Ribbed Circular Design

I've been studying the Timber Frame Joinery & JDWork Design Workbook for the last few months along with some other structural design material. I have access to quite a bit reclaimed timbers; most of which are 7 ft or less; and have come up with a design I am happy with that fits the timber lengths I have (with very little additional purchasing). Unfortunately, the book doesn't deal with this kind of design, in particular ribbed framing, that would accommodate the material and explain load calculations.

Attached is a link of a quick example in Sketch-Up (by no means a blueprint, but a rough estimation). Does anyone have any ideas or criticisms?

File can be downloaded here:
http://www.filedropper.com/ribbed

For your inspiration. This is what immediately comes to mind when I think of Ribbed circular roofs. I can't view your file so I don't know if this is what you had in mind or not.

This is a Baroque (I think it's baroque anyway) German technique known as Bohlendach, which literally means plank roof. The idea is that the roof truss structure is made of many short timbers instead of long ones, which allows them to make radiused roofs.

Those are beautiful! But not what I had in mind, the material I have goes over this type of architecture. The image above probably better explains what I am trying to accomplish but I haven't seen anything related (quite possibly because its not do-able).

something like this would make your design pictured conceivably doable. If you can't tell, this has each 'rafter' set comprised of 3 layers of wood planking pegged or bolted together.

This one is the same basic idea, but better shows how it is incorporated into the rest of the structure.

The key to this design is the multiple layers of overlapping planks, which effectively transfer loads from one part of the curve to the next. You won't get that with simple end joined timbers. This of course uses 'planks' and not 'timbers' The first roofs pictures use timbers, but are a combination of a vaulted interior and a straight sloped exterior.

You could accomplish the same thing with larger timbers, The trusses shown here are technically light trusses, in function similar to stick frame trusses. If you would make them heavy trusses, which support purlins, then I think you could use the same basic principle with heavy timbers

Here is what I had in mind, a very very rough sketch using a simple semi-circular arch.

The timbers would not be simply end-joined. The underlying timbers would have a special bladed joint, and would be pinned/bolted from the bottom and the sides for strength -a lot of pegs here.

Now here is a thing about arches that you have to keep in mind. An arch has a tremendous amount of outward thrust at the bottom, that is the reason for the massive buttresses of medieval vaulted churches. The Romans understood this and actually used arches to brace each other. The building that you have pictured would topple over under its own weight, because there is no way to handle this force.

There are 2 simple solutions:
1: Use a tie beam, which has the obvious disadvantage of interrupting the interior space.
2: Incorporate external aisles with simple shed roofs into the design, if designed properly these would buttress the arch structure.

Keep in mind that my design is a quick sketch, you could use more curve section to achieve the desired radius. The top timbers might be best to have a specific shape (i.e. not be straight on the top) to both form a smoother outside curve and to transfer loads more effecively. I will work out some ideas...

also, the shallower your arch the more outward thrust there will be.

There are a few geometric arches that will handle this situation a little better than the simple half circle, if you are interested I can put up some pics of them and how to make them.

Either way, the simplest arch is not the best in this situation.

I'd be very interested in purchasing the book with the illustrations you posted.

Sprichst du Deutsch?

The illustrations are from a German website http://www.payer.de/tropenarchitektur/troparch047.htm

some of them are my own and have no publication.

Here I have worked out some timber details, a possible way of creating a heavy timber version of the plank roof. An adaptation of your pictured design. Something like this would be difficult to accomplish on a very large span. The outward thrust is going to be enormous.



The outer timbers are the longest, the longer these are the better. Their odd shape allows them to form a nice radiused exterior as well as aiding in load transfer. The cut off at the end help direct load paths.

The smaller timbers are joined into the larger ones, the dotted portion is a blade (not sure what you would call it exactly, a tenon? A blade?) that joins into the center of the larger timber and is pegged through to hold it in place. the orange markings show general peg locations.

The flat top is a possible issue, however it could be used as a good opportunity to put on a vented ridge.

There is a lot of joinery involved in this as you can probably see. This is adapted to functioning as a truss to support purlins.

Amazingly helpful information... thank you sir. It appears that this designs offers no benefit to a standard straight A-Frame design other than simple aesthetics. Ill certainly play with this some more. Again, thanks for the information. You're quite talented as well.

it does have the advantage of allowing you to use short timbers, which it sounds to me is your goal you have in mind.

Whereas most roof structures rely on trussing action and/or triangulation to bear roof loads, this uses the properties of the arch to do the same. Think of Roman Aqueducts to get a good idea of what arches can accomplish.

Instead of a tie beam, you could use tensioned steel cables/rods to solve the horizontal thrust issues. This would lessen the impact on your open interior.

Cheers, Chuck

Based on the info you guys gave me I made a full scale digital model. The structure is 24'x24' @ 18' high using 6"x8" timbers exclusively and makes use (if I understand correctly) of the ideas presented. The joints where a little more complicated than I thought they would be but I am confident my 3 axis CNC router can cut them properly since none of the pieces are longer than 6 ft. Obviously i'll have a structural engineer sign off on the structure first, but I wanted to get some feedback and any criticisms before I move forward with the math. Thanks again for all your help!

I have thought about the joint I had presented earlier, and have actually made a few modifications to it. The reason is that, as designed, it relies exclusively on the shear strength of the pegs or bolts. It seems to me that it is necessary to have an interlocking joint that resists shear by its own virtue, with pegs/bolts only adding to this quality. It's hard to explain what I am taking about, when I have the opportunity I might put up some drawings to show what I am talking about.

two things I notice about your design are ingenious:
1: The use of the lancet arch. This arch is the strongest form of the common arch. The Romans would have used it if they could have figured out how to make it. This is one of the greatest achievements of the Middle Ages.
2: The arch continues to the ground. If the arch is fastened directly to the foundation, the problem of outward thrust is totally eliminated. The ground itself will handle this for you.

The concept with my original design was that the interior faces of the timbers would form a continuous, uninterrupted arch. This was accomplished by applying two octagons on top of each other, 1 rotated in relation to the other. The reduced ends made a similar figure on the exterior and helped direct load forces through the arch. This design was adapted to a simple arch, and would require a lot of revision to suit a lancet arch.

Just a suggestion, but it might be interesting if you designed your structure with cross vaulting, which would give you a very interesting interior



The advantage of the groin vault is that the loads are 100% transferred to 4 spots, and the two crossed vaults oppose the other's outward forces. The result is you have almost purely vertical forces. The arches will hold each other together. This fact is the real reason why medieval churches were built cruciform, it reinforced the otherwise unstable long vaulted halls.

The disadvantage is that the framing would be quite complex.