Paddington Flyover

[Gary L]

Hi all

I’m working on the fair assumption (tbc later) that the weight of my 7.25” gauge Paddington is 200kg. This makes it more than double the weight of the previous loco to use the extraction equipment from my workshop, which was a bit wobbly when I used it to get the ‘undressed’ loco out, without platework. I’m sure that adding the extra weight has taken the setup beyond the limits of safety.

My main concern is the removable ‘bridge’ that spans an internal doorway and carries the locos over the workshop threshold. It needs to be 6ft long, and still carry the weight of the loco, ideally without an intermediate support.

For reasons of lightness, rigidity, and handling I would favour bright steel rectangular hollow section, and for coupling it to the existing fixed ‘track’ it would preferably be 20mm wide, though this isn’t essential. My question for you, the assembled panel of experts, is what section material should this span be?

Obviously I could just massively over-specify, but if I make it too heavy it will bring other problems.

I’ve tried to use the online calculators (if which there are several) but I get silly results when I test them against known behaviour of the existing 20 x 40mm section which can carry the weight of Bridget, so either they are badly programmed or I don’t know what I am doing. The latter is certainly true, the former may or may not be! Mostly they are intended for the construction industry, not this kind of relatively trivial application, so possibly I am taking them out of range.

So can anybody help me with this please? Any suggestions gratefully received...

Gary

[chris vine]

Hi Gary,

I have never used steel for this sort of thing, I have always used large planks of wood.

One I use is about 9" wide and about 2" thick and 8' long. I don't fix track to it, but put wooden strips between the backs of the wheels, a bit like check rails.
It does bend a bit, but nothing too alarming. You can cut it to fit in between rails at the ends.

For loading cases, into formulae, it isn't that easy! If you take the whole weight of the engine and put it into the middle of the span, you will get a much large deflection that will actually happen. the weight on the end wheels of the engine is nearer to the supports than the middle. Equally it isn't the distributed load case which most formulae allow. Some of the online ones allow you to put in different loads along the beam.

The other problem is to decide what deflection is acceptable to you! Is it 1 cm or 1 mm, or 0.1 mm. The section chosen will vary hugely depending on what you decide.

On a railway bridge, as you are making, I think that the deflection downwards in the middle is probably not the critical thing. I think that more important is when the engine is in the middle of the span and you are thinking about it tipping sideways. IE loading the span in "torsion" - except it isn't really torsion, because what stops the engine tipping over sideways is that one rail must not drop more than the other when the engine rocks a bit.
IE, you are really talking about the stiffness of the two rails/structure. The distance between the two rails is not all that great, so a bit of flexibility, up and down on one side or the other will create quite a big angle of tip. I think this is probably the thing you are most worried about.

Sorry not to give any answers, just some ways to look at the problem...

Chris.

[andyhigham]

To save weight you could use sat 20 x 20 x 3 square tube and add an under bracing say 75mm 0r 100mm in the center tapering up to the 20mm at the ends

[Deleted]

Hi Gary

Very difficult to comment without knowing all of the parameters such as available space around the area to be bridged. I'm a big fan on using triangulation, my lightweight loco stand being a case in question, this would remove any sag from the middle of the span. Possible off the shelf products of use would be alloy scaff tube and swivels to connect everthing, these can be found relatively cheaply on ebay. I use this for my electric hoist stand for supporting its swinging jib.

I'm no expert in any sense of the word but if faced with the same issue I would be looking at what I have suggested above.

Kind regards

Pete

[Gary L]

Apr 19, 2022 19:00:53 GMT chris vine said:

Hi Gary,

I have never used steel for this sort of thing, I have always used large planks of wood.

One I use is about 9" wide and about 2" thick and 8' long. I don't fix track to it, but put wooden strips between the backs of the wheels, a bit like check rails.
It does bend a bit, but nothing too alarming. You can cut it to fit in between rails at the ends.

For loading cases, into formulae, it isn't that easy! If you take the whole weight of the engine and put it into the middle of the span, you will get a much large deflection that will actually happen. the weight on the end wheels of the engine is nearer to the supports than the middle. Equally it isn't the distributed load case which most formulae allow. Some of the online ones allow you to put in different loads along the beam.

