RonMar said:
Span engineering can be tricky and can make or break the engineer that designs them. How many joices, spaced how far apart? How long do you intend on using it? Who is going to be qualified to inspect it for safety? How long are you willing to accept liability for it's inspection, safety and use? A horse is a difficult engineering problem in itself. Since it's weight is concentrated in a small area at the hoof, you will need heavier decking, adding to the overall stress borne by the span. 16' sounds like an awfull lot for your 3X10 beams. How deep is this stream that you want to cross. How deep is the canyon that it has carved(how high will bridge deck be above water)? What is the peak and average water volume(depth and width of the water throughout the year). how do your local codes apply to bridging streams?
Would it be possible instead of a traditional bridge span, to lay large gauge culvert pipe across the stream bed and then fill in over the culvert with gravel and stone on the flanks? This is a modern variation of some of the oldest stone arch bridges on the planet. Constructed by the Romans, there are still some in existence today. IF the stream flow and local codes will allow it, it would probably be the safest long term low maintenance solution.
large stepping stones and a foot plank for the pedestrians, and the horses and ATV"S get to get wet fording the stream is also another option.
I personally would probably pass on the liability of a traditional span for the situation/use you described.
Good Luck
Unless he is opening this up to the public, his liability is pretty limitted. I did run some math and I am not confident the even 3 beams of double 2x10's would be enough to satisfy me. At least no one suggested a house trailer frame yet!
This is what I looked at:
3 beams each constructed of 2 2x10's , each wieghing around 120 lbs
A wood deck 5' wide x 3" thick wieghing 700 lbs
2500 lbs additional load
Total load per beam = 1200 lbs
The beam deflection exceeded .5" with that load, I was looking for less than that based on an L/360 deflection.
I think you need to go a little heavier. As far as the small foot print of the horse goes, the concern there would be shear forces
I am around horses some so this isn't a blind guess.
A horse hoof is say around 12 sq in.
The horse and rider is probably in the 1500lb range together
The load per hoof is 375 lbs
The ground force is around 30 lbf/sq in.
I would not hesitate to build a wood deck, I would inspect it for rot every year, but it should be fine. I did not try any math with larger beams, but 2x12's seem a better choice. bolt them together with two bolts every 20-24" and it should act as a single beam. I am not a PE, but I am a mechanical engineer and I disagree with the attitude of most engineers that it is too risky to give advice.
