Log tie-beams

[Ernest T. Bass]

Hi folks,

Me again, with a question regarding the best method of employing tie beams in our upcoming vertical log cabin. The cabin will be 16' wide, and I'm planing on an open log raftered roof. I would like to use pretty beefy logs (no less than 6''-7'' in diameter on the small end) for the rafters, and space them 3-4 feet apart w/ exposed 2'' decking over them. Now, on to the ties.. Would it be possible to attach them to the lower third of the rafters themselves, rather than notching them into the plate logs at the top of the walls? Boosting the ties up a bit would give a more open feel to the ceiling, and would simplify the plate notching process. My concerns are, attaching the tie (round log) to the rafter (also a round log) securely. I also understand that the lower portions on the rafters would flex outward under the snow load if the tie is installed too high, but how high would be safe? As far as the joinery goes, would a simple carved lap joined, fastened with a heavy through-bolt do the trick?

Thanks for your input,
Andrew

[glenn kangiser]

How about a slot in the end of the tie log with the rafter in the center assuming board rafters?  Drill and put a rebar pin or two to hold it together.

[Redoverfarm]

Half lap would probably do the trick but I believe I would modify it to a 60-40 lap being the 40% on the rafter and the 60% on the tie. That would not take away too much of the rafter. If you are not worried about whether it is offset or not then any combinations will work. Then you could auger through both and drive wooden dowels or countersink the tie log, drill and lag screw to the rafter. Plug the hole afterwards. The only problem with doing something like this is that you will have to handle the tie logs several times (up and down, up and down) to make the lap cuts. The lag can be done with them in place.  I believe the use of two lags per end would eliminate an scissor action.  Three would be ideal but to eliminate that pivot but you could get by with two (not much room) with a small dia log.

[Ernest T. Bass]

Log rafters, Glenn. I thought that doubled 2x rafters with the tie sandwiched between would look nice and be easy to construct, but we'd rather do things the rustic, labor-intensive cheap way.

You don't think it would comprise the integrity of the rafter to cut a notch in it, Redover? If I wanted to get fancy, interlocking dovetail joints would probably be the strongest, but I don't know how easy that would be to accomplish with round logs.

Offset ties is fine. I'm not planning in a ridge board, so the rafter pairs could be fitted with the ties before being erected.

Approx. how high could I put the tie and still not have to worry about the rafters flexing?

[Redoverfarm]

Ernest the question of how far up could probably be answered better by Pegg. As far as the dovetail I don't really think this would be the place IMO. Better suited for end to end log joinery or middle with a 90 degree configuration.  If you have some way to make them in "truss" fashion and be able to get them on your plate than maybe thats the way to go.

I think I read before of using 6-8" ties. If you went with even a shallow lap 30/30 then I don't think you will be weakening anything. In fact you will have more than most lumber ties.  But this would mean that the ties will not line up with the rafters(offset).  You could run the numbers past Pegg.  I thought there was a structural engineer showed up on the board a while back. Maybe he quietly drifted off.

[glenn kangiser]

How about carefully laid out and bolted 1/2 notches in each log.  No guarantees bur if you have a similar amount of wood to the proper sized standard lumber it should work.

...in other words half side by side -- vertically coped to fit.

How about cope the end of the joist to go around the rafter - kind of a V hollowed on top ..then the rafter bolted through both it and the joist to make a kind of big A frame truss.  Joist could go to the outside of the wall and tafter pass tight over the corner - or joint moved in a little and birds mouth the rafter a little.  Maybe a 1/2 inch or so all thread to bolt it through both in the V area.

Make sense?

[peternap]

Here's a nifty little pipe fastener that has a lot of uses:






If you don't want to use a mechanical fastener, just scribe the joint and leave it round.

[MountainDon]

Interesting method of joinery, Peter.

Something seems familiar about the tubes, but I can't place it.    Hex head bolts or carriage bolts head in filed square holes?

[Ernest T. Bass]

Nifty fastener there! Not sure how it could help in this particular situation, but I can see a lot of other uses. Looks easy to make from scratch....

How about carefully laid out and bolted 1/2 notches in each log.  No guarantees bur if you have a similar amount of wood to the proper sized standard lumber it should work.

...in other words half side by side -- vertically coped to fit.

How about cope the end of the joist to go around the rafter - kind of a V hollowed on top ..then the rafter bolted through both it and the joist to make a kind of big A frame truss.  Joist could go to the outside of the wall and tafter pass tight over the corner - or joint moved in a little and birds mouth the rafter a little.  Maybe a 1/2 inch or so all thread to bolt it through both in the V area.

Make sense?

Are you saying to set the "A-frame truss" like a regular truss, so the ties are setting the plate, under the rafter? I had been thinking of bird-mouthing the rafters and placing the tie up higher, but perhaps this could eliminate the pesky birds-mouth and make a stronger joint... The underside of the ties should probably be flattened a bit where they sit on the plate to make a stronger joint and keep all the "trusses" at the same height.

Yes, Redover, these things could get pretty heavy... It's a pretty narrow cabin though, and we could always get help and put 'em all up at the same time.

[Redoverfarm]

Ernest I would have suggested the ties to the rafters and plate but I thought you wanted to set them higher on the rafter. I set wall ties in my 16' wide hortizontal logs by notching out the top sill log and then ship lapped the tie log. Then used 18" rod through them to the next log lower as well.  Doesn't do much for the rafters though.  I am not sure how many you were planning to do but if there was a substantial amount you could tie them to the rafters as well as the sill log. Either way you can still assemble them complete whether the tie is up high or down low provided yu can devise a means to left or move them.

[glenn kangiser]

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

[peternap]

The pipe and bolt attachment (which I picked up on the Forestry Forums) is very versitle. If I understand ypu right, you want to attach your wall ties to the rafters.

