Wednesday, October 14, 2009

Bracing Situation IV: Design for Compression

In the previous post I outlined the reasons why designing a brace for a hinged door or gate that relies upon a down brace (a brace in tension), is a poor idea and one that it bound to result in a sagging gate over time. It's a shame really that the weakness of wood in shear parallel to grain precludes designing around the tremendous strength of wood in tension parallel to grain. The next best thing however, is designing around the strength of wood in terms of compression parallel to grain. For those readers new to this thread who might be perplexed by the lingo I'm using, I might suggest taking a look back at the previous installments in this thread so as to get, er, on the same page. I hope that last phrase was not too murky for those readers who are not native speakers of English. It's a funny language.

As noted in the first post, the strength of wood in tension parallel to grain is 2-, 3- or even as much as 5-times greater than the strength of the material in compression parallel to grain. However, despite that, the strength of wood in compression parallel to grain still far exceeds, by a wide margin, its strength in shear or in tension/compression perpendicular to grain. So, designing around the use of compression is the next best thing, in terms of a descending list of wood's 'greatest strength qualities', and more importantly, is the very best thing overall in terms of the practical carpentry matters inherent in making a strong structure that resists creep deformation or shear problems without extensive recourse to metal fasteners.

When I took a look at the shortcomings of designing around tension, I first considered the simplest case in which the brace is simply cut and nailed/screwed bolted to its adjacent pieces (i.e., a rail and/or a stile). That form results in the strength of the connection being entirely dependent upon the fasteners (which are vulnerable to rot), and shear parallel to grain issues. With a brace designed the opposite way, namely for compression, and simply cut at the end to meet the adjoining pieces, we have an entirely different situation. In the compression situation, instead of fasteners doing the work, the entire end of the brace will bear the load, and transfer that load against the adjoining pieces. Any fasteners in this case will serve more to locate the connection than to actually fix it. Any down loads on the gate by gravity are immediately met by the end grain of the brace. If loads go up, say a heavy person decides to sit at the free-hanging end of the gate, then some deformation will take place - the brace will want to deflect and if the load is heavy enough and of sufficient duration, then one might expect the side grain surfaces of the rail/stile to crush to some degree. The main thing however, is that the up brace does its job as intended, and does so employing all of its fiber effectively, unlike a piece in tension which, as I noted yesterday, must be over-sized so as to have adequate material at the end to resist shear parallel to grain.

Borrowing again from the work of James Newlands, let's have a look at the up-braced form in its elemental simplicity:


Again, 'a' is the hanging stile, 'b' the rail, 'c' the brace, and 'W' denotes the load on the end of the gate. By the way,the term hanging stile refers to the vertical side of the gate on which the hinges are mounted - the opposite stile on the gate (if it has one) is termed the falling stile.

With weight, 'W' applied the the gate structure, the various parts and their connections are subject to loads - in the above case, the rail 'b' is put under tension, and the brace 'c' in in compression. As for the connection points in relation to the brace, the joint at each end will be in compression. The rail 'b' will have tension joints at both ends. On the surface, it may appear as if this arrangement is scarcely better than the one described yesterday, in a similar manner, for a brace acting as a tie.

However, one needs to consider the way in which a gate is going to be framed and mounted. The problematic piece in the puzzle is the rail 'b' which is in tension. The hinges of the gatepost typically - I should say ideally - have long straps to attach them to the gate itself- thus the upper hinge strap, which can be made to overlap a good portion of the rail, will act as a means to effect tension support at that location. As for the other end of the upper rail, the weakness at that location is that of shear parallel to grain. Like the photo from the previous post showing an example of a shear failure in a bridge post, the brace in this situation has the potential to shear off material from the end of the rail. The solution to this issue is to connect the brace to the rail at a good distance back from the end of the rail, so as to provide a sufficient amount of relish to resist shear loads.

Here's a drawing of a well-designed gate embodying the principles mentioned above:


Notice at the right of the picture the long upper hinge strap, and the set back of the brace connection at the upper rail. A distance of 10~12" would be sufficient set back for the brace in most cases. A falling stile, 'b' in the drawing, has been added, along with horizontal bars, e, f, g and h. The gate posts have been well-buried and bolstered with packed stone. If the bottom of a direct-buried post is well-drained and kept away from soil, it will last longer. Charring the post to make it less appetizing to insects and fungi is also a wise move. Typically, in such a construction the hanging stile is a larger section of wood than the falling stile.

