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?
I see that you are from the UK. In the states we use a little different 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.
The basic problem I see with your design is angular bracing. As I see your design, it would seem that over time the unit would wobble and the joints loosen up, with ultimate collapse.
There would be two ways to prevent this. The simplest would be to add pieces in the corners 45 degrees to the up rights. This could be plywood triangles or short piece cut at 45 degrees so they would fit into the corners.
If you want to keep the openness of the structure I would half lap all of the joints. Half laps are quick and easy to cut and are very strong joints. They may not take any more time that cutting and installing all of the 45 degree braces above.
It you really want to get fancy you could make mortise and tennon joints, but I think that is over kill, unless you want the practice for a future project.
This is a very, very, very old problem. Trees didn't come with directions. Indeed, all trees are different and many of them are probably not very happy about what you have in mind! : )
More specifically with regard to your question, I'm reminded of the words "form" and "function". Ignoring "basic rules" there are entire cultures of ideas of how to transform the flesh of a tree into something more noble.
I like this web site--after numerous visits, I still haven't seen eveything on it.
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
Being such an old problem, I don't know why there seems to be little advice. All woodworking books I've got my hands on spend the their time describing the million-and-one ways of making a joint. None give advice on how to structure the pieces you are joining. Most don't even explain when to choose one joint over another.
I guess I'm interested in function more than form. How you design something with to use minimal effort/material but still be confident it will do the job. Up till now I either hope or overengineer it. I'd like a better method.
On 5/10/2013 1:36 PM, Alexander Lamaison wrote: ...
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You're looking in wrong places...
What you seem to ask is more to do w/ engineering and is what architects and structural engineers do. It's not common in the recreational woodworking literature because there's such a widespread conventional set of dimensions used for virtually all such pieces that amateurs or semi- to moderately-advanced woodworkers tend to build that there's little real need for actual stress or sizing calculations. Really experienced will have enough through their experience to be able to extrapolate or will consult w/ knowledgeable folks who can do that. Otherwise, it's really all about the design for appearance.
OTOH, the other place where there's real engineering for wood products is w/ the manufacturers who design for production and efficiency and cost.
As for the complaint of joint selection for purpose and the like, I would recommend Tage Frid's series of books published by Taunton Press probably 30 yr ago. While somewhat dated in their appearance by today's standards of publishing, they're very well done and not much better than I know of yet.
And, of course, for structural design, there are building codes that provide minimum requirements for structures that include things like live/dead loads, deflection limits, wind and snow loads and such things as hurricane/tornado/earthquake enhancements for locations affected. Don't know the UK equivalent, in the US it's known as the Uniform Building Code.
I'm not trying to build anything as engineering-critical as a building ... yet. Just every day items: log store, yard gate, kitchen cabinet, TV table, wash stand, window frame etc.
I'm not aware of a "conventional set of dimensions" so maybe that's where I'm struggling. I didn't even realise there was such a thing. Perhaps you could enlighten me. Let's say I wanted to build a simple table to hold a small television; how would you size the pieces that make this up?
It must be good. Even old versions are still expensive on Amazon :P
Scanning the contents list it still reads like the others: let's go through a load of tools, let's go through a load of joints. Hopefully its interspersed with higher-level information about good structure as well.
The strength come from the fact that the weight of the cross member is borne by half of the up right. The sides of the cut prevents the joint from twisting, and keeps it square.
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These are a lot fancier that what you need but gives you an idea of what I am talking about. A brace is a piece of wood placed in the corner to create a triangle.
I suspect our problem is I am speaking English in America and you are speaking in English in the UK.
On 5/10/2013 3:07 PM, Alexander Lamaison wrote: ...
Well, it's been experienced carpenters and the like that take the kinds of work for the more and scale it down to the lesser...I've certainly seen a number of DIY'er type books on racks at the places like the BORGS and all in the States--I'd presume they're there, too.
