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bridge for Island
2 used pontoon boats and you are across the pond. Not sure how much weight you need to move over the bridge, but pontoon boats might fit the bill. You can get pontoon decks pretty inexpensively if you look around. Anchoring them would be simple enough, spud them or drop anchor. for sure you would want to either spud, or pre-load drop anchors. Pre-loading the drop anchor will make the deck very stable. A little imagination, and dressing to be somewhat appealing to the eye wouldn't be too difficult.
We've put 275 ton cranes on pontoons. Once you spud them, they are very stable.
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bridge for Island
Never done it, but have heard of old pontoons filled with foam. They don't even try to keep them from leaking any more, fill them with foam and keep going.
A marina may be able to help. I Don't know what kind of foam was used, and how much bouyancy is retained, but it has been done.
55 gallon drums make great pontoons for docks. Each drum has a bouyancy value of about 3200lbs. That could be an option. All you need to do is frame the deck. I would still recommend pre-loading drop anchors to give the bridge stability.
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bridge for Island
Simply said, you pre-load your anchor ties for about the amount of weight each tie would see. If the load influence to a tie down point would be..say..500lbs, you would pre-tension the tie down to about the same or slightly less. The pre-load keeps the float from moving under load, so you have a stable structure. If you used barrels, you tie down at each barrel. The more tie downs you use, the less pre-load (and smaller anchor)is required at each anchor point.
The exact same philosophy is used for floating off-shore drilling structures. You float the platform out, then anchor to the sea bed and draw it down. The draw down pre-load is based on a lot of factors, including dynamic loading from wave influence. The floating platform is then "stable" enough to set drill points. A bunch of stuff you don't have to worry about in your pond.
Of course pre-loading the tie-downs isn't required if you don't mind the bridge bobbing a little as you drive over it.
We did the wall around our pond a couple years ago. It was more out of necessity that appearance. Needed to stop the erosion from eating up the property.
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bridge for Island
OOOPS, sorry, bouyancy of 55 gallon barrel - 420lbs. Was thinking of something else. In any case, they make good pontoons and are cheap.
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bridge for Island
This remains an intersting project.
I am a structural consulting engineer. The facination with bridges traps all of us.
Here's what you have. Considering that you would want the bridge to meet building code, in case you try to sell or someone gets hurt, for an 8' wide bridge, you will need to carry a uniform load of 200lbs/ft. Add the dead load of the bridge girders and deck, by code, your bridge girders would need to carry about 260lbs/ft. This will vary depending on the construction material.
For a 32' span, with a uniform load of 260lbs/ft, your maximum Moment will be 33ft-kips. If you use A36 structural steel, you would have a required section modulus of 20in-3 with lateral bracing to keep the section compact. Since the moment is a function of the square of the span, adding a center support reduces the maiximun Moment to 8.25ft-kips, and the required Section modulus to 4.7in-3.
Here's what it means in Steel. If you had a double girder steel bridge, you would be looking at 2-W10x26 or 2-W14x22 steel Wide Flange Beam, with a braced top flange every 10'. The number after the x is the weight per foot. A good rule of thinb for deflection is the half the span in inches for the depth of the girders. Therefore, the W14 would be a better choice.
Total weight of girders - 1408lbs
At todays market prices - $535 for the steel, plus fabrication costs..
Here's what it means in Southern Structural Pine:
Section Modulus required for No. 1 dense structural lumber 246in-3. You can see the difference the reduced allowable stress makes. To get the required section modulus, you would be looking at a 14" deep x 8" wide timber. More realistically for a timber bridge, you would add the center pier to cut the span in half. To carry the code loading at a 16' span, you would need 2 (no.1 SP) girders 8" deep and 6" wide. Getting close to common timber sizes. These girders again need a deck that will laterally brace the top of the girder.
You may want to look into finding some steel bar joists left over or in a scrap yard. 32' is a pretty common commercial building span. You could get lucky. Again, the seck is an important part of the span, and must be able to laterally brace the joists.
Happy to answer any questions.
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bridge for Island
Yep, takes an engineer to make a qualified mess out of anything. I stayed out of it for as long as I could. Just had to jump in at some point.
Excuse the engineer in all of us.
Beagle
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bridge for Island
Here's the plan-
This post has been great to watch, but has gone a lot of different ways. Give me an idea of what material you are comfortable working with, what is most readily avalable to you, and I can tell you what you need to "nail" together and meet code. I really believe whatever you do needs to be to code. With the lawyers in this country outnumbering the insects, you can't be too careful. The codes are there to protect land owners if you use them right.
I can do it out of steel, wood, or floating decks. It's up to you. Let me know.
Beagle
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bridge for Island
I think we can make this work, and have a decent looking bridge. The 12x12 timbers are border line for the 32 foot span, whick means they are in the stress range, but will probably feel soft under load.
What I would suggest is doubling the 12x12 for the center 23 feet of the bridge. If you can picture this, span the 32 feet with the 12x12 timber, then reinforce the BOTTOM of the center 16 feet out of the 23 foot drop. You can use the whole 23 feet (recommended) if you want. We can get a good shear connection between the two timbers with through threaded rods on 24"C/C, and three rods at each end. In other words, your bridge girder will be 24" deep for the center 16 or 23 feet, whatever you decide.
If that sounds good for the girders, we can start looking at the stringers and deck.
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bridge for Island
Ok, let's try again. you need to but both 55 foot timbers. You'll have 2 12x12 timbers spanning to the island. For the center 16 feet of the 33 foot span, you will add another piece of 12x12 to the bottom of each girder. So for the center 16 feet, the girder will be 24" deep. We'll take a pice of what we cut off the 55 foot timber and add it to the bottom for the center 16', through bolted to the 33 foot long timber.
