11-17-2009, 05:54 PM
There's an article in the July 2009 issue of RMC on building a pile bent trestle. The support joists beneath the rails (below the ties, of course) consist of three 10"x16"s, on edge and bolted together, under each rail. The two outer members of each grouping start and end on the same bents at each end, while the centre member starts and ends atop an intermediate set of bents - in other words, except at the ends, each support member spans three bents spaced approximately 14' apart. The outer members start at the beginning of the bridge and extend 28' to the second bent. The centre member extends only to the first bent (14'). The next set of outer joists extends from the second bent to the fourth, while the centre member extends from the first bent to the third, a total of 28'. The joints are thereby lapped. (Three 10"x16"s bolted together are stronger than would be a single 30"x16" member (if you could find one) due to the dissimilarities in grain structure.) 
The bent cap across the top of the driven piles which supports these joists is a single 14"x16", on edge, and the drawing shows six 14" diameter piles under each cap - four of them are more-or-less directly beneath the area of the 10"x16"s, while the other two are slightly outboard of the ends of the cap timber.
Wayne
The bent cap across the top of the driven piles which supports these joists is a single 14"x16", on edge, and the drawing shows six 14" diameter piles under each cap - four of them are more-or-less directly beneath the area of the 10"x16"s, while the other two are slightly outboard of the ends of the cap timber.
Wayne