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Discussion Starter · #21 ·
Didn't have any balsa, or the patience to model it. Just drove around and looked at other small bridges and went up a size or two on the lumber and added a piece. The heaviest thing will be my wife and her lawn tractor, and she is heavily insured.


Later
Rich
 

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Sure, picture a heavy load at the center point of the crossing. SOME load should be delivered up the vertical to the intersetion of the 2 diagonals and distributed out towards the ends of the structure...

Did you model this out of balsa first? :laughing4-giggles:

I love it.
I was thinking along those lines, but my only visual ideas were from suspension bridges which load like the cables directly hung from the towers.

So, what you saying basically is once the load is at the center of the bridge, when it bears down the force of the weight is sent "up" through the verticals and then spread out by the diagonals. Almost a direct reversal to having an arc underneath the bridge to bear the weight? :confused1:
 

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I was thinking along those lines, but my only visual ideas were from suspension bridges which load like the cables directly hung from the towers.

So, what you saying basically is once the load is at the center of the bridge, when it bears down the force of the weight is sent "up" through the verticals and then spread out by the diagonals. Almost a direct reversal to having an arc underneath the bridge to bear the weight? :confused1:

SOME of the load is transferred up/then outboard, exactly.

An arch style support from below accomplishes the same (distribution) thing but in many cases transfers the load to earth rather than to other portions of the structure. Arch style is much more capable than A-frame in most cases.

http://gstructures.com/images/bridge.jpg
http://www.westcoastroads.com/california/images151/ca-154_cold_spring_bridge_10.jpg
 

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Very cool project, Rich! Thanks for posting the progress and pics. All you need now is a troll to live under there and recoup your costs.
 

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SOME of the load is transferred up/then outboard, exactly.

An arch style support from below accomplishes the same (distribution) thing but in many cases transfers the load to earth rather than to other portions of the structure. Arch style is much more capable than A-frame in most cases.

http://gstructures.com/images/bridge.jpg
http://www.westcoastroads.com/california/images151/ca-154_cold_spring_bridge_10.jpg
Okay... got it. The Romans were one of the first to use the Arches to construct their aqueducts. It is based on the theory of a circle of the eggs. It it hard to crush a circle or arch onto itself - this actually strengthens it.

However, I believe for this construction, Rich picked the right design. An arch would have been significantly more work, and more cost. He would probably have had to do some work on the embankment too to ensure properly placement of an arc.

Thanks!
 

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Tirone is my middle name
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Wim, sounds like you got it but think of a heavy load in the center of just the flat bridge, and the bridge is bowing down in the center under the load. now add those diagonal pieces and visualize how those would move if the center was bowing down (they would tip in towards each other). they push against each other, keeping the decking flat, instead of bowing downward.

Awesome job Rich
 

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Discussion Starter · #28 ·
Thanks for the good words guys. My dogs are leary of it, with 1 1/2 inches between the boards they can see the water flowing below. So they are very cautious, funny to watch.

Later
Rich
 

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Warning... I dont do wood design but I am a structural engineer... so I do have a clue.

Figuring your exact safe load on there will need the wood type, location of any joints in your 2x12's, bolt patterns, and exactly HOW you bolted the A frame supports to the outside beams. Honestly the A frames aren't doin you a bunch of good unless they are pinned pretty good and personally I would be more worried about the 2x12's not being flat against each other in the 'beams' which MIGHT allow for a bit of twisting. If you nail/bolt (2) 2x12's together they are hella strong. Now put a 1.5" gap inbetween them with nothing really keeping them acting as one... and it weakens your design. Deck boards will help that a bit - but only if you nailed each deck board into each 2x12 at each crossing. Typically fasteners are cheap, wood isnt. Unfortunately from your pics... I cant see ANY bolts.... and how the spaced 2x12's are being held ontop of the pilings. Just from the picture... a small side load will spell dissaster for that bridge. I am PRAYING there is a clever connection in there someplace. Its looks clean... I just hope its sufficient.

Thats the bad news...

The good news is... your good for several 1000 #'s safely.

Like I said I would have sammiched those 2x12's together... came up with some decent heavy duty 'curb' along the edges... I saw a really cool bridge at the golf course in Florida that had about a 12" high curb that was STOUT. If you hit it with a golf cart head on... it would stop you dead!

Lastly I would make the deck boards closer as well to try and assist with any twisting you got going on.

Let us know how the wife fares on her first test trip accross!

FILM IT for insurance purposes....
 

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Discussion Starter · #30 ·
The 2x12s are nailed approx every 6 inches along the edges and 8 inches down the middle.I pipe clamped them together whilst nailing.The decking is 2x6 and at each crossing there are 3 nails for a total of 9 nails per board.This weekend I was going to see how much deflection I get with 1000lbs in the middle.Like I said this is for mainly a 200lb rider and a 250cc dirt bike. I doubt that the lawn tractor is more than 300lbs and a lady never discusses her weight.
Later
Rich
 

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.... and how the spaced 2x12's are being held ontop of the pilings. Just from the picture... a small side load will spell dissaster for that bridge. I am PRAYING there is a clever connection in there someplace.
I thought I saw an "insert tab A into slot B" type system when I questioned the same... The piling has a 2X(Y) vertical extension which passes into a void in the beam(s) ... looks pretty clever!

Rich would have to confirm.
 

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Wim, sounds like you got it but think of a heavy load in the center of just the flat bridge, and the bridge is bowing down in the center under the load. now add those diagonal pieces and visualize how those would move if the center was bowing down (they would tip in towards each other). they push against each other, keeping the decking flat, instead of bowing downward.

