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Highway 1, the fabled road that runs along California’s coastline, makes for some dramatic scenery — and at times, dangerous road conditions. The aptly named Devil’s Slide region is one of the more treacherous stretches of the highway, and it’s often the scene of rockslides and road closures. After a particularly debilitating incident in 1995 (the section of Highway 1 between Pacifica and Monterrey was closed for 158 days, racking up $3 million in repair costs), the California Department of Transportation (Caltrans) realized it was time to come up with a permanent solution.
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That solution — a bypass consisting of two tunnels bored into San Pedro Mountain and a 296 m bridge spanning the valley north of the tunnels — is currently under construction on Highway 1. Construction on the $272 million project began in August 2006, with work on the twin bridges preceding construction of the tunnels. The balanced cantilever design of the bridges with a main span of 136 m consists of two sets of twin piers, from which concrete box sections are being constructed outward in an attempt to preserve the environmentally sensitive valley below. Aesthetic details such as tapered piers and curved struts will allow the structure to blend in with the surrounding environment. When it came to tackling this bridge-building project, general contractor Disney Construction was faced with a whole series of architectural challenges: In plan view, the two parallel bridges start with a straight section before moving into a 260 m turning radius. What is more, the superstructure has a longitudinal gradient of 4 % and a transverse gradient of as much as 10 %! The Top 50 system’s flexibly locatable steel walers and “H20 top” beams give it the ability to adapt to any type of architectural requirement, form facing and formwork pressure tie-hole pattern – an ability which definitely came in handy on this project. The unusual geometry on this bridge design made detailing each pour crucial for maintaining proper control.
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To help build the bridge in a fast cycle, general contractor Disney Construction opted to have a formwork solution planned and supplied to it for safe, swift and efficient balanced-cantilever construction of the box girders. The foreland zone of the Devil’s Slide bridge was shored from ground level by Disney Construction and formed using the company’s own equipment. When it came to the 136 m long span between the piers, however, a cantilever construction device from Schwager Davis Inc. was used in conjunction with an ingenious formwork solution from Doka. All the formwork elements were precision-assembled by the “Ready-to-Use Service” at Doka’s Riverside, CA branch and delivered to the site ready for immediate deployment. Throughout the extensive planning work for this unusually challenging formwork-technology assignment, Doka USA was able to draw on know-how and professional backup from the Doka Expertise Center for balanced-cantilever bridges in Amstetten, Austria.
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One of the first challenges that the Doka Formwork Experts had to surmount was the tight turning radius of the bridges. The problem here was that if the formwork had been too long, it would not have been possible to press it tightly enough against the previous casting section, owing to the curvature of the bridge, resulting in an unclean joint. Extensive planning work was needed here to calculate the optimum length of formwork element – 5.30 m – at which it was still just possible for the cantilever forming traveler to negotiate the curve. Some rather special formwork technology challenges were posed by the geometry of the bridge box girder, which is being constructed in 5 m long casting sections. Over the course of 9 casting sections, its cross-section decreases from a height of 5.90 m at the pier-heads to 2.70 m in mid-span. Spaced a constant distance apart, the webs are not permitted to deviate from the vertical along the entire length of the superstructure, for architectural reasons, and always have the same height within each casting section. Adaptation of the superstructure to the turning radius is effected solely by way of the inclination of the parallel cover slabs and bottom slabs of the box girder. As the webs are not permitted to incline out of the vertical, they have to be located at different heights in relation to one another. For this reason, Doka designed and furnished a horizontally and vertically precision-adjustable inside formwork for the cantilever forming traveler. This makes for optimum forming of the different heights and inclinations of the bridge superstructure.
The bridge does not feature the typical wings that most cantilever bridge designs do. Instead, the top of each box-girder web gradually transitions into a rounded cantilever-arm. Doka achieved perfect accommodation to this architectural detail by using exactly shaped timber formers located directly under the form-ply. The formwork was attached to the previous casting step by means of steel walers and stop-anchors. This solution ensured tightly closed transition joints between the casting steps, and exact compliance with the superstructure geometry. Doka’s ability to conform standard components to the unusual design of the bridge and adapt to quickly changing conditions made the transitions from pour to pour economical, safe and relatively simple.
The transition between the web and the bottom slab had to be effected by means of a haunch inside the box girder. Exactly fitting, easy-to-operate custom elements crafted by the Doka “Ready-to-Use Service” made light work of getting this shape right. To achieve the desired fast-track cycle, Doka also supplied suitable forms for the blister boxes.
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The amount of planning, attention to detail and on-site expertise that Doka was able to provide to the contractor also went a long way in helping to get this major project off the ground. Doka provided the contractor with detailed drawings, and a Doka representative was on-site to provide expertise and guidance during the initial erection. The project’s success was largely dependant on good coordination and communication between Doka, the contractor, the traveler supplier, the bridge designer and the project’s consulting engineers. Multiple meetings and discussions were held to control and maintain accurate, fast, safe and efficient construction of the bridge.
Rick Disney, President of Disney Construction, stated “DOKA’s performance on our project was exceptional. They delivered a great product, which included detailed shop drawings with an eye for detail. In addition, the pre-assembly and on-site support was a great benefit in getting the project started properly. We look forward to partnering with them on future projects.”
Thanks to the dedication of the entire team, the $40 million bridge is on track to be completed in the summer of 2008. It will have to languish without traffic for a few more years, though, until tunnel construction is complete in 2011. Once the Devil’s Slide bypass is in use, the old section of highway, along with 28 hectares of state-owned land, will be set aside for public access and recreational use, ensuring that those who travel Highway 1 in the future will be able to enjoy both the beauty of unparalleled scenery and the security of safe roads.
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| As the webs are not permitted to incline out of the vertical, they have to be located at different heights in relation to one another. For this reason, Doka designed and furnished a horizontally and vertically precision-adjustable inside formwork for the cantilever forming traveler. This makes for optimum forming of the different heights and inclinations of the bridge superstructure. |
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| A 296 m bridge is currently under construction on Highway 1, the fabled road that runs along California’s coastline. The balanced cantilever design with a main span of 136 m consists of two sets of twin piers, from which concrete box sections are being constructed outward. |
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