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Riverside Deck

 
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Old 04-17-2007, 12:34 AM   #1
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Riverside Deck


Hi guys, I've been asked to quote on a deck along a river frontage. The existing frontage is poured concrete which has been there for years and the new deck is to sit on top of this. The problem is that the river does occasionally flood and if this happens the deck is likely to be a couple of inches underwater, albeit for only a few days. Now the problem as I see it is what effect is this likely to have on the deck with regard to trying to lift it up and how best do I counteract this. The deck will be built from red cedar. If anyone has dealt with this before I would much appreciate any advice. Thanks.

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Old 04-17-2007, 09:29 AM   #2
 
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Re: Riverside Deck


The key is wind exposure. Look around the spot. What is the longest distance of open water? Wind-driven waves slapping the underside of a deck present an almost irresistable force.

The frame members should be interconnected to the ground with at least 5/8 inch bolts. Your agreement should specifically exclude severe weather.

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Last edited by bill r; 04-17-2007 at 03:25 PM.
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Old 04-17-2007, 12:40 PM   #3
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Re: Riverside Deck


The river is actually a small side stream of off the river Thames and is only 30 foot across and very sheltered so the only real problem as I can see it is flotation. Bill, have you done this kind of stuff before ? Am I right to think that this deck, if submerged, will try to rip itself away from the ground fixings or am I maybe over-estimating the forces involved ? Its a new one for me and it does kinda worry me.
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Old 04-17-2007, 02:49 PM   #4
 
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Re: Riverside Deck


the actual bouyancy force would be merely the volume of water you displace. Water weighs 65 lbs per cubic foot. So 1 board foot of lumber would have a bouyancy of (slightly less than) 65/12 lbs. If no active surface waves, you just need to allow for the bouyancy. So a 2x8x8 deck board measures 1.5 x 7 x 96 inches after planing which is 1008 cubic inches or 1008/1728 cubic feet of wood, and would exert an upward floatation force of, say, 30 pounds on the structure. However, even a 1-foot tall wave can give the underside of that deck a tremendous wallop. And it doesn't take much wind or fetch to produce a 1-foot wave. Even a ski boat could produce this. Yes, I am somewhat familiar with it, but hereabouts we usually deal with much larger streams. Is the deck for fishing or boating or just relaxing?

In US, a board-foot is 1 inch by 12 inches by 12 inches

fetch = the farthest distance the wind has continuous access to the surface of the water

Your other concern is 100% relative humidity always eats up metal fasteners. You want hot-dipped galvanized or stainless

The only other cautions I can think of:
a) if the stream rises and falls with the tide, be sure you observe the site at high tide
b) Be aware that as the water rises, the stream gets wider. As the stream widens, it gives the wind more time to perturb the surface into waves. If you can't see more than 2-3 hundred yards of water surface in any direction at flood time, then it sounds indeed protected. If more, it becomes much more susceptible.

Last edited by bill r; 04-17-2007 at 05:53 PM.
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Old 04-17-2007, 06:20 PM   #5
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Re: Riverside Deck


Bill Mon ,,thanks!! that is some very usefull info a lot of things I would have never thought of. I do some work for the mucky muck rich folks on the lakes we have here,I know the adverage picture of Oklahoma is the dust bowl but belive me there is a lot of water over here in places and it comes down all at a moment notice.

Thanks Again!! John
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Old 04-17-2007, 06:27 PM   #6
 
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Re: Riverside Deck


Good John, I think I'm the only pier builder on here ? so I finally have a chance to 'pay my rent'. The actual bouyancy is (weight of displaced water - weight of dry wood) I was neglecting the wood weight because he's using redwood -- very light. If I can ever help ya, just shout.
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Old 04-18-2007, 01:37 AM   #7
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Re: Riverside Deck


Thanks bill, apologies for not spotting the "marine" tag. From your description this is a protected spot so I have to deal with the flotation issue only. If you can imagine the deck is along a small frontage about 60 foot long by 16 foot deep (from the edge of the concrete jetty back to a brick wall). He wants me to construct square frames using 8" x 6" cedar beams, fix these down to the concrete and then infill the sqaures (each approx 16'x16') with 6"x2" cedar planks, obviously with some other addition joisting under. From your explanations this seems like an awful lot of timber to try and hold "under", do you think its feasable and if so what kind of fixings and how many would you reckon to need.
I really appreciate your help Bill, I have visions of a real nice cedar raft floating down the Thames towards Wndsor,
Thanks
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Old 04-18-2007, 02:18 AM   #8
 
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Re: Riverside Deck


Yeah, Tom Sawyer and Huck Finn, floating down the river. It'll do it, too, unless you get her tied down really good.

Sounds like you're talking about 4000 board feet of lumber = (4000)x(65/12)=10 tons of bouyancy. Significant, but not impossible. 5 cubic yards of concrete would be the theoretical minimum ballast; given that flood water is often moving, I'd probably double it at least. Now all we've got to do is make sure that your lumber remains attached to this 10 yard blob of concrete. I hope someone is checking this arithmetic. Can you use steel attachments? It depends on the salt level and liklihood of proper periodic inspection. The lumber is good for 20 years, maybe. Steel, in the wrong environment, may not be. "CASTLE HO!"

