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Old 04-30-2010, 02:15 PM   #1
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Default Re-engineering my dock - advice requested

I just left the lake house up in GA and the floating dock apparently tried to separate from the fixed dock since the last time I was up here. The gang plank is what connects the two and it is bolted to hinges that are bolted to a 2x8 that runs across the front of the fixed dock. Well, the 2x8 is held on by nails which had pulled out and was more than 50% separated from the rest of the structure. It looks like somebody might have run into the side of the floating dock with a boat an the leverage tried to yank the 2x8 off.

I temporarily tied off the floating dock to the fixed dock in case it does come completely loose. I'm headed to Raleigh, NC for my brother's wedding so I didn't have time to screw with it. I need to contrive a beefier mounting arrangement for the 2x8. Right now it is only held on with nails driven into the ends of other 2x8 stringers that approach at 90 degrees. I'm thinking 90 degree framing straps would help. Any other ideas? Mechanical engineers or building framers?

Before pics of dock (there are no after pics):
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Old 04-30-2010, 06:13 PM   #2
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Looks to me like that floating dock would provide alot of torque on that front 2 x 8 all the time. and this movement helped push the nails along.

I would look at.. on the underside... making some angle iron pieces that would bolt the front board, to the joists on the underside underside of the dock. Add to the ends, and especially along to where the ramp attaches. Your looking for thru Lag bolts to attach no nails or screws, BOLTS. You want to better tie that front board into the frame of the rest of the dock, and hopefully move the strees back into that wall.

Pop a few boards off the floor of the upper dock, and you shoudl be able to attach the iron pieces from the top side. Bolt thru and SOLID.

Basically this idea is a glorified framing brace... just stronger.

Goodluck.

(disclaimer : I am not a structual engineer, or professional. I did stay in a Holiday Inn once. and thats how I would approach it)
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Old 05-01-2010, 06:18 AM   #3
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Dang, any movement parallel to the shore of the floating part will cause a hell of a moment at the point where the incline connects to the stationary dock. For example, say a boat tapped the dock at F, creating 100 lbs of force. Assuming a distance from F to M of about 25 ft, that creates a torque around 2,500 ft lbs.

Just by thinking for a min, I would do something like this, where you add either one or two supports parallel with the incline, same length, with pivots on the ends:

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Old 05-01-2010, 07:59 AM   #4
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Here's my Illuskration. Two solutions. If they doesn't make sense right away then I failed to understand the problem.
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Old 05-01-2010, 09:30 AM   #5
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I'd be adding steel cable (x pattern) from the corners of the upper platform to the corners of the lower platform thus sandwiching the bridge. If need be you could add a steel pole from upper to the lower platform underneath the bridge in it's centre.

Ideal (strength wise) would be also having steel tethers anchored to the ground veeing out from the floating dock. This may be too compromising of your use of the dock and ugly. At my cottage we leave our floating dock in the water over winter with the above vee cable method and it's only ever off centre by 1' which our mounting can accommodate. Also it's not anchored and it's in a lake 2 hours north of Toronto (it sees lots of ice in winter).

The other alternative would be to sink posts at the end of the bridge to affix it to, this would help torsionaly and aesthetically vs my cable suggestions.

Not a PEng (but I'd like to be).
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Old 05-01-2010, 09:55 AM   #6
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How long has it been assembles with nails? If it has been a while I would perhaps throw some reinforcement on each side and use some stainless bolts with large washers to go through the whole shabang. The piers I had my boat tied to were set up like this.

Also they had anchors coming off each side via steel cabling and they made a criscross under water (forward anchor was under the aft corner and aft anchor was under the forward corner). That is what really kept those bitches in place, the joints were mainly for people purposes.
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Old 05-01-2010, 10:53 AM   #7
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Man, you're asking a lot of that joint at the walkway. I would put cables from the dock back to the shore at 30 degrees or so. This will take the torque stress off the walkway pivot and make it mostly just compression.
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Old 05-01-2010, 12:56 PM   #8
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I wouldn't support the dock with the walkway. There's other ways you can do this.

My buddy does seaweed removal around the lake and last year he asked me to help him anchor a floating dock with him.

We ended up using some concrete column forms that we have at the shop to make 4 circle lolly columns. They probably weighed in at about 300-400lbs a piece, maybe more. We put a tarp down on the dock, with the forms on top on the tarp. Mixed up the concrete and poured them 3/4 of the way full. Once they were set up enough we put in a bent piece of rebar at the top of each column.

