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

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.

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

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

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!

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

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

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?

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....

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.

you'll need iron (or steel) at the arch to rafter and arch to arch/collar connection or buttresses at the location of the xframes. My read is that in the 2nd configuration, the lower arch pieces are decoration and the upper arch braces are in tension. you will also need deep rafters because you will be loading the rafters in midspan with the tension connection from the arch.

we did a hammer beam roof that sat on masonry walls with buttresses once. the buttresses were poured concrete and were basically full height walls 12" thick that stuck out at 90 degrees to the main walls. They stuck out about 8' if I remember correctly. Big old buttress. But the span was about 56'.

Hope this makes sense, I'm a bit rushed -- dinner plans.

Well - I got the span, what spacing are they on?

Point load the king post with ridge? Purlins? Common rafters?



You'll definitely need tension steel in there. Roof surface and load will tell you how much. I have a shallower pitch version that we did (sitting on posts as bents) @ 42' span. I can post a pic when I get back to the office next week.

The closer you get the lower curve to the end of the rafter the less shear you'll have - so piece sizes can be reasonable.

Note too that not all masonry can take outward thrust. You can certainly design for it - but in 1 case I designed for I couldn't get the engineer to give me any value for the stone...

Hey Guys,

Thanks for the insight. Gabel, you say that the lower arches are decoration. I think you mean that the load of the roof is carried by the rafters at the plates, and the lower arches in a sense are just "hanging" there. Is this correct? I wonder if there is a way for those to actually carry the load of the roof more than the principal rafters? This might somehow be convoluted, and to indepth for a forum, but if you load the lower arches in compression and have it's previously mentioned tenon butt against the upper arch tenon can this negate some of the tension in the upper arch? Anybody have thoughts on this? Im thinking like Thane. I see a dome or "arch" under there and all of an arch is in compression.


Mike, by the way, we just got 8" of snow this past weekend! HA!
Id also like to see the pic if you have time.

Only if the lower arch is well supported and the wall resists outward thrust. Otherwise some steel gussets at all joints will be required to hold tension. This way the wall and the truss can share the resulting forces.

More to your PM inbox, but this is a New Energy Works project from years ago. Steel framed walls with masonry, tension tie steel across the trusses, oak.