The Story goes like this

The Bridge was originally a 60 TON bridge, built to Take logs from the forestry to the saw mills around the district.
through lack of maintainance it was reduced to 10 Tons, then some Clown with a very Irish name decides to drive across with 20 Tons of logs and the bridge gave a very loud crack heard by many people, and they closed the bridge for some time.

You still get Truck drivers with trucks of over 6Ton going across even though there are signs restricting vehicles to 2.6tons