"Single Arched Braced Truss"
#22634
02/12/10 12:33 AM
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mo
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Howdy, I got a question for you good people. What do you all think of Chappell's "Single Arched Braced Truss"? Has anyone built one of these? When looking at its strength my intuition is at a loss. Im assuming that the arches are in tension holding those feet together. I would think the pegging there should be offset to prevent one line of a tension on some grain. Other than that what do you all think of this configuration? Would some forged iron be needed here? Conditions: (this is for you Mike, ;)): All things being equal lets say 20' span Anybody want to run through the members and tell me whats in compression and whats in tension, and...where the greatest magnitude of those forces would be? Thanks
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Re: "Single Arched Braced Truss"
[Re: mo]
#22638
02/12/10 02:38 AM
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Thane O'Dell
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Tension or compression would depend on the stiffness of the walls. For example: If the walls were made of concrete or stone like in an old church then the truss would only be in compression. Likewise if the walls were flimsy and relying on the roof to hold them together then the truss would be in tension. The arches would almost have to be made from two pieces which means there will be 4 joints being pulled apart. If there were keys and bolts fastening these joints then I could see this working as a tension truss. This is just what my intuition tells me.
Life is short so put your heart into something that will last a long time.
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Re: "Single Arched Braced Truss"
[Re: Thane O'Dell]
#22639
02/12/10 02:53 AM
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mo
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Hi Thane, Masonry walls. Guess I need to ask the mason what the walls can stand. 16" dia should make those arches one. I guess it was a trick question. If these are indeed "trusses" there should be no thrust by definition only a gravity point load. I think it will work. Since they are masonry walls... maybe this
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Re: "Single Arched Braced Truss"
[Re: mo]
#22640
02/12/10 02:58 AM
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TIMBEAL
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First, I would say it depends on the wall construction, and the deck in the back would give some assistance. It almost follows a hammer beam configuration but minus some pieces. What is worse than a hammer beam truss? A hammer beam truss missing some pieces. With the recent thread on drift applied and my dislike for buildings without ties at the proper level, I give it a thumbs down. Maybe put some sheet metal on it somewhere Now at 20' you can push some luck or see a qualified engineer. I see the rafters bending just below mid point from foot to tie. The ends of the arched braces are thin and wood was removed from the rafter for the housing and the mortice, less section there. Tim
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Re: "Single Arched Braced Truss"
[Re: TIMBEAL]
#22641
02/12/10 03:06 AM
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mo
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Hi Tim, Ties at the proper level are not an option, at least over the loft. Im assuming you posted about the first concept, which frankly I don't like either. Now the second one reminds me of a plumb compression ring, much better I think. I forgot about sheet metal!
Last edited by mo; 02/12/10 03:07 AM.
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Re: "Single Arched Braced Truss"
[Re: mo]
#22642
02/12/10 03:20 AM
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TIMBEAL
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Yes the first.
I was looking through English Historic Carpentry, Cecil Hewett, and found a few examples more looking like the second posting. Knowing the walls are concrete and the box outs, as well, helps the issue.
On page 219, fig 199 of said book, is an example I like. The arched braces are thicker and full pieces running from below the plate to the tie, really beefy. Kind of half crucks.
Tim
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Re: "Single Arched Braced Truss"
[Re: TIMBEAL]
#22643
02/12/10 03:46 AM
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mo
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I really need to get that book.
In the second, I think I would like to have the corbels carry most of the load. That being said, there is also the point where the principals hit the plate. Im thinking the rafters will have a bearing nose that falls into the housing at the plate and that surface and only that surface should carry any load from the rafter. If that means I need to cut the level cut over the plate 1/4" proud of the plate then so be it. I just would not want any shrinkage or other factors to have that rafter rest on the top of the plate creating some shear forces. Still there seems to be a fact that the rafters (given to bearing points: 1 plate, 2 corbel) there might be some forces trying to pull the joint between principal and arch away from each other. Perhaps the bending of the rafter will negate this. Anybody have thoughts on this?
Thanks
Last edited by mo; 02/12/10 03:50 AM.
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Re: "Single Arched Braced Truss"
[Re: mo]
#22644
02/12/10 03:48 AM
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Don P
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c'=(Etb/a)/[2(l+u)(g/p)+K2/(bt)^2] well for the sheet metal diaphragm of course in the first picture I'm picking the arch and the kingpost as the tension members. Why not put the rafters' feet on the arch?
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Re: "Single Arched Braced Truss"
[Re: Don P]
#22645
02/12/10 03:51 AM
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mo
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Don,
Good one! Good question. That sounds like a good possiblity. It would almost turn into a scissor truss at that point. Maybe a scissor truss is the way to go.
Now that I think about it. That is a great question. It seems to be better to terminate the compression member into the tension member, instead of the other way around....
Last edited by mo; 02/12/10 03:54 AM.
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Re: "Single Arched Braced Truss"
[Re: mo]
#22649
02/12/10 12:03 PM
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Thane O'Dell
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If at the point where the lower arches (not shown in the original picture)contact the wall, there was a block protruding to provide a shelf so to speak, then this truss works like a dome with the rafters and tie holding it together at the joints. Actually, with the addition of these lower arches and some possibilities, I rather like this concept.
Life is short so put your heart into something that will last a long time.
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