(Troutdale, OR)  --  Authorities believe a fire that destroyed a popular playground structure in Troutdale was deliberately set.  The Multnomah County Sheriff's Office says the blaze was reported at Imagination Station on Cherry Park Road around 4:14 Saturday morning.  No other structures were damaged.  Troutdale Mayor Doug Daoust told KGW-TV the playground was built in 1994 and financed with donations from thousands of community members.