Design principles

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Are there any basic rules governing the design of a wooden structure?
I'm trying to improve my wood working skills, and my next project is a log store for the garden. My design so far is here [1], here [2] and here [3].
Usually I decide on the structure by vague 'intuition' about weight and load, but I'd much rather apply some tried and tested strategy. For instance, how do you decide on the relative size of the members? And which members transmit load to others?
Any advice is greatly appreciated.
[1]
https://dl.dropboxusercontent.com/u/6028779/Wood%20store%201.png
[2]
https://dl.dropboxusercontent.com/u/6028779/Wood%20store%201.png
[3] http://sketchup.google.com/3dwarehouse/details?mid ·71b29ae4bc6fd91153797f3361d543
Alex
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http://sketchup.google.com/3dwarehouse/details?mid ·71b29ae4bc6fd91153797f3361d543

terminology. I assume that this is for firewood. That is wood that will be burned in a stove or fireplace? Is that correct?
Rather than get all concerned about design principles and strength, I would first define EXACTLY what the structure is used for. Then go about building something to suit that purpose. As I have built a number of "woodsheds" and "firewood stands", I will tell you what I think.
1) Structures to house firewood simply do not have to meet a higher standard used for houses, etc. They are often built out of whatever is lying around or recycled materials. And if the only thing that is needed is to keep the rain off, you don't need much.
2) I find it interesting that you have two layers. I have never done this or found it necessary. Having it open front and back is nice to air out the wood and let it dry. The increased circulation, especially in a wet climate, would definitely help with that. But when the rainy season begins, you may need to cover the front and back to protect the wood.
3) The last "firewood structure" I built was simply a platform to keep the wood off the ground with a simple 2 X 4 frame at the ends and over the top. I then put up some heavy duty plastic tarps over it and tied them to the frame with the embedded grommets. I built the frame based on the size of available tarps. This was nice because I did not have to wrestle with the tarps when retrieving wood. I just pulled the front flap aside and got out my wood.
4) Unless you have some severe size restrictions or need to create something unusually pretty, you are really over thinking a simple, basic structure. By not having the second level you substantially simplify the structure. This makes it much simple and cheaper to build.
5) A lot of the time, I just put some wood down to keep the fire wood off the ground. And then covered it with a tarp. That is as simple and basic as it gets. You want to do more, fine. I just don't think a pretty little cottage is all that necessary. But you do what you (or the missus) want.
Any way, just tell us a little more information. Exactly what you are trying to do and what the structure will be used for. We can then fine tune out advice a little more.
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"Lee Michaels" <leemichaels*nadaspam* at comcast dot net> writes:

Quite so.

The reasoning is that the logs are not all coming from the same source or at the same time. The layers allow new unseasoned logs to be added without making it hard to get already seasoned logs out.

Originally it had a back but, as I'm going to be putting it against the brick wall of the garage, I got rid of it to cut the cost. It's pretty much always rainy season here but if it gets especially nasty I could attach tarpaulin to the front which rolls down - seen that done a few times.

I realise I'm thinking about this too much given what the project it, but that's because I want to learn things I can apply more generally to future projects, not just this little old log store. Even if I go with a simple platform+tarps jobbie, I'm curious what I should be thinking about if I were building something more substantial.
But let me narrow the requirements: - Keep logs dry - Allow air to circulate round logs - Hold weight of logs when full - Last, say, 10 years with preservative occasional sprayed on it
Thanks for the advice,
Alex
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On 5/10/2013 2:24 PM, Alexander Lamaison wrote:

I don't think the structure will be strong enough. I think the boards for the shelves are not oriented correctly on the bottom. The top has a skirt that helps but all boards in my estimation should be set on edge to hold the heavy weight of the logs. That way they won't sag.
Your corner and middle supports can be downsized to conventional stud lumber. They are vertical and the stresses are less than you think. It's the shelves that need the most.
I would also put some plywood gussets in the top corners to strengthen the OPEN structure. tie them into the vertical and horizonal supports and it will not rack.
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Jeff

