Hand rails and guard rails used in highway/bridge construction, have criteria available from the sources below.

AITC’s Glued Laminated Timber Bridge Systems Manual.
http://www.aitc-glulam.org/

Wood Transportation Structures Research Website: Forest Products Laboratory (304) 285-1591. na-wit@fs.fed.us

Also see General FAQ “Where can I find information on timber bridge design and construction?”