Designing a Removeable Overbridge Structure

[ISW]

I'm in the process of building the overbridge / station combination for my layout based on the 1883 Burton-on-Trent station buildings. For reasons of access and maintenance, I want to be able to lift out the overbridge, so the overbridge is a basic 9mm plywood sheet with timber risers, as below:

 

Installed on the layout:

 

Underside:

 

For the sides, these will be 'submerged' in ballast/soil to hide the join and provide necessary alignment.

 

My problem is that the design of the overbridge includes support columns between the tracks, as seen is this photo:

 

 

So, do I:

1. Attach the columns to the overbridge or,
2. Attach the columns to the baseboards?

 

In (1) I have the problem of getting the columns to align with 'holes' in the ballast at the foot of each column. In (2) the problem is getting the column to align with the 'steel' cross-members that'll be attached to the underside of the plywood. In both cases, I have the problem of adequately attaching the column to the overbridge or baseboard, such that they won't get knocked.

 

In either case, it seems to me that I'd have to be impossibly careful placing the overbridge back onto the layout without damaging those columns. As I have to stretch 'over' the baseboard to lift/place the overbridge, an entirely 'vertical' landing is not possible.

 

Any ideas on how to overcome this problem?

 

Ian

 

Edited May 9, 2023 by ISW
removed duplicate photo

[Flying Pig]

I don't think you need more then a couple of columns in each row if you want to maintain the illusion that the track continues in a straight line.  Could you perhaps make the break above the wider base of the columns and fix the upper parts to the overbridge?  With adequate alignment of the bridge you might not need any location between the two parts.

[Jackfrost83]

Do the columns play any structural role? If not, how about using some of those small, but strong magnets recessed into the baseboard/column base to hold them in place?

[doilum]

Fix the columns to the baseboard. Create a strip of thin ply or thick plastikard which runs along the top of the columns. This should make them much less vulnerable and will be invisible once the bridge is in situ.

[Les Bird]

1 hour ago, doilum said:

Fix the columns to the baseboard. Create a strip of thin ply or thick plastikard which runs along the top of the columns. This should make them much less vulnerable and will be invisible once the bridge is in situ.

I used a similar arrangement to allow for the removal of a long platform canopy. It works well.

[Michael Hodgson]

Sheet of perspex over the top of columns ?

I don't think those bridge giders would be visible from a normal viewing angle.  We tend to get a birds eye view.

[ISW]

7 hours ago, Flying Pig said:

I don't think you need more then a couple of columns in each row if you want to maintain the illusion that the track continues in a straight line.

Unfortunately, while the track 'should' extend in a straight line to reflect reality, space constraints meant that the tracks have to curve to avoid falling off the baseboard!

 

3 hours ago, Jackfrost83 said:

Do the columns play any structural role?

Thankfully no. The 9mm plywood is more than adequate to support itself and the 'flyweight' building is made from cereal packet cardboard and inkjet printed textures.

 

2 hours ago, doilum said:

Fix the columns to the baseboard. Create a strip of thin ply or thick plastikard which runs along the top of the columns. This should make them much less vulnerable and will be invisible once the bridge is in situ.

Now I like that. It means I can make sure the columns are clear of the KE (kinematic envelope) of the trains, and get them in place without the Overbridge being 'in the way'.

 

Yes, it'll be hard to see the 'detail' under the Overbridge, but I'm trying to incorporate sufficient detail that it doesn't look too bad when a loco is pushing  a CubeCam around the layout.

 

41 minutes ago, Michael Hodgson said:

Sheet of perspex over the top of columns ?

I don't think those bridge giders would be visible from a normal viewing angle

Either perspex or styrene. In reality (see photo in my first posting) there is such a longitudinal beam.

 

======================================================================

 

Many thanks to all who've taken time to respond, it has really helped me out.

 

I was originally tending towards attaching the columns to the Overbridge, by simply drilling vertical holes (if I can keep the drill orientated correctly!) through the Overbridge plywood, and gluing styrene or wooden rods into the holes. This would have made alignment with the Overbridge beams easy, but would have been hard to get them correctly aligned with the tracks (and almost impossible to rectify afterwards!). This doesn't sound good.

 

However, your replies have shown me that I really should connect the columns to the baseboard (using the same drilled hole through the plywood method) and install the longitudinal beam on top. The lateral beams, that'll be attached to the Overbridge, will be simple cut out shapes of cardboard (aka cereal packet) painted black (ish), and so the alignment can be 'close enough'. It's also fairly easy (and cheap) to rectify any misaligned cardboard beams.

 

Ian

 

[Michael Hodgson]

1 hour ago, ISW said:

However, your replies have shown me that I really should connect the columns to the baseboard (using the same drilled hole through the plywood method) and install the longitudinal beam on top. The lateral beams, that'll be attached to the Overbridge, will be simple cut out shapes of cardboard (aka cereal packet) painted black (ish), and so the alignment can be 'close enough'. It's also fairly easy (and cheap) to rectify any misaligned cardboard beams.

 

Ian

 

Yes, if you're worried about precise clearancwe, there's less risk of drift in the wrong direction if they are fixed from below clear of the KE.

The thing about pillars is they can be pretty robust, even if you don't need them structurally - over-engineering a bridge support won't do any harm.

[ISW]

2 hours ago, Michael Hodgson said:

The thing about pillars is they can be pretty robust, even if you don't need them structurally - over-engineering a bridge support won't do any harm.

True, but my columns are purely 'decorative' and only serve to fool (?) the viewer into thinking they are functional (a bit like most of a model railway!). Either way, they need to be clear of the KE to avoid unnecessary derailments and 'scraping' noises ...

 

I'll be making them between 4mm and 6mm diameter, depending on the gap I have between the adjacent KEs of the tracks. They will certainly be 'robust' ...

 

Ian