# Thread: Beam strength for miniature railway bridge

1. ## Beam strength for miniature railway bridge

G'day all,

I have a question regarding beam strength and loading which someone may be able to help me out with.
I am currently building a 7 ¼” gauge miniature railway and am at the stage of building a bridge to cross a dry creek bed. The total bridge span is 7.5 meters but the bridge will have two trestles supporting the span part way in from each end. The middle span is the longest at 3 meters. I have determined that the heaviest load over the bridge will be 600 kg per meter so the longest span will have a maximum load of 1800 kg over it (not including the actual bridge weight) and I would like a safety factor of around three times on the bridge. Each end of the bridge will be supported by large rock abutments which are already in place.
See: http://www.smex.net.au/bouldercreek/Photos/Track21.htm
And: http://www.smex.net.au/bouldercreek/Photos/Track39.htm
I intend to use two “I” beams or parallel flange channels as the load carrying beams for the bridge - (one each side of the track)
What I need to know is what size beams will I need to use to carry the loads with a three times safety factor and with minimal deflection with that load?
I have looked as various strength calculator websites but the calculations seem as clear as mud too me Hopefully someone can help me out.

Regards,
Dave.

2. I can't help you with the figures but since your in to large model trains I have a site near me that claims to have the worlds largest 1/4 scale set up.
It is very nice.

3. Ok for a 3m span with 600kg/m
Thats 300kg/m per rail

So using a liveload factor of 1.6 we get 4.8kN/m lineload per rail.
So moment at midspan is 5.4kN.m
Using a 150UB14 (thats the smallest and lightest commonly used "I" beam)
Deflection would be 16mm at midspan, should be ok.
Moment capacity of 150UB14 is 23.98kN.m,
Safety factor for strength is 23.98 over 5.4 = 4.4

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