I forgot to mention that we have looked into the Bentley Lars Bridge Rating Software but based on its capabilities the software is only capable of rating steel trusses and not timber.
There's a program called BRASS-TRUSS, developed by Wyoming DOT & NY State DOT. It's been around for years; never had a copy though so I can't say if it's any good.
Army Corps of Engineers, for combat and construction MOS's, has a field manual (FM) that covers this very well. I forget the number, but I used it forty years ago in the service to rate timber trestle bridges.