So, Nick, there are some ideas from the colonies. I think prudence dictates that you probably discuss with your local authorities. Over here, we have gov't specialists who work with shoreline improvements. Take him a printout of this if you like. They'll know the best methods of securing in your specific locale. DO NOT, of course, let the property owner talk you into some half-assed method of attachment, like lead compression fasteners, etc. Not for 10 tons. It WILL float away if its not secured. Good luck with it!

One more thought: That platform, mostly submerged, in the middle of the Thames, could present a significant hazard to navigation.

Well, here's one more: Don't assume that un-reinforced crete will hang together in one piece. The fastening scheme is difficult to design, because the blessed castings could easily pop under local stresses. So, I have pranced all around your original basic question, and still haven't answered it. Maybe we've 'gotten our heads around the questions' better at least.

I hate to say this, cause its gonna be expensive but you may need to resort to stainless for the anchoring. Or the locals may say that regular steel galvanized will survive for an adequate period.

Hey Nick, just found that western red dry cedar weighs 21 lbs per cubic foot.
So you can safely reduce my earlier bouyancy figures a little; say 30%.

Last edited by bill r; 04-18-2007 at 08:36 PM.
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Old 04-19-2007, 01:35 PM   #9
 
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Re: Riverside Deck


Nick, Hope I didn't scare you away from the whole project. I certainly did not mean to do that.
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Old 04-19-2007, 03:53 PM   #10
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Re: Riverside Deck


No you haven't scared me off, just confirmed the sort of worries I was having anyway, I'm going to go back to the guy and try to explain the difficulties involved and then maybe get some engineering design work done. The guy seems to think a few bolts and angle brackets is going to be fine.
I'm not the sort of person to do things on a wing and a prayer, I need to KNOW its gonna be right.
So thanks for your help Bill its much appreciated.
Nick
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Old 04-19-2007, 05:03 PM   #11
 
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Re: Riverside Deck


Best of luck with it, let me know if I can help. Yep, you probably need and engineer to tell you how to connect to the ballast. Might be very simple, might be very tricky, depending upon a few key factors mentioned. Should be nice working by the water! My number 1 man is a yellow lab. He loves it. Tell him I'm an engineer, but with a small 'e' -- no certs.
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Old 04-20-2007, 06:49 AM   #12
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Re: Riverside Deck


Bill, i've been doing a few sums, let me run them by you and see how it sounds.

Ive got to deck an area of around 820 square feet.
Deck boards will be 2 inch finished thickness.

Volume of this deck material will be 820/6 = 137 cubic feet

Volume of additional beams and joisting approx 27 cubic feet

Total volume 164 cubic feet.

Weight of water less weight of lumber = 44 pounds

Total bouyancy = 164 x 44 = 7216 lbs say 3 1/4 tons

Does that sound right to you ?

I guess from there its a matter of deciding how many fixings you can use, how much load each fixing will be taking, and from there what type of fixing will do the job. I suppose another problem is that if the main structural members are held down the rest of the deck has to be adequately fixed to those members to avoid them breaking away.

The more I think about it the deeper i'm getting !

Any thought will be much appreciated.

Nick
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Old 04-20-2007, 08:12 AM   #13
 
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Re: Riverside Deck


Hi Nick, Looks OK if you're sure about the amount of framing lumber.

My higher number ( for comparison only ) was as follows:

60 x 16 = 960 X 4 inches thick (to allow for 2 inch deck and a generous budget for framing) = (approx) 4000 board feet or 320 cubic feet. You're the one there with your plans; I would caution you to perhaps check over your framing (or undercarriage) allowance; seems to be a little light to me, given double circumference headers, or whatever. But as best I can see the you're handling of the 44 pounds per cubic foot correctly. You were initially mentioning 6" x 8" sleepers as I recall. One 6x8x16 would contribute 5.33 cubic feet! Be sure to include those as well. And then, if it is a stream under flood conditions, the water is likely to be rolling along down the stream with some velocity (aside from wind-effect) That down-stream veloocity is likely to impose quite a surprising lateral load onto your fixiings. Even at 2 knots, the lateral load would be hundreds if not thousands of pounds. As we "peel" the onion, we get a little closer to the truth; just keep a healthy consideration for these thoughts. Another thing that happens in flood conditions is that one renegade object collides with other objects and sets them free. Make sense?

Just keep these things in mind:
a) 44 pounds of boyancy per cubic foot net is good way to handle it.
b) don't under-estimate the side-wise loads from stream current.
c) make some healthy allowance for collisions from other run-away objects
d) I know it's hard to believe, but even 1" thick steel you put there may or may not be there in 10 years, depending upon salinity (saltiness). I surmise that you are well inland, but then I hear about very high tidal variations there. Does saltwater get up there to London? The other key variable here is: How far away from the water surface is that metal normally? If it is normally 10 feet above and normally high and dry, then corrosion concerns are trivial. If it's 1 foot away from the water, and it is salty, then you've got serious concerns.
e) Concrete shatters easily, particularly locally around fasteners.

