SCRATCHBUILDING A TIMBER TRESTLE

AND

WOODEN BRIDGES

Rewritten from Narrow Lines 26 – April 1984

Introduction

Railway builders on the North American continent built railroads at an enormous pace over vast distances. Many of the bridges were built as temporary measures using local material (nearly always timber) that could be quickly assembled. Today there are still many timber trestles to be found in the American North West and in Canada.

Most modellers look on a timber trestle as a spidery, flimsy structure that only a hardened scratchbuilder with a winter to spare is capable of building. One glance at that collection of vertical, horizontal and diagonal strips of timber has been known to scare the would-be trestle builder into thinking that a simple plastic kit for a bow-string or plate girder was just what he had in mind in the first place. However, no plastic kit I know can replace the feeling one gets when one’s first train edges out from solid ground onto a trestle of one’s own construction.

If you are skilful enough to

• prepare a simple full-size drawing of one cross-section,

• cut several pieces of wood to the same length

• stain all the stripwood and

• glue and pin them all together,

then you can build a detailed timber trestle, giving your layout a centre-piece that any modeller would be proud of – and it does require a little patience but doesn’t take that much time.

Trestle bridges usually came in three basic forms

1. A timber trestle bridge that is was a temporary structure to cross a gully/stream as timber extraction moved on from site to site, many such structures were rapidly built using whatever timber was to hand and later abandoned as an area of trees were cleared and the railroad moved elsewhere. These trestles would use untreated timber tree trunks with little more than the bark removed at areas where a cross piece was connected. Rather than dig around the base to find solid ground the ‘Bent’ uprights were often pile driven into the ground. Quite often these trestles looked as if they had been built by ‘Heath Robinson’!

2. The more common type of timber trestles used machine cut timber, the sizes of the pieces of wood employed and the amount used could differ depending on the weight of train that was to run over the bridge. The sizes of the timber employed depended on what was available

3. The third type of trestle not covered in this article is an all steel structure; probably one of the most famous steel trestles is the Keddie Trestle.

Preparation

You must first determine the maximum height of your trestle by measuring from the lowest point of your scenery to the top of the rail level. Be as accurate as you can in this, as it is far easier to double check at this point than to rip half the scenery out later.

Having established the dimensions, a drawing can be prepared for the highest upright frame or ‘Bent’ as it is called. This drawing will act for every bent in the trestle. (See figure 1) for a typical example. Trestles are very individualistic. Although they used the same principles of construction, the actual number of timbers used varies considerably. Some bents have four columns, some five and some six. (I have never seen more used.) Some have diagonal bracing across each column and some have diagonals going across all the columns. I prefer the latter for most of the stages as this makes for easier construction and does not look so cluttered. My drawings show a five-stage trestle of about 65 scale feet high, but your drawing must suit your site.

Translate the maximum height previously determined into scale feet and deduct the depth of the rails and sleepers (Ties) plus 16 inches for the top stringers. This gives the net height of each bent from bottom of the ‘base sill’ to the top of the ‘cap’. Next, calculate how many stages you require. These are rarely more than 15ft-6in or less than 2ft-6in apart. On my drawing they are at 12 feet intervals. The lowest stage often varies in height to follow the undulations of the terrain. On the prototype the length of timber available determines the lengths of the columns. 200 feet appears to be about the maximum timber length used so a trestle bent 60 feet high can be constructed from one piece of timber.

The number of bents that you require depends on the length of your trestle. Bents are normally between 10 feet minimum and 14 feet maximum apart. My side elevation drawing (Figure 1) shows them at 12 feet apart. To calculate the number of bents required you divide the total length by the spacing less one. Where the trestle ends onto solid ground there is no problem but occasionally where the ground is soft an end wall supporting the ground was added. (See Figure 4)

The original of my drawing was done on one millimetre squared graph paper. This helps the non-draughtsman to produce a scale drawing that is not only easy to prepare but also nice and square. Mark out a base line, pencil in a centre-line for the bent and mark the centre-line point of each column along the base. The distance between the columns at the base should be about ten (scale) feet for a trestle of this height, but there is no hard and fast rule about this. The slope of the columns from the base to the top varies according to the height of the trestle, but a good guide is to make the slope of the outer columns about 1 in 15 (i.e. slope in one unit for every 15 units rise).

