Cable stay designs are increasingly common for bridges in the United States. In an effort to promote the evolution of design and installation techniques,
Ben Forbes
Cable stay designs are increasingly common for bridges in the United States. In an effort to promote the evolution of design and installation techniques, post-tensioning and speciality reinforcement developer VSL continues to refine innovative systems and methods for cable stay bridge erection. Its newest system, the SSI 2000 anchorage, is designed for strand-by-strand installation, where each 0.6-in.-diameter strand in the stay is installed individually. The strands are tensioned one at a time during installation using a computerized, hand-held set of stressing equipment. This stressing system is called Automatic Monostrand Stressing (AMS).
The strand-by-strand installation, coupled with the AMS stressing equipment is part of a systems approach to stay technology, aimed at improving both installation methods and the performance of the finished stay. The result is a more robust system with improved corrosion resistance, individually replaceable strands, higher tolerances and easier installation.
With strand-by-strand stay installation, the stay pipe is lifted in place using a crane, with the first strand, or master strand, already mounted inside it. This master strand is threaded through the top and bottom anchorages and is stressed using the AMS ram to a predetermined force. Once the master strand is in place, it supports the stay pipe and the crane is no longer required. Subsequent strands are pulled into the stay pipe using a winch and are stressed in turn as they are installed. In order to reduce the chance of damage to the strand coating, the strands are pulled into the stay pipe off of bulk reels, around a deviator that prevents the strand from dragging on the deck. This also eliminates the need to pre-cut the strands, along with the associated space and handling requirements. Each strand is stressed to a different force during installation such that when installation is finished and the load is shared by all of the strands, equal force in each strand is achieved.
As the strands are installed into the stay pipe, both the weight of the stay and the tension in the stay vary. Since the sag of the stay pipe and the deflection of the deck and tower elements vary as strands are installed, the length of the stay changes throughout the installation process. To account for these changes, VSL has developed software that determines the correct stay pipe length for the final condition as well as the lengths for each strand during installation. These lengths account for the thermal expansion or contraction of the stay pipe as well as the varying sag of the stay at each stage of the installation. On hand is a table that shows the length of the protective sheathing to be removed from each end of the strand. The sheathing must be removed at the anchorage to allow the wedges to grip the strand, yet the bare strand must not extend past the seal at the exit of the transition tube out of the anchorage since the sheathing is the primary protection for the strand inside the stay pipe. A minimum amount of sheathing is stripped off of the cable so that when the strand is fully stressed the sheathing extends into the sealed transition tube behind the anchorage, but not far enough to interfere with the wedges. Once stray stressing and tuning are complete, the transition tube is pumped full of grease, providing a completely sealed, void-free encapsulation of the strand from wedge to wedge.
The key to achieving a uniform force distribution between the strands with strand-by-strand installation lies in the stressing system. Each time a new strand is installed and stressed, the bridge deck and tower deflect toward the stay. The increased weight and tension of the stay also affect the sag and therefore the arc length of the stay. When the arc length of the stay changes, the forces in all of the strands that have been installed previously are affected, thus every strand must be stressed to a different initial force to account for these factors. Furthermore, the force in any strand must be verifiable at any time during installation to ensure even force distribution among the strands at final tension. VSL has developed the AMS stressing system to handle these challenging requirements.
AMS STRESSING EQUIPMENT
AMS is an automatic, computerized stressing system for cable stay bridges. The system consists of a small, 2-gal. electric hydraulic pump; a 16-ton-capacity, single-strand hydraulic ram; and, a computerized control unit. The pump is equipped with solenoid valves, operated by the control unit to control the flow of hydraulic fluid. The pump is also equipped with pressure transducers that report the pressures in the advance and retract side of the ram to the control unit. The ram is equipped with a linear encoder that reports its position to the control unit, which contains all of the electronics and has a user interface screen that displays information about the current operation and allows for navigation among the different functions. The unit controls the operation of the ram based on the pressure and position of the ram according to a set of pre-programmed criteria.
The capabilities of the AMS ram are designed around improving the accuracy and reliability of the stressing operation, while at the same time simplifying the operation for the field personnel. The two main features are automatic stressing and automatic lift-off verification. In the former mode, each strand in the stay is given a stressing target of either a prescribed force or a prescribed elongation. The ram quickly and accurately stresses the strand to the desired target and then stops automatically. When all the strands in a stay have been installed, the equilibrium of force among the strands is checked using liftoffs and the stay equalized if required. In lift-off mode, the ram extends until the wedge is lifted out of the anchor head and the current force in the strand is displayed. If the variance among the strand forces is outside of acceptable limits, stressing each strand to the same force can equalize the stay. When all of the strands have been equalized to the same force, any further stressing is accomplished by automatically stressing to an elongation versus a force target. In this case the ram automatically stresses each strand by the same displacement, preserving the equilibrium of the force in the strands. Wedge seating in the anchor head is accounted for by calculating the draw-in as a function of the stressing force and automatically extending a corresponding amount after reaching the desired target before retracting. At any time, the force in any strand can be measured using the automatic lift-off function.