The other problem is to decide what deflection is acceptable to you! Is it 1 cm or 1 mm, or 0.1 mm. The section chosen will vary hugely depending on what you decide.

On a railway bridge, as you are making, I think that the deflection downwards in the middle is probably not the critical thing. I think that more important is when the engine is in the middle of the span and you are thinking about it tipping sideways. IE loading the span in "torsion" - except it isn't really torsion, because what stops the engine tipping over sideways is that one rail must not drop more than the other when the engine rocks a bit.
IE, you are really talking about the stiffness of the two rails/structure. The distance between the two rails is not all that great, so a bit of flexibility, up and down on one side or the other will create quite a big angle of tip. I think this is probably the thing you are most worried about.

Sorry not to give any answers, just some ways to look at the problem...

Chris.

Hi Chris

Lots of very good points here. I'll try to respond in turn:

I'm not keen on using a plank even a very thick one as you suggest, partly because of difficulty in making an adequate secure connection to the lift at one end and the fixed 'track' at the other, but also because I have found with Speedy and with Paddington that the large distance of the guard irons from the wheels makes them very unhappy about negotiating concave changes in gradient, which is what would be the case with a bowing plank. A final consideration is that wood is a somewhat inconsistent material, and if it fails it has a nasty habit of failing suddenly and catastrophically...

The point loading vs distributed loading is a good point. Simplistically, I assumed that if I calculated on a 200kg point loading, it would introduce a 'failsafe' margin, but (judging from the enormous deflections I was getting from the online calculators) perhaps this is over-cautious. If anybody is interested enough to try the distributed load calcs, the wheelbase of the loco is 19.25in or 48.9cms, i.e. a little over ⅓ of the span.

Re deflection, I don't know what would be reasonable. One of the online tools suggested span/360 which gives 5mm. That sounds reasonable, but...

...sideways tilt (or twist) could be a significant issue, which I hadn't thought about at all. This is where a bit of practical experience (which I don't have much of) will come into the calculation as well. The tilt could be mitigated by arranging a central support strut, which would also reduce the section required significantly as I understand it. (It is in fact what I do at present with the lightweight bridge that carries Bridget, but it is a bit Heath Robinson and I had hoped to either eliminate it altogether or construct something altogether better engineered.) I expect it could be done with a pivot so that it folds away when the bridge is stowed. Alternatively, it follows that the less the static deflection, then the less is the potential for this tip/twist behaviour.

As you say, the problem is more complicated than I first thought!

Gary

[Gary L]

Apr 19, 2022 19:36:40 GMT @greenglade said:

Hi Gary

Very difficult to comment without knowing all of the parameters such as available space around the area to be bridged. I'm a big fan on using triangulation, my lightweight loco stand being a case in question, this would remove any sag from the middle of the span. Possible off the shelf products of use would be alloy scaff tube and swivels to connect everything, these can be found relatively cheaply on ebay. I use this for my electric hoist stand for supporting its swinging jib.

I'm no expert in any sense of the word but if faced with the same issue I would be looking at what I have suggested above.

Kind regards

Pete

Hi Pete and Andy

Thanks for the truss/triangulation suggestions. You are envisaging something resembling an old-fashioned carriage underframe (but with two struts instead of four), or a spreader on a mast. I admit it is something that had briefly crossed my mind. In its favour is weight and therefore cost saving, while against it is the extra storage space it would require. The current bridge stows out of the way vertically against a wall, but with a protruding triangulation strut this would not be possible because it would foul the doorway. It isn't out of the question to find an alternative stowage for it, but the current location is very convenient.

Pete, is there a picture of your lightweight loco stand online?

Many thanks to you both for the suggestion

Gary

[Deleted]

Hi Gary.... this is the stand/weighbridge, the model here is 244lbs so about half your 200kg, I expect the model to be closer to 300lbs when finished. The bracing used here are alloy 25 x 5 mm lengths (it's available in larger sizes for stronger applications), drilled/bolted together as seen here. It is very strong and as I said has no deflection. This particular design has the extra lower bar so a bit like a ladder frame as used by riggers when assembling something extra strong with scaff.




For your purpose, you could have two downright structs spaced say a foot apart in the middle which are individually braced forming a boxed structure and then have triangular pieces coming from either end of the bridge down to the bottom of the two verticals. For storage, this doesn't need to be a permanent structure, if everything is bolted together with pivot points at either end of the bridge, with a few bolts removed it could be folded flat for storage.

