Contrary to what some may believe this bridge still carries logging traffic as evidenced by the picture below. It goes by the moniker "6000 Logging Road".
Your photo of the end post clearly shows empty holes at the top, which appears to prove that this was indeed once a through truss. It also shows the outriggers are bolted, since they were added at a later date than when the bridge was first built.
Your theory about someone not wanting to worry about height restrictions makes sense. A 152 foot pony truss is not normal, especially one from 1902 and pin connected. Someone must have had a desire to eliminate overhead bracing.
They certainly could have been removed when the bridge was relocated and/or reconstructed for the logging use.
Remember when I said that Weyerhaeuser built their own rail and road system so they would not have to use county or state infrastructure? The idea was that they could stack their cars or trucks as tall, wide, or heavy as they wanted to something restricted on the others. Is it possible that instead of being cut out they were removed when the bridge was moved and assembled this location? I ran into someone who raised that question.
At first glance this appears to be a through truss with its struts removed and outriggers added. But I couldn't find a detail photo to show where the struts were cut, or any evidence that the outriggers are not original. Attached is the DAHP survey form with some history about the bridge. I disagree with their assessment however, they fail to discuss whether the truss outriggers are original or not, and condemn the bridge's non-original deck and abutments, which are generally not a consideration in truss evaluation. If this bridge really was designed as a pony truss like this it would be extremely rare.
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