…in a good way. No splits have appeared, and it no longer creaks and gives conspicuously. Because it is an arch, the stresses are spread along the span, it is exposed to sun above and humidity below, and I think it’s kind of settling-in, in a carpentry sort of way. You no longer hear alarming sounds as you walk across it. I make several trips a day, tending to filter cleaning, etc, and hopefully it will be good as it weathers and ages. We’re not sure what its weight limit is, but so far it seems good for around 200 lbs, and is feeling steadier than it did.
oh phew I thought for a moment not in a good way. it certainly seems to save you time going around the pond!
Hi, CJ.
I ran some calcs on weight limit back when you were talking about the design ‘back when.’ If you can give me some specifics I can rerun those. But I will say first that the limiter is most likely the slats or treads or crosspieces (the bits you step on!), which ISTR you named as 1X4s. I would have recommended 2X4s as treads for nearly any span you named, BTW. One-bys are pretty bendy for people to walk upon… especially as a board advertised as a 1X4 is really about about 3/4 inch thick, perhaps less–and pine (or Doug fir) ain’t all that stiff to being with. How wide is the bridge?
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Anyway, if you want to, email or post with dimensions (actuals, if you please) of the thickness of the longitudinal timbers and the vertical dimension of those timbers where they leave the support of the footings at each end, that’s the weak spot for shear at least. Also I need the actuals of the slats/treads: width, length unsupported, and thickness. I can provide at least nominal strengths of the basic structure, but can’t vouch for the rigidity in torsion, for example, without a pretty detailed look at how it is built and supported.
Span Length, 12 feet, 2″ thick, actual structural shape about 6″ wide, carved out of a 2″x12″, with a foot-wide footing, ie, where it sits on the bank. So—6 feet from base to top of arc, 6″ wide, then descending 6 feet to opposite bank.
2×4 planks would gap between slats. 1x4s lie flat with a 1/8th inch gap.
Length of decking planks, 30″, with 1/2 inch overhang on either side, so effectively the planks are 29″ resting on the 2″ width of the arch on either side. Reality: the 3/4″ thick slats are resting on the 1.5″ width of the arch (due to the size lumber REALLY is). So the unsupported center of the slats is 24″, [real dimension 25″, by subtracting the inch of ‘cheat’ in an actual board thickness]. Because the slats are flat to walk on, ie, your weight can rest on more than one at a time, you are constantly supporting your weight on more than one slat at once, usually two with most of your weight and a third with more of it. Is that crazy enough? The screw, the stress point, actually is dead center of the 2×12 edge, and there are 2 in each slat end, 4 screws per slat.
In Jane’s pix, you can see the actual arch. We don’t have a protractor that size but we think it would take about 6 2x12s to make a full circle.
Some food for thought courtesy of the Mythbusters:
http://www.youtube.com/watch?v=7xODgR2FEKo
and perhaps even more relevant:
http://www.youtube.com/watch?v=-yof-pWr6Go
Who needs wood when you have Duct tape 🙂
Douglas fir is quite strong in addition to being flexible, but 3/4″ is minimal for unsupported decking and you should inspect it periodically for damage or decay.
I can see two failure modes for the overall structure. One is a failure at the ends of the span from either compressive or sheer forces. A short ~ 2′ doubler on the inside of each beam starting at the bearing points would add a lot of strength to these critical areas. Due to the short arc length and obscured location, you can probably get by with a 2X6 or 2×8 instead of a 2×12 to make an adequate doubler.
Secondly, as I understand it, the bridge is essentially a shallow U-shape projected along an arc. Is there any bracing between the beams in the middle of the span? If not, an overload could cause the beams to twist thereby cutting their vertical strength 50+% through attempting to support the vertical loads with the beams’ highly flexible faces rather than the much stiffer edges. If it were I, I’d put in a couple pairs of diagonal braces like the ones placed between floor joists that you see in basement ceilings and then complete the box with slats attached to the bottoms of the rails at those locations. A set of braces 4′ from each end would reduce the scope of any torque on the beams by a factor of 3. I’m sure that you would find the bridge to be quite a bit steadier with the additional constraints on lateral flexing.
I concur, brennan–a little caution is a Good Thing. That cross-bracing would dramatically improve torsional rigidty of the beams.