Hope that makes sense?

Pete

[springcrocus]

Just to throw an alternative into the mix:

A 2 metre length of 40 x 20 x 3mm HE30 box section will support a greater weight than a 2 metre length of 20 x 20 x 3mm steel box section for about half the weight.

Regards, Steve

[Gary L]

Apr 19, 2022 20:26:57 GMT @greenglade said:

Hi Gary.... this is the stand/weighbridge, the model here is 244lbs so about half your 200kg, I expect the model to be closer to 300lbs when finished. The bracing used here are alloy 25 x 5 mm lengths (it's available in larger sizes for stronger applications), drilled/bolted together as seen here. It is very strong and as I said has no deflection. This particular design has the extra lower bar so a bit like a ladder frame as used by riggers when assembling something extra strong with scaff.




For your purpose, you could have two downright structs spaced say a foot apart in the middle which are individually braced forming a boxed structure and then have triangular pieces coming from either end of the bridge down to the bottom of the two verticals. For storage, this doesn't need to be a permanent structure, if everything is bolted together with pivot points at either end of the bridge, with a few bolts removed it could be folded flat for storage.

Hope that makes sense?

Pete

Thanks Pete, that's very elegant, and it has given me an idea...

Gary

[thumpersdad]

Hello Gary

What is the wall thickness of your 20x40 section?

Eric

[Gary L]

Apr 20, 2022 7:09:40 GMT thumpersdad said:

Hello Gary

What is the wall thickness of your 20x40 section?

Eric

Hi Eric

It is 1.8mm

Gary

[andyhigham]

Another suggestion would be to use your existing flyover with a "prop" in the middle to reduce the span to 3ft.
This is what we do at RVLS when bringing a 5" Duchess over the loading/unloading bridge

[Gary L]

Apr 20, 2022 18:41:51 GMT andyhigham said:

Another suggestion would be to use your existing flyover with a "prop" in the middle to reduce the span to 3ft.
This is what we do at RVLS when bringing a 5" Duchess over the loading/unloading bridge

Hi Andy

Yes indeed. It is what I already do to load Bridget with the existing lightweight bridge, albeit the ‘prop’ in the middle is not exactly a miracle of engineering (or indeed engineered in any sense at all!) Trouble is, I don’t trust the existing bridge with Paddington’s weight, even with the central support in place.

I have no data to back up my lack of trust; it just feels altogether too flexible to be safe.

I had hoped to do away with a central support by reinventing the bridge in a more robust and scientific fashion. What you say is very true; AIUI halving the span will more than double the strength of the bridge. I would prefer the span to be strong enough in its own right though; then adding a central support would become a ‘belt and braces’ and/or a backup option in the (not very likely) event of the bridge being used for an even heavier loco than Paddington.

Gary

[smallbrother]

Hi Gary,

I made a 5 feet long bridge for my Bagnall in steel angle. It is extremely heavy and one of the reasons i am struggling with engines of this size.

I believe it is 3" x 2" angle so pretty robust.

Pete.

[thumpersdad]

Hi Gary

The bending moment for a load W in the centre of a beam length l is WL/4. So for 200kg in the middle of a 2m long beam you would have 1,000,000Nmm.

The bending stress for moment M is My/I where y is the maximum distance from the neutral axis (20mm for a 40mmx20mm section) and I is the moment of inertia, which I get to be 40754mm^4.

So that gives a bending stress of 490MPa. I suppose you have two parallel beams so that would be 245MPa each.

You should be able to expect a yield stress of at least 200MPa for structural steel. So it looks like your beams may be overloaded without a central support.

OK assuming the load is concentrated at the centre of the span is pessimistic, but at the same time assuming that the load is spread evenly between two spans may be optimistic.

Eric

[Gary L]

Apr 20, 2022 21:04:11 GMT thumpersdad said:

Hi Gary

The bending moment for a load W in the centre of a beam length l is WL/4. So for 200kg in the middle of a 2m long beam you would have 1,000,000Nmm.

The bending stress for moment M is My/I where y is the maximum distance from the neutral axis (20mm for a 40mmx20mm section) and I is the moment of inertia, which I get to be 40754mm^4.

