Double level layouts

kristopher1805 said:

Hmm the main girder would be the front one of a line of them behind and not simply supported by the single girder at the front, naturally I do not show these extra supports

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Yes, that is true. In your case, there would be multiple girders parallel to the outer girders, possibly constructed as multiple spans next to each other. You may or may not see the girders between the parallel tracks, depending on how it was constructed. There would also be cross-member girders spanning between the parallel main girders.

The challenge with this of course, is that you have pointwork on your bridge which would have to support trains transitioning between parallel bridge spans.
In practice, the permanent way engineer would try to avoid placing turnouts on a bridge if possible, because such layouts would impact the construction (and cost) of the bridge. However, if there was a requirement that it had to be done, the engineers involved would need to engineer a viable solution at a reasonable cost.
This is the sort of thing that when it is 'done right' on a model layout, it 'looks right' and adds to the overall appearance of accuracy - makes it look like the modeller 'modelled what they see' rather than what they 'think they see'.

kristopher1805 said:

View attachment 21941


One for you Graham, this is the Great Ducie Street Bridge in Manchester at the top end of Deansgate it carries a number of lines, was heavily loaded in the last 150 years so just why is it still standing?

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Kris,

I don't know the location, but looking at that picture, that bridge looks like it has been widened. It looks like the original span is the one further away with the supporting arch structure.
The later addition appears to be the closer span which, as I think you are alluding to, has no intermediate supports. The answer to this is simple: look at the height of the side girders themselves - they are huge - and that is where the strength is and evidently, the engineering deemed that it was sufficient not to require intermediate supports.
Also look at how the girders sit on the stone buttresses each end - the girders are not 'cosmetic' attachments to the sides of the deck as most modellers model them!
You can also see the cross members underneath which support the track bed. I modelled all of this on my girder bridge: Modelling a Steel Bridge - Model Railways On-Line

From experience, this type of metallic extension to an existing masonry arch structure probably looks something like the below in elevation and section:




Note however from the above that whilst it looks like the RHS ('inside') girder bears onto the masonry, it does not - it is supported by the abutments on either end of the arch instead. As a general rule you don't want to apply point loads to an arch if you can avoid it...

Note also that the above structure utilises longitudinal wheel timbers to support the track, a feature not often modelled...

Regards,

Cameron.

Hi

Apologies, as usual I have over engineered this

Here is my reasoning, a vertical alignment is a series of straight grades connected by vertical curves. However I have yet to see a layout where the rail line undulates to follow the landscape, we mainly have flat areas connected by grades. Although an undulating layout would be interesting.

Vertical curves can be defined as
L = K x A
Length of VC (m) = Length of VC for 1% change in grade x Algebraic grade change (%)

So in rail design the K value is about 610 for sags and crests. This will more than likely vary depending on the design requirements of the stakeholder.

For roads the same applies, however depending on the speed you do not need a VC for a minimum change in grade. This is a bit contentious as the scale speed does not equal real world speed (that's another thread).

In road design for 40km/h the minimum change in grade I can have is 1% before I need a VC. As the speed increases so the change in grade decreases. Technically I'm not sure if this applies to rail design, I'm thinking no, but this makes sense to me.

The 250mm I got from the length of the wagons I'll be running, so each wagon will be on a different grade ( ie 1% and 2%) rather than a wagon being on a 1% grade and the other sitting on two grades 2% and 3% although now that I have typed it, I should extend that to the loco length.

By not using a VC I can shorten up the transition from one grade to the next, this only really works for low speed environments, which is what I will have.

I agree with what you both have said, I'm just skinning it differently. Since what is done in the real world does not necessarily translate to HO scale.

Julian, in your case I would more the start of the transition back so you do not need a steeper compensatory gradient between the transitions, but that also depends on how much space you have.

Cheers