We got some long *** chains and bolted one end to each column. We dropped the columns down into the lake with a separate chain on each one, making sure not to lose the other end when we dumped them. Let me say that holy **** were these things heavy! One of them caught the tarp and dropped my buddy right on his back on the dock. lol

Anyways, once they were in we hooked up the chains to the dock. We used some rope to pull it the way the people wanted it to face and then tightened all the chains. This thing was solid man. There's no way that dock is moving unless they disconnect those chains.

Seeing that you have a walkway right to your dock this shouldn't be too bad. The dock we worked on was about 100ft out past their boat dock and was a free floating one. We had to raft all the bags of cement and forms out there without getting the bags wet. It was a lot of work, but it was fun doing it. The people were happy with the end result and he never got any callback about it moving.
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Old 05-01-2010, 01:03 PM   #9
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vash has the win on this one. i would add a few guide cables just for insurance but the anchor system is the best idea so far.
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Old 05-01-2010, 05:28 PM   #10
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Will a mod please change just one vowel in the thread titie I mean title to make this thread an even bigger win?
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Old 05-01-2010, 05:43 PM   #11
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Here I go again...
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Old 05-01-2010, 06:26 PM   #12
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Like this, the crossed lines keep it from shifting side to side.

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Old 05-02-2010, 12:22 AM   #13
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What cardriver said was the first thing that came to my mind. That gangplank isn't going to give you any strength against racking forces.
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Old 05-02-2010, 12:31 AM   #14
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Where's Hyper? He could re-engineer it for you!
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Old 05-02-2010, 12:37 PM   #15
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Quote:
Originally Posted by cardriverx View Post
Dang, any movement parallel to the shore of the floating part will cause a hell of a moment at the point where the incline connects to the stationary dock. For example, say a boat tapped the dock at F, creating 100 lbs of force. Assuming a distance from F to M of about 25 ft, that creates a torque around 2,500 ft lbs.

Just by thinking for a min, I would do something like this, where you add either one or two supports parallel with the incline, same length, with pivots on the ends:

Yeah, I'm really glad the 2x8 ripped out actually. If it hadn't ripped out the whole fixed dock might have collapsed. Leverage is a real bitch. I had not considered solid beams as an option previously. The cost might be a little more than criss-crossed cables but cost isn't my primary concern.
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Originally Posted by inferno94 View Post
I'd be adding steel cable (x pattern) from the corners of the upper platform to the corners of the lower platform thus sandwiching the bridge. If need be you could add a steel pole from upper to the lower platform underneath the bridge in it's centre.
This is a viable solution as well. Which is better for strength, cables in an x pattern or hinged beams? The only mechanical conflict with the cables would be their tight physical contact against the bottom of the gangplank. I don't anticipate any sawing motion, so I guess that isn't a problem.
Quote:
Originally Posted by inferno94 View Post
Ideal (strength wise) would be also having steel tethers anchored to the ground veeing out from the floating dock. This may be too compromising of your use of the dock and ugly. At my cottage we leave our floating dock in the water over winter with the above vee cable method and it's only ever off centre by 1' which our mounting can accommodate. Also it's not anchored and it's in a lake 2 hours north of Toronto (it sees lots of ice in winter).
This would not be a feasable solution due to changing water levels. December through March the lake levels are lowered about two feet below when that picture was taken. Normal lake levels are about two feet higher than when pictured, and after torrential rains upstream levels have been as high as one foot above the fixed dock elevation.
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Originally Posted by inferno94 View Post
The other alternative would be to sink posts at the end of the bridge to affix it to, this would help torsionaly and aesthetically vs my cable suggestions.
This is obviously a common choice among the professional dock builders but would involve the gangplank not being attached to the poles but riding between them. This is a problem for me with regard to being able to physically position and drive the posts as well as being aesthetically less desirable.
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Originally Posted by inferno94 View Post
Not a PEng (but I'd like to be).
You are doing a fine job.
Quote:
Originally Posted by jedduh01 View Post
Looks to me like that floating dock would provide alot of torque on that front 2 x 8 all the time. and this movement helped push the nails along.

I would look at.. on the underside... making some angle iron pieces that would bolt the front board, to the joists on the underside underside of the dock. Add to the ends, and especially along to where the ramp attaches. Your looking for thru Lag bolts to attach no nails or screws, BOLTS. You want to better tie that front board into the frame of the rest of the dock, and hopefully move the strees back into that wall.

Pop a few boards off the floor of the upper dock, and you shoudl be able to attach the iron pieces from the top side. Bolt thru and SOLID.

Basically this idea is a glorified framing brace... just stronger.

Goodluck.

