With a public holiday in Brisbane on Wednesday this week, I spent some time working on my Pratt Truss Bridge to go over the Richmond River at Cassino. I needed to make a few extra truss girders for the top of the bridge, as my bridge has 8 box sections instead of the 6 in the standard kit. So I scratch built these using the ones in the Central valley kit as a template. Mine are near enough, and you will not be able to pick them out when the bridge is painted and viewed from 3 feet away.
Once these were installed, I took the bridge down to the shed and rested it on two pieces of pine and inserted a piece of flex track through the bridge. I then proceeded to move some locos through the bridge. The bridge base deflected by about 1cm as the three locos were pushed through it. That was not good! My bridge has the top section total removable from the base to allow access to the track for cleaning. I think I will have to somehow connect the top truss structure to the base via a couple of screws from underneath. That way I think I will be able to further reduce the deflection of the bridge when it is fully laden.
Today I went further in adding a length of flex track either side of the bridge and jumpering that to my DCC system. I then ran my triple headed consists across the bridge – back and forth. The deflection this time seems to be about 5mm. I have taken a couple of videos of the travel and they are below. Everyone hold on to your hats – my wife was driving the train while I videoed. Will miracles never cease!
Three locos crossing the bridge in a real stress test.
And another from a slightly different angle.
You can see in the video – particularly the first one, where I have manufactured additional components for the truss structure and the base. This is necessary because my bridge has more box sections when compared with the Central Valley kit that the bridge is kitbashed from.
I think I will look at installing two small screws up through the bottom of the bridge into the trusses and see if I can get the deflection down to zero. At that point I will add the cross girders to the truss structure and then let a tender to the painting crew to start at one end and go to the other, painting the bridge a base grey colour. Then I can work on the transoms and lay what I think will be hand laid code 70 rail across the bridge.