The idea I had was scribe the end of the tie after cutting the angle so they fit flush against the round rafter. Then install the pipe bolt by inletting it into the tie and bolting straight down through the pipe. The end that attaches to the rafter would go straight through the rafter and be drawn tight. In other words, a T bolt.

Explaining is a tough but hopefully you get the idea.

[peternap]

Glenn beat me to it but I was using the pipe/bolt because I thought you wanted it higher (something like a low collar tie).

[Ernest T. Bass]

Glenn, is there a good reason for cutting a birds mouth in the rafter at all with this idea? I was kinda envisioning something like this:


Scalloping out the end of the joist/tie like you said, and spiking the rafter to the top plate through the joist, all in one shot perhaps. (The joist could just get temporarily attached to the rafters on the ground to make installation easier.)

Yes, I had been talking about raising the ties up higher, but if this avoids some tricky joinery.... Scalloping the ends of the ties doesn't sound like the easiest thing to do, but perhaps a helpfull jig or a good system could be devised... That's the tricky part about working with round logs. You always have to be creative and use lots of jigs to get good results, but once a system is developed it isn't always so hard to do..

Peter, do you think that the pipe joint would be strong enough in this application? Maybe I'm not understanding you right, but couldn't that pipe get ripped apart if there was enough stress on it (ie. heavy snow load)?

[glenn kangiser]

The pipe joint is a good joint.  It puts the brakes on slipping at the joint.  The birds mouth would help to do similar.  The only thing that would be holding in the method you show would be shear of the wood on the bolt or pin.

The wood may want to split as it pushes against the pin.  The ridge wants to go toward the ground, and the pin would be the only thing resisting the force. 

It is a bit more of an engineering question, but without an engineering requirement or desire to use one, you need to make it plenty strong

[Ernest T. Bass]

Yes, I can see where a birds mouth is going to use gravity to minimize the shear on the pin/spike used to secure it at that location, but isn't the bolt attaching the joist to the rafter in your diagram  the only thing holding the building together? It seems that the weight of the rafter on the tie (my diagram) would help lock the tie in place, where as the birds mouth (your diagram) helps prevent the rafter from slipping down. Perhaps the to methods could be combined, with a small birds mouth or some other means of better pinning the rafter to the tie, perhaps with a pipe joint or something.

You wouldn't be able to use a pipe joint in conjunction with the scalloped joint though, could you? Seems that you'd need a flat surface running perpendicular between the two members in which to place the pipe. Also, not to get picky or anything, the exposed pipe isn't the most attractive piece of hardware around...

[glenn kangiser]

Yeah - I didn't take time to show the connection the top plate or beam or bond beam.  It would need that too.

Some connections used the 1/2 coped I mentioned before - vertical cope, then had a pipe ring sandwiched between the two - maybe 2 or 3 inch.  The pipe wouldn't work well with the scallop.

The weight is minor compared to the leverage pushing down and out. 

Seems we need to figure something better out.

[Ernest T. Bass]

So, couldn't you just flatten the underside of the rafter a bit and put a pipe in like this:


If the pipe was a little short (recessed), perhaps the holes could be plugged if it looked too bad..

EDIT:

Actually, you wouldn't see the joint at all if it were installed as I pictured it... But, The tie could be moved up the rafter, right? A birds mouth would need to be used in that case..

[glenn kangiser]

Looks like that would work -- probably need nuts on both ends of the bolt -- all thread  to get into the pipe without having problems. 

Just jamb a couple wood plugs in the end of the pipe. - I doubt you'll be able to see in there anyway

[Ernest T. Bass]

After thinking a bit... Isn't the design I have pictured going to be stronger than if the tie were moved up the rafter? It seems that, right now all of the weight of the roof is going to be trying to slide the rafter down, which the pipe effectively prevents. However, if the tie were up higher and the rafters were attached to the top plate, won't the forces be "pulling" on the joint more, rather than sliding on it? Maybe it would still be fine, but the pipe seems stronger in the previously mentioned scenario..

What size pipe would be adequate, do you think? 

[glenn kangiser]

It is strongest at the  bottom.  Higher up, gravity tries to pull the peak down which pushes the legs out -- if the rafter is capable of bending.  At some point you will have a meeting of your required strength and snow load etc.  Above that point flexing will occur- too much above - failure will occur.

[glenn kangiser]

Pipe size - 1 1/2" ? 2"?

[Ernest T. Bass]

Thanks for the ideas; it certainly gives me a lot to chew on. I'm going to have to think about how these joints could be easily and accurately made... I had a great plan before for a rafter peak and birds mouth angle jig, and now I'm going to have to start all over.

[peternap]

Earnest T, 6 months ago I was sitting here asking the same questions. What is happening is that we are getting away from stick framing and into post and beam with an occasional timber frame idea.

Timber framing and post and beam are the original and to my way of thinking, the better way of building. Stick framing came along as a fast, cheap mwthod to get the houses built.

The pipe joint for instance, is a modern method to combine the two disciplines.

I consider the tie, the most important member that goes on top and the rafters can be delt with easily without a birdsmouth. What your doing is more difficult but workable. As with most things, there are a hundred methods to solve one problem. You just need to find the onw you like.

You need to start looking at cathedral ceilings since that's essentially what your doing by raising the ties.

Again, a lot of solutions...you just have to find them and pick one.

This joint is a very old and not very well known solution to that area. For some unknown reason, it's called a sprocket. Just one more idea to consider.




Now if you go with a heavy enough ridgebeam, properly supported at both ends, you don't have to worry about the rafter spread at all.

[mvk]

peter

Sprocket joint. Thats pretty interesting where did you find it?

Mike