Saturday, 9 December 2017

Murrumbidgee River bridge – part 2 – the overhead support beams

I am not sure when the Whitton truss bridge across the Murrumbidgee River at Wagga had its overhead arch girders replaced with steel beams, but it was before 1970.  And I do not know the reason, as the double track Whitton arch bridge at Albury over the Murray River still has its arches

Shortly after being built in 1879, this NSW Archives picture shows the Wagga Wagga bridge, and the timber trestle

In any case, I needed to model the Murrumbidgee bridge as it appeared in 1970, and that meant scratch building the overhead “U” support beams.   The first part of the Murrumbidgee bridge blog post is way back on the 26th June, 2017.  I finished that blog post saying that I was waiting for ABS styrene from China, or a source of brass “C” channel or the correct size.

The bridge as it appeared in my June 2017 blog. 

To cut a long story short, the styrene “C” channels from China were substandard, and I was unable to locate the correctly sized brass “C” channel.  However, I don’t like to leave projects sitting around for too long, and I wanted a distraction from the pain of shingles (yeah, that is another story), so a start was made. 
The styrene “C” channels, whilst poor, could be modified.  I cut out 7.5 cm sections that were more or less straight from the 50cm lengths supplied; and reduced the height down to approx 1.5mm.  These I put into a jig to hold them at the correct distance apart, whist I glued the styrene spacer plates on.  Unfortunately, removal of the jig stressed the joints out, and the whole lot disintegrated.  Better to find that out now, than after I had spent any more time on them.

The failed girder attempt with styrene

As no suitable brass “C’ channels were available. I had little choice than to fabricate my own from strip brass.  In this case I was very lucky.  I had in my supply of North Yard brass strip, enough 1.5 x 0.25 x 150mm, and 3.0 x 0.25 x 150mm strip pieces  to fabricate the 3.15 metres of the “C”  channels of the correct size for the bridge.  Yes, that is a lot of brass strip

Parts needed to make one 150mm strip.  This completed strip will be cut into two 7.5cm sections. This is a fortunate measurement, as it eliminates waste of the brass.

Once the “C Channels” were made, I fitted them into the distinctive “U” shape of the prototype, and matched a pair back to back, and soldered spacers to hold them a scale foot apart. 

Three 7.5cm sections make one half of the "U" beam

One completed "U" beam.  The spacer plates are 3mm strip brass.

  All these measurements have been guesstimated from the prototype photos I took during the bridge replacement over a decade ago

New concrete support piers were formed whilst the Whitton Truss bridge was still operational. The actual replacement of the bridge took around 3 days. The spans of the truss were cut into sections, and removed by crane. The old piers were then removed, and new concrete beams were laid onto the new concrete supports, and the track was then re-instated.

After being gas axed, the bridge truss sections were craned to one side awaiting removal. This view shows the interior construction of the girder quite well.

To match the prototype, I will have to fabricate 7 “U” beam supports, 4 on the first girder, and 3 on the second. 
 “U” beams were secured to the bridge using triangle gusset shapes.  These were made from sheet brass, but soldered to the bridge with 144 degree C solder, rather than the 60/40 solder I had used earlier.  I did this for 2 reasons – a)  I did not want to desolder the assembled “U” girder upright, and b) 144 degrree solder can flow with my small 25 watt iron on the large heatsink of brass that is the Whitton truss bridge side

The "U" beam fitted to the end of the bridge. The bottom had been secured to the bridge via a brass plate, although the triangle gusset plates on top are still to be fitted.  I was also concerned that I may have reduced the clearance too low, but I tested it with a USA outline diesel, and things should fit, provided I use lower profile sleepers.  But, I won't be able to run double-stacks through the bridge, 

One span, with two "U" beams fitted, and braced, and after removal of the temporary brace

The result was very strong, and I felt confident to remove the temporary brace, designed to stop the 2 bridge sides from collapsing inwards with handling.
I made a minor change to the assembly order of the subsequent “U” girders.  Basically, the 2 uprights were installed, soldered, and braced, before the overhead beam installed.  This was a lot simpler than my earlier method.

Detail soldering of one of the venter uprights

Angle braces were fabricated, and soldered to all 4 “U” beam assemblies.  These are purely cosmetic on my model.

I have been using fluxes to aid the soldering, and these need to be removed.  I have scrubbed the girder with kitchen cream cleanser, and washed the lot in water.  Further treatment will be needed prior to painting, but at least the model is no longer a clothing stain hazard. 

I have not yet finished the first girder.  The distinctive rivet overlay supplied in the kit will need to be cut to size, and glued [soldering heat would cause distortion of the thin rivet overlay]   to the flat brass on either side of the center uprights.  In some ways these uprights actually correct a problem in the kit, as the rivet overlay is around 6 mm too short to cover the area on the upper, and lower bridge chords where it was expected to go. I do not know if this was corrected with the newly re-released kit

As you can see, I still have to fit "U" girder supports to the second span.  Followed by painting, and then the sleepers and track can be added.  

This process is very time consuming.  I have not added up the hours I have spent – but my last “U” beam assembly took just over 5 hours to fabricate, and add.   I am sure that talented modellers might have used 3D printing to make the “U” beams - I am not in that category

Happy to report that after 3 weeks, the shingles pain has much reduced.  Whoever said Model Railways is therapeutic must have had me in mind.  I will continue with fitting the “U” beams, and with luck, will have painted the bridge for my next bridge blog post.

Happy modelling 


  1. Nice work Rob, still gathering items for my next bridge. I feel they can be frustrating.

  2. Well done Rob, I'm sure this bridge will become the scenic stand-out on your layout. Hope the New Year brings a break from any pain for you. At least our model trains are a welcome distraction to help us stay positive.

  3. I have found the date when the overhead beams were replaced. 1928. The reason for this might be the anticipated introduction of the 57 class, where the loading gauge was affected. History shows that the 57 never ventured south of Junee, but that didn't stop the modifications to the infrastructure