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Those are the best bridges, and they're an easy take down.
Any Engineers?
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Those are the best bridges, and they're an easy take down.
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It would have made me more comfortable if the 2 by 12's would have been bolted together rather than nailed. I don't think I'd send my wife across there for the first test.
Rich there wasn't a whole lot of time between the time you asked for help and the time you had it built.
Rich there wasn't a whole lot of time between the time you asked for help and the time you had it built.
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are the beams actually (3) 2x12's each the whole way. I'm having a hard time telling from the pictures. The last picture we can see at the end there is a 2x12, a gap, and then another 2x12 for each of the beams. Is there another 2x12 in the center of each beam after where the support is, other than where the supports come into it?
adding blocking underneath and connecting the tops together would help a lot if you think you need to add more to the bridge.
adding blocking underneath and connecting the tops together would help a lot if you think you need to add more to the bridge.
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OK the center beam is 3 2x12's all the way except for the last foot the center is open to slip over the notched 6x6.The 2 end ones are triple 2x12s up to the diagonals which is approx 2ft ( or less ) from the end.The diagonals are 2x8s.
And Sandy has crossed it already that's her in the pic.
Later
Rich
And Sandy has crossed it already that's her in the pic.
Later
Rich
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Only single file and we only have one atv the Kodiak. Kids are pretty much
restricted to mountain bikes. My youngest is 21, I've been more worried when
they ride the dirt bikes through the creek because they are always flipping over.
At least they always wear full gear.
Later
Rich
restricted to mountain bikes. My youngest is 21, I've been more worried when
they ride the dirt bikes through the creek because they are always flipping over.
At least they always wear full gear.
Later
Rich
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Rich,
What that spec is saying is that in order to build a floor that supports 40 psf, over a 32' span would require one 5" x 19" beam every 8'...or in your case...2 beams, one for each side.
A 5" x 19" glue-lam is a tremendously strong and rigid, also tremendously heavy and expensive. Specs for built up dimension lumber beams don't even go beyond 20' for spans....which should give you an idea of their suitability for longer spans.
The gap in the middle of the 2x12 beams you build up is....I hate to say...at the worst possible location. Typically in the triangulated overhead bracing like you are using...the vertical center members would go outside the deck...and connect to a beam the crosses the middle of the deck supporting the 2x12 beams from underneath...then the two "45's" would go from the top of the center vertical and connect to the side of the deck as close to the support piles as possible.
The problem with the overhead bracing is it put the lumber braces in compression load with no bracing....the weight of the bridge pulls down on the center pole, the center pole transfers the weight to the "45's", the 45's bend under this load without bracing (like the wall of a house with no sheathing). If all the bracing was on the underside of the bridge, the weight of the bridge would push down on the centerpole, the center pole would transfer the weight to the 45's by putting them under "tensile" load (pulling)...they would need no extra bracing if they are under tensile load...( imagine trying to push something with a string....then try to pull something with the same string...... same string...but it works way better in one direction than the other).
JohnnyB
What that spec is saying is that in order to build a floor that supports 40 psf, over a 32' span would require one 5" x 19" beam every 8'...or in your case...2 beams, one for each side.
A 5" x 19" glue-lam is a tremendously strong and rigid, also tremendously heavy and expensive. Specs for built up dimension lumber beams don't even go beyond 20' for spans....which should give you an idea of their suitability for longer spans.
The gap in the middle of the 2x12 beams you build up is....I hate to say...at the worst possible location. Typically in the triangulated overhead bracing like you are using...the vertical center members would go outside the deck...and connect to a beam the crosses the middle of the deck supporting the 2x12 beams from underneath...then the two "45's" would go from the top of the center vertical and connect to the side of the deck as close to the support piles as possible.
The problem with the overhead bracing is it put the lumber braces in compression load with no bracing....the weight of the bridge pulls down on the center pole, the center pole transfers the weight to the "45's", the 45's bend under this load without bracing (like the wall of a house with no sheathing). If all the bracing was on the underside of the bridge, the weight of the bridge would push down on the centerpole, the center pole would transfer the weight to the 45's by putting them under "tensile" load (pulling)...they would need no extra bracing if they are under tensile load...( imagine trying to push something with a string....then try to pull something with the same string...... same string...but it works way better in one direction than the other).
