Over the past year, I have been extending my G Scale layout to include a standard gauge GWR branch line. The track is 63.5mm (2.5") and the scale is 1:22.5 making it Gauge 3. On the approach to the main station, there is an eight metre gap, one meter deep. A viaduct was therefore called for. Being a GWR line, it had to be a Brunel Fan Trestle Viaduct.
First step was research; to find drawings of the prototypes. With the help of friends and Google and the model at Pendon, a plan emerged. An old Railway Modeller article gave me some dimensions and surveying work started. This was complicated by the fact that the track needed to climb at one in fifty while the land below was at a variable gradient, averaging about one in twenty five. This lead to some very tricky calculations of trestle sizes. In the end, I decided to play safe and cheat.
First I made a formwork mould for the concrete piers. When each pier was poured, I inserted two vertical treaded steel rods, on to which I was able to accurately locate the decking using supporting nuts. Using a laser beam, I was then able to adjust the decking up and down until it was level crosswise and at the correct gradient. The decking consists of two thicknesses of Harditex Blueboard, a cement sheet, sandwiched between two 12mm square strips of Tasmanian oak; all glued together with external Aquadhere.
Next came the trestles. I drew up two templates, one for the inner trestles and one for the outer. Again, 12mm square strips of Tasmanian oak were cut, pinned and glued to produce each trestle. Then they were pinned and glued into position in the gap set by the steel rods. Finally cross bars were bolted in place each side and all the woodwork was painted with decking preservative.
So far, three piers are complete and I have five to go....not a quick project this.
Peter
First step was research; to find drawings of the prototypes. With the help of friends and Google and the model at Pendon, a plan emerged. An old Railway Modeller article gave me some dimensions and surveying work started. This was complicated by the fact that the track needed to climb at one in fifty while the land below was at a variable gradient, averaging about one in twenty five. This lead to some very tricky calculations of trestle sizes. In the end, I decided to play safe and cheat.
First I made a formwork mould for the concrete piers. When each pier was poured, I inserted two vertical treaded steel rods, on to which I was able to accurately locate the decking using supporting nuts. Using a laser beam, I was then able to adjust the decking up and down until it was level crosswise and at the correct gradient. The decking consists of two thicknesses of Harditex Blueboard, a cement sheet, sandwiched between two 12mm square strips of Tasmanian oak; all glued together with external Aquadhere.
Next came the trestles. I drew up two templates, one for the inner trestles and one for the outer. Again, 12mm square strips of Tasmanian oak were cut, pinned and glued to produce each trestle. Then they were pinned and glued into position in the gap set by the steel rods. Finally cross bars were bolted in place each side and all the woodwork was painted with decking preservative.
So far, three piers are complete and I have five to go....not a quick project this.
Peter
