ABOVE GROUND POND / TANK (load bearing question)

[Catfishnut]

Today the weather was a little nicer (warmer) so I did a little surveying at my cabin for my "big bad bait pond" (tank) installation.
Many years ago, I started to construct an outdoor shower house / restroom / pump house / changing room / storage building at my cabin.
I never completed this construction because there was some doubt regarding the lot line at the time, but I had already installed nine concrete foundation pilings in a rectangular layout.
Now that the lot boundary issue is resolved, I can proceed with any future development.

The nine pilings are 18"-24" diameter and set 56" below ground level with rebar and wire reinforcement.
Above ground, atop each piling, I have stacked 14" x 14" x 7 1/2" chimney blocks. Total height of 46" (six blocks per stack).
These chimney blocks are mortared together and filled with concrete and rebar.
The dimension of the overall rectangular footprint is 168" x 148" (outside to outside of the blocks).

My idea is to fabricate beams from 2" x 6" PT lumber that would set atop the chimney blocks and run N-S and E-W.
The N-S beams would be a sandwich of three 2" x 6" boards, 14 ft long, with 2" x 6" x 14" lengths of board sandwiched in between at the very ends and at the center and all bolted together.
The E-W beams would be constructed the same way, but would be shorter and installed perpendicular to the long axis beams.
These E-W beams would rest their ends on the edges of the chimney blocks and be joined to the long axis beams with angle brackets or joist hangers or both.

Once this whole framework is assembled, I thought to lay 3/4" PT plywood atop and use deck screws to secure it to the joists and then coat the entire surface with industrial or garage floor epoxy adhesive sealer and set my pond atop this floor.

The maximum capacity of the pond (tank) is 970 gallons, but I will never have that much water in it. However, let's assume 1,000 gallons as the maximum capacity (to figure in a safety margin). That equates to 8,330 lbs of water.

Do you think that the mechanical support of this "deck" would be sufficient for the load of this much water?

EDIT: The weight of the water is the most critical here as there will be no concrete or rock involved. The rest of the weight will only come from the weight of the tank and some minor wood structures and the "people" who may be standing on the deck. By overestimating the total water volume as 1,000 gallons, I think this should account for the total maximum weight. However, just to play safe, let's say it was 10,000 lbs total.

Catfishnut

 

[Waterbug]

I'm not exactly sure of what goes where, text is harder to understand than plans.

My idea is to fabricate beams from 2" x 6" PT lumber that would set atop the chimney blocks and run N-S and E-W.

The N-S beams would be a sandwich of three 2" x 6" boards, 14 ft long, with 2" x 6" x 14" lengths of board sandwiched in between at the very ends and at the center and all bolted together.

This stood out to me, but I'm not sure so just to be sure...If you're saying the beam would have gaps like the following:

[sharedmedia=gallery:images:2843]
That wouldn't be the strongest option. The standard method is to glue and nail the 3 2x6's together using 16d nails. In cases, like for headers, when a specific width is needed, a 1/2" piece of plywood or OSB is glued between the 2x6's but the entire length, no gaps. You're basically making a gluelam beam and openings between the 2x6's are weak points. You want all the wood to act as a single unit. It doesn't sound like you need an exact width. The 1/2" fillers would also add strength, but that isn't normally their primary purpose.

In general having three 2x6's vs two 2x6's isn't a big improvement for spanning. Better to use two 2x8's or 2x10's which would cost about the same as three 2x6's but you get more strength.

Bolting wood together isn't as strong as glue and nails. It spreads the load.

Max load...
It's complex to figure out. The position of the tank is really important. 10,000 lbs / 168 sq ft is only 60 lbs per sq ft. But the tank isn't spread out over the entire floor space. Placed mid span or over a pier is a big difference. When over a pier you might be able to use a single 2x6.

The type of wood is a factor especially if you're spanning 14'. You kind of have to consider the entire structure, not just a single beam.

Off the top of my head a 6x6 beam spanning 14' doesn't sound good, should certainly be computed. I'd be thinking more like using two 2x12" instead, but I wouldn't feel good just winging it with even that. My first choice would be to add 5 piers so the max span is 7'. Then I think two 2x6 would work.

