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Author Topic: Bridge Design Help  (Read 508 times)

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Offline toll.grant

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Bridge Design Help
« on: February 05, 2019, 02:40:50 PM »
Howdy, 
New to the website --- but love everything I'm reading and learning. I need help with a bridge design, or at least need folks to provide some insight from past experience. Looking to build a bridge over a creek on the back 40. Will only be used for my tractor(s), mowers, maybe my pick up. So that puts me in the 9000-10000 lbs max range. The span needed is 22-24'. Original discussions led to a design that had 6 precast concrete abutments on each side; 2 - 14" web steel I beams, and 4x decking. Probably overkill. Unfortunately, without a service road, no large track hoe is going to try to trek down to the creek. So I'm left with my own two hands and my tractor. My new design calls for 2 - 24" inch diameter sonotubes fill with concrete, 4' below ground as the footers on each side of the creek and then two glulam 6" x 18" x 24' beams, spanning the distance with again 4x used as the decking. I've never used a glulam beam before, nor can I do the load calculations. Has anyone worked with them before, thoughts on this design? The footers could be 6 or 7' apart... i'm open to whatever holds the weight. Need a third beam? 

I appreciate any insight. 

Online Old Greenhorn

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Re: Bridge Design Help
« Reply #1 on: February 05, 2019, 03:18:53 PM »
You will want to get in touch @nybhh  as he has already built just what you speak of. He has a thread here somewhere with photos. I visited his pace last month and saw it, he made it a bit heavier than your to handle an excavator, and he made precast abutments. Go look that thread up and you will be off to a good start. He milled all the lumber for his beauty.
 Good Luck, and have fun here on the Forum!
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Online Old Greenhorn

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Re: Bridge Design Help
« Reply #2 on: February 05, 2019, 03:30:09 PM »
Here is the link to that thread. Also I forgot to mention that @doc henderson knows a thing or two about bridges himself. You will likely find much help here from the smart folks. It would be helpful if you filled out your profile info. Nobody knows where you are and it just might be that someone is local to you that can offer help or have something he/she did that you can go look at. Happens all the time here, but if nobody knows where you are...
 Again, welcome and Enjoy
Tom
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Offline nybhh

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Re: Bridge Design Help
« Reply #3 on: February 05, 2019, 06:17:02 PM »
Hi Grant.  
Disclaimer:  I'm not a professional engineer and know just enough to be dangerous ;).

Every time you double the span, your deflection increases by 8x with the same size member so a 24' span is going to have 3x the deflection of the 16' span I designed for if using the same size/species.  In my case, had my span been 24' instead of 16', my (4) 10" x 10" beams would have had to increase to (4) 14" x 14" beams to support the same load with the same deflection.  (4) 12x12s would still pass L/360 though.  

The main reason I used concrete was to get my wood timbers up off the ground for decay resistance.   If you have plentiful access to white oak, I'd suggest using that for longevity and although white oak is stronger, it still requires a 13x13 white oak to match the same deflection of a 14"x14" EWP at those spans.

Hope this helps.  Good luck!  Post pics.



Woodmizer LT15, Kubota L3800, Stihl MS261 & 40 acres of ticks trees.

Offline Don P

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Re: Bridge Design Help
« Reply #4 on: February 05, 2019, 09:54:55 PM »
Some resources;
Maybe start with chapter 7 here,
http://www.dot.state.mn.us/bridge/pdf/insp/USFS-TimberBridgeManual/index.html

Glulam/steel substitution tables, roughly, this is for buildings rather than bridges but gives somewhere to start;
http://www.aitc-glulam.org/substitution.asp

Individual footings on creekbed soil should be thought through carefully for bearing size and whether or not they will settle differently.
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A craftsman uses his brain and his hands
An artist uses his brain, his hands, and his heart

Online doc henderson

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Re: Bridge Design Help
« Reply #5 on: February 05, 2019, 11:12:33 PM »
any pics of the proposed site.  what is the usual width of the creek.  What is the nider (lowest point relative to the sides, think 24 foot string line across and measure to the lowest point, prob. in the bottom of the creek) of the depression.  So is the plan for 24 feet with a creek that is 20 feet wide,  or is the creek 6 feet wide with steep banks. so 24 feet is to go flat across side to side.  Given what you want to cross (vehicles) , what is the smallest width that you would feel comfortable with.  

Offline toll.grant

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Re: Bridge Design Help
« Reply #6 on: February 06, 2019, 09:46:27 AM »
I appreciate all the responses. Quick follow up -- started to update my profile.I'm in Loveland, OH just outside of Cincy. Got a good chunk of land. I didn't plan for that to happen, but had to stop a neighboring developer from destroying my peace and tranquility. Figure the kids can go to junior college like their old man.  :D

Thanks for the thread for @nybhh... great to see someone else tackle this, already making me think. As for Doc's questions --- 24 foot is the span that gives me ample clearance for when the creek runs high out of the banks; which will happen ~5 times year. The creek bed itself is about 11-12 feet wide with 3.5-4 foot high embankments. It runs year round, but most of the time just a foot or two deep down in the bed. I've attached a couple pictures from this morning, obviously we are high right now, coming off our wettest year ever and having all the recent rains/snow melt. below is the link to my pics, I think the link will take you to them. In the pics you will see two twigs with yellow ribbons on the near side, that is where I want to have the bottom of my bridge timbers. Those twigs are 10 feet apart (they don't have to be). Across the creek you may or may not see the other two twigs, they do not have yellow ribbons, but they are 20 ft from the yellow ones. 