I may be erring on the conservative side; from my perspective, I must.

Let me know how I can assist,
Regards
Bill

Last edited by bill r; 04-20-2007 at 09:56 AM.
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Old 04-21-2007, 07:59 AM   #14
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Re: Riverside Deck


Thanks for all your input Bill, its most appreciated.
I've just had "the conversation" with the client and I think he gets the picture and has agreed to consult with a structural engineer before we take the project any further.
As an aside, I was chatting to my two sons about this particular job, they are both in construction, anyways my youngest spent a winter working at a river in the Dominican republic and said that everytime the river went into flood the jetty lifted without fail, a real pain the ass.
Another amusing point was that when i was trying to explain to the client about the problems of bouyancy he said he thought the gaps between the boards would let the water through so there shouldn't be a problem, he got it eventually though.
Thanks

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Old 04-21-2007, 06:20 PM   #15
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Re: Riverside Deck


Man,Bill, I have it made!! all I have to worry about is geting to the job when the lake is down. No lie most of the projects we do are in town where the creet creeks are installed with big gate screw down control valves/over flow protection. One time a year they lower the leval for all the Wallets with maintainace on line problems. We just rush over there with in the 7 day time span and get er done.

Let me run this by ya>>> Those lakes are like composed of at least 4' of total slime on the botom, they are a water shed composed for flood control combined with the natural water shed pretty good idea really. but installing creet piers to get thru all that stuff..ghesssssss. My work so far involves banding up glav flashing into a cilinder drifting on out there on foot or in a boat and driving the clinder into the muck looking for a rufusal rate. Then driving regular rebar along the sides just to keep the form in place, bracing the flashing form with 2x4s filling the thing with really low slumb creet<<

We wait for the creet to push down and build off the pier. With the given the rebar is going to go away as the rust sets in at least the pier is in the general area for the deck/boat dock. I use all .60 pt material and SS fasteners. Most of the time we only have to go out 16' or less for the deck then consturct a moving ramp hindeged into the existing dock that goes up and down with the major change in the lake height.

I havent tried the foam filled products I find on line they just seem too flimsley to me. John
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Old 04-23-2007, 09:39 AM   #16
 
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Re: Riverside Deck


John, I've never used concrete for the posts, I don't have experience in getting them right. If they're done well, I would think that they would out-last wooden poles. I use pine poles (debarked young tree trunks) that have been pressure cooked in preventive chemicals called CCA (2.5). I hammer them down with a pile driving hammer from my barge. Yeah, I know inland lakes are sometimes controlled. Unfortunately, they rarely lower the lake level of the Atlantic. The muck is often 3 to 10 feet deep depending on how the tidal currents hit the site; and of course the pole needs to be hammered into at least a couple of feet of clay beneath. I've often driven a 25 foot pole 15 feet into the creek bottom in thick silt... The muck is what makes site access so difficult, with or without water.

I would think that the corrosion issues discussed above might be much less severe on an inland lake compared to the salty water I have to deal with.

With the cast-in-place process you described, I would think you might be able to evacuate the silt from your form using high volume water jet and evacuation pump. Never tried that. Over here, if the post site is covered with a few inches of water, water jetting is often used to settle the post into the creekbed, as follows: erect the post over it's permanent site, then use a high volume water jet in 3/4 steel water pipe around and beneath the butt of the post as the post is held erect. As the water washes the silt from beneath the post, the post will settle in. Here, after the post is 'jetted' down, the constant tidal flow will gradually fill in the hole and firm up the post. This technique is used frequently around here and successfully, you just have to recognize that it won't be very solidly planted for the first year or two. After a couple of years, I personally can't see any difference in the final result of the two techniques. If I were using concrete posts, I might pre-cast them ashore and then use this technique to plant them. You can go as deep as you wish until you hit rock. Its just slower than a hammer. I know of piers installed this way that have outlasted pile-driven piers. Good depth is what you've got to accomplish here, against hurricanes. The bane of my existance is that the long, deep poles are extremely hard to keep positioned perfectly, they tend to migrate laterally as they are being installed.

Last edited by bill r; 04-23-2007 at 09:49 AM.
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Old 04-23-2007, 05:59 PM   #17
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Re: Riverside Deck


That is intersting Bill with the water power jet hmmmmmmm we could get a gas powered rig on board. I bet that sucker makes a mess but hey we are usually covered in mud anyway!!!

It is a deal with the creet and the forms the lakes are formed shallow in the begining 5' or so then drop off sharpley to 20/60 ' we dont go out that far but fourming up in 5' is a chalange. I usually wait for 2 days for the creet to move down where it will we dont use that for grade just foundation then frame up from them with the hilti two part mix and threaded rod thru the plate on the pier. It would also be intersting to form up and drive a pt post into it then fill around it.

Its not even close to square but that seems ok out in the water. These lake decks are the only project that I defently use pt lumber no rail of course they are just a glorified boat/party dock.

Man is that muck nasty or what???? John

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