Or, since this is being built for a specific purpose, you could build a foundation just for the tank. That way you'd only have to add 1 or 3 piers. An existing pier could be under one tank corner and you could add another pier to catch the opposite corner. 2 more piers could be added to catch the other 2 tank corners if needed, depending on tank size.

Options are endless.

On the other hand complete failure is unlikely. If you saw bending you could hack together additional support from the ground and just have to keep adjusting that as it sagged. Just depends on you goals.

 

[Catfishnut]

Thanks Waterbug.

Your attached drawing is exactly what I was describing and you are correct about the strength of sandwiching them together this way.

I will change this to 2" x 8" (or 2X10) with the boards glued and screwed together. I wanted a little more width than just two boards, but instead of messing around with cutting a 1/2" OSB to sandwich in between, I'll just use three 2x8's (or 2x10's) instead of two. I will make up three of these to span the longest three sections (N-S) and they will rest upon the chimney block piers having to span only 6.5 feet in between each pier.

Then I will make up six more, but much shorter - only ~5 feet each, to set perpendicular in between the three long beams for cross support at each pier. Then the open gaps in between could be filled with 2x6's joists on 10" centers.

Currently there are nine piers and the maximum span between them is ~6.5 feet (N-S) and ~4.5 feet (E-W). The tank bottom is 63" x 111". Two of the long beams would end up being directly below the long sides of the tank and one shorter, cross beam would be right at center across the short width of the tank. This should be good for support.

Catfishnut

P.S. My original posted dimensions had a typo. The pier blocks make a rectangle 198" x 148" not 168" x 148".

 

[Waterbug]

I don't know if you care for Tommy Silva on This Old House but he says to not use screws for making beams because they're too brittle. I do pretty much what ever Tommy says so I use nails. I've also never had a problem with nails. Plus for 2x10's you'd be talking about north of 300 nails or screws. That's a lot of screws. If you don't put 1/2" ply in between you can use 10d nails. I normally use 16d because I always have a lot.

For built-up beam made with 3 pieces of 2x lumber you have (suppose) to use 20d nails, 4" of goodness.

I'd be concern about the 3/4" ply floor. I'd probably be thinking like using more 2x4 rather than 2x6 10"o.c.

Seems like a lot of work and expense compared to concrete block.

 

[Catfishnut]

Waterbug,

I hadn't heard that about screws vs nails. Nails would be cheaper and easier (that is if I could use a pneumatic nailer).

What would be your concern regarding the 3/4" ply floor? Any suggestions?

For the floor joists below the tank, I think 2x6 should be used. For the other floor areas 2x4 would suffice and I could put them 18" OC as they would only need to support a person or two.

Catfishnut

 

[Waterbug]

I'd be worried about the plywood sagging between the joists.

What I meant about the 2x4s was instead of adding 2x6 10" on center I might think about keeping 16" o.c. and using 2x4 blocking between the joists and have those 10" o.c. at least under the tank. But maybe that's just over thinking it. Might not actually save any wood and sure would be more work. It's a idea, but maybe not a good idea.

Probably not much worry of sagging if 10" o.c. like you said.

 

[Mucky_Waters]

The screws vs nails thing really depends on what type of screws you use. The cheaper screws are brittle and have a low shear strength compared to cheap common nails, but screws in general hold a lot better than nails, especially when you are trying to laminate beams together. In any case, if you are concerned about the integrity of your wood framed structure consider gluing everything together as you build it. All quality furniture, cabinets, floors that don`t squeak, engineered beams, etc... are all glued together..
Glue and screws, nails can work too, but screws are much better at sucking two pieces of wood together to get a nice bond until the glue sets.
I have framed a lot of houses, and often had to use twisted lumber for making beams and lintels, we didn`t bother to glue those beams, we just nailed them, but when the boards were twisted we would try and nail the boards together with the twists opposing each other so the end result was a straight solid beam, but to get the opposing twist together we would use a clamp and screws to get it to bond together straight, then nail it up as usual, but the screws work best for laminating the twisted boards together and the nails did the work of maintaining the shear strength of the beam.

 

[Waterbug]

Built up beams can be made many ways, however all of these ways has to be engineered. Here's some box beams that use screws. Skipping the engineering step and winging it can certainly work, wood is very forgiving. Most builders I've worked with do think they can skip lots of steps. They know better than engineers, building inspectors, the owner, basically everyone. They confuse luck with knowledge and that why building inspectors are needed.