(disclaimer : I am not a structual engineer, or professional. I did stay in a Holiday Inn once. and thats how I would approach it)
I will definitely use drilling and thru-bolting with large washers at the end posts. While I like the idea of strengthening the 2x8 with angle to prevent flex, I think the calculations showing 2500lbs per 100lbs force demonstrate the need to reduce the leverage exerted being more critical than strengthening the existing point at which the force is concentrated. That is, if I am understanding them correctly.
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Originally Posted by astroboy View Post
How long has it been assembles with nails? If it has been a while I would perhaps throw some reinforcement on each side and use some stainless bolts with large washers to go through the whole shabang. The piers I had my boat tied to were set up like this.
Stainless thru-bolts at piers are a definite thing.
Quote:
Originally Posted by astroboy View Post
Also they had anchors coming off each side via steel cabling and they made a criscross under water (forward anchor was under the aft corner and aft anchor was under the forward corner). That is what really kept those bitches in place, the joints were mainly for people purposes.
I can't do bottom anchors due to water level fluctuations.
Quote:
Originally Posted by jaydee View Post
Man, you're asking a lot of that joint at the walkway. I would put cables from the dock back to the shore at 30 degrees or so. This will take the torque stress off the walkway pivot and make it mostly just compression.
JD
I'm thinking about cables to fixed dock and then anchoring fixed dock to shore inland about four feet with buried cables and buried mobile home screw anchors.
Quote:
Originally Posted by Vashthestampede View Post
I wouldn't support the dock with the walkway. There's other ways you can do this.

My buddy does seaweed removal around the lake and last year he asked me to help him anchor a floating dock with him.

We ended up using some concrete column forms that we have at the shop to make 4 circle lolly columns. They probably weighed in at about 300-400lbs a piece, maybe more. We put a tarp down on the dock, with the forms on top on the tarp. Mixed up the concrete and poured them 3/4 of the way full. Once they were set up enough we put in a bent piece of rebar at the top of each column.

We got some long *** chains and bolted one end to each column. We dropped the columns down into the lake with a separate chain on each one, making sure not to lose the other end when we dumped them. Let me say that holy **** were these things heavy! One of them caught the tarp and dropped my buddy right on his back on the dock. lol

Anyways, once they were in we hooked up the chains to the dock. We used some rope to pull it the way the people wanted it to face and then tightened all the chains. This thing was solid man. There's no way that dock is moving unless they disconnect those chains.

Seeing that you have a walkway right to your dock this shouldn't be too bad. The dock we worked on was about 100ft out past their boat dock and was a free floating one. We had to raft all the bags of cement and forms out there without getting the bags wet. It was a lot of work, but it was fun doing it. The people were happy with the end result and he never got any callback about it moving.
Lake level changes will cause me to need to pass on this design, but it sounds quite robust.
Quote:
Originally Posted by turotufas View Post
Here I go again...
That isn't going to let it rise and fall like it needs to.
Quote:
Originally Posted by astroboy View Post
Like this, the crossed lines keep it from shifting side to side.

Did the little guy's arm come off when the 1000lb weight went off the dock? I appreciate the work that went into the drawing, but I'm limited to a design that will accommodate lake level changes.

The most promising designs are the hinged beams and the crossed cables. Which is best? Is there a better solution?

You guys have been very instructive in your explanations and very helpful with your suggestions. Thank you for your help. Keep them coming.


Now that I think about it, if any of you Atlanta area guys feel like helping a brother out on Saturday the 8th, I'll make plenty of beer available for your efforts and be very grateful for the help and company.
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Old 05-02-2010, 12:41 PM   #16
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That is a nice place you got there.
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Old 05-02-2010, 12:52 PM   #17
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nvm, misread thread title
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Old 05-02-2010, 01:23 PM   #18
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That is a nice place you got there.
That mortgage is the primary reason my Miata has no turbo still. Even with the pay cut and the bad sales environment I could have still been turboed if I didn't have it. But I really enjoy being up there.
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Old 05-02-2010, 03:16 PM   #19
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What is the height change? You can accommodate that with lines that are long enough at the highest of tides and would be slack when the water level is lower. You could even go so far as to using chain with a clip (insert non-***** style clip image here) so when the time comes to slack the chains you can and when the water level lowers you can take up the slack in them. Personally I would not tether it to another wooden structure because anchoring it and changing the length of the lines going to the lakes bottom would ensure your other deck/dock thing will remain undamaged.
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Old 05-02-2010, 04:47 PM   #20
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Since you have suggested the direction of flow in the image. Then just go upstream and drop an anchor and attache it to an eye bolt.



You might also want to run two cables from the gangplank attachment board on the main dock to the Primary concrete wall. This would prevent that front board from ripping off
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