New technique forms I-84 bridge
Top-down building method will shave months off construction
On Wednesday morning, April 4, a steel girder measuring 70 feet long and weighing 140,000 pounds arrived on the back of a flatbed truck from Utah to the Sandy River in Troutdale.
One steel girder down, a few dozen more to go.
It's one of 52 steel girders, ranging from 55 feet to 120 feet long, that are being used in the construction of the new eastbound Interstate 84 bridge over the Sandy River.
To place the mammoth girders across the bridge, the Oregon Department of Transportation and crews with the Springfield-based Hamilton Construction are using a method known as top-down beam placement - the first time ODOT and Hamilton have used the method in bridge construction.
Each girder is lifted with a gantry crane, which rises hundreds of feet above the bridge. The gantry crane rolls along a track across the bridge's span and sets the girder onto temporary supports and bridge piers. The girder segments are bolted together, and the temporary supports are removed.
The girders are shipped directly from the manufacturer, the Utah-based Mountain States Supply. About five girders of varying sizes arrive each week.
Wendell Snook, a project manager with Hamilton Construction, said construction crews have already set more than 20 girders across the bridge using the top-down method.
Next, a 167-foot girder, consisting of three separate pieces that will be assembled at the worksite, will be set on the bridge with both gantry cranes, Snook said.
Jyll Smith, a spokeswoman with ODOT's Major Projects Branch, said bridges are usually built with the bottom-up method, where a temporary work bridge is constructed alongside the bridge span. A crane on the work bridge is used to put the steel girders in place.
The top-down beam placement method is more expensive, Smith said, adding about $10 million to the project's total cost of $83 million.
The top-down beam placement method also enables crews to work throughout the whole year, cutting about two years off the project's timeline, Smith said.
Federal regulations related to fish runs set limits for how long construction crews could work in the Sandy River. With the bottom-up method, crews planned to work just 26 weeks a year, Smith said.
The top-down method also means that crews don't have to assemble a work bridge in the river.
'By eliminating the work bridge, it saved us having to put in a bunch of pilings and bridge supports, and that reduced the potential for a flooding impact,' Smith said.
Project planners have cited flooding as a major concern on the project, as the temporary bridge piers have created an increased risk for the vulnerable Sandy River Delta.
That flooding risk came to pass in January 2011 when heavy rains and snowmelt from Mount Hood caused the Sandy River to swell about 21 feet above its normal level.
The fast-rushing water carried logs, trees, remnants of buildings and other debris down the Sandy River, creating a log jam at the bridge. Construction crews used heavy equipment to remove the debris in order to prevent more flooding in the delta.
ODOT also has had to reimburse flood insurance premiums for nearly 100 residents along the lower Sandy River whose properties have been put at a higher risk of flooding because of the project.
The new I-84 bridges will have a total of six piers in the river instead of 18 piers currently holding up the eastbound and westbound bridges, reducing the potential of storm debris getting caught up on the bridge piers, Smith said.
Construction crews expect to finish the eastbound bridge and open it up to eastbound and westbound traffic by September.
When the eastbound bridge is completed, it will consist of two lanes for traffic and have a combined 16-foot-wide bicycle and pedestrian crossing alongside it.
After that, construction crews will close the westbound bridge, which is currently open to traffic, and start construction in 2013. The temporary eastbound bridge now being used by traffic will become the work bridge for the westbound bridge.
The I-84/Jordan Road project is funded by the $2.46 billion Oregon Transportation Investment Act passed in 2003 that supports a 10-year series of major transportation projects that include repairing or replacing hundreds of bridges across the state.
Crews expect to finish the I-84/Jordan Road project by October 2014.