Forbes Creek Bridge in Kirkland, Wash., is just 18 ft tall and 240 ft long.
It has no towers or suspension cables like those on the Golden Gate Bridge; no arch ring like those medieval English bridges that adorn calendars. It doesn’t even have a trestle like the railroad bridges of traditional Americana. But this simple, utilitarian bridge does have one design quirk that distinguishes it from many reinforced slab bridges built in Washington jurisdictions in the 1970s.
“The columns don’t look like the typical bridge columns that go straight up,” said Rob Richardson, bridge and structures manager at HDR’s Bellevue, Wash., office. “The piers flare out substantially from the bottom to the top.”
The six-sided columns flare from about 6.5 ft x 5.5 ft at the base to 9 ft x 18 ft at the top, where they connect to the bottom of the bridge deck.
The purpose? “I think it’s just aesthetic,” Richardson said. “They look cool. You don’t see that kind of bridge in this area. I’ve done a lot of retrofits and I’ve never seen anything like it.”
For nearly four decades, looking cool was the flares’ primary responsibility.
In 2015, however, the columns at Pier 3—the bridge’s middle-most pair of columns—received a more important assignment: Help the bridge survive a 1,000-year earthquake. Its size was one of the characteristics that could help it achieve this.
Workers prepare for columnar work on the load-bearing Pier 3.
Four decades earlier, engineer Ray Upsahl designed Pier 3 to be different from its fellow piers. While Upsahl used semi-isolated, zero-moment connections at the bases of Piers 2 and 4 to allow them to flex and rotate with lateral force, he designed Pier 3 to be stiff, static and strong.
“Force takes the stiffest, strongest path to the ground,” Richardson said. “So Pier 3 attracts most of the bridge’s load. [Upsahl] knew the load was going there [to Pier 3]. He knew he needed to make it beefier and stronger. And he did. The problem is, it’s not beefy or strong enough to handle our current seismic design values.”
When an earthquake overwhelms a bridge designed to withstand it with strength alone—rather than flexibility—the whole structure can collapse. That’s what happened on Jan. 17, 1995, when tectonic plates collided 17 miles beneath the Japanese port city of Kobe. The Great Hanshin earthquake killed more than 6,000 people, damaged 400,000 buildings and compromised $2 billion worth of infrastructure. It also toppled 10 spans of the Great Hanshin Expressway, which, up to that point, conveyed 40% of the region’s traffic.
And to a less catastrophic extent, this type of event is what a May 2014 report said could happen at the Forbes Creek Bridge.
“The column displacement capacities are severely limited because the columns are shear-controlled and will not exhibit a ductile response to seismic demands at all,” HDR’s engineers wrote in the report. “This means that we expect that under the Upper Level Event, the columns at Pier 3 will experience a sudden and dramatic shear failure that will likely result in a domino effect that culminates into a complete collapse of the bridge.”
Obviously, neither the state of Washington, nor the federal government, wanted this to happen. To prevent it from happening, the state has awarded nearly $100 million in federal grants since 1991 to retrofit state bridges west of the Cascade Mountain Range. This investment paid for the complete or partial retrofit of more than 400 of the state’s most vulnerable bridges.
The Forbes Creek Bridge is one of them. The federal government awarded the city a $1.4 million grant, managed by the state, to retrofit it—just enough to pay for what Richardson called “an elegant solution.”
A diamond-bladed wire saw put to work under the bridge deck.
An elegant solution
Richardson’s solution was to capitalize on the size of the column bases to make the two columns of Pier 3 less like the stiff, affixed columns of the Great Hanshin Expressway and more like the columns at Piers 2 and 4. A couple of high-strength steel shear dowels installed vertically from the bridge deck would prevent the deck from sliding off the columns. To reinforce the strength of the bridge deck itself by increasing its flexural capacity, Richardson planned to epoxy sheets of carbon fiber to the underside of the bridge deck, between the columns. High-strength steel shear dowels also would be installed at Abutments 1 and 5 to resist low-level seismic ground motions and keep the bridge in place for those events. (Piers 2 and 4 already had steel pipe shear keys in place as part of Upsahl’s original design.)
