Help with bridge planning

[old_guy]

My daughter and her family just bought their first house.  It is in a nice neighborhood in Marietta, GA.  We visited them from Michigan over Thanksgiving and rebuilt the deck.  Next on the list is a bridge over the stream which separates the lot, isolating them from 1/2 of the back yard.  We intended to build a bridge (60" wide) from 20' treated 2x12s resting on a footing on each side of the stream.  An inspector came out to answer some questions and said that the bridge could not be placed on the only narrow part of the stream, as there is a sewer line crossing the river there with a fairly wide easement.

We hope to build them a bridge next summer, and it will need to be approximately 30' long from footing to footing.  I would like to hear some suggestions on how to span that distance without breaking the bank.  The bridge is planned to be 60" wide to accommodate the lawn/garden tractor, will need railings with 4" or less between spindles(2 young children), and cannot have any mid-span supports in the riverbed.  If I could build sub-assemblies at home and transport them there to reduce on-site time it would be better, but is absolutely not necessary.

I have some ideas but they seem ungainly and/or, uh, hideous.  Any suggestions or guidance will be welcome.

Thank you.

[busted knuckles]

Steel beams?  http://atlanta.craigslist.org/eat/mat/4751820927.html
http://atlanta.craigslist.org/nat/mat/4717002971.html

[Don_P]

In wood, LVL's ~18" deep every foot across as the deck joists should be close. The supplier can check the beam sizes. Keeping that dry would probably require a covered bridge approach. At which point they usually built a truss. I'd bet the truss shop could design a pair of ganged parallel chord trusses ~8' tall x 30' span spaced 5' apart that would form the sidewalls and main structure. Cross joists the 60" way and ~7' trusses on top for a roof.

[MushCreek]

I built a bridge from plans from www.mathemati.ca. Interesting, pretty bridges, and they are claimed to very strong, but if it has to be inspected, I don't know how you would prove that it is strong enough. They have plans for bridges up to 50 feet.

[Don_P]

Neat, that looks like DaVinci's reciprocal bridge  .

[Adam Roby]

Yeah, I was thinking triangles but could not give any design feedback so didn't reply.  This seems to use triangles in the way I was thinking (sort of). 

It would be a good idea to look at local businesses that might do this kind of work to get an idea of the cost.  While you are there get a look at what they do and if you decide to build yourself, borrow from their design.

[old_guy]

I built a bridge from plans from www.mathemati.ca. Interesting, pretty bridges, and they are claimed to very strong, but if it has to be inspected, I don't know how you would prove that it is strong enough. They have plans for bridges up to 50 feet.

Thanks, Mushcreek.  I had looked at their bridges and plans.  Many pictures of completed bridges, interesting design.  Bridge does need to be permitted and inspected.  We will find out what they think of these.  You built one.  Did it feel strong?

[Don_P]

Off on a tangent, this is a fun, simple, truss design program.
http://pages.jh.edu/~virtlab/bridge/truss.htm

[MushCreek]

We had about 10 people on it, and it didn't even wiggle. Mine was a little 15 footer made out of 2X4's.

[MushCreek]

Just remembered- this kind of bridge requires a substantial footing on each end, as there tends to be a lot of thrust with a load.

[old_guy]

Just remembered- this kind of bridge requires a substantial footing on each end, as there tends to be a lot of thrust with a load.

Excellent point.  Most arched bridges require good footings to resist thrust.  The bridge cannot sag as long as the ends of the arch cannot move.  The thrust resistance of the footings could be replaced by some sort of "tie" connecting the 2 ends of the bridge - rather like wall ties resisting the thrust of rafters.

[old_guy]

In wood, LVL's ~18" deep every foot across as the deck joists should be close. The supplier can check the beam sizes. Keeping that dry would probably require a covered bridge approach. At which point they usually built a truss. I'd bet the truss shop could design a pair of ganged parallel chord trusses ~8' tall x 30' span spaced 5' apart that would form the sidewalls and main structure. Cross joists the 60" way and ~7' trusses on top for a roof.

Since LVLs cannot be left out in the weather (hence the cover), what about glulams?  I believe that they are available treated for this type of application.  A single glulam beam on each side of the bridge, connected by joists running side to side to support the decking and prevent the beams from twisting, might be the ticket.  Getting the beams in place across the river would be a challenge.  Heck, if it was easy anybody could do it.

[Don_P]

I have seen them used in some pretty impressive bridges.
This is the glulam tech folks, there are span tables here;
http://www.aitc-glulam.org/capacity.asp
More stuff here, I think there is deck strength and layout info;
http://www.aitc-glulam.org/Shopcart/index.asp

[old_guy]

I have seen them used in some pretty impressive bridges.
This is the glulam tech folks, there are span tables here;
http://www.aitc-glulam.org/capacity.asp
More stuff here, I think there is deck strength and layout info;
http://www.aitc-glulam.org/Shopcart/index.asp

Thank you, Don.  I had seen those capacity tables, but got distracted and missed the technical info.  Glulams, or a truss of some sort with the top chord at (or near) railing height, sound the most appealing right now.  I have now found many beautiful examples online of bridges with glulam beams.

There are a couple of months available to continue on my ignorance reduction quest, then a couple more for final planning and permits.  Many thanks to those who responded, and to the forum in general, for helping me get pointed in a productive direction.

[Desiredsilver]

The cheapest way to do this is to find a 70ft mobile home that needs tore down and take the frame Cut it to the length you need. you can put the bridge pillers as far back as you need with a 70ft frame.

You may get lucky and find a frame on craiglist.