I did neglect to mention the two 2×4 braces of 29″ that are installed on each end a foot apart, parallel with the slats but down on the frame. These 2 braces are near the ground on each end, keeping the bridge from torque, and just under the slats, just about a foot on, preventing spread of the 2 sides as the bridge takes weight. I should have said, re those.
Those braces will keep the ends from spreading which is a very good thing. Unfortunately, they will offer very little protection from lateral shear forces which could cause each of the beams to rotate in the same direction without varying the distance across the top or the bottom of the box and turn your crossmembers into crowbars that pull their own fasteners out of the beam. Opening out the top and bottom of a cardboard box and pushing on oppositer corners is a simple demonstration of the effect. The screws will try to keep the slat parallel to the edge of the beam, but the leverage on them is huge. Even though the chord is only six inches, I would expect diagonal bracing to noticeably stiffen the bridge even for only one occupant. Adding the lower box member to the diagonals would contain and stiffen the braces and I’m sure that you would notice a difference there as well as you approached the load capacity of the bridge. I think we’re talking perhaps about 20 feet of 1×4 and 16 screws between the eight bords required. As far as installing them, if the pond ever warms up, I would suggest borrowing a floating chair so that you can work at a gentler angle than directly over your head. Sorry to go on, but years of building fences and ad hoc structures on a ranch give one an acute appreciation of shear forces and the required diagonal bracing because its really no fun to go out and do the job over again because the wire created slack by pulling the post sideways or some critter has leaned on something and flattened it.
Interesting. Our pond being only knee-deep, we can go in at any time—but it’s slicker than oiled glass. We’re collecting ideas, and we’ll be alert for problems. Your bottom containment is certainly something we could do.
This reminds me of a milder version of my college buddies (mostly engineers, I went to Texas A&M) having fun throwing engr problems back and forth to quiz each other. Only they were tossing around more high-level calc and physics than here, interspersed with occasionally more ah, salty words. 😉 I knew enough calc after Calc III to halfway follow it. Mostly, I enjoyed them having fun with it. Me? I started as an English major with heavy interest in foreign languages, French chiefly, and ended up trying to switch to computer science. so academically, I’m a mutt of a cat. I’m suited to both.
I take it there will be some shoring up of the bridge’s underpinnings and footings, still within Japanese style. No telling what the koi make of it all, most alien and incomprehensible to them. One hesitates to say, unfathomable. Possibly one did not hesitate sufficiently….
Best of luck with whatever you do to the bridge. Wonderful imagination and humor, and honor to Japanese styles and thought.
Well, the curious phenom is that last week, the bridge would flex very visibly, nearly a full inch, as one of us crossed it, and creak and crackle. This week Jane crosses it and you’d have to work to detect any flex. Whether this is good or bad, presaging a full collapse, we have no idea.
Hi, CJ.
In re: the bridge creaking and flexing last week and not this, what may have taken place is a settling of the points of support–it looks from the photos as if these are just cinderblock pavers laid upon the ground. Especially with the damp Spring we have had, the ground may have compacted here and there enough to even up the load–not an entirely bad thing.
I share (as may have been apparent) Brennan’s concern for what to an A/R engineer (is there any other kind?) seems a somewhat ad hoc design. I speak not of the appearance but of the weight-bearing capabilities. If your attractive rectangular cross-section arch goes at all parallelogram on you, it could, nay would, be unpleasant. I, at 220 lbs, can balance on one foot atop an empty Coke can without apparant trouble or damage to it, then reach down and gently tap the side of the can, which at once will collapse and crush completely because it has lost its perfectly columnar shape. Your bridge must maintain its rectangular cross-section to be strong enough to hold anyone standing atop it. Please do not forego the cross-braces (or blocks, see below) out in the span, the braces do not even need to be all that hefty, even 1X4s would suffice because the wood is so strong in compression along the brace’s length. Ideally these would be in pairs at midpoint and perhaps the one-third points of the span. They will be invisible unless you’re one of the fishies, looking up from underneath…. a simpler version would be square-cut blocks 6″ “tall” or slightly less, 2X6 perhaps, cut to just fit snugly inside the beams. These blocks would maintain the frame rails in that necessary rectangular shape. You could either use corner brackets and 1″ to 1-1/4″ screws into beams and braces inside, or pre-drill & drive 2-1/2″ screws from the outside of the beams into the blocks. The screws only need to hold the blocks in place, don’t need to provide any real strength; simply by being there they prevent any rotation–“parallelogramming”–of the beams.
