Usually it’s just one person on duty in the control room per shift, he said — cameras and automation have streamlined what used to be a three-person operation. Up to three more team members are present during daytime weekday hours to address maintenance issues.
Coast Guard rules give river traffic priority over road traffic, so the bridge team needs to always be ready to quickly halt the freeway and raise the drawbridge span for approaching vessels, Gross said — although the rules limit lifts during rush hour, and boats can be told to wait if a vehicle accident has left cars stuck on the lift span.
There are up to 350 bridge lifts per year for river crossings or maintenance purposes, plus up to 50 more maintenance-related traffic stops without a lift. Overall, traffic stops tend to be about a 50-50 split between river crossings and maintenance work, he said.
The bridge lift frequency varies depending on the time of year; lower river levels in the late summer typically allow more vessels to slip underneath without a lift.
The northbound span opened in 1917 and originally carried two-way traffic. The southbound span opened in 1958, initially carrying two-way traffic while the original span underwent an extensive retrofit. The spans shifted to their present-day northbound and southbound configurations in 1960.
The age difference means the “twin” bridges aren’t quite as identical as they might appear. The northbound bridge was built at a time when heavier and more rigid steel was the norm, and it was designed to carry streetcars. The old rails are still there, Gross said, entombed beneath the roadway.
The southbound span was built with only vehicle traffic in mind, so it uses a similar design but with a shallower deck and lighter steel in the trusses, resulting in visibly smaller counterweights that weigh in at only 650 tons apiece compared with the northbound span’s pair of 900-ton blocks.
The original bridge was level across its whole length, but the southbound span was built with a sloped “hump” section near the middle, and the northbound span was retrofitted to match. The added vertical clearance — up to 72 feet — allows a wider range of vessels to pass under the bridge without a lift.
That can be tricky for some vessels, Gross said, because the BNSF Railway bridge located a little less than a mile downstream doesn’t have an equivalent hump section. Vessels short enough to fit under the I-5 Bridge hump but too tall to fit under the rail bridge must follow the “S-curve,” he said, turning quickly from the center of the river to the north side so they can use the rail bridge’s swing span.
There are seven operations-related buildings on or near the twin spans, according to Gross, although some of them are either no longer in use or have been repurposed over the years.
The two machine rooms sit atop the lift span trusses and house the motors that power the lift system. The motors turn cable drums to wind up or release cables connected to the deck, lifting or lowering the spans. A second set of cables connects the lift span to the counterweights, offsetting most of its weight.
“The motors are doing some work by pulling, but it’s probably a difference of about 10,000 pounds,” Gross said.
The bridges use 6.5 miles of cabling for the lift system, all coated in a thick layer of grease that needs to be re-applied annually. The bridge uses about $40,000 worth of grease each year, Gross said. Extra buckets of grease are kept on standby in the machine rooms for spot treatment.
The cables stretching between the tops of the towers aren’t part of the lift system, Gross said — they’re telecommunication lines that the bridge carries across the river.
Standing inside a machine room can be a slightly harrowing experience for visitors who are unaccustomed to how much the bridge vibrates as freeway traffic barrels across below, but Gross said the operators quickly get used to the shaking.
It could be worse, he added — the northbound bridge’s machine room originally served as the bridge house, meaning the main control room. Back then, the bridge tender on duty would stay in the box for the entire shift, even riding with it up to the top of the towers during bridge lifts.
The modern control room was a later addition, perched between the northern towers of the two spans. Subsequent upgrades added cameras and sensors throughout the structure, allowing operations to consolidate in one place.
The operating crew originally included pairs of flaggers who would manually stop traffic and close the crossing gates. The two roadside gate houses for the flaggers are still standing between the bridges at either end of the northbound lift span, although the northern house now serves as an electrical room.
The generator room is built into the foundation beneath the north end of the southbound bridge, providing a backup power source to keep the lift system running in case of an outage. The generator sees fairly frequent use, Gross said, but usually only for short periods at a time.
The final structure is a small white building located immediately east of the bridge’s north landing site, which originally helped supply power for the lift system, according to Gross. The building was built in 1917 and housed Clark County’s first electrical substation, according to Clark Public Utilities media specialist Dameon Pesanti, but it ended service in the 1970s and is currently unused.
Routine maintenance costs take up about $1.2 million per year, but both spans occasionally require big-ticket repairs that need to be budgeted and planned as separate projects. The most recent example was last year’s $10 million trunnion replacement, which closed the northbound span for a week.
The northbound span’s bridge deck will need to be resurfaced by 2027, according to ODOT spokesman Don Hamilton, at a likely cost of around $33 million. The southbound span will need a new coat of paint relatively soon — probably about $75 million — and some of the bridge bearings are also due for replacement, although there’s no cost estimate yet.
The average lifespan of a bridge in Oregon is about 85 years, according to Hamilton, but both I-5 Bridge spans are still in good condition and can remain in service for many more years as long as they continue to receive regular maintenance.
A replacement for the I-5 Bridge would eliminate the need for further repair work, but the Interstate Bridge Replacement Program is still at an early and uncertain stage, Hamilton said, so ODOT can’t afford to delay maintenance in the meantime.
“Until we’re ready to break ground, we’re not holding off on the work that needs to be done,” he said.
The existing bridge does have one problem that none of the upcoming maintenance projects will address: seismic vulnerability. The bridge has some built-in flexibility and has weathered small earthquakes with no damage, Gross said, but a major Cascadia Subduction Zone event would very likely be fatal.
The towers are only designed to withstand forces caused by up-and-down movement of the counterweights, according to Interstate Bridge Replacement Program communications officer Kelliann Amico, so lateral motion caused by a major earthquake could cause them to fail.
The piers beneath the bridge are similarly vulnerable to horizontal loads.
The riverbed includes a layer of soil that is vulnerable to liquefaction during an earthquake, extending about 70 to 80 feet down. Both spans’ piers rest on wooden pilings that don’t quite reach all the way through the soil to the bedrock below, Amico said, so liquefaction could cause the pilings to sink and take the piers with them.
“Soil liquefaction is one of the most serious potential consequences of an earthquake, as it has the potential to undermine bridge foundations and lead to collapse of the structure,” she wrote in an email.
ODOT has never performed a full engineering analysis to examine the cost of upgrading the bridge to modern seismic standards, but it’s “safe to say it would cost hundreds of millions of dollars,” according to Hamilton.