33'
Land--------------------------------------Island
--------------------
16'
Side View of Girder
Hope fully the diagram above helps. The girder is 24" deep for the center 16'.
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bridge for Island
The little skecth didn't work. All got left adjusted. The girder is 12" deep for the first 8 1/2 feet. 24" deep for the center 16', and 12" deep for the last 8 1/2 feet.
That's what I was trying to show.
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bridge for Island
Just so you understand, we can make that work out of the 2 55' timbers he has available.
If that works out, use 4"x6" stringers at 2'0 c/c between the girders, with 2x8 deck boards accross the stringers for the length of the bridge.
Beagle
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bridge for Island
Thanks, I didn't think of that. Would definitely be easier than babbling back and forth. It's that whole picture and a thousand words thing.
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bridge for Island
I'm sure you understand that the tapered poles won't span the 32'. Curious as to what you are going to do with them. With the taper, we could use them as masts for a cable stayed span, but not sure that's what you had in mind.
Feed my curiosity. What ya going to do with the tapered timbers?
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bridge for Island
They won't free span the 32'.
We could make them work if we add a center pier and span them 16', but not 32'.
We needed a 24" deep girder for the center of the span if we go the whole 32'. Thats why I was doubling up the 12x12" timbers we discussed last week. The taper makes them difficult to double up.
If you would like a sketch of the girders, let me know and I will post one to my pictures this evening. I don't think the words are explaining what you need very well.
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bridge for Island
OK, check out my picture 20. Very rough sketch of the side view of two different bridge concepts. One is a 32' free span with 24" deep girders, the other is for 2-16' spans, requiring 12" deep girders.
Take a look at the sketch and see if we are together so far. We can continuously add detail as we get further through this.
I had to reduce the sketch, hopefully you can still read it.
Beagle
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bridge for Island
OK, the 2 - 55 foot 12"x12" timbers would work for the 32 foot free span sketch. Cut each timber to 32 feet, then take the 23' we cut off and add to the bottom of the 32' piece to create the 24" deep girder. That's what my 32' free span sketch shows.
The through rods I talked about last week is what connects the 2 12"x12" pieces together to create the 24" deep girder.
How are we doing so far?
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bridge for Island
I think once we get our vocabulary figured out, the rest will be easy.
If you want to go with the 2-16 foot spans, it might be easier to get the materials and keep the bridge light. The center pier would be some work, probably 4 "poles" jetted into the pond bed about 4 feet, with 2 2x12's as a pier top across the tops of the poles. The sketch kinda shows the pier for the 2-16 foot spans.
Tell me again, what is the heaviest piece of equipment you want ot move over the bridge?
We're not going anywhere.
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bridge for Island
If either of you is interested, I can tell you what the load capacity of the arch is with a little more information. With an untied arch, the foundations could govern the capacity of the arch. If the arch is tied at the bottom, the capacity is internal to the arch structure.
If you let me know if the arch is tied, and if not what the foundations are, we could determine the capacity.
Maybe someone did it for you before it was built.
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bridge for Island
Wait a minute. He used TS4x4x1/4 wall tube for his girders. Not counting the added length for the arch, the girders weigh 342 lbs each for a 28 ft span. TS4x4x1/4 wall tube weighs in at 12.21lbs/ft.
The capacity of the bridge isn't necessarily directly related to the weight of the girders. Assuming he used two girders, the bridge weighs 684lbs, plus the stringers and deck.
Relatively light actually.
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bridge for Island
You know, I went back and looked at his post. It does say 4', or 4 foot 1/4 wall tubing, but I think he meant 4" tubing, which is a common size in square tubing, so I used that for the weight. I don't think he built a 28' span bridge out of 4 foot deep tube.
TS, or more recently HSS, is used to designate cold rolled steel tubing. The old TS designation was "Tube Steel", the more recent HSS designation is for "Hollow Structural Sections". Just a sexy industry designation for steel tubing.
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bridge for Island
Pretty good stuff, could be a solution, but also could be pricy, be carefull of a couple things when picking a bridge type:
Un-tied arch bridges rely heavily on their foundations for capacity. The foundation needs to be designed to accomodate the horizontal thrust from the arch. The bridge structure will be lighter and less expensive, but you will pay for it in foundation cost.
Tied arches are extremely efficient, and require less foundation work. There aren't horizontal reactions to worry about in tied arch bridges.
Pick a wood bridge with the least number of pieces in it. Connections of pieces in woof structures is where all the deterioration starts. The fewer the number of pieces, the less places for the structure to rot.
When picking a bridge type, make sure you know what kind of foundation you need for your span, you could end up putting a lot of money in the ground.
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bridge for Island
SG, I'll check the 10' x 10" if you can tell me what type of wood it is, (ie southern pine). THe allowble stresses vary quite a bit between species.
Remember we are doubling up the girders at the center, so we sould have a nominal 20" depth where we are doubled up.
The other problem with wood structures is "creep". Wood tends to "stretch" over time when under load, so we want to stay in a low stress range. Otherwise your bridge will have an ugly sag in it after a couple years.
Se if you can find out what type of wood the 10" x 10"'s are, and I'll check them. They will probably work.
Beagle
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bridge for Island
Thanks, I'll check itr for southern pine. I trust you still have the sketch of the girders we drew a couple months ago.
I'll let you know on the 10x10's.
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bridge for Island
That is what we needed with the 12x12 timbers. Give me until noon today and I'll let you know what you need for 10x10", southern pine.
Got a couple thingws we need to get out this morning, them I'll check the 10x10's.
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bridge for Island
10" x 10" Southern Pine timbers will work at the 32' span, with 24' of double timber girder as shown in Pic. 20.
Beagle
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