dyna link anchor box system - dywidag systems€¦ · of a concrete pylon, post-tensioning in...

DYNA® Link Anchor Box System

Contents

Key Features �� 4

Bridge Pylons and Saddles �� 5Bending and Transverse Stresses �� 5Slip Limit State �� 5Friction Factor �� 5Construction Stages �� 6Service Stage �� 6Seismic Design �� 6Bridge Design �� 6Bridge Construction �� 6Cable Inspection and Maintenance �� 6

3


Key Features

The DYNA® Link Anchor Box System is based on a conventional steel structure in which stay cables are anchored with standard DYNA Grip® anchorages� It features many advantages in comparison to conventional saddle solutions in which strands are guided through the pylon�

The DYNA® Link Curved Anchor Box is economically designed using conventional steel construction standards to ensure capacity, serviceability and excellent fatigue characteristics� Testing is therefore not required.

The key features of the DYNA® Link Anchor Box System are: ■ No friction problems; horizontal forces are transferred by the anchor box

■ Cable anchorages located outside permit slender pylon shapes

■ The pylon does not need to be accessible

■ Stay cable assembly is just as flexible as in the case of common stay cables with anchorages that are located inside the pylon

■ It is even possible to replace a complete strand bundle only on one side of the pylon

■ There are no limitations in terms of deviation radii or differential forces; consequently, no limitations in any national regulations need to be taken into consideration

4


Bridge Pylons and Saddles

In many contemporary cable stayed bridges the cables are anchored inside the pylon section� In these bridges the tower cross section was made hollow to allow access to install, stress and service the cable anchorages� The hollow pylon section also had to be reinforced to transfer the local cable forces and resist the opposing cable forces on either side of the pylon� This resulted in large dimensions of the tower section and heavy reinforcement either by structural steel frames or, in case of a concrete pylon, post-tensioning

in addition to congested passive steel reinforcement� In many instances this substantially increased the cost and the construction time of these pylons�

To mitigate the issues discussed in the previous paragraph, cable saddles may be used� Their use will result in compact solid pylon sections that are more economical and faster to construct than hollow pylons� DSI has supplied some of the first saddles used in cable stayed bridges (Clark Bridge, Missouri, USA, 1993) and the largest saddle used

up to this date (156 strands for Maumee Bridge, Ohio, USA, 2003) in addition to many other bridges�

Although saddles offer some advantages, it has become apparent that they also create many complications in the design, uncertainty in the performance and reduced efficiency in the construction of cable supported structures, as discussed below:

Bending and Transverse Stresses

Due to the curvature of saddles, bending and transverse stresses are introduced in the strands� The bending stresses will reduce the axial strength of the strands� The transverse stresses may result in fretting effects between the strand wires or between the strands and the saddle that will result in reduced fatigue strength of the strands�

Slip Limit State

For cable supported bridges to behave as intended, slip of strands inside a saddle must be completely avoided� This is an additional limit state that must be investigated and verified by the bridge designer� This limit state in not related to the strength of the cable components but depends on the friction between strand and saddle and the magnitude of the cable forces at both ends of the saddle� Additional load cases and combinations may need to be investigated, using advanced analytical methods, to ensure that this limit state would not be violated� This is particularly critical when seismic loads are being investigated as discussed below�

Friction Factor

Slip in saddles is prevented by friction between the strands and the saddle� Current design recommendations (such as PTI recommendations and fib Bulletin 30) require that the friction factor be derived from tests on specimens representing the parameters of the actual saddles� These tests are conducted in the laboratory where it is dry and under controlled conditions� Due to possible contamination or condensation the conditions in an actual saddle on site may be quite different than the test conditions� This creates uncertainty as to actual friction factor available to resist differential cable forces during the construction and service stages of the bridge�

5


Bridge Pylons and Saddles

Construction Stages

Most cable supported bridges are constructed by the cantilever method� In this method, a structure with saddles is very vulnerable to slip due to unbalanced loads during the construction stages� Considering the possible variability of friction the risk of slip must be carefully considered and mitigated� During the cantilever construction of the Clark Bridge a heavy counterweight was positioned on deck and moved to counter the effects of unbalanced loads and preclude the possibility of slip in the saddles� It is noted that unbalanced loads may be due to construction activities and the vertical components of unequal wind loads acting on opposite cantilevers that may be required for some sites�

