Upper tier bridge (part 1)

The upper tier bridge construction has gone well. I ended up using several new (for me) techiques but I think in the end it's worked out.

First, the planning. I held off on coming up with a final design until I could see how the rest of the staging layout turned out. This has the advantages that 1) you can see everything in 3-D when completing the design and 2) you can employ lessons learned while constructing the rest of the layout. But, it has the disadvantage that you may have unintentionally created difficult or impossible constraints while building the rest of the layout. I tried to avoid this by thinking through the likely upper tier bridge designs as I made progress on the other tiers, and by deferring any tasks that might impede the upper tier bridge construction as long as possible. In this instance, the process worked without the disadvantages.

The first step was to build cardboard cutouts of two possible routes for the upper tier bridge track to take, as shown in this photo:

The outer route utilized an 18" radius curve (my staging minimum for curves) and thus allowed for longer straight sections. I thought the longer straight sections might ease construction and also make hand access to the switches easier since it put the bridge back as far as possible from the aisleway. The other route was shorter utilizing a 30" radius curve to connect the two end points. This route was close to the original design and had a few advantages. First, the wider radius curve would tend to help operations: long trains on the upper tier will have to go around the 18" radius loop -- about 5/8 of a circle -- then reverse direction after a 7" straight section. This shouldn't be a problem, but a wider radius further improves the chances of smooth operation. Second, the 30" radius route only crossed one switch directly, although the other switches were behind it so access to them would be slightly impeded.

What to do, what to do? (as Dana Carvey once said on SNL). Neither was an ideal solution, as both would make switch maintenance a problem. And I'm sure switches will need maintenance, even if only once per year (assuming regular track cleaning runs with the track cleaning cars). So, eventually I concluded that no route was acceptable for a fixed position bridge. The only acceptable solution was to make the bridge removable for occasional maintenance. A lot more work up front, yes, but likely to save tons of time and frustration in the long run.

So, now that I have experience using 5.2mm lauan plywood for a bridge (the middle tier bridge), I concluded the most reliable, stable design for this curved bridge is as follows:

1. The bridge would have a triangular form, allowing for straight edges on the sides. Each side would have straight edge, similar to the middle tier bridge, but taller for the additional strength the larger bridge would require.


2. The bridge would extend over the end points to allow it to be fixed in, adding stability. Both endpoints would be supported by level blocks of wood, set on end (similar principle to the spline roadbed mentioned earlier).


3. The center of the bridge would be supported by nuts and washers attached to long bolts that are anchored to the center of the middle tier.


4. I'm not sure yet on how the track connections will be set up to allow for the bridge to be removable -- this is usually the trickest part of removable track due to the lack of rail joiners to keep the rails in line -- but there are a number of methods people have used for this so I'll figure this out later.

The first step in construction was to get another 4x2 section of lauan plywood. I have some sizable scraps left, but none was big enough to cut out the whole bridge in one piece. One 4x2 sheet is just $4.24 after tax -- very reasonable.

Once bought, I put the cardboard cutouts onto the plywood for use in drawing the lines for the bridge floor:


Once cutout the bridge floor is put in place on the layout for sizing and trimming (I intentionally allowed more space than needed along some edges to permit later adjustments):


Fortunately it fit nicely with little adjustment. Next step was to build the nut/bolt/washer supports for the middle of the bridge. These are the materials and tools used:


1/4" coarse thread bolts (12" length) and bulk packages of 1/4" coarse thread nuts and washers. The saw has a blade intended for metal cutting. The two bolts shown on the right side are what is left after the cutting has been complete. The picture should also have included a metal file, which is needed to get rid of the excess metal around where the bolt is cut. Ironically, once filed the cut side of the bolt is easier to fit a nut on than the non-cut side.

Here my 13 year old son, Daniel, is busily assembling the bolt support tiers:




Meanwhile, my 8 year old daughter, Emma, is helping out by cutting insulated rail joiners:


In the next post I'll show the results of what Daniel was assembling.