The corrosion was revealed by bridge inspections carried out by Washington State Department of Transportation (WSDOT).
An in-depth review and assessment found that heavy rust has penetrated large bolts that provide the connections of the steel trusses to the piers at the north and south ends of the span.
“We were surprised at the depth of corrosion we found within these bolts during our inspection,” said bridge preservation engineer Harvey Coffman.
Bridge engineers at WSDOT headquarters in Olympia have designed a temporary counter balance to reduce the stress on the bolts while plans and specifications for permanent repairs can be formulated.
The crossing has now been reduced to a single lane and only one vehicle over 4.5t will be allowed on the bridge at a time. Wide loads and vehicles over the legal weight will not be permitted.
WSDOT said that it may take up to six months for the repairs to be completed.
The 457m-long steel-truss bridge was constructed in 1948. It is 8km south of the US/Canada border and carries about 500-800 vehicles a day, with trucks comprising about 40% of the total.