Miniature railway bridge

[Boulder Creek]

G’day all,

Over the past 12 months or so there has been some great progress on the track building on the Boulder Creek Tramway and the track now extends from the workshop siding up to the top reversing loop which is now almost complete. The major obstacle being the bridge crossing Boulder Creek. I did have someone who was going to do some calculations on my bridge design however they have not had time to do anything and I really need to get the bridge built so the top loop can be completed.
The bridge will be a plate girder type bridge of typical railway design.

edit - (for some reason the 2 links below don't work properly as the highlight stops at the : symbol in the link. to view the link, copy and paste it into you browser)

Such as:
en.wikipedia.org/wiki/File:PlateGirderBridgeUType.jpg

And:
images2d.snapfish.com/232323232%7Ffp536%3C2%3Enu%3D3363%3E33:%3E57%3B%3EWSNRCG%3D33323:3%3B3%3B336nu0mrj

It will span Boulder Creek in three sections with the middle section being the longest span at 3 metres. The two outer sections will each span 2.4 metres.
I have done a simple CAD drawing of my preposed bridge which will be of steel construction.
The plate girders will be fabricated I beam sections that I will build on site. The preposed design makes the girders 900mm high and 2.4 or 3 metres long respectively. My design of the girders uses 3mm sheet steel as the vertical member of the I beam with 100 x 10mm flat bar welded top and bottom to form the horizontal sections. To stiffen the vertical sheet I plan to weld a piece of 25 x 25mm angle iron vertically from the top flat to the bottom flat on both sides at 600mm intervals.
The rail will be supported by 75 x 40mm PFC or 75 x 50mm RHS cross members welded on top of the bottom flat beside the vertical braces at 600mm intervals. This will support a timber deck which would have the track laid on it.
The trestles supporting the spans will be fabricated from steel using 90 x 90mm RHS for the two main posts with a 150 x 75 PFC top member which the plate girders will sit on. The trestles will also have a 75 x 75mm RHS horizontal brace towards the bottom and 25 x 50mm RHS diagonal bracing crossing through the centre.

Much like this:
www.corbisimages.com/images/IH064677.jpg?size=67&uid=6C765FEC-4853-4087-8C56-1A651108675A

Or:
www.bobcarol.com/images/2007AU1834.JPG

The bridge abutments are all ready in place and can be seen here:
www.smex.net.au/bouldercreek/Photos/Track21.htm

And here:
www.smex.net.au/bouldercreek/Photos/Track39.htm

What I need to know is, how much will this carry? I prepose a working load of 600 kg per metre which I imagine would cover the heaviest of 7 ¼” gauge locos. The middle span at 3 metres would therefore need to carry 1800 kg however I would like a safety factor of at least 3 times this for my bridge. If my designed bridge will carry, say 3000 kg per metre, I will be happy to build it as designed but if it will only carry, say 800kg per metre, I will need to do some re-designing.
I have searched the web to find some sort of a bridge design calculator however I have not had any luck finding what I need. I did find a site giving calculations for plate girder sizes which had every calculation imaginable covered except the load it was to carry?
I am hoping that someone may be able to point me in the right direction to find a suitable calculator or maybe give me some advice on how to calculate my girder strength.
Any help would be very much appreciated. I can email a copy of my bridge design to anyone if it helps explain what I have designed.

Regards,
Dave Webb.

www.smex.net.au/bouldercreek

[AndrewP]

Great project Dave, I have forgotten almost all of the structural calc stuff I did at College - it always seemed like guesswork anyway - guess a beam section then analyse it to see if it fell down
The Model Engineers Utilities that can be downloaded from here includes beam calcs which may do what you want although with an on-site welded beam I would add a significant extra safety factor, but that could just be my welding!

Cheers, Andy

[arch1947]

G'Day from sunny Launceston Dave,
Like Andrew I have forgotten more of the structural engineering than I remember but what you describe should be pretty strong in bending. I don't think it would be too hard to determine the moment of interia (your CAD package may do this for you)of your beams and hence the maximum theoretical load capacity but, if I understand you post, the beam is 900 mm high with a 1/8 web. If so, you biggest problem will most likely be localised buckling of the web. My recollection for testing this is to consider a column 1/8 thick by one unit wide (say 25 mm) and 900 high subjected to a point load equivalent to the to the distributed load for 25 mm.
Good luck Dave send me an email of you beams design if you like, I'll see if I can do something with it.
Cheers,

Arch Robinson (ELRSS)

[bambuko]

Jan 6, 2010 22:06:39 GMT Boulder Creek said:

edit - (for some reason the 2 links below don't work properly as the highlight stops at the : symbol in the link. to view the link, copy and paste it into you browser)

Such as:
en.wikipedia.org/wiki/File:PlateGirderBridgeUType.jpg
And:
images2d.snapfish.com/232323232%7Ffp536%3C2%3Enu%3D3363%3E33:%3E57%3B%3EWSNRCG%3D33323:3%3B3%3B336nu0mrj...

try these links ?
plate girder bridge
and
and another bridge

[Boulder Creek]

Thanks Guys,

Andrew, I downloaded the Model Engineers Utilities and entered the sizes of my girder. With a length of 3 metres and the sizes mentioned in my first post the girder has a safety factor of 14.4 times with 1800kg applied to the centre of the beam! And that is just one beam! Obviously I need two beams - one each side of the track - but I still have no idea if what I have come up with is correct?
Arch, the CAD software that I have is actually a CAD/CAM package that I use for programming my CNC machines but I can use it for drawing. Unfortunately it isn't capable of any calculating for anything other that the CNC program. I'll send you the drawing of what I have designed for you to have a look at.
Thanks for the link corrections bambulo. How did you do that?