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On 5/11/2013 3:05 PM, woodchucker wrote:

I hadn't actually looked at the links...I tend to agree. Being US, I look at the sizes and see roughly 5' width w/ only a couple of supports in the center at front and rear.
The drawings don't show proposed dimensions of the material for the shelves; agree doesn't look like it's more than perhaps what would be 1x stock here which would have finished thickness of 3/4". That'll certainly sag w/ time on a 30" span even though it'll likely hold as he's only got 30" maximum shown for vertical opening at the front that isn't going to hold a whole lot of round firewood--it just won't pack that densely in the round.
But, lets see what we might get--doesn't have any dimensions on the structural members as said but the uprights look like might be on the order of what a tubafor would be in the US so let's subtract 10" from the overall for interior and half that would be each opening about 25" also. So let's assume 5" diameter log and straight enough that could get five on a first row and then alternate 5/4, 5/4, etc. That produces 3 layers of 9 logs in an opening. Ok, now how much is that?
V= 3.14*(5/12)^2 * 2 * 27 ~~> 3*(25/12)*(2/12) * 27 ~ 3*~2/6 * 27 --> 1 * 27 ==> 27 cuft/opening
If assume oak at roughly 45 lb/cu-ft, that's 1200 lb on each of those shelves which translates to an average loading of ~135 psf. That's pretty healthy load; more than I'd have thought. Of course, it's not likely that the actual firewood will be so accomodating as to fit so well, but it is a point...
Now one can go look at deflection tables and so on and make some choices on sizes.
Just looking at a minimum I'd put a solid piece of 3/4 ply or the like in the middle between the two supports plus their definitely will need to be support across the width of the openings under the shelves.
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You mean flip the shelves 90 degree to make them stand like joists rather than floor boards? That would make them strong and let more air in (both good). How should I join them to the supporting beams without losing the strength of having them run accross the top, because now they'll be a bit thick to drive a screw through the top of them, even if I reduce the vertical dimension a bit?
Or did you mean something else?

Nice. That more than halves their cost.

Yes, sorry, should have mentioned the dimensions. The shelves are 25 x 100 finished size (1" x 4" ish).

Now this is getting fun :) Some engineering.

I've not come across deflection tables before. Do you have an online reference to the one you're using? The ones I'm finding online are all for large structural timbers for house building and don't cover the kinds of pieces we're talking about here.

I'm not sure where you mean. Across the shelves, knitting the 5 pieces together like another support beam?

I was avoiding this in order to allow air underneath. Perhaps I can compromise using a smaller piece that both provides support and leaves an air gap.
Thanks for all the advice guys. It really is apppreciated.
Alex
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On 5/12/2013 6:31 AM, Alexander Lamaison wrote:

Yes, he means run as joists. That's certainly one way altho it will cut down the opening size significantly unless you raise overall height.
You could support them simplest by adding a ledger board on the ends to rest them on and only w/ a little more effort notch them to hide it.

Could even go to two 1x joined in tee and for the vertical will still be plenty stout-enough...
...

Well, all you have to do is set the dimensions correctly and use appropriate values for the various material and geometric properties...
I use the beam calculator quite a lot...but for stuff like you're doing the "sagulator" is probably the easiest tool as it hides a lot of the complexity by making all the assumptions about material properties and assumes rectangular pieces...
<http://www.woodbin.com/calcs/sagulator.htm

Well, if I read your drawing correctly, it's open the full width w/ the exception of there being a vertical support at the front and rear in the middle but no support in the middle internally. Since there would be a full 5-ft span w/o those that's obviously not possible/reasonable to consider removing it; it will prevent any use except of the two "bins" so you're not losing anything by putting a divider between them for vertical support across the full width. Ply would be the easiest way to accomplish that; solid verticals would also work. You could either use the full length shelving and fit the verticals or use solid one-piece verticals and separate shelves--your choice, same result.

