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Fabricating the truss


Construction of the truss is some- what complicated. However, this intri- cacy is essential in keeping the bridge’s visual character. There are actually four wood trusses, two on each side. Each wood frame is flanked by a set of iron tension rods that make up a sort of “web” along with the bottom chord, which is in tension. In a Howe wood and iron truss, the wood members are in structural compression and the iron rods and bars are in tension. Wood cross bracing is provided at the top to prevent lateral collapse to the side. Further iron tie rods are provided at the end of the cross bracing to insure that the members remain in compres- sion. The connection of the iron and wood components required special cast iron shoes to support the ends of the timbers and provide connecting points for the iron tie rods and pins. The layout and design for bridge No. 5 are shown in the detail photos and draw- ing. Examining the plans revealed that three different sizes of square and rec- tangular iron members were used for the tension rods and bottom chord, and it be- came apparent that in HO scale the dif- ference between ¾″ square and 1″ iron rod would not be visible on the model. Fortunately, Detail Associates does make suitable square and flat stock brass wire. The criterion was to select the appropri- ate size within a half an inch of the ma- terial called out on the plans. A major is- sue was how to make the “eye” at each end of every tension rod or chord at the model’s scale. I decided I could use Preci- sion Scale Company brass lift rings cleaned, tinned, then mounted in a jig to solder each piece of the ironwork into a coherent structure. After experimenta- tion, it became clear that it was better to leave the casting pins on the eye bolts to position them for soldering, then clip them off later. Eight complete sets of iron tension rods and chords were required, four for each side. The jig was carefully drilled out for pins fabricated from aluminum nails to position the end points precisely and avoid adhering them to the sol- dered parts. The eye bolts were then dropped over the pins and anchored at their tails by small model railroad spikes. Wood blocks were also glued to the jig to hold the brass shapes in align- ment. The most difficult joint was at the bottom chord where four pieces came to- gether for each of the eight components of the ironwork assemblies.


While I suppose it would be possible to find or make very small washers ¾″ thick in HO and do each rod with an eye at each end, I decided that I valued my eyesight and blood pressure more. I also found that the wire could be bent


RAILROAD MODEL CRAFTSMAN 53


around the top pins where there was only a pin or one other piece of iron- work. This reduced the amount of sol- dering and actually was visually more prototypical per prototype detail photo- graphs. Still, this was a tedious task. I will emphasize that all surfaces to be soldered were cleaned and tinned prior to placing them in the jig. Once this was done and all the wire


was pre-cut to the exact length to fit be- tween the pins on the jig, the soldering itself was relatively straightforward. All the wires and the eyes were held in place by the jig and small lead weights. For me the real satisfaction came when the lacy


ironwork was added to the timbers and the bridge character became visible. The eight sets of tension rods and lower chords were then airbrushed with rust colored paint. I used Floquil since the lacquer base offers better coverage on metal and solder joints.


There were a couple of tense moments when one of the solder joints came un- done after assembly. However, with a few choice words and a combination of Barge®


contact cement followed by a


touch of cyanoacrylate cement the prob- lem was solved. The extra bit of “blobbi- ness” was covered by weathering later. Using the previously prepared jig,


Extra bridge revenue came in the form of pipelines carrying oil to several piers. These required add-on out-rigging as shown in the sketch on page 52. Oil in the lines was heated to facili- tate flow to the harbor; hence insulation was required and protected by an aluminum jacket. Heavy, four-foot iron caissons filled with concrete formed the original bearing points for the bridge. These weathered features are quite prominent on the model.


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