Service Stage

For cable supported bridges with anchorages at both tower and deck, it is normal to investigate the live loading patterns that would result in the maximum and minimum forces in each cable so that its axial strength and fatigue resistance may be verified� For cables with saddles it is required to additionally investigate loading patterns that would result in the largest force differential in a cable at both faces of the tower� This additional design effort is essential to preclude the occurrence of slip in the saddle during service�

Seismic Design

Cable slip in saddles must be avoided at all cost to maintain the stability and safety of a cable supported structure, particularly during a seismic event� Standard seismic design methods are not sufficient to ensure that cable slip would not occur� The designer must resort to advanced analytical methods such as time history and push over analysis to ensure the safety of the structure� To produce reliable results these methods require complex and sophisticated modeling of the structure, including the interaction of its foundations with the surrounding soil� In addition, the designer must select and use a ground motion input that hopefully would be similar to the future seismic event� In some instances a detailed study of the seismology of the area and site conditions is needed so that a site specific ground motion may be produced for use in the analysis� Regardless of all the above efforts, a considerable degree of uncertainty remains in predicting the behavior of saddles during seismic events�

Bridge Design

The use of saddles results in equal number of strands on both sides of the tower for each cable� This outcome might not be the most efficient or desirable from the economic and behavioral aspects of the structure� This is evident for end spans that

have considerably larger stiffness than adjacent main spans and particularly for cable anchored close to end supports� There is also uncertainty on what is the effective cable length to be used in the structural analysis� This uncertainty stems from the fact that a certain length of the curved strands inside the saddle will be subjected to varying axial strains� This effect may be significant for the shorter cables of Extradosed bridges�

Bridge Construction

The use of cable saddles results in doubling the man power and equipment required for the installation and stressing of cables� In addition, saddles require that the pace of cantilever construction on both sides of the tower be always symmetrical before a cable may be installed� This results in increased construction costs and inefficient utilization of resources�

Cable Inspection and Maintenance

Saddles do not allow the inspection of stands inside them� This may be critical because the strands are anchored by friction inside the saddle pipes and are subjected to multi-axial stresses and differential movements due to live and dynamic loads� These conditions may adversely impact the design life of the strands and their corrosion protection system; and this uncertainty may not be acceptable to bridge owners�

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V E N E Z U E L A

North AmericaDYWIDAG-Systems International USA Inc� 320 Marmon Drive Bolingbrook, IL 60440 USA Phone +1-630-7 39 11 00 E-mail dsiamerica@dsiamerica�com www�dsiamerica�com

South AmericaPROTENDIDOS DYWIDAG Ltda� Rua Iaiá, 150 – 10° andar - Cj�102 Itaim Bibi 04542-060-São Paulo/SP Brazil Phone +55-11-21 31 37 00 E-mail vendas@dywidag�com�br www�dywidag�com�br

EMEADYWIDAG-Systems International GmbH Siemensstrasse 8 85716 Unterschleissheim Germany Phone +49-89-30 90 50-100 E-mail info@dywidag-systems�com www�dywidag-systems�com/emea

APACDYWIDAG-Systems International Pty� Ltd� 25 Pacific Highway Bennetts Green, NSW 2290 Australia Phone +61-2-49 48 90 99 E-mail civilsales@dywidag�com�au www�dsicivil�com�au

ChinaDYWIDAG-Systems International SPP – ASIA Ltd� Units 905-907 Prosperity Millennia Plaza 663 Kings Road North Point Hong Kong Phone +852-28-33 19 13 E-mail jeff�liaw@dywidag-systems�com www�dywidag-systems�com/emea

IndiaDSI-BRIDGECON Pvt� Ltd� 265, Okhla Industrial Estate, Phase-III 110020 New Delhi India Phone +91-11-4183-2060 E-mail info@dsi-bridgecon�com www�dsi-bridgecon�com

www.dywidag-systems.com/emea

Please Note:This brochure serves basic information purposes only� Technical data and information provided herein shall be considered non-binding and may be subject to change without notice� We do not assume any liability for losses or damages attributed to the use of this technical data and any improper use of our products� Should you require further information on particular products, please do not hesitate to contact us�

dyna link anchor box system - dywidag systems€¦ · of a concrete pylon, post-tensioning in...

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