Sunday, August 18, 2013

Bridging the Gap

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.
Another crossing.

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.


  1. Craig, Why not run some brass tubing across either side of the bridge to represent some service pipes for water or Sewerage etc. This will add strength to the bridge and will look the part. No one will know that it shouldn't be there, only me, I have the same bridge.

  2. Jim,

    Come to think of it, there is a water service that runs across the bridge. I will be adding that. So that will help. I will look at making this from brass tubing. I have a photo I took from on the bridge many years ago that shows this pipe.
    Thanks for helping me remember this.

  3. Craig,

    I am no bridge engineer, but I suspect the reason you are getting a deflection is because you have not made the truss section an integral part of the bridge. It relies on the trusses to support the girders. I would be making the bridge a solid unit, as I suspect over time, the deflection will place stresses on the bridge and one day may fail!! If you are worried about access for track cleaning, you can always make a cleaning car that can be dragged through.

    Just a suggestion.


    Ian Millard

  4. Ian,
    You are absolutely right. I did want to keep the two pieces seperable. If I install some screws up through the base into the trusses, it will transfer some of the load to the trusses. I already have a number of home made cleaning cars. My layout rarely gets a clean though, only after I do some scenery work and spray glue about.

  5. Craig,
    I don't think screws will help any as the trusses are not that strong and will still deflect. The original kit has 2 flat metal strips as support. Another thought would be to run a timber support across the length underneath in the centre where it would be out of sight. you would only have to remove one end and slide it in then glue it up to the underside of the sleepers.

  6. Craig,
    You can delete my comment August 19 at 4.02pm that about the timber it won't work after looking at my bridge. I think that the trusses are still too weak to carry a triple headed train. Use a one piece brass tube and tie it into the sides of the girders supporting the track. If you have to use several pieces make sure the joint has a neat telescopic and of a sizeable length so there is no deflection in the tube.