On18 Trestle – Pt V

deckHere’s a preliminary rending of the trestle. I have calculated the necessary sizes for the secondary stringers and support bents to allow passage of On30 track to the left and a road to the right. The height of the openings isn’t finalized yet – I need to provide adequate clearance for the trains and trucks but that will be relatively minor.

The caps are postioned where they need to go. The specifics of the bents that support them depends on the exact positioning of those caps relative to both the road and the On30 track. That will most likely be determined as the bents are installed.

[1] These were introduced because often “… the bearing surfaces at the ends of stringers and caps in railroad trestles (were often) too small.” In the end objections to their use was variously – increased cost, increasing timber against timber which increased the liability of decay – a somewhat interesting argument but over a century old. Ultimately, they were a stop-gap .. better selection/design of the stringers and caps made them obsolete since their use stopped.

I can make use of them though, in my trestle. The trestle has about 2/3 of it’s length on a curve .. consisting of approximately 3″ long segments. Where the join is a probable week point. Corbels can reinforce these joints .. and what the heck .. they look cool tool.

There’s no set size. –

For a 7-3/4″ x 15″ x 14′ stringer the corbel was 7-1/4″ x 9″ x 2′
For a 8″ x 16″ stringer the corbel was 8″ x 15-3/4″ x 4′

Fig_176[2] Trestle Superstructure Corbels. – In both frame and pile trestles the superstructure consists of everything above the cap, viz., corbels, stringers, cross-ties, and guard timbers. The first sketch in Fig. 176 represents a side view of the superstructure at the bent, and the second sketch shows a longitudinal view of part of the superstrucure, aa being the ap.
The corbels, or bolsters, bb for two 8 xby 16-inch stirngers, consist of a block 8 inches thick, 16-3/4 inches wide, and 4 feet long, resting symmetrically upon the cap aa and supporting the stirngers cc. The corbels are notche 1 inch over the cap, and should be drift-bolted to the cap; the drift-bolts have countersunk heads.

[1] Economical designing of timber trestle bridges – A.L. Johnson – 1902
[2] Elements of railroad track and construction – Winter Lincoln Wilson – 1915

Note: Bolts run through the corbel up through the stringer and through the ties .. one on either side of the rail. The corbel is the width from the outside on one stringer to the outside of the other.

My stringers are similar to those in Fig 76 being a pair. the width is .270″ or about 13″ in width. I have some strip-wood measuring 3/16″ x 1/4″ – equating to a 9″ x 12″ corbel. That should suffice. I need a total of 10 corbels, each 1″ (4ft) long and notched 1″ or so.

In the end I didn’t use corbels. I had actually glued some on .. and decided that the trestle just curves too much .. the corbels just made that apparent (more so) – and then looking closer I realized that because of the angle of the lower track that some of the bents have to be placed an angle .. and therefore those corbels would hang in the air. Oh well.

Assembling Bent
GluingBentThis .. *appears* to be going well – but I haven’t attempted to remove the assembly from the jig yet. Hopefully adding the diagonal bracing will strengthen everything.
Test Fitting Bents
test_fit_bentsWell. I decided to remove the corbels. The trestle curves very, very sharply – something like a 11″ radius. That’s no problem with operating an On18 engine on the curve .. but because of the angles that the trestle bents will have to go to fit between the On30 track and the road it means the angle between the bent caps and the corbels was too much .. too .. obvious.

I’ll build the additional bents I need to support the trestle under the long open spans.

Test Fitting Bents – 2
bentcheck2Test fitting the bents. Two of the bents for the long span supports – still need a third one to the left of the loco. Just placed in the approximate position .. for .. the heckofit.

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