I have planned a trestle on my layout on a 1 in 50 grade (and a curve, but I just like to make life difficult!) and I cannot find how trestle builders coped with a grade.
Although only about 20 inches long and six inches high, the track will rise about two fifths of an inch from one end to the other, and I doubt that this would be dealt with by increasingly thick corbels or wedges under the roadbed or stringers. However, every picture I can get of trestles seems to show that all the horizontal braces are just that, horizontal, not gradually sloping up with the grade.
Did they keep the lower bent sills all at the same level and just adjust the height of the top bent to increase the grade? I have the Model Railroader Bridges and Trestles book, but this does not help, nor can I find anything on the internet that helps. So I am just hoping that there are several of you out there who have faced just this problem and know the answer. If so, thank you very much in advance for your help. (We did not have many trestles over here in the UK.)
Peter Cox
The trestle bents should be treaded no differently than if they were "risers" on a grade. The connecting girders between bents would act as the subroadbed. You have a very small grade, Cut out a template that is tie width and from stock that remains fairly flat. Bent position and girder length and "mitered" angle can also be marked/ figured from this template. (do remember that if the template is tie width that the suppoprting girders will be inboard of the ties. The girder placement can be drawn on the template to ensure tie overhangs and miters work out. Better to calculate this on a template that fix problems later. Your radius would follow the template for bent placement and fix the grade for bent height changes. A straight edge can be placed on the template to ensure that it is straight as you do set the bent height. A hint would be to use a template thickness that matches the girder height. This will allow you to locate to exact tie level (under tie) at the abutments on either end of the trestle. I would recommend using ME bridge flex track on the trestle. You can even clip ties off to allow the rails to continue on to the "standard" ties at the abutments.
Modeling B&O- Chessie Bob K. www.ssmrc.org
You can take several measurements of depth across the gap and then build your trestle up-side down with the bents rising toward you as you build. You can curve the deck if you wish. But the deck should be very close to flat and this would help you to insert it into a grade.
When you have your gap mostly roughed in, you invert your somewhat fragile treslte and test fit it. Trim bits off each bent piling as you need to, and use sculptamold or bits of wood or goop to shim and shore up the footing that will be your mud sill area. At some point you should be able to just lay your trestle ends across the abutment and the stringer tops should be flush with the adjoining roadbed at each end. Continue to stabilize your trestle by gluing the ends to the abutments so that they don't want to shift and also deal with the footings.
At some point you will know that you have a well-established and stable trestle, and you can then continue to lay your track across it.
Crandell
The pile or frame bents for trestles will always be vertical since you need to have the weight (forces) transferred straight down, never along the length of the bridge. (Of course the outer piles or timbers of each bent usually slope outwards to brace it laterally.)
The horizontal bracing will similarly be horizontal. On a steep grade they might step up every few frames but it is just as easy to set them at a compromise elevation to keep them level throughout the length. Also bear in mind that a trestle at a valley crossing may be in a dip and close to level anyway.
The normal practice is to tailor the length of the timbers for each frame bent to the actual site conditions. For convenience they may try to keep some consistency in the relative elevation of the mud sills so several frames have the same dimension but that is of only minor advantage. Pile trestles have the piles driven to refusal and then sawn off at the top at the required elevation. You will occasionally see wedges or shims between the cap and the stringers. These might be because of a miscalculation or more likely because that bent has settled or the entire rail profile has been raised.
John
ptccox Although only about 20 inches long and six inches high, the track will rise about two fifths of an inch from one end to the other, and I doubt that this would be dealt with by increasingly thick corbels or wedges under the roadbed or stringers. However, every picture I can get of trestles seems to show that all the horizontal braces are just that, horizontal, not gradually sloping up with the grade. Did they keep the lower bent sills all at the same level and just adjust the height of the top bent to increase the grade?
Did they keep the lower bent sills all at the same level and just adjust the height of the top bent to increase the grade?
I have another book called Bridge and Trestle Handbook by Paul Mallory. In the trestle section they show sketches of trestles with the stringers supported directly on top of the upper bent cap, as well as stringers that are supported on top of corbel blocks which in turn are supported by the cap. In the paragraph describing these construction options it states "the corbel timber not only supports and connects the stringers, but it can be notched easily to adjust for variations in the heights of the bents".
