Plank bridges

[Gern]

Hi guys. I got to build a small plank bridge for the 15mm dio I'm working on. I've got some pre-made handrails which I can use and some small I-beam sections. For a simple bridge I want to put a couple of I-beams across the gap, lay some planks on top then fix the handrails on top of those.

I've got some coffee stirrers I can use to lay across the beams for the roadway. As it's in 15mm scale, I'd like to get the joints between the planks nice and tight. Is there any way I can clamp them together while the glue sets, and am I OK to use PVA to glue them together?

[spanner570]

Dave. I don't think the the boards would ever be 'Nice and tight', particularly on a bridge, due to them warping in the sun etc.
I would be inclined to just glue the top edges of the beams and simply lay the boards onto these, butting the boards loosely together.

PVA is fine and dandy if the whole structure is wood. If it is wood/ plastic, then a general purpose adhesive is best. I use UHU, but I guess any G.P. one will work. Also, gluing just the beam edges prevents any glue oozing onto the top surface of the boards. If you glue the board edges, the glue will show up as patches when you stain / paint them

When you lay the boards, get some sort of weight (Not heavy) just enough to prevent them from lifting off the beams and gently place on the bridge structure.

In fact, thinking about it, the planking will look better and more natural if one or two do lift a bit.

The above is just my method, but I'm happy with it....

Ron

[scottie3158]

Dave,
I would do the same as Ron.

[minitnkr]

Depends. Was it built by "the bloke down the way" or field expedient (any materials at hand)? Then the above apply. Allied & German engineers used dressed quality lumber from stores of bridging companies, they would be tight & squared away. PaulE

[Allen Dewire]

Hi Dave

I'm with Ron and PaulE on those methods of bridging…. I can only add that if you build a small bride with a load class of 5 tons, please don't put a 30 ton tank on it. It wouldn't look right.......

Bleib Gesund
Allen

[Gern]

Well the I-beams I have are 2mmx2mm (I'm planning on using three beams, one each side and one central) and the planks about 2.5mmx1mm. Scaled up they would be about 200mmx200mm and 250mmx100mm, so I'm thinking they're quite substantial beams with railway sleepers as the roadway. I'm no engineer but I'd say that would be strong enough for cars and small trucks, so I'm hoping armoured cars and half-tracks would be OK. I wouldn't be looking to put tanks on there unless they're small like the Panzer II or T-60. What do you guys reckon? Does that sound reasonable?

Forgot to mention, the total unsupported length would be about 60-70mm with about 10-15mm supported at each end which scales up to 6-7 metres and 1-1.5 metres respectively.

I see your point about warping and planks moving Ron, I just don't want them moving too far at 1/100 scale. Even a 1mm gap would represent a 100mm gap in reality. I think if I just get them close and sand any larger gaps I should be OK though. I've got a tube of UHU floating around somewhere. You reckon just a bead of that on top of the I-beams will hold everything OK?

[spanner570]

Yes, UHU will do nicely.
If you are concerned about the strength/ weight ratio, just add a couple of pier supports mid span. Possibly round dowel, tree trunk sort of thing.

That's what I did on that snowy bridge dio. I did not so long ago.

[JR]

Agree with my learned friend 570 and fellow woodbutcher completely.

[Gern]

Yes, UHU will do nicely.
If you are concerned about the strength/ weight ratio, just add a couple of pier supports mid span. Possibly round dowel, tree trunk sort of thing.

That's what I did on that snowy bridge dio. I did not so long ago.

Can't go wrong with central supports! I haven't built the groundworks yet, but once that's done, I'll have a look how 'natural' the bridge looks. I'll decide then if I need central supports. Don't know if I'll use wood or stone, but as I've got a rocky scene in mind, I may even use a rock outcrop in the middle. Just waiting for my road material to arrive before I start all that - meanwhile, back to playing around with painting the buildings!

[spanner570]

Rocky outcrop.

Top idea, Dave!