Measure up the centre line the height of the bent and draw two lines parallel with the base a scale foot or whatever size you use for the bent columns apart. These two lines are for the ‘cap’ piece which should be a scale 13 feet long (for 0n30). This could be longer for standard gauge trestle. On the lower of these two lines mark for the upper ends of the columns which have centres at 2 feet and 4 feet either side of the centre-line. Pencil in the centre-lines of the columns by joining up the upper and lower marks. Mark on the main centre-line the number of stages that you have chosen and pencil in the centre-lines for the horizontal sills. The diagonals can be marked on the drawing, but it is only necessary to draw those for one side. When the drawing is used in the construction stages the diagonals on the reverse side are fitted by lifting the bent off the drawing and reversing it. It is however wise to mark the reverse side diagonals on your drawing as this will help when it comes to knocking the ‘Jig’ pins into place.

Draw the various parts in properly by inking permanent lines a scale equal distance either side of the centre-lines to complete the drawing. The timber sizes used in the construction of a trestle vary, but the following dimensions are typical:

• Main columns – one foot square minimum but often larger

• Cap – same size as columns

• Base – same size as columns

• Sills –12 inches by six to eight inches

• Diagonal braces – 3 inches by 10 inches

• Stringers – 16 inches wide by 24 inches high. Usually there are five running parallel, on a straight trestle these would be the longest lengths of timber available, on a curved trestle the lengths would cross two or three bents with the joints staggered.

• Struts – 6 inches by 12 inches

• Longitudinal braces – 3 inches by 8 inches

The timber for the model trestle can be purchased in two forms. It must be hardwood – balsa is too soft. For the main columns, sills and stringers it is best to buy stripwood but for all the bracing it is cheaper to buy 1/16 inch sheet obechi and cut it into strips yourself. On the last trestle I built I came across stripwood used in tea shops, MacDonalds and Wetherspoons that are used as tea/coffee stirrers and managed to purchase a box of these for only £5, cheapest modelling material I have come across for a long time. Most of the stripwood used for the rest of the construction came from the range of “Hobby” shops, you get a pack of 50 half metre lengths in various sizes for under £5. All the wood must be stained or painted before it is assembled. Colron wood dye is an ideal stain to use but the stain will not take if glue has been allowed to get on the wood. The colour of the stain may vary but I found That a shade of Teak looked the best. It must be remembered that over a period of time the wood would go grey. The pins should be the finest you can get; I found PECO track pins ideal and I found that with cutting them into two they were long enough .

Making a Jig

Because the building of a timber trestle involves a lot of repetitive modelling it is common sense to make a simple jig. This takes only a few minutes to make but it will ensure that all your bents are identical and save you hours of time. The completed drawing (or a photocopy of it) is glued to a piece of blockboard or ply which needs to be larger than the drawing by at least 2 inches all round. Add a layer of kitchen cling film over the whole drawing to stop any excess glue attaching the bent to the drawing. Some 1 inch panel pins, the thin type are firmly secured hammered into the blockboard at the junctions of the columns and the sills. Cut off the heads so that the assembled bent can be removed from the plan. Two strips of wood could be permanently fixed to the plan, one below the base sill and one above the cap, or alternatively fix a few more pins in a row.

Bent Construction

1. Cut two pieces of stripwood to fit permanently below and above the base strip and the cap and firmly attach them on the jig. Cut two lengths for the base and the cap along with four lengths for the main columns and lay them on the jig, a small drop of adhesive at the top and bottom of each column helps. Use the panel or household pins on both sides of the columns to locate them. With bent columns in place I prefer to knock a fine pin through the base and cap and into the columns. Note on adhesives – As most of the construction of a trestle uses wood then you would expect to use PVA White Glue but in many cases I prefer to use Superglue Gel along with ordinary thin Superglue, the reason for this is that the stripwood I use for the Sills and Diagonal Braces are made from tea-stirrers that you can acquire form many cafes that have a coating that does not adhere with PVA, I bought a box of 1000 for £5, considerably cheaper then buying model stripwood but they do require working over with some coarse sandpaper before staining!

2. The sills on the prototype are bolted across the face of the bent, one each side. On the model these where laid on the jig, fine holes drilled through them to stop the stripwood from splitting and with a small drop of superglue gel attached to the columns and secured by knocking a pin through the sill into the column, I found PECO track pins ideal for this, one pin cut into two will do two joints.

3. When all the sills have been fixed into place on one side the diagonal braces can be added using the same technique.

4. Lift the assembled frame gently off the jig and lay the bent assembly back onto the jig and add the Sills and Diagonal Braces to the other side, note the Sills MUST aligned with opposite side ones.

5. The first bent is now complete. Repeat this for the number of bents required. All you need to do when you want smaller bents as the valley gets shallower is to move the base sill further up the jig.

