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bridge for Island
There happens to be a pond with island (two) near me. One belongs to a friend and he built the bridge. Also put a (okay the exact term left me) shelter on it for such as picknicks. That bridge still is an over kill.
The other guys uses a very simple bridge built of wood for his automatic, leather covered, self propelled lawnmower or as some of us call them...goats. I really like his thinking. With this is the lawnmower breaks, you can eat it.
Not being smart but you could put tile in or not and just dump dirt back in to build a road. Since it is a pond I am guessing the water is nor running around the island.
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bridge for Island
I have been presented with a plan. 4" X 6" 16'wooden beams knotched and attached with two steel 3' X 6" 1/4" plates bolted through the four by sixes. Built in a roof truss design and set on concrete slabs on each end. If necessary it could be supported in the middle with one 4 X 6 post on each side. The decking would be 2 X 6 4' long. 4' hand rails would be part of the load bearing structure. All Items will be treated. The price is around $800.00 to $1000.00. Anyone see where I may run into problems with this now or down the line.
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bridge for Island
SG,
I looked back at your orginal post. You said it is about 35 feet to the island. If you use 16 foot lumber are you planning on those slabs making up the length difference or have two joints on each side?
I suggest you get engineer specs on the size of lumber you will need for this span if you go that direction. A 4 by 6 laying down might be sufficent but your decking will itself weigh a lot. Based upon the specs I was given for headers for a shed I think the 4 by 6 is not sufficient.
If you put post in the middle I think they will need some sort of footing so they don't sink in the pond bottom over time.
kt
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bridge for Island
Call me a cynic, but, here I go anyways.....
From the very limited amount of detail you can give in 3 or 4 sentences, and IMHO (as an engineer), it won't work.
For lot's of reasons.
First of all, the shear volume of wood itself in the structure. Bear in mind the first thing any structure has to do is hold ITSELF up, before you can even think about loading it further.
Assuming with where you're located, that it will be built using Southern Yellow Pine, and we know that dry SYP has a weight of 45 pounds per cubic foot of wood.
You said the beams will be 4" X 6" X 32' (in total), and there are 2 of them, as well as the decking of 2" X 6" X 4'wide. The beams will weigh about 475 pounds, and the deck another 960 pounds. We're up to about 1435 pounds, and we haven't even thought about the sides, railings, the steel, or the fasteners.
I would guess-timate that the whole structure would weigh in the area of 2,000 - 2,500 pounds.
Best of luck.
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bridge for Island
Kt and Murf,
I was given this plan, and did not think it had a chance in hell of working. Just for note he wanted the hand rails made out of 4 X 6's. I work with cranes and weight everyday and this was way out there. I got just the kind of reaction I wanted from this post good logical thinking and guidance toward a workable bridge and have a little fun along the way. I was thinking of 2 X 10's with a plywood sandwich for strenght. Three of these spanning the pond supported in the middle with a post on the bottom. decking to be added with regular decking boards. Please don't quit on me now.
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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
Huh?
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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
I am as much a country boy as country boy is. Beagle what the hell do I have to nail together to get me and the lawnmower to the Island in style. You are right resale value has a lot to do with it. Coastal marsh front is not cheep. A bridge to that island will look good.
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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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