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Well, virtually everything in the US for face frames, rails, things like table aprons, shelf uprights and shelves, etc., etc., etc., are nominal
3/4" stock w/ widths generally adjusted for appearance on most small stuff. Larger spans/heavier loads means wider aprons to provide additional bending moment/load capability. That's about it for structural design.
After that for joints, furniture is generally a combination of M&T and possibly some dowels for traditional craftsmanship; commercial stuff uses many shortcuts or machine-cut joints for efficiency and cost-savings. OTOH, fences and the like generally are just nailed/screwed together w/ perhaps a lap or slip tenon or something again pretty simple.
Again in the US standard lumber sizes are nominal 1x, 2x, 4x and occasionally 6x timbers for structural members like deck legs and the like. These are, as said, nominal rough/green dimensions whereas the actual stock as sold is 3/4, 1-1/2, 3-1/2, etc., ... Also, widths of construction lumber in the use are nominal less 1/2" pretty consistent.
Ply used to be all english units as well but now has unfortunately succumbed to the metric of being 19 mm (I believe it is otomh) which is just under 3/4" enough to be a real pita in thickness compared to actual lumber. It's still 4x8 sheet size, however, owing to the universal stud spacings of 16" for 2x4 or 24" on much 2x6 now so that at least sheet lengths and stud spacings still work.
I am sure there are standards similar in dimension but w/ metric numbers over there as well. It doesn't take much thought to realize that when one can by 1x4 or 1x6 off the shelf that 99%+ of projects will use one or the other but only a very pricey custom piece of work will actually specify 1x5 because it is exactly what a load calc says is need or the aesthetics are pleasing since it would require buying the 1x6 anyway and then onsite cut to fit.
Hardwoods or clear pine/fir/etc. for cabinet work or furniture are _not_ sold in prefinished sizes like construction lumber, however, but as roughsawn thickness (in 1/4" increments generally) and random width/length and priced on a board-ft basis. Some retail outlets will sell dimensioned stock but it will come at a very premium price markup as compared to market prices.
As for the Frid books, indeed Tage does talk a lot about using what when altho it's _not_ home repair or yard projects or fencing he's talking about--he was a furniture maker/highly skilled cabinet maker and that's the audience he's after, not the DIY'er homeowner.
If that's your target, you might look at the Taunton site again, but at the Fine Homebuilding site instead of Fine Woodworking. I can't make recommendations; I've not had interest in the genre so don't know about the selection.
There are also the trade journals and such like Journal of Light Construction (or very similar title) and there are, of course, trade school and college design texts altho my collection there is so dated that I'm sure none are still in print so won't name any even though the content is still valid for the most part for what an individual would find useful. What's missing in them are all the new engineered materials such as glulam, engineered trusses, joining systems, etc., etc., etc., ...
I can't help much on that front other than the generalities--I'm another who just picked it up from having grown up w/ such things so it just seems second nature. Doesn't hurt I suppose that am also engineer by training so structural and mechanics are sorta' routine even though I was a NucE/Physics guy primarily still had to have the rudiments of statics/dynamics/strength of materials, etc., ...
Actually, we call them the same thing. It's nice to have explained for once what lap joints are good fore. The books tend to skip that bit.
I'm familiar with braces, but I've never know when they are necessary and when they aren't. Once you start adding braces in one place, its tempting to add a few in others and hard to know when to stop.
There is no one path to learning "design", but there is one sure fire method to get you started on your own designs for just about anything you want to build out of wood:
Stand on the shoulders of those who came before by carefully researching the item you wish to build, then incorporate appealing elements and combinations of form and function, gleaned from your research, into your own design.
This can be as simple as clipping ideas from magazines, or using technology like Google searches using the "image" feature and/or websites like Houzz.
Then either sketch, or use a 3D modeling program like SketchUp (with which you already appear to have some proficiency), to finalize your design, paying particular attention to incorporating appropriate, traditional joinery methods that have stood the test of time
The more research you do, the larger your store of design ideas becomes, for both current and future projects.
I would like to chime in here. 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.
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?
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.
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.
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...
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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...
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.
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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
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 b=base 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.
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.
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