Log tie-beams

[peternap]

Here's the link Mike:
http://www.forestryforum.com/board/index.php/topic,29637.0.html

[Ernest T. Bass]

Neat design, Peter. I have a lot of timberframing books, but many of the designs rely heavily on intricate joints made by highly skilled craftsman, working with square timbers. It's a lot harder trying to find ideas for building a cabin with round logs and a chainsaw.. I'm sure I'll have to accept a level of imperfection in my work, but hopefully my chainsaw skills will improve and the resulting structure will still be sound.

I want to come up with some quick'n easy techniques and systems for building this cabin with only basic resources that can be applied to a lot of other similar structures that we may be building in the future...

The cabin is 32' long with very few interior walls, so I'd have to use posts to hold the ridge beam. This complicates the design, and I think the posts would really get in the way with such a narrow cabin. The other option is fewer trusses and a purlin design, but from what I've read that doesn't sound so fast and easy either..

The ceiling will be open regardless of where the ties go, but since the rafters will be spaced every 4' or closer, there's still a lot of beams right overhead that kind of take away from the open concept.

Glenn, do you think one could safely place the tie 1/3 of the way up the rafter (about 3' from the eve, in our case)? The rafters will naturally be more stout towards the bottom, which should help..

[Redoverfarm]

Ernest the biggest problem that I can see with either a log truss system or a rafter system is being able to make a birdsmouth or a flat cut for the rafters to sit on your sill plate.  And in doing so that your roof is level.  You will have two obsticles to overcome. One being the uniform elevation of your plate log and the other being the uniform thickness of the logs rafters. I had a problem with incorporating two different cabins under one roof.  In essence I had 8 corners instead of 4 and thats not counting the built on with the other two corners, But I could compensate that portion as it was built out of stick frame.

To solve this problem since I was using convientional framing for the roof I sandwiched  the top log with 2X8. That allowed me to level the top plate to the same elevation which was 5-1/2" difference from the highest to lowest elevation. Then I used a 2X10 as a top plate which was attached over the top log and nailed to the two sandwiched 2X8's.  But with yours there will be alot of adjusting to bring the top of the log rafter the same for your entire roof.  You might consider a rip face cut for the top of the rafter and then adjust your birdsmouth or flat cut accordingly off that face cut.  This way your sheeting would be level. I guess you could actually make your jig using that flat surface to makeup a truss and since everything would be measured off that flat cut they could be done consistant with each one being your plumb cuts and birdsmouth.

As for the ties I would probably attach them after I set the rafters if you are going to set them individually.  Get you rafters in place. Raise your tie log to the desired heigth and clamp in place or attach a 2X6 at the desired heigth below your rafter with 2-3 (3-3-1/2")deck screws. Then lay your tie beam on top of those to hold in place and clamp tight . Mark and scribe the locations of your lap cuts on both your tie and your rafter. Then with a chain saw ( Glenns favorite tool) make your lap cuts. lift it back onto your supports and bolt in place.

If I seem to be rambling on it is just because I can see how it's to be done but not easily translated. Make any sense?

[Ernest T. Bass]

Makes sense to me. No problem here with a level plate; I plan on cutting all the vertical log walls level with a chainsaw guide and a 2x4 temporarily tacked to the logs. The top plate or bond beam will be a course or two of 2-sided logs. I'd rather get away with only one course, but two courses with staggered joints seems a lot stronger to me. I guess it'll ultimately depend on how we decide to do the rafters/tie beams.

The rafters will be flat sided by a friend of ours with a portable sawmill down the road. That'll definitely help to make consistent cuts.

As for attaching the ties later, it would be more work but we'd probably get better fitting joints. If I wanted to put the tie on the plate and attach it to the rafter with a pipe joint as per my latest sketch, I'd have to fabricate the whole "truss" before setting it up.

Would the pipe joint be all that effective on a tie that is positioned a little ways up the rafter, though? Like I said before, it seems that the forces would be a little different. If the rafters are attached at the plate and the tie is attached closer to the peak, it seems that the leverage will be such that the rafter pairs will be trying to rip apart the tie under a load. So basically, if your bolt rips out of the pipe, the rafters are free to spread. Whereas if the rafter is only attached at the pipe joint (my pic) on the plate, the main force will be shear as the rafters try to slide over the joint. The pipe seems much stronger in this configuration..

I guess what I'm getting at is, is there any reason to go the pipe joint route if I place the tie higher up, or would it be better to lap the tie along the side of the rafter and just bolt it through?

[Redoverfarm]

Ernest don't know what to tell you about where to place the ties.  But if you look at convientional ties and their attachment I don't see why using logs ties with a motised lap and lag screws will not work just as well.  The surface area to bolt will allow at least two bolts for attaching, The mortise if it is 1/3 will enhance the shear along with the bolts.

I am sure Peters pipe is a proven design but it looks like alot of work to me. 

[peternap]

Earnest, I threw in a couple of different ways that it could be done. That doesn't mean that they are the best ways it can be done. Redover is right. It is a lot of work. While I can point out the weakness of lagging them in place...the fact is that it is probably plenty strong and it is easier. You've chosen a difficult route anyway. Don't make it harder just for the sake of it being different. Chose the method that will work, and take the least amount of time and energy.

[Ernest T. Bass]

I certainly agree with you there.. It doesn't look like the pipe joint would be at all difficult, though; drill a hole through the joint for the pipe and two other holes for the bolts. However, by the time you carve a nice lap, lag bolting (or through-bolting?) would be a bit easier. So, as long as it's strong enough I'm ok w/ it. <--- (I've been meaning to comment on him for awhile... Kinda makes you feel like a goof every time you smile. )

[peternap]

A time saving tip you may want to try for cutting the lap joints on the round logs is"

Set the depth on your Skil Saw and make a series of passes...(Old trick) nad use your power planer to finish (New trick). Cuts the time in half and gives a cleaner bottom.

[glenn kangiser]

I was saying like this unless you only meant collar ties.

Seems the above one with about a 1/2 inch all thread or larger through bolts and large washers on both ends to stop spread and  splitting over the bolt from too high a fiber stress would be pretty simple -- cope or flatten as desired for a good fast joint.  set it on the joist with the rafter passing over.   It would be pretty much a truss type setup.  I assume you are going with purlins anyway - can't remember if you stated that.

[Ernest T. Bass]

No, I haven't been planning on purlins or a built up roof, like most vaulted log ceilings.. Too much extra lumber and work, and since it's only a cabin we don't care if the roof is only around R-18 or so..

The rafter spacing just depends on how far our 1 1/2'' spruce decking can span. It's not t&g'd, but since we'll be installing 6'' of rigid foam insulation on it and the metal right over that, I think the weight distribution will be good enough that it wont make a difference.