JohnnyB
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Just thought of a way you could add quite a bit more rigidity. Build a railing/wall about 30" high on both side of the bridge...just like the wall of a house, with studs 16" on center. Then sheath both sides with 1/2" plywood, with the outside layer of plywood overlapping the outer beams by about 4". You'd have to stagger the plywood like bricks in a wall so that there was no full 30" seams in the same spot.
That's basically how old time covered bridges worked...they used the stiffness of the sheathed exterior walls to provide the support for the deck surface.
I'd still opt for four more support piles down close to the creek bed with braces that run from there to the center of the span. I'm sure it would be a pain in the ass, but it would make for a very strong bridge.
What you have is also workable if you came up with some way to improve the overhead braces. One way would be to lay another 2x6 on top of the braces to they form a "T" cross section. The top of the "T" would keep them from flexing in one direction the bottom of the T would keep them from flexing in the other. The center pole would be in tension so that would be ok as long as it was fastened well at the top and bottom (bolts would be good).
Yes I love bridges.
JohnnyB
That's basically how old time covered bridges worked...they used the stiffness of the sheathed exterior walls to provide the support for the deck surface.
I'd still opt for four more support piles down close to the creek bed with braces that run from there to the center of the span. I'm sure it would be a pain in the ass, but it would make for a very strong bridge.
What you have is also workable if you came up with some way to improve the overhead braces. One way would be to lay another 2x6 on top of the braces to they form a "T" cross section. The top of the "T" would keep them from flexing in one direction the bottom of the T would keep them from flexing in the other. The center pole would be in tension so that would be ok as long as it was fastened well at the top and bottom (bolts would be good).
Yes I love bridges.
JohnnyB
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The most minimal setup I'd think would do the job. Should have a ridge board across the peak of the two trusses though....keep them from wobbling around. Also...the beam ends should be supported by another beam that goes from pile to pile.
JohnnyB
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I just looked and at 32ft... you dont have a continuous beam... I see now the joints in your fabd beams. Assuming you didnt calc where those would be best to place... You might have an issue there.
I understand NOW that you do have 3 2x12's gang nailed together but as JohnnyB said you stopped that before you got to the most critical point... the connection to the piling. I still want bolts at that connection too.
So now that the beams are mostly solid... inbetween them as they suggested still needs to have some bracing from a side load.
WIM - a side load and can be implied in the most interesting ways you hav enever thought of. Sure wind is an obvious choice... but even while delfecting the boards will want to twist and on their own... get to a sliding stage. Lets say your driving over th ebridge in the 4wheeler and hit the breaks. One side grabs harder than the other... instant twisting motion applied to the beams which arent tied together. Matter of fact... back to the nails at the conneciton points... they may slide right off the end with a directional load. BOLTS!
JohnnyB - LOVE the model!
I say take down that overhead support thingy you got going on and save the weight... put on some underneath lateral braces between the beams... use 2x12's since you can get them and do it right. And then get going on a railing system that will tie back and help as well.
I understand NOW that you do have 3 2x12's gang nailed together but as JohnnyB said you stopped that before you got to the most critical point... the connection to the piling. I still want bolts at that connection too.
So now that the beams are mostly solid... inbetween them as they suggested still needs to have some bracing from a side load.
WIM - a side load and can be implied in the most interesting ways you hav enever thought of. Sure wind is an obvious choice... but even while delfecting the boards will want to twist and on their own... get to a sliding stage. Lets say your driving over th ebridge in the 4wheeler and hit the breaks. One side grabs harder than the other... instant twisting motion applied to the beams which arent tied together. Matter of fact... back to the nails at the conneciton points... they may slide right off the end with a directional load. BOLTS!
JohnnyB - LOVE the model!
I say take down that overhead support thingy you got going on and save the weight... put on some underneath lateral braces between the beams... use 2x12's since you can get them and do it right. And then get going on a railing system that will tie back and help as well.