So that gives a bending stress of 490MPa. I suppose you have two parallel beams so that would be 245MPa each.

You should be able to expect a yield stress of at least 200MPa for structural steel. So it looks like your beams may be overloaded without a central support.

OK assuming the load is concentrated at the centre of the span is pessimistic, but at the same time assuming that the load is spread evenly between two spans may be optimistic.

Eric

Excellent, thanks Eric, that confirms my suspicions.

Could I ask what sort of section would be safe at this span and load? By 'section' I guess I'm talking primarily about depth, because section width has little influence, and there isn't usually a lot of choice when it comes to wall thickness.

Gary

[kipford]

Gary
S235 30mm x 50mm x 2.6mm thickness steel section as a simply supported point load has a max stress of 97MPa, which is well below the 235 Yield and deflection at mid span will be 6mm. The mass of each beam is 14.5 kg without any structure tying them together. This is essentially a stiffness issue as if you limit the deflection the stress takes care of itself.

If we look at Aluminium, you have to remember that while a material like 6082-T6 is as strong as S235 steel, 255 MPa yield and a third the mass. It is only a third as stiff so the equivalent structure for the same deflection has to be more substantial. 25mm x 60mm x 3mm thickness box has stress at mid span of 73MPa and a deflection of 12mm and a mass of 6kg per beam. 75mm x 25mm x 3mm thickness has a deflection of 7mm and a mass of 8kg per beam. I can send you an excel spread sheet that works it all out if you want.
Dave

[92220]

Why not use 2 pieces of 2" x 1/4" angle iron as rails and the bridge at the same time. 200Kgs will be nothing to that section, and it can be made light enough to handle easily by using wood to join the 2 angles at the right gauge. I've use 1.1/2" x 1.1/2" x 1/4" angle for my 5" 9f and the angle handles it with hardly any deflection.

Bob.

[thumpersdad]

Hi Gary

I recently lifted a new lathe in my workshed using a pair of 40mm square x 3mm (nominal wall) hollow section beams to take the load. The lathe weighed somewhere between 150 and 200kg and the span of the beams was about 1500mm, so a bit less onerous than your problem, but everything went well at least in regard to supporting the load.

These beams actually have a wall thickness closer to 2.8mm, which would make the moment of inertia 96638mm^4 and the bending stress 103MPa for each of two beams for a total central point load of 200kg and 2000mm span. The maximum deflection if 8.6mm (I think). So one beam could probably take the full weight, which gives some comfort.

A bit of cross bracing between the beams would also help by giving a bit of torsional rigidity.

Eric

[Gary L]

Apr 21, 2022 8:24:18 GMT 92220 said:

Why not use 2 pieces of 2" x 1/4" angle iron as rails and the bridge at the same time. 200Kgs will be nothing to that section, and it can be made light enough to handle easily by using wood to join the 2 angles at the right gauge. I've use 1.1/2" x 1.1/2" x 1/4" angle for my 5" 9f and the angle handles it with hardly any deflection.

Bob.

Hi Bob (and others, especially Dave and Eric)

That's very helpful, very many thanks. All the proposed sections are well in excess of the current set-up, which is exactly as I expected (as you have gathered the issue is really about how much heavier to make everything). I'll reply individually, but in the meantime you might be interested to see the current arrangement in the (staged) photo here:



Note the internal and external doors, which are what causes the span to be an irreducible 6 ft. On the left is the end of the fixed raised track of 20 x 40 steel, used for storage. This was originally also of 6ft spans, but I have reduced it to 3ft spans, which Paddington now sits on quite happily.

The string at the RHS is not part of the normal configuration, just holding the bridge roughly horizontal for photographic purposes! When being used in earnest, the RH end of the bridge is fishplated to the hydraulic lift trolley, moved outside the door and raised and locked at full height. (Shown in the photo fully retracted and parked under the bridge) A central support is put under the bridge to halve the unsupported span when Bridget is carried.

This current bridge as shown is all aluminium (mostly 1" x 1" x ⅛" angle), still carrying a redundant 5" gauge 'rail'. So Pete (Greenglade)'s suggestion of adding a triangulated bracing structure to it is a definite possibility. However simplicity is often best, so a pair of big section steel runners remains a strong contender.

Gary