ATV/Riding Mower bridge using materials laying around...request thoughts

[cpy911]

I am wanting to build an ATV/Riding mower creek bridge with some of the materials I have lying around.
The bridge would be 16' long and 4' wide, with an unsupported portion of about 12'.

I have been toying working with a windfall fir tree that I cut in half for the beams. These are shown with dimensions in the images below. I have a bunch of 2X6 boards that were lying around for the decking. I also have PT 6"x8" "posts" that I could use for center supports, but not sure how well those posts will set in the creek (I could set them in concrete in the summer time when it is dry, but not sure how they hold up under constant water). What are your thoughts? The beams would be about 3'-4' apart with the 2x6 cut into 4' lengths going cross wise the entire distance.

 

[ovrszd]

Interesting project.

Free span. Do whatever you have to do to remain free span. Understanding that everything you add above the free span beams loads them. So the decking should be minimized as much as possible.

I built an ATV bridge using steel beams and steel decking. I also built a bridge I can drive everything I own across using steel I beams and wood decking.

I'm not smart enough to tell you how to calculate what's needed. Others will chime in with that information.

My biggest suggestion is to build the width in relationship to the load bearing capability. My ATV bridge is too narrow to drive anything across it that would overload it. My larger bridge is 8.5 ft wide. So for anyone that stumbles onto my bridges they can't overload them.

I'll post up pics for reference as you need them but don't want to "swamp" your thread.

 

[Doorman63]

Here’s mine

 

[jaxs]

Your beams are more than capable of spanning 16' and supporting 2k in live weight. No way of predicting how long before rot weakens them. If you have White Oak it lasts much longer than Fir. Redundant load bearing can be had with couple scraps of 6"- 8" cee purlin at middle. Center support posts aren't neccessary so I wouldn't add posts that stand in water.

 

[Rustyiron]

Well my "seat of the pants" testing would be to support your 2 beams at a 12' span on some blocks, RR ties or something similar and make a ramp and makeshift platform for actually driving the heaviest of the 2 you mentioned and observe the deflection.
That should determine whether you test it the first time, or your brother in law.
Seriously that should give you a pretty good idea of the "beams" strength. Keeping it from rotting is another thing. Maybe a hillbilly flashing of self adhering roof under layment like "Winterguard" on top of the logs and under the decking.
I'd place the beams so that they're directly under the wheel track or as close to a happy medium if there's a big difference between the two.

 

[Doorman63]

2 utility poles
Approx 20’ span
Approx 5’ wide
Center piers of cutoff utility poles that were slathered with roofing tar

2x6 decking lagged too the poles

ATV/riding mower no problems at all

I clean the debris a few times a year that collects against the center pier.


Built approx 2yrs ago

 

[cpy911]

2 utility poles
Approx 20’ span
Approx 5’ wide
Center piers of cutoff utility poles that were slathered with roofing tar

2x6 decking lagged too the poles

ATV/riding mower no problems at all

I clean the debris a few times a year that collects against the center pier.


Built approx 2yrs ago

View attachment 783104

Can you tell me about your center pier a little more? Is it set into the ground, in the water?

 

[cpy911]

Your beams are more than capable of spanning 16' and supporting 2k in live weight. No way of predicting how long before rot weakens them. If you have White Oak it lasts much longer than Fir. Redundant load bearing can be had with couple scraps of 6"- 8" cee purlin at middle. Center support posts aren't neccessary so I wouldn't add posts that stand in water.

Can you elaborate on how the cee purlin would be used? Are they vertical, supporting the beams? I am not following. Thanks.

 

[Doorman63]

Can you tell me about your center pier a little more? Is it set into the ground, in the water?

Sitting on some flat slabs of rock
Limestone maybe? Not sure

Creek bottom is very rocky in that area

 

[Doorman63]

Better view of center pier

 

[cpy911]

Better view of center pier

View attachment 783107

Makes sense now. Thanks.

 

[cpy911]

Better view of center pier

View attachment 783107

How is your pier attached to the underside of your beams?

 

[Doorman63]

How is your pier attached to the underside of your beams?

Each pier is rebar pinned to the horizontal pole it’s under.

2x4 x brace between piers

2x4s horizontally between piers as well

 

[WranglerX]

Have two simple bridges, both based on two 4x6x12 foot timbers with 2x6 inch cross decking... Actual free span is about 10 feet.... It supports a 520 pound riding mower and 200 pound operator just fine.... Think I would go bigger with main timbers before trying to cross with 1400 pound tractor though....

 

[Oaktree]

I like doorman63's suggestion of utility poles. They'll support a lot of weight, and will likely outlast you before they rot, unlike the fir.
As far as center support (should you go that route), how big is the stream at its highest? A strong current or high level is going to put a lot of stress on those supports unless they're well anchored.

 

[ovrszd]

I like doorman63's suggestion of utility poles. They'll support a lot of weight, and will likely outlast you before they rot, unlike the fir.
As far as center support (should you go that route), how big is the stream at its highest? A strong current or high level is going to put a lot of stress on those supports unless they're well anchored.

Yep. I'd also worry about debris lodging against the center support.

 

[taem]

Since you've already got your stringers, this may not help, but around here (VT) there are a lot of skidoo bridges built with old trailer frames or mobile home frames. Cheap, rot resistant and strong

 

[Doorman63]

Yep. I'd also worry about debris lodging against the center support.

I just deal with the debris.
So far no “structural” issues

As far as bouncing…..
I’ve got another bridge-same length-approx 3’ wide just for foot traffic.
It’s is a bit bouncy……..I always comment to people……how many bridges do you have?

 

[cpy911]

Have two simple bridges, both based on two 4x6x12 foot timbers with 2x6 inch cross decking... Actual free span is about 10 feet.... It supports a 520 pound riding mower and 200 pound operator just fine.... Think I would go bigger with main timbers before trying to cross with 1400 pound tractor though....

Your bridge frame is about the specs of my beams (smaller beam is 5" on one end and 7" on the other, similar to a 4x6). Free span will be about 12'.
I am sure you used PT beams.

I am thinking of burning the outside and then putting a natural oil on the fir beams to help preserve them. Then, also add a pier or support in the middle (I understand the drawbacks) as the fir beams will lose strength due to not being pressure treated.

I have tried to find utility poles around here, but no luck.

 

[cpy911]

Since you've already got your stringers, this may not help, but around here (VT) there are a lot of skidoo bridges built with old trailer frames or mobile home frames. Cheap, rot resistant and strong

I have looked at the trailer route, and may do another bridge based around a trailer, but people around here want a mint for their junk. Part of the west coast extra cost tax I guess.