gregh
electronics, computers and scratchbuilding
Following on from rebuilding my big timber trestle, I’m now working on rebuilding my 3.3m (10’) long ‘steel’ girder bridge.
Originally it was just a piece of 100 x 75mm timber used to support the track. In 1998, I added fibro sides with plastic angle to represent the girders. (In case it’s new to you Poms, fibro is a fibre-cement sheet, 6mm thick. It used to contain asbestos, but the new non-asbestos stuff is more brittle)
Now the fibro has all cracked and the angles are falling off, so it’s time to start over. This time I’m using 2mm thick styrene for the web. I can buy it in 750 x 1500mm sheets for 22AUD, so it’s pretty cheap. But as I have discovered previously when using it as a baseboard – it has a LARGE thermal expansion. I decided to make each side in two halves of 1650mm each. I cut them in the shade and then laid them in the sun where my thermometer went off scale at 50C and it was 1660mm long – a 10mm increase for say a 25C change. So I need to allow for maybe a 20mm change in length. (0C to 50C)
So I’ve fixed each of the halves at its midpoint, so it can expand +/-5 mm in each direction from 25C. The moving ends have slots with loose screws, hopefully to allow it to slide.
I used a water based silicone sealant to glue the plastic angles to the styrene, which stiffened the ‘girder’ nicely – just like the real thing.
The top of the main 100 x 75 timber was rotten for about 5mm so I chiseled all that off and placed plastic ‘damp course’ on the timber, under the track to keep water off. It has a ‘sort of’ chequerplate look. I`m also experimenting using this dampcourse between the rails to stop leaves getting under the rails.
Originally it was a single span of 3.3m – much too long for a prototype steel girder. I would have liked to made into 3 spans but one of the new piers would have fouled the track beneath, so I added just one new, central pier. This gives two spans of about 35m full size – about the longest prototype ever built (up to the 1950s when I model anyway) . The prototype would have needed girders about 3-4m high (scale 120-160mm) high, so I’ve used 120mm.
I’ve left the small angle-iron stiffeners off (black areas in above pic) until I’m sure it expands OK. So far it has survived a 35C day, but I’ll wait for a 40C before I cover up the screws with the plastic angles.
Originally it was just a piece of 100 x 75mm timber used to support the track. In 1998, I added fibro sides with plastic angle to represent the girders. (In case it’s new to you Poms, fibro is a fibre-cement sheet, 6mm thick. It used to contain asbestos, but the new non-asbestos stuff is more brittle)
Now the fibro has all cracked and the angles are falling off, so it’s time to start over. This time I’m using 2mm thick styrene for the web. I can buy it in 750 x 1500mm sheets for 22AUD, so it’s pretty cheap. But as I have discovered previously when using it as a baseboard – it has a LARGE thermal expansion. I decided to make each side in two halves of 1650mm each. I cut them in the shade and then laid them in the sun where my thermometer went off scale at 50C and it was 1660mm long – a 10mm increase for say a 25C change. So I need to allow for maybe a 20mm change in length. (0C to 50C)
So I’ve fixed each of the halves at its midpoint, so it can expand +/-5 mm in each direction from 25C. The moving ends have slots with loose screws, hopefully to allow it to slide.
I used a water based silicone sealant to glue the plastic angles to the styrene, which stiffened the ‘girder’ nicely – just like the real thing.
The top of the main 100 x 75 timber was rotten for about 5mm so I chiseled all that off and placed plastic ‘damp course’ on the timber, under the track to keep water off. It has a ‘sort of’ chequerplate look. I`m also experimenting using this dampcourse between the rails to stop leaves getting under the rails.
Originally it was a single span of 3.3m – much too long for a prototype steel girder. I would have liked to made into 3 spans but one of the new piers would have fouled the track beneath, so I added just one new, central pier. This gives two spans of about 35m full size – about the longest prototype ever built (up to the 1950s when I model anyway) . The prototype would have needed girders about 3-4m high (scale 120-160mm) high, so I’ve used 120mm.
I’ve left the small angle-iron stiffeners off (black areas in above pic) until I’m sure it expands OK. So far it has survived a 35C day, but I’ll wait for a 40C before I cover up the screws with the plastic angles.