Here's a cross section detail of the same ideal gate form, looking at the hinged end and in cross-section:


There are numerous subtleties to the design of this gate which may not be immediately apparent to the reader. Given that the hanging stile is a bigger section than the falling stile, usually about 0.5" fatter, the top rail is tapered along it's length so as to cleanly attach at each end. At the ends it will be tenoned into the stiles. This tapering is additional work to be sure, but the purpose is to lighten the gate at it's extremities. Similarly, the brace itself would taper - from say 4.5" tall in section at the lower end to perhaps 3" at the upper end. The bars e, f, g and h also taper as they move from the hanging stile to the falling stile, say 1~1.5" over their length. The upper rail is over-sized, as it is in tension, and the greater width of the piece compared to the brace, lower stile, and intermediate bars below it, and thus serves to provide a small amount of weather protection to the lower parts of the gate. The upper tie would further be beveled or rounded on top so as to drain water more easily - a process known in western carpentry as saddle-backing. The joint connecting the brace to the upper stile has some vulnerability to the weather, so the abutment in that joint would be ideally sloped so as to throw water from the joint, or, even better in my view, caulked on the long grain sides.

Note that the lower hinge does not need to be as long as the upper hinge, for the hinge, and the gate frame in that area, are subject to compression loading. The cross bars are horizontally oriented so as to reinforce the gate structure a small amount - vertical bars would only add weight and provide no resistance to sagging over time, as is evidenced on a great number of picket fences one sees out there.

The brace should be properly let into the tie so as to bear against it with a good portion of its end grain. Here's an example of that from a State Park gate I photographed up in southern Vermont recently - look closely and you will see the outline of the joint:


A slightly-improved version of the joint in the picture would take a little less meat out of the underside of the tie and reduce the stress riser effect, by tapering the top abutment of the brace. In fact, there are several better versions of that sort of connection than the one shown above, like these as three examples out of perhaps a dozen or more variations:


I certainly wasn't expecting to see that sort of joinery on the park's gate - it is soundly made but not a highly-refined sort of item.

Here's a look at the entire gate, note the lengthened hinge straps (unnecessary on the lower end):


I have looked all over the place for examples of well-made and soundly designed braced gates to take pictures of, and so far this is the best example I have come across in New England. That's a bit sad to say I guess, given the commonness of the braced gate form. I'll keep looking.

And what about 'X'-braced gates? At first glace it might seem like a certain minor amount of support might be gained from the portion of the 'X' that is in tension, however, as mentioned in the previous post, the tension tie component is really pretty much useless at 'bracing' anything. There's more fault to find in such a construction however.

The 'X' brace might be composed of either 2 or 4 pieces. If it is of 4 pieces, that is, a continuous brace, giving one leg of the 'X', with two smaller pieces forming the other leg, and the continuous brace is oriented in the up-brace manner, then this is the best possible case for 'X' braced hinged gates. In the best case, the non-effective brace components have become simply decorative, which is another way of saying they are useless work. They are something trying to look structural but actually aren't, and in fact the extra weight of the parts serves only to add more load on to the hinges.

If, on the other hand, the 'X' brace is of two pieces, which in order to be in a common plane need to be half-lapped into one another, then not only has useless work been competed, but the strength of the only useful member of the 'X', the up-brace part, has been weakened by half. Thus it is a inferior way to brace a gate or door. I think that such is the case in this picture, a close up of the one shown in the first post of this thread:


Here's another example of an 'X' braced gate/door, however this time there's nothing wrong with the construction from a structural standpoint:


The difference here, despite the similarity of the 'X' bracing in that above door to other gates and doors, is that this door is not hinged, rather it is track-suspended. The loads on the door frame structure are completely different in such a case. I wanted to show this as a good example where unthinking imitations of a form, as evidenced by hinged gates with 'X' bracing, can lead to unsatisfactory outcomes.

This concludes a look at hinged braced gates and doors - I hope the reader found it worth the read.

Next up: polygonal hoppers.

21 comments:

  1. Hello,
    That about the orientation of the crossbars was helpful. The whole series was good. Doing some fencing around the place myself just now. Even though I'm using sweet chestnut posts I'm going to try that charring.

    Thanks

    Don Wagstaff

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  2. Hi Don,

    glad you found the read worthwhile. The enemy of wood is the soil, since it is set up by nature to contain organisms that like to feed upon, or house themselves in wood.

