Matthew (OV) said:
The Slate Creek will have two bridges on the ROW.... and it's looking like one will be a wooden trestle. This trestle will be very short (like everything else on the railroad....) probably three feet long (or thereabouts.)
I’ve figured out most of how it’s all going to work (and Richard Smith’s new bridge confirmed this) as far as how it’ll fit into the benchwork… but I’m left with a couple of questions…
Is it more common to have a “retaining bent” at the end of the trestle, where the “ground” is as high as the whole bridge, or to support the end on a small concrete pier with the ground sloping down to, say, the next bent? To me it seems like if you did the method with the concrete pier and slope, there would be no place to attach the “Grits” at the end of the bridge, as the part where they’d be attached to the end bent would be underground…right?
Also… does a trestle that spans a stream (real or otherwise) require concrete footings in the streambed, or was that based on the situation?
What’s the standard practice on refuges, with respect to spacing and clearance?
While we’re at it, what’s the usual width of bridge ties, spacing of guard timbers, etc? Would like to make the bridge as believable as possible.
Matthew (OV)
Matthew,
I’ll answer what I can…
For anchoring the end of the trestle…
One way is to have an end bent in conjunction with a timber retaining wall to hold the fill behind it in place. A variation would be to have a retaining wall behind which are laid crosswise heavy timbers. The trestle stringers are extended beyond the retaining wall and on top of the crosswise timbers and are partially buried in the fill. Concrete or stone masonry abutments can also be used. Usually if the bents are supported by concrete or masonry footings the abutments will be of the same material.
For a stream crossing it depends on depth and current. For a simple slow moving stream a grill can be used. This is constructed with standard ties arranged about a foot apart and connected with two heavy timbers on both ends of the ties the same length as the bottom sill on the trestle bent. The grill can then be floated into place and the trestle bent set on top evenly between the two connecting timbers thus sinking it. The bents are held in place with cross bracing to each other and not connected to the grills at all. For modeling you wouldn’t need the grills unless they can be seen beneath the water and could simply anchor the bents below. In the case of a pile trestle the piles may be simply driven into the stream bed.
I have also seen pictures of rock filled wooden abutments with pointed ends facing upstream upon which the bents are placed. Of course for a fast running river or stream you might need concrete or masonry abutments once again pointed towards upstream.
Note: abutments as mentioned above should have been piers (senior moment or two). I’m adding this addenda instead of merely correcting the post to draw attention to it for those that may already have read it once.
Refuges are installed between every 80 to 200 feet. I would assume the shorter distances would be used in the case of higher train speeds while the longer for trestles with slow orders as there’d be more time to get out of the way of trains. Higher vs lower speeds would be relative as most wooden trestles require a lessening of speed on them. A rough size would be 4’ wide x 3’ deep and a little larger for a handcar. Too I’m sure that there was much variation the farther back in time you go. Thus a modern era trestle would have to adhere closely to accepted standards whereas an 1880 logging line might not have any standards at all or even any refuge bays.
Regular length ties can be used on the trestle except where a refuge bay is needed. Then extra length ties are used at that location(s). The guard timbers are placed on the tie ends and serve the dual function of protecting from derailments and keeping the ties aligned. Guard timbers are typically 6x8 and bolted to each tie end.
Best wishes on your bridge and … need I say … photos!!! 8)
