Portland Oregon, at the confluence of two rivers with a population of over 600,000, is a bridge-dependent city. Each of its thirteen bridges spanning the Willamette River are vital to keeping transportation flowing. In 2011, the city of Portland began to prepare for the inevitability of destruction from a predicted major earthquake without any usable bridges.
Major Quake Destruction Predicted for Portland
The Oregon Department of Geology and Mineral Industries sees 1 in 3 odds of Oregon experiencing a magnitude 8 or 9 Cascadia Subduction Zone earthquake sometime within the next 50 years. The last Oregon quake of that strength was in 1700, with a magnitude of 8.7 to 9.2. Today, a quake of that strength would be enough to destroy buildings and roads, take down power lines, block streets, rupture gas lines, and break water and sewer lines, causing many areas to be uninhabitable.
In that scenario, the State of Oregon predicts pockets of isolation where people may be stranded due to broken transportation infrastructure for 72 hours or longer. The bridges of the Willamette River are the city’s transportation connection. In the aftermath of a devastating earthquake, first responders would need those vital links to administer aid to all areas of the city.
Majority of Portland Bridges Not Designed to Survive a Quake
Varying in age, each bridge spanning the Willamette River is unique in construction, with some built in the early 20thcentury to carry horse and buggy across the River. Even bridges more recently built were not designed by current standards to withstand an earthquake of the magnitude that geologists predict for Oregon.
“A majority of the bridges…were built prior to scientists’ current understanding of the regional seismic threat and prior to the engineers’ current understanding of effective seismic design,” said Portland State University civil and environmental engineering professor Peter Dusicka. “We have learned a lot in the past several decades; unfortunately, a majority of our infrastructure is significantly older,” he said.
Oregon bridge engineers did not design for Cascadia-level earthquakes of magnitude 8 or 9 until the mid-1990s. Even the relatively new Fremont Bridge, built in the 1970s, is not expected to be useful following a major quake. If the bridge survives, engineers predict that the ramps leading up to the bridge will not hold up.
Bridge Replacement vs Retrofit
The susceptibilities of Portland’s bridges are complex, each having different weaknesses, with some bridges prone to more than one type of seismic deficiency. Since the mid-1900s, new bridges have been constructed to account for the seismic risk to the region. Older bridges would need to be replaced or retrofitted to current seismic design standards.
There are two phases of seismic strengthening (retrofitting) of a bridge:
Phase One: costing millions of dollars, the first focus is to prevent loss of lives, not to save the structure. An example of a phase-one upgrade is under the Marquam Bridge, where cables strap the deck to its supporting columns. The Burnside Bridge has also received phase one upgrades.
Phase Two: more difficult and costing tens of millions of dollars, this upgrade includes repairs that bring a bridge up to current seismic standards, to allow it to survive a quake and be quickly reopened afterward.
Each bridge has a lifespan of 50-75 years. Multnomah County, in its 2015 20-year Capital Improvement Plan for six of the county’s bridges, says it wants to spend about a half-billion dollars to prepare the 89 year old Burnside Bridge for emergency crews to safety cross the Willamette River after a major earthquake. Burnside Street runs from Gresham to Washington County, and is a crucial roadway for response after a catastrophe.
The other five bridges are:
- Broadway Bridge (built in 1913)
- Hawthorne Bridge (built in 1910)
- Morrison Bridge (built in 1958)
- Sauvie Island Bridge (built in 1950)
- Sellwood Bridge (built in 1925)
Engineering standards for new bridges in Oregon are now among the toughest in the country. Oregon is the only state that requires bridges to be built to survive a once-a-millennium earthquake and to be serviceable within 72 hours after a quake.
The Sauvie Island Bridge, crossing the Multnomah Channel of the Willamette River, was built in 1950 and replaced in 2008, now up to current seismic standards, at a cost of $38 million.
In 2016, the new Sellwood Bridge costing $319 million replaced the 1925 bridge of the same name, and is seismically designed to open for emergency responders and the public within 72 hours of a magnitude 8 or 9 earthquake
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