour to the design assumptions including construction tolerances were also properly incorporated; Fig. 8: Temporary Bent Arrangements for Superstructure Erection At the early design stages, the designer and the contractor had extensive discussions and reached a consensus on the appropriate restrictions that are required in placement of superstructure steelwork and precast deck panels to limit the erection loads on the flexible foundations; To comply with the contractor's very aggressive construction schedule, the designer had to complete the design of the first tower before all studies and investigations (such as wind tunnel and technique shat load tests) had been completed. The designer made some reasonably conservative assumptions in design of the first tower based on the available information and prior experience. The first tower design was then confirmed, and design of the other towers, anchor piers and superstructure was progressively optimized when findings from the studies and investigations became available. The end result is a durable structure that will grace the Louisville Skyline for years to come.