Many functions of the AMS ram are automatic. A compact flash memory card in the control unit allows all of the input files for the entire bridge to be preloaded onto the card. The initial stressing targets for every strand of each stay are loaded into the memory card so the operator does not have to input them during stressing. During strand installation, the operator simply uses the number pad on the control unit’s face to specify the current strand. The control unit then looks up the target force for that strand and stresses the strand until it reaches the pre-programmed force for that stage of installation. When the target force is reached, the ram automatically compensates for the wedge seating and then retracts. The force and elongation at the target are recorded on the memory card.
Liftoff is accomplished the same way. The operator selects the strand to lift off, and then the ram extends until just after the wedges release, transferring the force in the strand to the ram. After reading the liftoff force, the ram lets the strand back down at the same force. In order to accurately determine the liftoff force, a multilevel algorithm is used which monitors several parameters at the same time to automatically record the liftoff when it occurs. When the liftoff is detected, the ram stops and records the force in the strand on the memory card and then retracts.
The results of each stressing operation are recorded automatically on the memory card when the strands are stressed or liftoffs performed. To safeguard the stressing data, the results files easily can be copied from the memory card to a personal computer using the included card reader. Typically, this is executed at the end of each stressing day to ensure backup of all the files. Once the strands have been stressed, several different reports can be generated automatically using the AMS software. To generate a stressing report, the user opens up the AMS Stressing Record program and specifies the stay and the system of units for the report. The stressing results are read directly from the memory card or from the hard drive of the PC. Once the results for that stay have been loaded, reports are generated that show either the most recent activity recorded for each strand, every action performed on each strand since installation or all activity on a particular date. The force and elongation are shown for each entry, and the system of units may be specified as U.S. or metric, regardless of the units displayed during operation.
The AMS has safety features for preventing the system from exceeding its design parameters. These features reduce the likelihood of incorrect or accidental operation and ensure the operator has the ability to override the control unit and stop the ram quickly at any time. Pre-programmed limits on the maximum allowable force and pressures will automatically stop the stressing operation if they are exceeded. The control unit constantly monitors the functioning of the pressure transducers, and the user is alerted if a problem is detected. A dead-man system on the pump motor allows the operator to release the switch at any time to stop the ram immediately, even during automatic stressing.
If the construction sequence requires multiple stressing stages for the stays, multiple stressing targets can be pre-programmed into the memory card. In the event of multiple stages, the control unit keeps track of the current stage of each stay and which stressing targets to use.
The system is flexible and can be used even if conditions change from the pre-programmed sequence. If the desired stressing targets are not pre-programmed on the memory card, they can be entered on the user interface screen at any time. Often stay adjustments are determined from the results of deck surveys performed after the installation of some or all the structural members. Since the exact stay adjustments will not be determined until after the final survey, the desired adjustment target is simply entered on site immediately before the adjustment is to be performed. If desired, the bridge stiffness parameters can be modified after the first few stays and the input strand tensions recalculated using the revised data. If the input files are modified, it is not necessary to return the control unit for re-programming since the contractor can e-mail and copy to the memory card any revisions to the input files.
In order to provide the greatest versatility to both the designer and the field operator, the user can switch between U.S. and metric units at any time without having to revise the input or results files.
SYSTEM ADVANTAGES
Strand-by-strand installation with AMS stressing offers many advantages, the most obvious being that the ram is much smaller than the multistrand ram that is required to stress all of the strands simultaneously. The AMS ram weighs less than 50 lbs and is easily carried and operated by hand. All other components are easily handled as well. The AMS ram can be used easily in tight spaces that would be difficult or impossible with a larger ram. If desired, it can also be used at the bottom of the stay for an alternative stressing location and the small hydraulic pump is powered by readily available 110-volt power. The non-combustion power source allows use in confined spaces such as inside the pylons where a gas powered hydraulic pump would require additional ventilation and where high voltage power, such as 480 volts, may be unavailable. Crane hoisting is not required for stressing operations nor during strand installation once the master strand has been stressed.
AMS system operation is simple and repetitive. A two-person crew can operate the system very efficiently, and a single person can operate it if required. Although basic knowledge of hydraulic equipment is beneficial, extensive experience is not required to use AMS. With a small amount of training, a crew that does not have experience stressing cable stays can learn the equipment operation and installation procedures very quickly. Because automatic stressing with a small crew can be accomplished very efficiently, contractors benefit from labor savings.