Both finished bridges look lovely! I’ve just been enjoying the slideshows up on Jane’s blog, and the front garden is really taking shape, and coming into balance with the back garden. You two have a good eye for composition. The bridges look as if they’re from the same family, and help link the two gardens, and they are really gorgeous.
I’m very happy Jane is better but a bit worried about the enormous amount of heavy work you two have immediately started up again, moving stones and concrete bridges.
Just an idea, from someone who is not technical at all, about the creaking of the big bridge at first now having stopped. I read somewhere the wood for the bridge-timbers had been in your garage for 2 years, because of all the other things that have happened between starting to plan it and being able to finish it. If that is correct, it was probably very dry. Now the bridge is set up outside, over the pond, in the dew and evaporation as well as possible precipitation. Maybe it’s rehydrated a little bit, thereby tightening all the joints and screw-connections just enough to take care of the roominess that gives opportunity to move, flex and creak? What do you construction-savvy readers think?
If you do end up needing to do some of the engineering-type things to the bottom of the bridge, is there room underneath the bridge, above water level, to work in? In the pictures on Jane’s blog it seems to float quite close above the water.
Could you wait for summer to get hot and ask one of the church’s sturdy young men if he’d come wading in your pool to fix those things underneath, instead of doing it yourself? I fell in my garden pond (one and a half times, even) and know that the pond liner gets awful slippery, it would be very easy to fall and hurt yourselves doing that.
You and Jane like doing things yourselves, but after the health scares you’ve had it seems a bit much. Also, lifting it up or tipping it to reach the underneath would be very heavy, now it’s fully planked: better to ask those strong young men for help than having Jane’s blood trouble come back if the leaky spot gets over-strained. The story about moving the concrete bridge for the front garden sounds very worrying, from that viewpoint.
That worries me more than the chance that your quite sturdy-looking and now nicely settled bridge might move and drop you into the pond someday in the (far) future: I’m assuming you can swim, and most of the pond is not that deep (but deep enough to cushion a fall), so as long as it isn’t deep winter when you could fall through the ice it would be unpleasant if it happened but doesn’t seem very (immediately) dangerous. Also, both ends are nicely settled on firm ground now, with no forces pushing on them that might make them move parallel to each other: putting weight on from above shouldn’t make one side move forward compared to the other, in my completely non-technical expectation, so why would the bridge shear or start twisting? It’s not likely to get the kind of wind shear or vibration to make it do that, like those high suspension bridges in the video-links, being low to the ground and water, and not fixed to its foundation. If the wind ever got that powerful it might shift the bridge as a whole, and that might damage it, but it’s heavy and low enough that’s not very likely: you’re not in tornado country, are you?
Still I’m very glad that you are all doing so much better, well enough that you’ve so enthousiastically started again on the garden and the writing. Succes to all your endeavors!
Hanneke’s idea, of the wood swelling just enough to stop the creaking, sounds good to me. soft wood always moves with the weather.
Hanneke’s idea about the rehydration and swelling/tightening seems likely. Good thinking.
I agree with the idea of not pushing yourselves too hard, even though to a degree, that’s quite healthy, and despite that you’re both eager to get it done.
Hurray for sturdy young men. ‘Nuff said.
Do fishes dream of cross-braced planks? Plankton, maybe….? Looking up at the alien goings-on above their watery ken.
Ken, lowercase, not a name, you hyper-correct spellchecker….
Another way to keep the bridge from twisting might be with tension rather than crossbracing. You already mentioned that you had a couple of internal braces under the bridge, making a series of rectangles. If you can lay hands on some thin steel cable with loops on the ends and a couple of turnbuckles, you could drive a eye lag into each ‘corner’ of the bridge underneath, and run 2 cables with turnbuckle diagonally corner to corner. Tighten them up, and the tension should prevent the bridge torquing. Anything more elaborate, I defer to the avowed engineers present 🙂
Just returned from 10 days in Florida. I was fortunate enough to be present at Kennedy Space Center when the last flight of the Space Shuttle lifted off. I may write more on this later, but what a momentous, bittersweet event that was!
Indeed. I was there for the first. It took two tries and a motel stay but it got off.
While I remember to mention it: yesterday, when crossing in my bare feet, the slats noticeably flexed downwards, especially when I was near the top of the arch. My feet were perpendicular to the long axis of the bridge itself. No creaking, but the flexing gave me pause.