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JohnnyB and Midwest
Thank you for the time and thought you put into this.Johnny B that's quite a
rendering you did there it is very neat. What program did you use to do that
and how long did it take? I'll add some bracing as you guys suggest and see how
it goes.
The center beam also sits on a 6x6 in cement and I did intend to bolt those
together.
What would be considered acceptable deflection with 1000lbs sitting in the middle?
Again Thanks
Later
Rich
Thank you for the time and thought you put into this.Johnny B that's quite a
rendering you did there it is very neat. What program did you use to do that
and how long did it take? I'll add some bracing as you guys suggest and see how
it goes.
The center beam also sits on a 6x6 in cement and I did intend to bolt those
together.
What would be considered acceptable deflection with 1000lbs sitting in the middle?
Again Thanks
Later
Rich
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4,392 Posts
TJ--I'm having a time with a relatively simple (?) one. Level 10 on map pack 01.
Also, how realistic is this to actually building a bridge?
Help!
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Rich.... the more I look at the pics, the better it looks, you guys did a quality job on the construction. Which helps when design isn't the best.
I definately WORK UP, to 1000 lbs. I'd judge the load capacity by how much it deflects at lower weights. Start with about 200 lbs.
Bear in mind the defection won't be linear....it will deflect a decent amount at lower loads (say 1-2" for the first 200 lbs). Then the deflection rate will slow down....say only 3" at 400 lbs, then maybe 4" at 600 lbs. When it slows down it's finished taking all the easy deflection out of the bridge, proceed cautiously after that....listening and looking for signs of failure.
Definately put some blocking between those beams....if you put 1000 lbs in the enter of that span I think those beams will try to flip right over....just like they probably tried to do when you installed them.... it's where they naturally want to go.
I know I'd be confident walking across that bridge with another 200 lb guy no problem with some blocking. Add those blocks and something to stiffen up your upper bracing and it will probably handle 6-800 lb traffic with no problem
At least add some collar ties across the top of your bracing (like the letter "A").
Models were done with AutoDesk 3DS Max, took about 20 minutes.
JohnnyB
I definately WORK UP, to 1000 lbs. I'd judge the load capacity by how much it deflects at lower weights. Start with about 200 lbs.
Bear in mind the defection won't be linear....it will deflect a decent amount at lower loads (say 1-2" for the first 200 lbs). Then the deflection rate will slow down....say only 3" at 400 lbs, then maybe 4" at 600 lbs. When it slows down it's finished taking all the easy deflection out of the bridge, proceed cautiously after that....listening and looking for signs of failure.
Definately put some blocking between those beams....if you put 1000 lbs in the enter of that span I think those beams will try to flip right over....just like they probably tried to do when you installed them.... it's where they naturally want to go.
I know I'd be confident walking across that bridge with another 200 lb guy no problem with some blocking. Add those blocks and something to stiffen up your upper bracing and it will probably handle 6-800 lb traffic with no problem
At least add some collar ties across the top of your bracing (like the letter "A").
Models were done with AutoDesk 3DS Max, took about 20 minutes.
JohnnyB
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Johnny B
Actually the 'guys' was me.The extra hands were there only to move the beams into position.I think I went through a dozen cans of Cutters Deep Woods on
this project.
I'll let you know what happens this weekend.
Thanks for your input
Rich
Actually the 'guys' was me.The extra hands were there only to move the beams into position.I think I went through a dozen cans of Cutters Deep Woods on
this project.
I'll let you know what happens this weekend.
Thanks for your input
Rich
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Brev - There are plenty of clues in the form of sample bridges on the original site I linked to. As far as it's applicability to actual bridge construction... i'm not going there. :turned:
Some of the physics are accurate, so there is something to be learned for most people who play with it.
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I'll have to check out the other link. That is a very cool game! I'm hooked. And I should have been more concise by asking about the relative stresses that are shown on the game. Physics elude me, educationally speaking, so I was wondering about the accuracy of the weight-induced stress and how each member reacts under it's own weight, how the weight transfers, etc...
I figured it wasn't too real when a train crossed a bridge I had built, and continued up the side of the hill as the bridge was falling. I passed that level, though! :lol:
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I made one bridge that looked like tig bitties!
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