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March 21, 2013

Accordion-like truss system plays a big role at Capitol Hill transit station

  • The collapsible steel structure supports the form work for concrete pours.
    Special to the Journal

    Photo by J.K. Walser/Sound Transit [enlarge]
    Turner Construction is using the Peri truss system to build nine 40-foot sections of the station box.

    As Sound Transit expands its coverage of Puget Sound, light-rail is a key component to moving passengers.

    But Sound Transit is also relying on a different kind of rail system to build a key component of the regional transit system's light-rail extension.

    Using a system proposed by Turner Construction, Sound Transit is deploying a collapsible steel truss structure that creates the framework for the Capitol Hill light-rail station concrete pours. Turner is the general contractor for the station.

    The steel truss system is an alternative method for pouring concrete in part of the massive Capitol Hill station — one that is designed to save time and money for Turner and Sound Transit.

    The truss system sits on a set of rails that has rollers on it, and once the concrete is poured and is curing, workers can disassemble the system and slide it to the next section to prepare for the next pour.

    Chad Brown, Sound Transit construction manager for the Capitol Hill light-rail station, said the system is similar to an accordion.

    “It's basically a maze of steel trusses that spans across the width of each section we're working on,” said Brown. “They are able to hydraulically squeeze and adjust each side.”

    Photo courtesy of Turner Construction [enlarge]
    The system retracts and moves along rails to the next pour site.

    Brown said Turner proposed the system early in the project's planning to provide the most efficient way for pouring concrete. This is the first time that Turner has used it.

    Rob Vreugdenhil, Turner's project manager for the concrete component of the station, said the system pays for itself in terms of labor savings.

    “The formwork is mounted to both sides of the truss system,” said Vreugdenhil. “The labor saved outweighs the cost of the formwork through reduced crew sizes.”

    At the end of each pour, the steel truss system collapses about two or three feet from the walls of the station — just enough to move to the next location.

    Brown said the rails for the truss system simplify moving it.

    “It's definitely the most efficient method for Turner,” Brown said. “It doesn't require a lot of labor to move the system into the next position, whereas the traditional (concrete forms) would have to be broken down and moved into place for the next pour and rebuilt. It's very quick, and takes less than a day to slide into position.”

    Turner is using the system on nine 40-foot sections of what is called the “station box” or platform area.

    Each 40-foot section takes about a day to pour, Brown said. Once a section is poured, it takes about a week for the concrete to cure before the truss can be compressed and slid down for the next pour.

    Tom Grose, Northwest regional manager for Peri Formwork Systems of Maryland, which designed the truss system, said the system was customized for the Capitol Hill station project.

    Grose said Turner is using Peri's Variokit, an engineered construction kit that allows both sides of the station to be poured simultaneously. Hydraulics are used to raise and lower the system.

    Image courtesy of Turner Construction [enlarge]
    Formwork is mounted to both sides of the truss system.

    “It condenses the overall scheduling for the concrete pours ... and allows other trades (such as electrical and plumbing workers) to get into the station and complete their work at much earlier phases than normal,” he said.

    Vreugdenhil estimated that the truss system shaves at least 25 percent off the time a conventional framework system would require.

    Although crews are early in the process of pouring concrete at the station, the system is working as planned without any glitches, he said.

    “It is definitely more efficient as far as eliminating labor for moving the system along and putting it back into place,” Brown said. “It helps speed up construction. It's a very nice product and been very efficient as far as a production method for the whole process.”

    The station is expected to be completed by August 2015, with an extra year needed for commissioning.

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