Awesome job Rich
Thanks Matt! Totally makes sense to my non engineer mind. So, the diagonals add support and load bearing by essentially being compressed together. So, to figure out load bearing, one could calculate how much compression force the beams could bear individually, multiply by four and you have a good estimate of how much the bridge could bear in the middle when loaded. Sound right? :confused:
 

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Warning... I dont do wood design but I am a structural engineer... so I do have a clue.

Figuring your exact safe load on there will need the wood type, location of any joints in your 2x12's, bolt patterns, and exactly HOW you bolted the A frame supports to the outside beams. Honestly the A frames aren't doin you a bunch of good unless they are pinned pretty good and personally I would be more worried about the 2x12's not being flat against each other in the 'beams' which MIGHT allow for a bit of twisting. If you nail/bolt (2) 2x12's together they are hella strong. Now put a 1.5" gap inbetween them with nothing really keeping them acting as one... and it weakens your design. Deck boards will help that a bit - but only if you nailed each deck board into each 2x12 at each crossing. Typically fasteners are cheap, wood isnt. Unfortunately from your pics... I cant see ANY bolts.... and how the spaced 2x12's are being held ontop of the pilings. Just from the picture... a small side load will spell dissaster for that bridge. I am PRAYING there is a clever connection in there someplace. Its looks clean... I just hope its sufficient.

Thats the bad news...

The good news is... your good for several 1000 #'s safely.

Like I said I would have sammiched those 2x12's together... came up with some decent heavy duty 'curb' along the edges... I saw a really cool bridge at the golf course in Florida that had about a 12" high curb that was STOUT. If you hit it with a golf cart head on... it would stop you dead!

Lastly I would make the deck boards closer as well to try and assist with any twisting you got going on.

Let us know how the wife fares on her first test trip accross!

FILM IT for insurance purposes....
Hey Mid - Sounds like overall, Rich did a great job on this thing. From what he says, he has at least a 200 Ib+ buffer zone as far as weight goes based on your 1000 Ibs load ability.

So, I understand this better. You are concerned that the horizontal beams which are the main support might flex laterally? I don't know how that might occur since all the load will bear "downwards" when the bridge is being used. Educate me brother. Also, what if he either:
1/ Puts beams or wood between these supports and then pass a bolt through the three and torque it together?

2/ What if he could like a "U" shaped metal thingy, place it upside down between the beams and bolt them together thus minimizing or eliminating the potential for lateral flex? Am I making any sense here?

WIM.
 

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Instead of using all that complex civil engineering theory, run this experiment. Keep adding 500lb to the center and take a deflection measurement each time and continue until failure. With the data I will derive a very accurate curve fit model. Then rebuild it exactly the same way it was. :lol:
 

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WIM, this is for you!!

Follow this link and download the bridge builder game. You'll see a link in the center of the page near the top. DL the 2006 version, i've confirmed it will run on Vista64bit so it should run anywhere.

http://www.bridgebuilder-game.com/


Here is a direct link: Free download Bridge Building Game (2006)

It's lots of fun. Pay attention to the stresses, shown in red, when you "stress" the bridge. Then add additional supports and watch them redistribute the load. Test it when you think it's strong enough then run the train over it.

Have fun!
 

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Il bambino e un cani
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Looks pretty nice...
My suggestions would be...blocking between the 2x12 beams beneath the deck...every 6' would probably do (just like joist blocking in a house to prevent twist).

I'd consider adding some trussing on the outside of the outer two main beams, but extending from your peak pole, closer to the support piles, with 45's off the middle of those back to the base of the peak pole.

32' is a hella span for built up dimension lumber beams. They've got quite a load just supporting their own weight.

Here's what my reference data says:

For built up wood beams, supporting a live load of 40 psf:
Max span listed ....20', requiring TWO - 3" x 14", spacing 6'

For Glue-Lam beam, supporting live load of 40 psf.
32' span ... spacing 8', requires 5" x 19" beam

Ok, first you probably don't need 40 psf, that's typical house floor rating. But if you were building based on 40 psf, and figure you fully loaded the center 15 feet of the span, you'd be looking at 3000 lb live load (plus the weight of materials). ( 5' wide bridge x 15' of length x 40 psf). If it was my bridge I'd want to be able to carry about 1500 lbs, spread evenly at the center of the span. I'd say you are in the ball park....but those built up beams bother me. (gluing them would have been excellent).

If you really wanted to ensure some seriously safe capacity..... go down to the creek bed....say 6' or so below, and a few feet out from your support piles, and make two more support piles on each side of the creek, run some 4 x 4 beams from those piles to the center, underside of each side of the span. Angle the new piles so the 4 x 4 beams are pressing the piles into the ground parallel with the piles. Add a brace or two on the 4 x 4.

As time goes by and things settle, you could come up with a way to drive wedges, or jack the 4 x 4's to bring the center of the span back up to level.

I can do some basic drawings if you want.

You are close I think....but bear in mind you already have probably 1000 lbs of load on the center of the span from the weight of materials. You add a couple hundred pounds of moisture to the wood, get three guys jumping up and down on the center of the span....and I think you could have a problem. The deck design is good....but you need some means of transferring more weight from the center of the span directly to the support piles, either through more extensive above deck trussing, or some below deck supports to addional piles.

Either way...you've already done the hard part.
JohnnyB
 

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Discussion Starter · #39 ·
Johnny B

For Glue-Lam beam, supporting live load of 40 psf.
32' span ... spacing 8', requires 5" x 19" beam

Are you saying the 5" x 19" beams are 8 feet apart for a 32 foot span?
I have 3 triple 2x12s approx 28 inches between them.
Boy where were all these suggestions when I asked if there were any engineers hanging around?
This weekend I'll see how much deflection I get with 1000lbs on board.

Thanks for the input.
Later
Rich
 

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I'm mechanical, but I haven't done load analysis in so long I'd be better off to tell you to build two ramps and jump the damn thing.
 
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