I like the glue and nailing scheme because it's well documented. Someone else has already done the engineering for me.

 

[Catfishnut]

Thank you guys for the responses to my questions.

I will probably end up using both good quality screws and nails. Using a few screws to help to suck the boards together and help keep them from coming apart til the glue dries. Most of the rest will be less expensive nails.

I will keep the 2x6 joists at 10"OC, but mainly under the tank area. I may install some 2x4 blocking in between as well.

The rest of the deck areas will serve as a walk-way and access points. Under these areas, I can get by much cheaper and use just 2x4s @ 16"OC. There won't be much weight on these areas, maybe a couple of people at a time, but usually just me and a 5 gallon bucket of water.

I am hoping that this setup will turn out really neat. I would like to install all my filter barrels under the deck so that I can hide them from view as well as be able to close the area in from sunlight and the weather. That will help in hot summers and cold winters. I have a limited height to work with under the deck, but I would be able to access the plumbing works fairly well. To access the top of the tanks, I was thinking about installing trap doors set in the deck flooring.

Eventually, I will cover this area with a roof and screen the sides in. It will make a nice "bait house".

Catfishnut

 

[Waterbug]

I was just watching Modern Marvels Engineering Disasters 15 and a building collapse because a rebar stand that was 1" too short was used. No doubt some Yahoo on site said "those will be fine".

I mention this because I'm planning on making my own ceiling beams using OSB primarily for a shop that I want to put a pond on top of. It's a difficult thing to figure out because there's not that much load data for DIY beams and I'll have a pretty heavy load. I've been doing a lot of research, but I'm pretty nervous about it.

 

[Catfishnut]

Waterbug,

That sounds interesting and intriguing, but I certainly understand your feeling of nervousness. My project isn't very large or elaborate but I don't want it to fall apart. Building a pond on top of a shop would be another matter entirely. You wouldn't want the roof to collapse when you are working inside the shop, that is for sure.

Catfishnut

 

[Mucky_Waters]

I've heard of, and seen, a few problems with hot tubs installed on decks that weren't designed to carry the load. I got called back on one particular job were I stuccoed the post and beams supporting the deck, they called me back because the stucco was cracking on the posts. Some of the post were 10 ft tall and just two 2x4s, which by itself spells trouble, but then they went and installed a hot tub on the deck. The outcome was inevitable, the long skinny post flex under the strain and it doesn't take much movement to crack stucco. I told them I'd come back and fix the posts once they talked to the house designer and made the necessary engineering upgrades to support the hot tub, I never heard from them again.

 

[Waterbug]

It always amazes me what wood can take. I've seen lots of cracked joists, rafters beams, huge sags but I've never seen a what I'd call a complete failure other than during construction when there wasn't bracing.

I am a bit nervous about working in a shop with a ton of water over my head. I only need maybe 6-12" of water, but still.

 

[Catfishnut]

Waterbug,

I'd be interested to hear what kind of a pond you are building up there... and why on a roof?

For my build, there won't be anyone under the deck except just to check on the filter systems once in a while, so I'm not too worried about the safety aspect. But, I'd be very wary with something over a workshop or some living quarter. Personally, I think I would rather opt for steel I-beams or open web steel joists. http://en.wikipedia.org/wiki/Open_web_steel_joist

We used these last summer to support my nephews new cabin. Put them under the floor joists and sill plates and set them on concrete slabs.



Catfishnut

 

[Catfishnut]

Mucky


Here's a good one for you. One of the production personnel at my work bought a brand new home (which he obviously could never afford on his salary). The house had a huge deck above the walkout patio of the lower floor. He and his wife bought an above ground pool, four feet deep and maybe 10-12 feet across and set this on the deck.. They put the garden hose in it and left to go shopping.

Yep! They had an above ground swimming pool alright. In the middle of the living room on the floor below. The deck gave way where it attached to the house and the pool slammed down, through the sliding glass doors below and into the living room, ruining just about everything inside as well as the entire side of the home, the door, the deck, and the pool! Yep, not too bright. He no longer works for our company.

Catfishnut