Regards,
Dave.

[bambuko]

Jan 8, 2010 0:43:08 GMT Boulder Creek said:

...Thanks for the link corrections bambulo. How did you do that?...

the code is as follows:
{url=here enter web address}here enter your title for the link{/url}
just use square brackets ie [ and ] instead of { and }
Chris

[ettingtonliam]

As a former bridge designer, I have the following thoughts:-
why is the beam so deep? If its just to stop you falling off when crossing, save yourself a lot of design and fabrication work and use a standard rolled section with a handrail on it.

It isn't just a case of checking that your beam has adequate bending capacity for the load, there are other things to be checked such as web stiffening especially over the bearings where the shear is at its highest, and compression flange instability (Because the top flange is in compression, it has a tendency to want to move sideways) Your proposed beam is tall and narrow with a very thin web, so web buckling and compression flange instability are definite possibilties. This is another reason for using standard sections, their proportions are not accidental, they have (generally) been arrived at so that if they are OK in bending they are probably OK for the other matters, but it should still be checked, especially for web buckling over the bearings. Full size practice is to weld web stiffeners to the web, not jusat to rely on weld ing the stiffeners to the flanges. Your proposal to seat the cross members on the inside of the bottom flange also introduces a torsional effect on the beam which is already very tall and narrow. There have been instances of full size bridge failure where seating the cross beams on the inside of the bottom flange has been found to be a contributary factor.

All my reference works and standards are, unfortunately, now either in store or have been given away, so I can't undertake the check for you, but hopefully there is someone out there still currently working who would do it for you - it isn't as simple as it might seem to do it thoroughly.

Sorry to seem a wet blanket

Regards
richard

[Boulder Creek]

Thanks Richard,

Those are the sorts of things that I need to find out from “someone in the know”
I am building my railway as a 1/3rd scale miniature of a 2 foot gauge railway and for that reason I want everything to "look right" as much as I possibly can. This rules out having a bridge with a handrail however, rules and regulations stipulate that due to the height of my bridge I must have a 900mm high hand rail or barrier just like a deck on a house has to. I would prefer to build a scale trestle bridge with no hand rail at all but I don't want to build something that is going to be a liability in years to come so the 900mm high girder is a good compromise.

Regards,
Dave.

[S.D.L.]

Design a bridge in std sections then add your design to the top as a boxed in handail.

Steve Larner

[Boulder Creek]

I could do that Steve, but i still have to get someone to do the calcs on the standard sections so i'm no further ahead

[smallbrother]

I have to echo the point about the very deep sections you propose. Would it be more economical to select shallower, standard joists that do the job and dress the thing up to the appearance you want?

I have also disposed of my Steel Designers Manual but may be able to borrow one next week. I don't specialise in this but would be able to check standard sections for all the necessary bending, shear, buckling etc.

What is it going to sit on? The photos show earth behind a rock, unless I've missed something.

Pete.

[S.D.L.]

Jan 8, 2010 13:13:10 GMT Boulder Creek said:

I could do that Steve, but i still have to get someone to do the calcs on the standard sections so i'm no further ahead

It should be easier to get the structural part checked if std sections. designs out of the norm always require more checking, if someone says not sure need to do FEA (Finite element analysis) it gets really expensive.

Steve Larner

[Boulder Creek]

That is a good point Steve. I will give that some thought.

Dave.

[steam4ian]

G'day Dave

One section you could consider is a Warren Truss, not the politician, but a lattice section. This is a standard type deep open section for which you may find a structural engineer who can verify it.

The other thing is just use a deep section universal beam as the bottom chord which does all the work. 3 metre span is not very much in real terms. If you use a simple section it will not cost an arm and leg to get it designed and certified. I guess, about 4 hours at AUD200 per hour. Professional Engineering is such these days that no engineer is willing to give back of beer mat advice; the courts have made sure of that.

Regards
Ian

[Boulder Creek]

Pete,

Sorry, I missed reading you post. Yes I might be better off using a couple of say 250mm x 90mm PFC beams and then building a truss bridge looking hand rail on them to gain the 900mm height.
Anyone know where I might find a calculator for standard beam strength?
The span will be supported at each end buy the rocks and will also have two trestles supporting it in the middle. The middle span being the longest at 3 meters. As can be seen here: www.smex.net.au/bouldercreek/Photos/Track16.htm and www.smex.net.au/bouldercreek/Photos/Track21.htm the rocks are huge and are as far in the ground as what they are sticking out so I'm sure they will make a suitable bridge abutment. I will probably pour a flat concrete foundation on top of them to get a level surface to support the bridge on. Another possibility would be to mark the height with a dumpy level and cut the top of the rocks level with a diamond saw.

Regards,
Dave.

[smallbrother]

Dave - I should be in the office this week (in spite of the snow!) and will borrow a steel designers manual. I can probably analyse any standard section with that.

I would defo construct a decent concrete pad behind and above the rocks. Thickness 225mm with some mesh reinforcing steel. The beams can be bolted to these pads but allow for expansion at one end.

Are you intending for the bridge to be sloping or level? The latter would obviously make it more stable more easily.

Also are you trying to make a miniature of the original or re-create a small section at original size?

Pete.

[ausdan]

this one any use www.engineersedge.com/beam_bending/beam_bending1.htm

[Boulder Creek]

Thanks Ausdan,

It probably would be if I had some idea of the figures that I had to enter into the calculation. Unfortunately all I know is the span and the load...

Dave.