    Another worthwhile step is to tar the end grain of the post. On the top of the post, it is better to cut it square than to bevel it, as beveling increases the end grain surface which increases moisture transport which leads to shrinkage, cracks, water ingress and premature rotting.

    Where did you get the chestnut from - is it salvaged?

    ~Chris

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  3. The bracing question is a good question, so I liked the posts.

    But nothing is simple, living in a rainy climate, my main concern for the braced doors is rot. I expect bracing to be inside and only visible when the door is open. For gates this is different and for traditional external window shutters the bracing is outside during the day. The diagonal parts will catch all the rain and release it in the joint. The result is that a correctly braced door (with external braces) will have the wood around the lower hinge rotting away first.

    Damien

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  4. Damien,

    you're quite right about keeping the braces on doors to the inside if at all possible, and I regret not mentioning that point, as the post more or less focused on gate structures.

    Gates and doors with exposed up-bracing will ultimately suffer from the problem you mentioned in regards to the lower connection of the brace to hanging stile. Better that though issue, over the long term, than the prematurely-sagging down-braced type of problem.

    Any wooden structure outdoors should ideally have a roof on it - otherwise the wood is pretty much out there to die a quick (typically no more than 20 years can be expected for most species) death, as it were, and should be considered as almost, dare I say it, disposable. Of course, it can't always be avoided - sometimes the wood will take a beating from the elements and a roof is not an option. In that case, careful design at the joint intersections, separation from the soil, and the use of highly rot-resistant woods (Black Locust, Ipé, Purpleheart, Mahogany, etc) is the way to go, or using less suitable woods perhaps and a commitment to regular replacement of the vulnerable parts (ie., the 'sacrificial' route).

    Thanks so much for your comment. There was more yet I wanted to add to the topic of braces and hinged gates/doors, but I decided to cut it a little shorter after 4 posts, which was more than initially envisioned.

    ~Chris

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  5. Damien,

    one point I failed to make in reference to this comment:

    "The diagonal parts will catch all the rain and release it in the joint. The result is that a correctly braced door (with external braces) will have the wood around the lower hinge rotting away first."

    While that is true, the same situation equally affects the inferior arrangement of a down-braced gate. Water will run down the brace and may seep into the brace connection at the bottom of the falling style and cause it to fail in time.

    I suspect the down-braced gate will already have begun to sag long before rot terminates it structurally, so again the up-braced gate is the logical choice.

    ~Chris

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  6. A very useful series of articles; thank you for spending the time to produce these.

    A couple of points which may be 'old wives tales' but when I was a young chap working on a farm we always used to 'plant' gateposts upside down (on the belief that the wood was likely to suck moisture up out of the soil if the root end was uppermost) and, if we had fair warning of the job would stand our gate posts in a barrel of a mixture of used engine oil and creosote for a month or two to act as a timber preservative. The tops we used to paint with a cold bitument paint (tar paint)to keep the weather out.

    Not much carpentary involved..!

    I'm planning on making a new back gate for my yard and will probably try to get hold of some Ash planking. It'll cost more but the softwood we get here in the UK is total rubbish and shows signs of failure in a few years.

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  7. David,

    glad you enjoyed the articles in this series.

    I am puzzled by the idea of 'planting' gateposts upside down (on the belief that the wood was likely to suck moisture up out of the soil if the root end was uppermost). For one thing, most of the moisture transport in a tree trunk happens from the ground on up, so one would think that the tree would 'suck moisture out of the soil' more effectively if it were planted root end down. also, why would you want the wood to suck moisture up out of the ground? The ground will regain the moisture from elsewhere, however once the wood become saturated you are setting up a situation promoting rot. finally, as tree trunk fibers are arranged in a manner akin to a series of stacked cones, as is the bark, the material sheds water well from top to bottom but not so well in the other direction. Especially with a sawn timber, placing it crown-end down in the ground would expose all the sliced wood cells in a position where they could most readily absorb moisture running down the surface.

    I always place posts in the orientation in which they grew, that is, crown end up.

    I'm surprised you are considering ash for your new back gate. Here in the US, ash (Fraxinus Americana) is considered non durable and perishable in outdoor applications. Maybe european ash is different somehow (?).