One way would be an on-edge "X" from corner to corner fastened to the bottom of the shelves w/ adhesive and screws to "stiffen-up" the shelves just as does an edging or a table apron.
If I take your design to the sagulator I get (using one of the pines that would be typical lumberyard material here; pick a species that is somewhat like what you would have for your material obviously)
Shelf Characteristics          Shelf Material              Shelf attachment     Fixed X Floating     Shelf load     200         per foot     Load units     lbs     Load distribution     X Uniform load Center load      Build shelves with less waste     Shelf span         30    in     Depth (front to back)                  Thickness              [Optional] Edging Strip     Material     None     Width              See note # 10     Thickness                  New Apply WoodBin lab correction?     yes x no      Sag    total 0.27 unit 0.107 in per foot         Target sag: 0.02 in per foot
Just for comparison I took to the
<http://www.engineeringtoolbox.com/beam-stress-deflection-d_1312.html beam calculation engine I use quite a lot and while it takes a little more effort, I'm always comfortable in knowing what actually happens...
For it for the same assumptions
Imperial Units
16.666 q - Load (lb/in)
30 L - Length of Beam (in)
.333 I - Moment of Inertia (in4)
2x10^6 E - Modulus of Elasticity (psi)
0.5 y - Perpendicular distance from to neutral axis X (in)
Unit Load - q : 16.7 (lb/in) Total Load : 500 (lb) Length of Beam - L : 30 (in) Moment of Inertia - I : 0.33 (in4) Modulus of Elasticity - E : 2000000 (psi) Perp. distance from neutral axis - y : 0.5 (in) Support Force - R1 : 250 (lb) Support Force - R2 : 250 (lb) Maximum Stress - : 2815 (psi) Maximum Deflection - : 0.26 (in)
I get a max deflection of 0.26 instead of 0.27 -- pretty doggone good agreement.
Now, that's based on the previously estimated 1200 lb total load divided out to the average uniform load on a 1x4 laying flat. You can see the difference if you turn it around on edge or change various other dimensions and or loadings, etc., etc., etc., ...
As I had presumed initially from just gut feelings, the sag would be noticeable but a 1x would likely be able to hold the load w/o actually breaking but it's well under-sized that way.
BTW, the I for a rectangular section is bh^3/12 where bºse and h=height. In English units it generally has units therefore of in^4. For your 1x4 flat that gives 4x1^3/12 = 1/3 in^4. You can see why on edge helps so much if you turn those dimensions around then it is 1x4^3/12 = 16/3 = 5.333. That's 4^3/4 = 4^2 = 16X times the stiffness for only 4x the thickness.
Note typical E values for wood are roughly 1/10th that of common steel.
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On 5/12/2013 10:15 AM, dpb wrote: ...

Or, you could simply use a horizontal support fastened to the two uprights w/ "enough beef" to support the load--tubasix would do nicely and still be thinner than it appears your verticals are now...
I just went w/ the solid center support (or you could replicate the ends in the middle as well) as it's simple and adds both vertical support as well as racking by forming a truss member that is pretty much lacking anywhere else at all (as others have also noted).
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Ok, new words.
Truss:
from a quick Wikipediaing, am I right in understanding that, to get a stable structure, you should join the planes together? And this is called a truss. The brace and gusset previously mentioned, as well as your suggested solid support, do this by joining two planes together. Do this in both directions, and you have a truss?
Racking:
Googling was no use here. It just tried to sell me shelves. What is racking?
Thanks,
Alex
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On Tue, 14 May 2013 09:40:08 +0100, Alexander Lamaison

Racking is when some type of shelving structure starts to lean over in a particular direction and then usually collapses. It often ends up in a cascade failure.
http://www.youtube.com/watch?v=fIP24ywBAdU

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snipped-for-privacy@none.com writes:

Haha, that's spectacular. :D Thanks
Alex
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On 5/14/2013 3:40 AM, Alexander Lamaison wrote: ...