With a multistrand ram, the forces in each strand in a stay should be close since all the strands are stressed simultaneously, but if there is excessive variation it is not possible to check or correct if necessary. Because the AMS ram stresses each strand individually, the agreement of the forces in the strands of a stay can be easily checked and equalized if necessary.
If it becomes necessary to replace a strand, either during construction or at a later date, there are procedures for detensioning the strand using the AMS ram. The strand can then be removed, replaced and stressed to the correct force with little difficulty.
Due to the automatic nature of the ram operation, the accuracy of the results is improved with significantly less chance of operator error during the stressing operation. The ram stops automatically at the target or at liftoff, eliminating the chance of overstressing the strand. Since all of the stressing targets are pre-programmed into the memory card, the operator doesn’t need to look up forces or elongations for each strand. Also, there is no need to refer to calibration charts, since the control unit performs conversion of pressure to force and the interface screen reads directly in units of force.
The user interface is navigated using an intuitive menu-driven structure. The operator uses the number pad and four function keys to enter required information and to change from screen to screen. When a stressing phase for a strand is completed, the resulting force and elongation are automatically written to the memory card, eliminating the need to stop and record the data manually. Automatic recording of the force and elongation is more accurate than typical field measurement tolerances. The included stressing report generator makes report creation easy and accurate.
Finally, one of the most important advantages of the AMS system is that at any time, the force in the strands can be confirmed using a quick and automatic liftoff. This provides the ability to demonstrate that the stay has been stressed to the correct parameters and meets the design criteria. Liftoff verification can also be performed at a later date if there is ever reason for an inspection to confirm the force in any of the strands. The long-term performance of the stays may be monitored and the force verified with a relatively simple operation. Liftoff of the anchorage using a multi-strand ram is a much more involved process compared to lifting off one strand at a time using the smaller AMS ram.
AMS and strand-by-strand installation offers advantages over other methods of stay installation and stressing. The result is a more efficient process that can be performed with smaller, less specialized crews. AMS also offers more reliable and accurate control of the stressing operations, which ensures that the engineer’s design is executed properly the first time and every time.
Ben Forbes is Project Manager with Baltimore-based VSL, a leading designer, manufacturer and installer of post-tensioning and specialty reinforcement systems for bridges, buildings, tanks and specialty structures. VSL is part of Structural Group, a specialty contractor whose other businesses are Structural Preservation Systems, focusing on structural repair and strengthening; and, Pullman Power, concentrating on chimney, silo, and stack construction, maintenance, and repair.
GREENVILLE BRIDGE
The second major application of AMS technology in the United States is the Greenville Bridge over the Mississippi River Û a structure connecting Greenville, Miss., with Lake Village, Ark. The new Greenville Bridge contains more than 2.5 miles of bridge deck and two concrete towers measuring 425 feet high. With a main span of 1,378 feet, the Greenville Bridge is the fourth longest cable-stayed span in North America.
The project specifications called for strand-by-strand installation and stressing Û requirements made possible with AMS technology. AMS provided the ideal solution for the Greenville Bridge because of its relative ease of use with great accuracy. Alternate strand installation and stressing techniques would have required a very large ram, which would have proved difficult within the confines of the pylon.
Instead, using AMS technology, two crews of six to seven workers each worked on one pylon to install two stays before moving to the second pylon to perform the same sequence. These two crews continued this sequence throughout the duration of installation, which lasted approximately eight to nine months. VSL supplied the cable stay system as well as technical assistance for using the AMS technology to install and stress the cables. The new Greenville Bridge will open to traffic in 2009.
U.S. GRANT BRIDGE
The first major application of AMS technology is the U.S. Grant Bridge connecting Portsmouth, Ohio, over the Ohio River to South Shore, Ky. The original U.S. Grant Û a suspension bridge Û was demolished in order for contractors to build the new cable stay structure. VSL provided the cable stay system as well as technical assistance to install and stress the cables.
Project specifications called for strand-by-strand installation, and the AMS technology provided the most practical option in terms of labor because overall installation is considerably simpler than installing all the strands and then stressing with a large ram. While VSL provided a considerable amount of technical support to assist the crew throughout the process, AMS allowed the U.S. Grant crew, which did not have specific cable stay experience, to install stays with a high level of accuracy.
For this project, one crew of six to seven worked on one pylon at a time and installed two stays on each pylon. Workers could install only three strands out of balance for each stay before moving to the next stay on the opposite side of the bridge. Installation took approximately 10 months with one crew.
The U.S. Grant Bridge opened in October 2006 as the Ohio Department of Transportation’s first completed cable-stayed bridge. Overall, the project proved successful and AMS found favor with the contractor as the crew became familiar with the technology’s ease of use.