A reader writes:
I may have delusions of grandeur, but I want to build a fairly long and more importantly tall trestle bridge. I have read your comments about trestles and a couple other useful sites. But I want to know how the trestles are constructed when the posts for the bents are not long enough the height of the bent. In other words, how are posts spliced together on a bent. Given prototypical trestles have posts probably no longer then 40 feet. How are those bridges constructed when the height of the bridge is 80 feet tall?
Sorry, I don't have anything about that in the information I've collected. If nothing else, they probably staggered the places where the bents connected. Based on other large timber construction. They MAY have spliced them by cutting away half of the material several feet back from the end of each piece, so the pieces would interlock like beam boards in an Amish barn, then attached them with bolts and possibly iron bands.
Relatively few of the really tall trestles are still standing This one at Holcomb Creek has places where it looks like the bents actually "jog," making me wonder if that's where one bent leaves off and the next one starts. There's no way to know how they managed to keep those joints from being weak or "wobbly," though most of the pressure on them is obviously from the top and they are cross-supported in every direction.
Sorry I can't find a clearer photo or more dependable information. I'll keep my eyes out and let you know if I see anything more specific.
holcomb_creek_trestle.png [ 695.84 KiB | Viewed 3356 times ]
holcomb_creek_trestle_jogging_posts.png [ 695.94 KiB | Viewed 3356 times ]