Another Bridge Thread

[ballegre]

Hi All,

I've been designing a bridge to hold my tractor using information I collected from here. The overall design is 12ft long with beams 5ft apart matching the wheel width on the tractor. The beams are to be 2 x laminated and bolted 12ft long, 2x12s which will give me a 12ft long 4x12 beam on each side. According to this web site http://www.forestryforum.com/members/donp/beamclcNDS2.htm, should hold at least 3250 lbs at each point for a total of 13,000 lbs using No 2 loblolly pine.

My kubota L3010 with cutter and fully loaded front loader should never exceed this lb limit. However, the devil is in the details.

My problem that I am having is due to the way I want to set the bridge beams on the concrete columns.



I'm designing to avoid having wood touch dirt to prevent rot (and yes I'm using pressure treated wood). So the colums will be at grade (grade slopes down toward the small creek) and, as you can see from the diagram, I want to set the beams on the columns and cut out a short 2 ft ramps in the beam to get on and off the bridge.

So my question, to anyone who has an opinion, is by cutting out the ramps on the beam, have I weakened the entire beam (I assume to the 7" depth that I have indicated)??

Of course I would never hold anyone liable for any information they provide and release you from any and all legal responsibilities for any opinions you offer

 

[bigtiller]

I'm not an engineer but it doesn't look like a good idea to me. How about building a separate ramp to the bridge?

 

[swick1]

I would build a seperate ramp and dig out your approach. Backfill with crusher run gravel and compact it. This will keep water away from the wood. Make sure you have a swale, ditch, or curtain drain to keep water away from the bridge approach a few feet before the crusher run

 

[radioman]

Yes I have a big problem with that. You need to recalculate everything based on 7 inch beam, not 12. The compression strength is at 7 due to the cut of slope regardless what size beam you are using. I would move the start of the slope at thee end of the concrete column to get the 12 inch beam strength you wanted. I know you wanted a nice smooth ramp up - for tractor, but you are better off using crusher run for the ramp to have a smooth ramp up, but still allow for drainage.

 

[ballegre]

I'm not an engineer but it doesn't look like a good idea to me. How about building a separate ramp to the bridge?

Wanted to avoid a separate ramp because of the extra work. I can do it if it's necessary.

 

[ballegre]

I would build a seperate ramp and dig out your approach. Backfill with crusher run gravel and compact it. This will keep water away from the wood. Make sure you have a swale, ditch, or curtain drain to keep water away from the bridge approach a few feet before the crusher run

A 12 inch deep approach seemed difficult to me and I didn't even think of the drainage issue (thanks!) which makes it even more difficult.

 

[swick1]

A 12 inch deep approach seemed difficult to me and I didn't even think of the drainage issue (thanks!) which makes it even more difficult.

If you have a loader this should only take a day. Use it to truck in your materials. Consider renting a skid steer or full size backhoe for a weekend to help.

 

[ballegre]

Yes I have a big problem with that. You need to recalculate everything based on 7 inch beam, not 12. The compression strength is at 7 due to the cut of slope regardless what size beam you are using. I would move the start of the slope at thee end of the concrete column to get the 12 inch beam strength you wanted. I know you wanted a nice smooth ramp up - for tractor, but you are better off using crusher run for the ramp to have a smooth ramp up, but still allow for drainage.

I did calculate at 7 inches and it does not hold. So I won't be going with this design.

 

[bcp]

I would do this:



Bruce

 

[ballegre]

I would do this:

View attachment 320705

Bruce

Thanks. I like this. Minimizes the amount of digging and solves the anchoring problem which was my next question.