“It’s dirt cheap,” Richardson said. “And it accomplishes everything you would want to accomplish with current seismic design thinking. And it does so with minimal impacts to the traveling public and no permanent wetland impacts.”
To achieve this, the city of Kirkland’s contractor cut 6 in. from Pier 3’s concrete columns just below the flare and filled the gap with bearing pads. Those bearing pads would allow Pier 3 to rotate and flex, like Piers 2 and 4.
Cutting the concrete columns and the No. 14 rebar with a diamond-bladed wire saw took five hours at each location. Installing the bearing pads required one day for each.
Not terribly complicated, but getting to that point was.
A little hiccup
Razz Construction’s first task was to prepare the work site—a wetland dammed up by beaver dams and floored in 3.5 ft of peat. The ceiling was just 9 ft above the ground.
Hardest of all, Razz Construction would have to support the bridge deck and the two columns of Pier 3—weighing 1.88 million lb—while its crews cut the columns, installed the bearing pads and lowered the bridge.
To do that, five of Razz’s workers devoted three weeks—about 600 man-hours—to building a bracing system that could lift and hold more than 2 million lb. They built half of the system off-site. This was the contractor’s last obligation before it could begin implementing Richardson’s solution—which began with the aforementioned 6-in. cut through Pier 3’s two columns. But the cut didn’t go as straight as expected. The diamond-bladed wire saw created a belly in the cut, which forced Razz’s crews to make a second cut.
“Each column was only supposed to have two cuts,” said Patrick Herbig, the city of Kirkland project engineer who managed the project. “But they ended up doing three cuts. To create a flush surface, Razz workers had to fill in the edges of the belly with grout.”
Herbig believes the belly in the cut derived from two sources.
“The blade probably got hot and stretched, and when it stretched, it lost its rigidity,” Herbig said. “But also, there was a coil of rebar running through there. We know the blade traveled up the coiled rebar. We knew the rebar was there, but we didn’t know exactly where it was.”
Cutting through the concrete columns was challenging enough with a coil of rebar and a hot blade. Cutting them with traffic vibrating the bridge would have been nearly impossible, Solestad said. So, during this period of work, crews closed 98th Avenue Northeast to traffic—Kirkland’s primary north-to-south arterial west of I-405—during overnights.
To prepare drivers for the closure, the city project staff published articles with photos in the local media, mailed flyers to residents and stationed variable message boards on either end of the crossing a month before the closure. They also created an interpretive sign near the worksite, where people who were walking or cycling could stop, have a look at the site and look to the sign for interpretation. All of these forms of communication provided a link back to the website, which staff updated regularly.
City of Kirkland Project Engineer Patrick Herbig holds up a cross-section of the rebar that was within the pier column and had to be chewed through by the diamond-bladed wire saw.
Hitting the mark
The Washington Department of Fish and Wildlife permit required crews to be out of the creek by Oct. 30, when salmon and bull trout typically return to it; therefore, once jacking began and facing three weeks of work, crews had exactly three weeks remaining in which to complete it, with no room for error or delay.
By Oct. 10, Razz Construction was ready to cut the columns and on track to return the creek to the species that depend on it. That day, however, a 24-hour rainstorm dumped nearly 1 in. of water throughout Kirkland. The water gushed through Forbes Creek and deluged the work site. Instead of finishing the cuts that night, the flood forced Razz’s crews to transport the equipment to higher ground and to repair the wire saw’s motor and clean out its connectors.
“The excavation for Pier 3 was flooded,” said John Lefotu, consultant inspector and engineer for Bellevue, Wash.-based KBA. “But we’re only talking about 5 ft deep. The soil was contained in bunkers.”
In all, the rainstorm cost workers about a day of work. Crews were able to double-down and finish the job by the Oct. 30 deadline. Forbes Creek Bridge can now withstand a 1,000-year earthquake with strength and flexibility alike.
“Old school seismic engineering philosophy says make it strong enough to resist those forces,” Richardson said. “But as we learn more and more about earthquakes, it’s pretty clear that [strength alone] is not a reasonable approach. We can’t make things strong enough to be earthquake-proof. The Kobe earthquake proved that. The current seismic design philosophy is to make it ductile so the bridge will bend and displace large distances of energy without losing the vertical load-carrying capacity.”