I agree. I was in the mud of language and confused about which direction Tim was describing.

That piece looks like a retrofit for sure. Cool though. Appears to be half lapped over the collar tie? And I assume the struts push down on a tie beam at floor level?


I guess I think it looks 'incomplete' when lopped off without interfacing with the balance of the structure. For a retrofit it doesn't make much difference - but if one is already running struts or principle rafters... why not let the KP span from tie to peak, assuming there aren't other compelling reasons to chop it short (either on top, or below)?

The king pendant is an interesting idea. It does offer the option of stiffening the rafters...

I'll be at the shaker village on the 25th and I'll see if I can get some more detailed pictures of this setup in the loft of this building.....

There are some different things going on in that building for sure:



Jim Rogers

STRUT. An axially loaded minor member in a truss or frame.

CHORD. In a truss, the major uppermost member (top chord) or
lowermost member (bottom chord). In a roof truss, the principal
rafters serve as top chords, the tie beam as bottom chord.

A couple more definitions for more clarity.

Mike you are calling the top cords struts. The struts are minor members, most likely supporting the rafters.

I think the confusion is in the definition, I am speaking of a truss not a roof truss, they are two different things, but could be one in the same.

I happen to believe that a 24' span of any member is too much, uncommon. In the beginning and through this whole process I have been sticking to that. The truss I proposed was only to support the center bent, allowing a clear span which the OP stated in the beginning as a concern, the first and third bents will most likely have walls under them, and no need of a truss. I further, do not see the need to build a roof truss for a 24' wide building. It is the economics of construction which lead me to that conclusion.

I was not the one who brought up leaving the king out. So I thought I would find an example of a truss where the king was compromised which is found in the portable document format which Mo and now I have posted in the same thread. It is there in a monastery, St Catherine's at Mt Sinai in Egypt (which Ken has pointed out has some concerns).

Furthermore, I would like to comment that a quick perusal of the pdf shows no truss with a span between members of more than 20' and unsupported from above, leaving 24' uncommon.

Tim

Hi Tim,

I tend to agree with your concerns about span and my experience is that when building widths go over 20 feet or so then additional support timbers are added generally in the form of aisles. I have seen buildings with roof trusses that clear span up to 68 feet but these are quite a rarity. Since this forum is providing help mainly to beginners then advice given on large spans is probably beyond the scope of this forum.

The St. Catherines monastry roof looks very odd to me. I have not seen any other roof that features these dangling king pendants / posts and given the great age it would be prudent to examine the whole building to establish what, if any, major modifications have been made to the timberwork in this building over the centuries (or is that millenia). Generally not many timber buildings survive beyond 6 - 700 years though recently I saw a TV programme that showed a Norse building in the Faroe islands that was over 1000 years old and this had been brought to these islands in "ready to assemble" kit form from Norway by the Vikings in their longships.

Regards

Ken Hume

I find it odd that we are concerned about designing a truss to hold up a 24' timber.

Yes, you can build a truss to hold up a 24' tie. But your argument of efficient building fails me here - build a truss to help hold up a single timber and then fail to allow it to do any other work in the structure? Without a loft or second floor in the design, the tie really needs to contain the spreading from the outward thrust generated by the rafters. Lower the pitch and this becomes a larger problem. Using that 'simple' truss that you designed solely to hold up the self weight of the tie... to do some real work up top... that to me makes more sense.

My point was that you are almost to the roof, and with little effort can solve the tension joinery required by allowing 24' of roof to push out on top plates and posts.

Strut, chord, correct.
A principle rafter truss like many in the PDF - the rafters are the top chords and internals are called upper chords. Anything internal to the rafters I would call a strut - mainly as that is what folks I've worked with / learned from would call them - and if they are not being used as principle rafters I would still call them struts - but your by the book definition works too. I have a hard time calling compression members 'chords'. Apologies for the confusion.

And note, from historical examples - aside from Jim's pictures of a retrofit - I think the common use of a truss is to hold floors and roofs. I'd love to see more examples of trusses that just hold up themselves. Those PDF examples are all integrated with the roof - mainly being principle rafter / chord systems with some arrangement of common purlins / rafters - much like mill and industrial trusses that I've seen in large masonry buildings from the turn of the last century and before that employ trusses on reasonable spacing holding up solid decking or common purlins and roof decking.

The only time I see trusses that are only holding themselves up (no structural ridge, no purlins, and no floor) is on new work, never old. I always assume that the designer either didn't understand what trusses are for or (more likely) he/she just wanted some eye candy. (which isn't necessarily a bad thing)

I see this most with architect-designed timber frames. (I'm not trying to start anything with that statement, just making an observation)

Hi Gabel,

I quite understand your experienced based observations however it is worth noting that the practice of sitting rafter couples (with and without collars), scissor braced trusses, principal trusses, tie beams or similar on wall plates without the aid of purlins, ridges, windbraces or the like is actually a very old practice nowadays referred to as "sans purlin". This practice can be found in very early buildings (11-1200's) through to about 1450. This type of roof is stabilised mainly by the use of masonary gables or hips and roof lath. It amazes me that this type of roof still survives but this simple approach to roof construction appears to be seriously long lived !

Regards

Ken Hume

Ken -

What holds up the sheathing / lathe / roof surface? Commons in between that sit on plates and butt or half lap at the peak?

Or are the 'trusses' packed together tightly to let the sheathing span?

Would love to see some pics or drawings.

Ken,

I, too, would love to see such a roof. I assume the trusses/rafters/etc are closely spaced, much like our modern conventional truss roofs?

Hi Mike & Gabel,

You can download a 3D panoramic photo rendition of such a roof from :-

http://cid-a449afe501ee8113.skydrive.live.com/self.aspx/Movies/Bishops%20Camera%20Roof.mov

This does not play like a regular movie but instead use your mouse to navigate inside the roof space.

The scissor couple spacing is 18" on centres but the gap between same is much less at about 10 - 12".

Some of the lower timberwork here is dated early 1300's but the upper scissor braced couples are a bit later at about 1375 possibly indicating a rebuild of the roof. Originally there was no ceiling at tie beam level and the roof was hence open to the camera below with only 2 tie beams crossing between the masonary walls.

Regards

Ken Hume