ORIGINAL DAWING

STAGE 1 CONSTRUCTION

STAGE 2 CONSTRUCTION

STAGE 3 CONSTRUCTION

Trestle Construction

There are at least two different methods of assembling the trestle depending upon the shape of the ‘valley’ that the railroad is crossing. If the valley has steep sides and a fairly long flat base then the trestle is best built in the conventional manner. On the other hand if the valley has steep sloping sides forming a sharp vee, or if the trestle is on a curve (quite prototypical – there were, and still are, many curved trestles), then I find it easier to construct the trestle upside down.

(a) Conventional Method

Start from the tallest bents and work outwards towards the ends. You will require a base to build on, such as a piece of floor-boarding, which must be long enough and wide enough to take your completed trestle.

1. Scribe a line to mark the position of the bents, making sure that these lines are at right angles to the edge of the board.

2. Determine where the first full-height bent will come and fix it temporarily in place on your board with modelling pins. If the board with the bridge on it is to be assembled into the layout the bents can be fixed permanently. Be sure that the vent is vertical and brace it with spare lengths of stripwood.

3. Repeat with the next full-height bent.

4. Slide all the inner struts into place, allowing them to extend equally either side of the two bents, these want to be as long as your valley is wide and therefore the higher up the bents you get, the longer they will be, then carefully glue each one to the two bents. It is easier to leave the outer struts until later.

5. Now it is just a matter of sliding successive bents onto the struts, positioning on the marks on the base, and gluing each one to the struts. Spacer blocks cut from scrap to the exact distance between your bents will help keep everything square, but always keep checking that the bents are vertical. As you move from the valley floor and start adding the shorter bents, use spacer blocks to maintain an even separation and pack the base up with scrap timber or expanded polystyrene blocks.

6. When all the bents are in place, add the outer struts, gluing them at each point they touch a bent.

7. The next parts to add are the longitudinal braces what are fitted to the struts, not the bents. Cut and fit one up against the trestle – try it in several places because the same length should fit everywhere except the top tier of the bents. These braces are fitted to the outermost struts only. When satisfied with the fit, cut enough for the whole job. Set the inside braces with a pair of tweezers – a touch of adhesive on each end of the brace will secure them in place. The longitudinal braces for the top tier are slightly shorter than the rest. Again, cut one, try it in several places, and when satisfied cut the rest and glue into position.

8. All that have to be added now are the stringers, ties (sleepers) and rails. The stringers are substantial pieces of timber that support the track. On the prototype, the stringers are cut into lengths equal to the spacing of the bents, centre-line to centre-line. There can be five to seven stringers crossing each bent and are usually evenly spaced. Some drawings I have seen have only two stringers placed under the running rails, but each one is made up of three lengths of timber. Glue and pin stringers into position.

9. The ties are glued to the top of the stringers and the rails in turn are glued to the ties using a contact adhesive (Thixofix or similar) Some trestles have four rails, the running rails and guard rails or check rails which are laid inside these. Using a track gauge or better still use at least two, lay the running rails (Flat bottom rail) equally either side of the centre line of the bridge securing the rails to the ties with spiked pins (Available from the Roy Link range of products) and add a drop of thin superglue, pre-drill the pin hole as this will help in not splitting the wooden tie and take care to make sure the rail sits flat on the tie, if you don’t the rail will twist and you will find it difficult to maintain a correct track gauge. Finally add two lengths of strip wood onto the ends of the sleepers as safety bollards, these strips of wood want to appear substantial but should not be higher than the track level.

Everything mentioned above is fine for a Straight Trestle but if you want a trestle on a curve or a S-bend then I assembled my trestles upside down.

(b) Upside-down construction

Construction of a tall, short trestle or a curved trestle follows the same basic method as described above except that the stringers are temporarily fastened to the ‘base plate’ first, the each bent is assembled upside-down onto the stringers. When building a curved trestle your will find that the stringers will need a fair amount of support to hold them in place to the required curve. In model form the Stringers can be full lengths of stripwood bent to the required shape and held in place with pins, in reality the Stringers would be shorter lengths of timber possibly as long as three bent spacing lengths but staggered so that the ends came onto the centreline of alternative Bent cap. To help maintain the curve, tap panel pins into the base at intervals between the bents. Do not remove the trestle from the base until all the struts and longitudinal braces have been glued and pinned into position. Also remember that the longitudinal braces on the outside of the curve will be longer than the ones on the inside.

Only when you remove the completed trestle from the base jig, irrespective of the method of construction you use, will you realise the enormous strength of one of these structures.