My impression from this is that if the trestle were planned to be level and someone goofed a little in the height of the upper bent, then they had the corbel timber to chew on to correct the mistake. But I don't think that they would have increased the thickness of the corbel timber to give themselves an upward slope. I believe that they would have adjusted the height of the upper bent (only) to increase the grade, as you mentioned. And then, if a minor adjustment were needed, they had the corbel block to bail them out.
In my opinion, the sills of any intermediate bents would have all been at the same elevation so that the horizontal bracing you mentioned would indeed be level. The sketches in the book don't show any horizontal bracing at the top of the upper bent. They all show diagonals that go from the side of one bent to the side of the adjoining bent, so any added height to the top bent wouldn't really matter. The diagonal would just be a little longer.
By the way, are you building a framed trestle or a pile trestle? You said that the height would be around 6 inches, which is 42 HO scale feet, more or less. According to the Mallory book, " the maximum height of pile trestles is limited by the length of piles available. Thus, pile trestles are virtually always under 25' high."
Thank you very much indeed for the time and trouble you have taken in your replies. I very much appreciate your help.
I am going for a frame trestle rather than pile, because of the height, and also because I think it will be easier dealing with rectangular timber rather than round. (There is difficult, and then there is 'life's too short'!) I shall build up, off layout, from a level sub-base, so that I can make sure that the bents are vertical and correctly spaced. As long as I get the grade right at the top I can then position it to exactly the right height by shimming under the sub-base, then scenic it into position.
From what I gather from your posts it looks as if I should keep each layer of sills level with each other (apart from the inevitable variation at ground level), and the braces between bents horizontal, just making the top section of the downhill-end bent shorter and gradually increasing this top section height from bent to bent to give me the grade. Gosh, it isn't easy expressing exactly what you mean, is it?! If I knew how to scan a sketch in I would do that.....
Does this make sense? Have I understood correctly?
Peter
ptccox From what I gather from your posts it looks as if I should keep each layer of sills level with each other (apart from the inevitable variation at ground level), and the braces between bents horizontal, just making the top section of the downhill-end bent shorter and gradually increasing this top section height from bent to bent to give me the grade. Gosh, it isn't easy expressing exactly what you mean, is it?! If I knew how to scan a sketch in I would do that..... Does this make sense? Have I understood correctly? Peter
I think what you are describing is correct. In my opinion, the tops and bottoms of the intermediate bents would all follow parallel, horizontal lines. The bottom of the bottom bent would vary due to varying ground level. Or you could make them all the same length and give the varying look by varying the amount of "earth" you apply during the scenery phase of construction.
Then the tops of the upper bents would lengthen slightly to follow the upward slope. Obviously, if the trestle is long enough or the slope steeper you would get to a point where another bent would be needed. But for the 2/5 inch you mention I don't think this would be necessary.
ptccox ...I shall build up, off layout, from a level sub-base, so that I can make sure that the bents are vertical and correctly spaced. As long as I get the grade right at the top I can then position it to exactly the right height by shimming under the sub-base, then scenic it into position. ...Does this make sense? Have I understood correctly? Peter
...I shall build up, off layout, from a level sub-base, so that I can make sure that the bents are vertical and correctly spaced. As long as I get the grade right at the top I can then position it to exactly the right height by shimming under the sub-base, then scenic it into position.
...Does this make sense? Have I understood correctly?
Sort of, but one thing you must not forget.....the bents must be, and look, vertical when they are on the layout, not when they are being erected and attached to your decking's underbelly. If you erect it all at right angles, invert it, lay it in place so that the decking is level with the oncoming roadbed, and the two abutments are not at the same height relative to the frame and terrain, your perpendicular bent frames are now going to be at an angle, canted as much as to reflect the tilt of the decking.....follow?
So, you must measure the height disparity between the abutments, take a vertical down to the terrain, and measure the angle somehow. Then, while attaching your bents to the underbelly of your trestle's deck, you must have the tops angled ever so slightly, and attached and supported until they are firmly adhered at that angle. Then, when you invert the trestle and go to install it, it will lie nicely in place with the bents all truly vertical.