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Kenmore City Council to discuss restricting weight on Sammamish Bridge
Kenmore city staff recommend that the City Council adopt an ordinance to modify the load limits on the West Sammamish River Bridge at its Jan. 27 meeting.
The suggested weight limits are 16 tons for three-axle vehicles, type 3; 25 tons for five-axle vehicles, type 3S2; and 27 tons for six-axle vehicles, type 3-3.
"The West Sammamish River Bridge is a local and regionally significant structure — it connects our community," said Kris Overleese, Engineering and Environmental Services Director at the City of Kenmore. "The bridge will have to be rehabilitated or replaced in the near future and we will begin the pursuit of funding this year."
Kenmore’s 68th Ave. crossing of the Sammamish River is two bridges. The bridge carrying southbound traffic is known as the West Sammamish River Bridge. This bridge was constructed in 1938 and crosses the Sammamish River on 68th Ave. N.E., which is .2 miles south of SR 522. It carries southbound traffic only and is located within the city of Kenmore. Over 13,000 vehicles a day (week day traffic) use this Bridge.
The adjacent northbound, East Sammamish River Bridge was built in the 1970s and carries over 12,000 vehicles per day. City officials say the number of vehicles crossing the bridge every day are greater than the bridge was designed for and the bridge is showing signs of deterioration such as cracking, leaching and spalling.
"Over 13,000 vehicles use this bridge a day which is more than half of Kenmore’s population," Overleese said. "The proposed weight restriction will prevent further over-stresses on the bridge which will help protect it while the city evaluates whether the bridge should be either rehabilitated or replaced in the near future."
In 2012, the city conducted a routine inspection of the West Sammamish River Bridge and identified additional cracking in the bridge concrete beams and potential soil erosion around two of its piers in the river. As a result, the city embarked upon a more regular monitoring plan, installing survey points on the bridge and surveying the bridge to determine if it has a settlement problem.
They hired a new bridge engineering consultant in 2013. Jacobs Engineering evaluated soil loss around the piers in the river, conducting geotechnical analysis and load rating analysis. The company recommended that the bridge be restricted for the axles/weights outlined in the fact sheet.
They do not recommended any emergency scour repair and believe soil loss around the piers is a result of dredging in the 1960s, said Overleese.
"Jacobs highlighted that the bridge is aged, is carrying more and heavier vehicles than designed for, has a heavier dead load on it from asphalt overlay and pedestrian protection barrier installed in the 1990s," Overleese said. "In summary, it’s an old structure nearing the end of its design life and is experiencing more stress than it was designed for."
Although deterioration is present, a structurally deficient bridge is safe. The bridge piers are well supported on firm soil at the bottom of the piers/pilings for the normal vertical loading. However, there is loose soil material surrounding the pier’s pilings in the river which is not adequate against major earthquake events.
"Safety is our number one priority and if a bridge is found to be unsafe, it’s closed. If a bridge can only carry certain loads, it’s restricted," wrote city in a press release.
Even though seismic upgrades were performed on the bridge in the 1990s, the current design earthquake is significantly higher than that of the 1990’s and a major earthquake could damage the bridge.
"If an earthquake occurs, inspecting this bridge will be a top priority," Overleese said.
Kenmore does not have another public crossing of the Sammamish River within the city. Kenmore staff is coordinating with the city of Bothell to determine the best southbound Sammamish River crossing. The recommended detour will be made available in the near future.
The city is currently performing a rehabilitation versus replacement study to determine the most feasible alternative for the bridge and this study will be completed this spring. Officials will begin applying for either rehabilitation or replacement funding this year.
"It is reasonable to expect construction to begin in the next 3-5 years and the weight restriction will be in place until then," Overleese said. "Increased monitoring frequency of the bridge will continue as well. We would like to stress that this structure is of regional significance and Kenmore alone cannot fund its replacement which is expected to be in excess of $10 Million."