Wills Viaduct Stability

[Jkelly]

So having finally gotten some time to model recently, I finished my viaduct. It is a single track Wills stone viaduct. I put it together with poly cement. I really took my time with it to make sure it is secure and strong, which it is. However, in order to bridge the 3ft long depression in the baseboard, I want to get straight to it and build the valley up with foam etc. and place the bridge snugly in - with no wooden supports. Would the bridge on its own be enough to hold the weight of the train? The track will lie on insulation board and then the bridge.
The reason I don't want to use a wooden support is that I initially tried it with a strip of 9mm plywood, (which is all I have to use at the moment), but it didn't fit. To get something more appropriate (i.e. thinner, maybe 6mm ply) would require even more time and effort just to obtain it (currently under a lockdown here in Ireland, so might have to wait for stores to open), and then to cut it up and put it in place. I'd rather get straight to it and put the viaduct in with no underlying wooden support. The bridge seems strong enough to me. Any thoughts? Should I resist my laziness and put in a proper support or just get on with it?
Thanks for an advice

[SRman]

Without supports under the piers, the arches could separate if your glue isn't up to the job.

Mine is supported, but I chose to strengthen the trackbed (double track), using some Plastruct I-beam girders along and beneath the whole length. If you were to do something similar, that would take the strain off the lower parts of the viaduct.

[Bufferstop]

As long as your viaduct only has to carry the weight of the trains, and the baseboard is strong enough not to flex you should be alright with no more than a Balsa or thin ply sheet deck. Here's a Metcalfe viaduct, built as per the instructions with just a balsawood deck added.

I originally built it as part of my previous layout around 1998. In 2007 I incorporated the whole section into my new layout, widened by two inches. I simply glued two inch thick polystyrene insulation to the front and attached a new profile board. It's still solid and rumbles nicely as the trains pass over it. The joint in the scenery passes just to the right of the front tree, cut up, used manila envelopes, PVA and static grass hides a multitude of bodges.

[Jkelly]

Thank you very much, that is very helpful. By the way, that viaduct looks superb!

[stuartp]

Agreed, the plastic sheet will take the weight provided the joints will. If it worries you reinforce them with thick strip along the joints inside.

[Bigmet]

I meant to comment on this one when it was first posted, and forgot.

There's an aspect relating to layout construction that wasn't mentioned, and that is the anticipated longevity of the layout. Now, I am a very slow and long term layout constructor, and if there is one thing I have learned it is that solid construction is the only way for a layout that will be in use (hopefully!) for twenty, thirty, or more years.

I wouldn't rely on a kit construction to provide a stable track bed indefinitely. My preference would be to construct the track bed of such a bridge or viaduct in the usual manner, wooden posts supporting the ply track base: and then 'dress' that construction with the kit, so the kit is purely cosmetic, oinly there to conceal the actual support structure: all the load is taken by the standard construction.

[gppsoftware]

I am with bigmet on this one: if there is any idea of longevity in a layout, then today's short-cuts nearly always become tomorrow's nightmares.

Trackbeds are probably the singly most important area not to take short cuts because to do so will result in derailments forever more and you'll be cursing yourself 'why didn't I do it properly in the first place ?'

Conversely, I went to the opposite 'belt and braces' extreme with my viaduct: http://www.mrol.com.au/Pages/Vu/ModellingaViaduct.
It was originally built in 1998/99 and I can confirm that it is still as firm, strong and level as they day it was built.