Detailing

Fire Stands

Extra detailing can be added to suit your own taste. (See Figure 3) Planking can be added on the ties on one side to provide a walkway. Some ties can be extended by about six feet, covered with planking and have a water barrel placed on them. Most trestles have a few of these. Where planking is laid on one side, it is not uncommon to have a handrail projecting up from the safety bollards. A few pins and some wire soldered together in situ will soon make this.

End Walls

Occasionally the end smallest bent in a trestle can also be a supporting wall in places where the top soil is deep and continual rainfall would wash away the trackbed adjacent to the end of the trestle. In these cases the end bent would have that outermost side filled in with planking.

End wall details

Roads, and even other railroads, quite often pass under trestles. Where a road passes under, just leave out the bottom sets of longitudinal braces and the first tier of struts. (See Figure 5). The bottom of the bents adjacent to the road would have a white band painted around them with reflective cats-eyes fixed towards the oncoming traffic – small brilliants as used in jewellery do this job admirably although I am not sure at what period these were introduced. Remember if modelling American that the clear or white brilliants go on the right and the red ones on the left as seen from the road. Where a watercourse passes under a trestle, and one or more of the bents extend into the riverbed, the bottoms of the bents are usually protected against damage from the water and debris by small stone walls built around the bent and filled with rubble.

Details of a trestle base

IRON WORK

In model form we are content with using pins and glue to assemble our trestles and bridges but in reality trestles had to be assembled using iron rods, these rods passed through all the joints and were secured with washers and nuts. One has to remember that many of these structures were built in the days when there was no such thing as ‘Off the shelf’ nuts and bolts, it was possible that because many of securing rods varied in length the threads would have been cut on site to fit nuts that were made in the railroad’s own works.

In model form I have used fine pine pins and glue to secure each joint then added cosmetic Nut, Bolt & Washer (NBW) on each joint, plastic moulded NBW are available from Grandt Line in various sizes but I prefer to make my own. For making each NBW using Evergreen styrene strip 40 X 80, drill a 0.8mm hole near the end and cut off square, some nuts used in construction were square but if you want hexagonal ones just trim off the corner, when you require a few hundred this sounds hard work but within an evening you can make quite a lot, I attached mine with a short length of plastic microrod through the centre to represent the bolt and superglue into position, once painted the NBW will add realism to your timberwork and no-one will notice if an odd NBW is slightly different in size to the rest.

‘Howe’ Trestles

There is an alternative to the above where the centre part of a trestle has to span a large gap; this is better illustrated in a trestle that I have just built where the centre part of the trestle has another railway running diagonally through it. In this case three of the centre Bents are replaced by a latticework structure, usually at least two ‘Pony Bents’ deep supported by additional Bents that are double or triple up to the base of the Howe lattice.

To built the Howe lattice I again returned to a simple jig, first drawn out on 1mm square graph paper then attached to a sheet of plywood or MDF, the photograph below illustrates the construction of the Howe centre, there are as many diamond shaped lattices spanning the gap between the Bents as there are upright columns in each Bent, in the example shown below there are four. Construction of a Howe trestle is basically two substantial beams running horizontally spaced with diagonal spars, in reality these diagonal spars are quite big planks and to assimilate on my model I used stripwood doubled. The pre-assembled lattices have to be carefully removed from your jig and are fixed into place working from the centre with the lower beams resting on the Caps of the additional strengthening Bents and the upper beams are fitted below the adjacent Caps, upper beams have Caps spaced out similar to tie spacing that carry the Stringers, make good use of a set square to make the Bents are vertical and parallel, it helps if you temperately fix the Bent base to a piece of wood or add strips across the base.

In addition to the diagonal spars there is usually iron rods running vertically between the upper and lower beams, quite often these in pairs and are secured to plates above and below the beams, some even have turnbuckles so the rod tension can be altered.

INNER HOWE SPANS ADDED

NOTE: Extra Bent supporting Howe span and temporary spar across the base to ensure all is kept square while the first parts of the trestle are assembled

Bridge in Collerado courtesy of Neil Fraser

Note the iron rods connecting the upper and lower horizontal beams

HOWE STYLE BRIDGE AN DTUNNEL MOUTH ADDED TO THE SCENE

Note the plastic box spacer sat inside the tunnel mouth is there to guarentee clearance so that all the stock will be able to pass through.

ANOTHER ALL TIMBER BRIDGE SPANNING A RAILROAD AND STREAM

Conclusion

The trestle can now be lifted into place on the layout, the rail level lined up and then, using scrap timber for packing, the whole assembly can be firmly glued in place. No plastic bridge kit match the excitement you will feel when your first train edges its way from terra-firma out onto the trestle which is not only all your design but also ALL YOUR OWN WORK.