Some additional thoughts -- I looked into multi plate arched culverts as well, and still contemplating that approach, but would prefer to do a bridge if possible. I do have roughly 30 acres of hard woods, all varieties, but I'm truly a tree nut and hobby arborist and would have a tough time parting with a beauty, but maybe. With that said I do not have the milling equipment even if I did decide to harvest. I went the route of looking at glulam beams because of the costs and availability to me. I'd be open to buying timbers too. 

@nybhh I like that you made your abutments as pre casts up in your shop. Are they anchored in any way? Instead of me doing vertical tube footers, which would be 133 bags of 80lb concrete mixed onsite. I wonder if I could also do something similar. I have a 39 HP Kubota to work with. 

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Thoughts?
(I very much appreciate the guidance)

Offline nybhh

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Re: Bridge Design Help
« Reply #7 on: February 06, 2019, 10:47:14 AM »
Beautiful spot you have.  Yea, you have a lot more water there to deal with than I do.  I only built the bridge because I got tired of fording the creek when the water was up.  

I considered driving some long pieces of rebar down through my abutments into the ground but I ultimately decided that was a fool's errand with the rocky soil we have here and probably not necessary since they are buried in a few tons of gravel.  I did dig down a bit with my FEL to place, level and compact a well draining gravel bed to place the piers on and then placed 15 yards of gravel around them to build up my ramps so they aren't going anywhere as long as the water doesn't rise enough to flow against the side of the beams and wash everything out.

My concrete piers are 8' (96") long by 12" wide so 1152 sq inches of ground bearing surface.  Even at 10,000 pounds of load per pier, that is less than 10 psi so my approach was to spread out and "float" the load rather than concentrate it at vertical sono-tubes. Our soil is 75% rocks with a little bit of soil filling in the voids so I do everything I can to avoid digging post holes. Concrete weighs about 150 lbs per cubic foot and I pretty much designed the size of the piers around my loader capacity.  I did use 3 pieces of 1/2" rebar along the bottom of each one to help prevent cracking if there are small voids in the gravel or uneven pressure along the bottom and I think that is an important step, especially if precast.

I don't really know much about glue-lam beams, especially for outdoor applications, but I'm sure they exist with the quality of the glues we have today.  I don't see any reason why they wouldn't work if you they were designed for that use.  Another thing I considered was to put a layer of ice & water shield along the top of each beam to protect from water infiltration into the cracks that form as the beam dries but I didn't have any on-hand and didn't feel like buying any.  I figure I'll probably need to replace the decking every once in a while and may add the I&W when I replace the decking if the beams show signs of deterioration.  This bridge didn't really cost me anything other than a couple of weekends so I can redo it down the road if I need to.
Woodmizer LT15, Kubota L3800, Stihl MS261 & 40 acres of ticks trees.

Online Old Greenhorn

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Re: Bridge Design Help
« Reply #8 on: February 06, 2019, 11:13:15 AM »
Looks like you are off to a good start here Toll. I figured you would hook up with some experienced folks. I'll just sit back and wait for the progress photos now. :) :)
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Online doc henderson

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Re: Bridge Design Help
« Reply #9 on: February 06, 2019, 12:22:48 PM »
Our bridge was 6 feet wide, 36 feet long.  It had a vibration to walking so we added a cent cross brace that made it feel like the earth.  The old bridge we replaced was 20 f.  Each bank sloped down to the water so it is seen in the picture as dark areas under the current bridge.  Each side was built up so you no longer had to walk down and then back up the other side.  It was flat across from one side to the other.  4 poles across the long way.  Handle 4 wheelers no problem and we had no illusion of anything heavier other than a bunch of boys.



 




 




 

Online doc henderson

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Re: Bridge Design Help
« Reply #10 on: February 06, 2019, 12:25:27 PM »
This is at camp Alexander near Colorado Springs.  The soil is rocky.  No moving water now but muddy at the bottom, with any rain or snow melt, the center brace will be under water.  it was 6 x 6 pressure treated.

Offline Iwawoodwork

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Re: Bridge Design Help
« Reply #11 on: February 06, 2019, 12:52:24 PM »
Why build? have you thought about using the concrete hiway center dividers 36"H x 12'L for abutments at the ends then set a 24' to 40' truck flat bed across them, a fairly cheap bridge, rent an excavator for a day and it is in, I purchased a 40' container hauler trailer for $1500 a few yrs ago and last year priced used concrete dividers at $100 ea, the trailer will easily handle 15m lbs at 24 foot span, I have seen 40' flat freight trailers go at auctions in last couple years for $1000 to $2000. just torch off landing gear and axles and have a ready made bridge.

Offline Sedgehammer

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Re: Bridge Design Help
« Reply #12 on: February 13, 2019, 12:39:23 AM »
I 2nd what Iwawoodwork says unless you're looking for something that looks unique to your property.

and not far from you


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