Strictly speaking, a truss is a coplanar system of structural members joined together at their ends to form a stable framework. The simplest form of truss is three members in a triangular shape--clearly you can see that pushing on any given corner is resisted by the compression of the two opposing sides and if the material is ideal it's perfectly rigid to loads in plane.
The suggested solid piece serves the function that a strictly truss-like mechanism would of an 'x' but adds the needed vertical support the shelves need in the middle so while not strictly a truss in the simple definition by being solid it provides the resistance to serve the purpose as well as the support.
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On 5/14/2013 8:23 AM, dpb wrote: ...

And, while repeating the end design in the middle would provide the vertical support, unless the attachment is very rigid there's little resistance forward/back to racking by doing so. A set of solid vertical 1x4s if the shelves were mortised into it very tightly would provide some resistance by the twisting moment of the shelves, but there's not an overall strong resisting member overall.
OTOH, either a solid plank or the ply has length on all sides for connection and ends up, therefore, providing that rigidity.
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Could I achieve the desired result, instead, by repeating the end design but having the planks run diagonally?
Alex
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On 5/14/2013 6:16 PM, Alexander Lamaison wrote:

Certainly helps. The question then hinges on how they're fastened to be as near a rigid connection as possible and how that connection will fare w/ time and use...
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In the existing design there were also two beams in the middle running between the front and back verticals, under each set of shelves. The extra support in the middle that you suggest, is that to provide extra support for the shelves, or to help keep the frame square?
As it happens, I was planning to tack some thing slats accross the middle anyway, just so that I could use each side of the bin independently. But I didn't include that on the diagram as it wasn't meant to be structural.

Wow, thanks for leading me through all of that. It's really helpful.
I'll adjust my design accordingly and post links again when I'm done.
Thanks,
Alex
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On 5/14/2013 3:26 AM, Alexander Lamaison wrote: ...

A) Didn't appear obvious to me they were there; but then again, I only looked at it very briefly to get the overall rough dimensions...
B) Both. See response above to note on 'truss' and 'racking'...
...

I see you did find the erratum posting that loading is much more like expected when one correctly includes the 1/2 factor in the diameter when computing volume so I'll retire unless have further specific questions...I just _knew_ something didn't make sense but kept overlooking it.
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On 5/12/2013 7:31 AM, Alexander Lamaison wrote:

So what you might want to do is use wood that is capable of notching the so the shelves would be notched into the support. Now you can toenail (nail at an angle) into the notched supports, or just notch and drive the screw from the top into the support. Do this with a pre-drilled hole half way throug the shelf so only half of the screw is in the shelf... either way will be fine the notching will keep them on edge, the screw or toe nailing will keep it tight... you will need less piece for the shelf so don't make 8 notches, 4 might be enough, 3 maybe.. I don't know.. I'm not concentrating on the dimensions.

I have a lumber rack for my woodworking. Made of 2x4 supports and it holds an unbelievable amount of wood thousands of pounds. People under estimate vertical support. Yes a 1x2 or 1x2 would not do it, but a 2x4 would SAE measurements.. A 4x4 is overkill which I assume is close to what you showed.

--
Jeff

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On 5/12/2013 10:17 AM, woodchucker wrote: ...

So would two 1x in 'L' configuration.
The key is maintaining verticality and short enough span (height) so buckling doesn't enter into it.
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On 5/11/2013 3:58 PM, dpb wrote: ...

Of course it's more than I thought/expected--forgot the divisor of 4 for radius. So the load/bin is more like 300 lb instead of 1200. That's a lot more in line w/ what gut feeling was; don't know why it didn't register before now...
V= (3.14/4)*(5/12)^2 *2*27 ~ 3/4*~2/6*27 --> 27/4 ==> ~7 cuft/opening
So, the deflections below are going to be ~1/4th those numbers which is getting marginally acceptable; if went w/ tubafor instead of 1X probably be fine w/ only slight modifications. Still this assumes the solid or near solid center support, though, of course.
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