    ~Chris

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  8. I finally found the answer I was looking for, upward bracing is better than downward. Not sure I understand the physics, but at least I have an answer. I built a gate downward because my guide was a picture I found online with the downward method! I have another gate to build, it will be correct.

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  9. Anonymous,

    great comment and I'd be happy to answer your question, however I don't post any comments without a persons real name attached. If you'd like to re-post your comment with your name, I'd be very pleased to address it.

    ~C

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  10. I have read that a brace angled at more than 45 degrees is useless because gravity is against it. I would be interested to hear your view.
    Baz

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    Replies
    1. Baz,

      thanks for your question.

      A brace takes loads, whether they are applied to it in a horizontal or a vertical direction, as a vector relationship to the load direction, and determining the relative load applied to the brace is no more complex than basic trigonometry. This is a simple way of looking at the situation but a useful one.

      Take the case of a load of 1 unit applied to a braced assembly. Whether that load is applied in a horizontal or vertical direction, the relative load borne by the brace is 1.4142 times greater. If the brace were inclined 30˚ relative to the beam (horizontal) and a horizontal pressure applied, the pressure transferred to the brace would be 1.1547 times greater. If the same brace were transferring a vertical pressure, then the brace transfers 2 times the load. Similarly, if the brace were inclined at 60˚ relative to the beam, and the same pressure of 1 unit applied, then the brace would be transferring 2 times the load if applied horizontally, and 1.1547 times the load if the load is vertical.

      So, the angle to place a brace depends upon the expected load path on the assembly. If the load might be expected to be both horizontal and vertical, then a 45˚ brace makes the most sense. If the loading is primarily vertical (as it would be in a bridge structure, absent wind loads), then inclining the braces 60˚ from the horizontal means they are having to bear less load (meaning they are effectively stronger) than if the braces were placed 30˚ from the horizontal (where they would be effectively weaker).

      If the assertion is that braces angled more than 45˚ are useless because gravity is against them, then we are thinking of braces which act only against vertical loads. In such a case, if the brace were angled at 30˚ to the ground, the brace would be bearing 2 times the load. That doesn't make it "useless", it makes it more highly stressed than otherwise. The question then becomes one of whether the section of the brace is adequately large to resist bending moments, whether the connections between brace and beam and post will be subject to loading in excess of the ability of the wood grain to not be crushed, etc.

      Generally, if gravity loads are to be expected only, then the strongest framing element to resist that is a post. It is rarely the case though that loads are borne by a structure in one direction alone.

      ~C

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    2. Thanks for the comprehensive answer Chris. That all makes sense.
      Cheers
      Baz

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  11. Chris, Still trying to get my head around bracing loads especially in regard to gates.
    I am a furniture maker so I have a good idea of timber structural properties.
    But I have been asked to build a long narrow gate. (1800 x 800mm).
    A long diagonal brace in this case will not work, (I can tell, it looks wrong, science apart :).

    I am wondering whether a series of braces might work. For example: NNNNN. With the right hand side of the N's being the hanging style.
    Assuming tight joinery, do you think this could work?
    Otherwise I might have to hand the job off to a steel fabricator. :(
    Thanks for your time, lurve your blog.
    Rgds, Baz.

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    Replies
    1. Baz,

      yes, you are thinking along the right lines with your idea of the 'NN' bracing pattern. You could also add a steel tension rod or cable sloping the opposite way.

      ~C

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    2. This "NN" style makes sense to me, but I wonder whether the extra weight from the second N outweighs (sorry) the benefits of the bracing it provides. Your thoughts?

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    3. I guess one could slim the second brace down, in the same manner as making the falling stile thinner and tapering the rails out towards the falling stile, to mitigate any problems with added weight. I tend to think, without doing any calcs, that the second outboard brace adds stiffness in excess of any weight penalty incurred.

      ~C

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  12. Cheers Chris. I like the tension rod, that should sort it out. Rgds
    Baz

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  13. I actually ended up using only one brace at 30d leaving about a third of the outboard side effectively cantilevered.
    Rails, Stiles and Brace was 140 x 40mm Western Red Cedar with loose tenon joinery,(Domino). 4 per joint.
    I have every confidence in the final result.

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  14. Thanks for this page, Chris. It does a real effective job of explaining the how and why of bracing a gate. Look like I'll be doing something pretty similar to Bazza.

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  15. Here's a link to the finished gates...

    http://barrymills.net.au/HeardGates.html

    Bazza

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