Bridge Question

[Sparks45]

At the other end of the spectrum, we're building two houses and the county requires a bridge that will support a fire truck (they also made us put in sprinkler systems but that's another story). The 10' walls sit on 3' footers. Span will be 20'6", with 15 12"x12" oak beams. Not sure what decking will be but asphalt will cover everything (also a county requirement).
Charlie

 

[EddieWalker]

The exact distance of the span is very important in determinin what sized lumber you need and how many you will need. Then determining what you will build as af footing to support the load of the bridge is needed to understand how the lumber will be assembled. It doesn't cost more to do it right, but it will cost a lot more to cut corners and do it quick.

Eddie

 

[NS Gearhead]

Ns gearhead
How are your boards attached at the end of the span?
As you drive a heavier vehicle across bridge , it appears the deck would fail before the beams would

Nice bridge for a four wheeler

It's made with 3.5" gal. framing nails. I made it like a deck, just beefed up where it needed to be. The layout should see the path of the tires travel over the decked beams, so in that instance the deck doesn't have to be that strong... although some room for error is always good. For example mine's decked with 2x6. When I built it I had an offroad Tracker that weighed about 2,500lbs and was right around as wide as the bridge. If I'd driven 4" off center the 2x6's could have supported that... much more and I'd be off the bridge anyway. LOL The center only needs to be strong enough for foot traffic or a dirtbike (in my case).

 

[NS Gearhead]

I'm not an engineer, but I have to question the soundness of that design. Both the beam layout and the manner in which is the secured to the banks.

Here's what I've been using for the passed two years


I think I'll be ok

Oh, and that little 2x8 frame, 2x6 deck, 12x6' bridge cost me just under $500

 

[Egon]

Can't help on the design but have a few suggestions .???

Treated lumber
There are proper bolts for timber construction.
Consider glueing plus bolting the beam members together.
Consider flanges on the beam. They will add to the allowable bending stress.
Lateral bracing, execially at center span.
After the decking add longitudinal planks where the tires go.

Consider gluelam construction with narrower boards stacked vertically and glued and with a deeper depth. Can be cambered. Can be bought also.



The rings provide more surface bearing area than a bolt.

 

[KennyG]

Here is the thread where I built a similar size bridge:

http://www.tractorbynet.com/forums/projects/289644-my-bridge.html

Beam tables are available on line to check the capacity. My thoughts: I would have the beams supported directly by the footings and not depend on connections or bolts. You can add more beams for the cost of a few bolts. Any bolts/nails, use galvanized or stainless. Use treated wood but realize the strength is slightly less than untreated wood.

Flanges were mentioned above. Not appropriate for wood construction. Flanges don't add much vertical strength, They are only meant to stop buckling on narrow section members. For timber construction use blocking instead to prevent buckling or lateral movement.

 

[RedNeckRacin]

I think the OP needs to clarify some things for us.

The span is critical! No beam calculations can be done without it. Using wood over a 9 foot spam vs a 12 span really effects design criteria. Torsional rotation becomes more likely, weight of the decking will become more of a concern, and the way you transfer the load to the foundation might become a factor. If the wood fibers begin to crush at the foundation, it makes no difference what the beam calculations say. Generally, the deflection controls the span btw. Also, if this bridge is going to be in service for a long period, water needs to be considered so the wood stays as dry as possible. If the lumber is laying in puddles, of water, trouble won't be to far away.

 

[Ken45101]

At the other end of the spectrum, we're building two houses and the county requires a bridge that will support a fire truck
Charlie

A good idea, IMO. When I was fire department pre-plan officer, I made the decision that we were not going to take our trucks across a certain bridge. The house did eventually catch fire and we had to fight it with the trucks on the Interstate using tankers and long lines.

Of course, if you can get a 20 ton fire truck across, you are on the road to providing access to moving and delivery trucks, etc. I've had a number of times where trucks were afraid to cross our (concrete) bridge but we have had loaded concrete trucks across it many times.

 

[Egon]

Here is the thread where I built a similar size bridge:

http://www.tractorbynet.com/forums/projects/289644-my-bridge.html

Beam tables are available on line to check the capacity. My thoughts: I would have the beams supported directly by the footings and not depend on connections or bolts. You can add more beams for the cost of a few bolts. Any bolts/nails, use galvanized or stainless. Use treated wood but realize the strength is slightly less than untreated wood.
Reader than the shear
Flanges were mentioned above. Not appropriate for wood construction. Flanges don't add much vertical strength, They are only meant to stop buckling on narrow section members. For timber construction use blocking instead to prevent buckling or lateral movement.

The flanges add a lot to the sectional modulus of the beam. Simple span the bending modulus will usually be greater than the shear stress.