32x48 timberframe plans.. In need of some review

[Ernest T. Bass]

If the upper purlin could handle a little deflection without failing, couldn't the lower beam as well? I suppose it wouldn't be that much more work to throw a few extra posts around the perimeter, but couldn't the roof easily be clearspanned in that case?

The upper beams would make it easier to lay the rafters out, but maybe temporary framing could be used for that and then removed after the connections were made? I imagine the upper rafters should probably be gusseted to the lower legs with a plywood sandwich or something, and the ties would have to be well attached, but do you think the truss would need any more framing than that? It just seems unconventional to have the lower "legs" sort of floating without anything to make a triangle down there..

There are also going to be a couple of shed dormers that would be simple to frame over an upper beam framework that might need a little more consideration in a clearspan design...

'Can't tell you how thrilled I am for all of your help and advice!

[Don_P]

Well I'm glad I haven't driven you to distraction  .

I remembered, in the guild's free download, Sobon's "Historic American Timber Joinery" Chapter 3 fig 20 and the accompanying text are a good read on joist connections.

I have a gambrel barn done with just the plywood gussets at the pitch break and tie with no posts or purlins, 24' wide and 1/3 your snow load, 15 years and ok so far. I framed the pile of "trusses" on the upper loft floor and stood them up.

By the numbers the outer wall beam failed in bending not deflection. I guess if there is nothing under a beam, a fiber failure is going to immediately preceed a deflection failure  . But, if it deflects it will land and bear on the cordwood wall. Is that wall good for ~1250 lbs/lineal foot? How far below the beam will it shrink to? Will having a beam bouncing on it harm it? If it is capable of being called load bearing then a 2x6 on top of it as a plate is all that is needed. If it is nonstructural infill then the 8x12 beams supporting 6 rafter pairs @~2600 lbs per, are overloaded. We do already have posts at 12' spacing that will not change in height, if the cordwood does change height in my view a beam spanning that should not sag between posts. That's where I'm coming from asking for more posts, making the beam an independent, stable, known sill to launch from. I can understand other viewpoints.

Remember the posts are not under each rafter pair, but every sixth rafter or so. If there were a wall or very stiff purlin plate then I would be comfortable saying most of the upper roof load goes down to the purlin plate and travels down those posts. I just made the outer wall very stiff so it is definitely attracting load. If the purlin deflects much at all the entire load goes down the lower rafter to the outer wall. The rafter pairs on the purlins very near the posts will send a portion of their load to the purlin posts, how much is beyond me.

Where I'm going, if you use upper posts do keep them aligned over the lower ones. In other words if the posts and any type of potentially load delivering purlin is there, treat it like it will take that upper roof load. We haven't looked at what it does with a wind hitting the side. I can see in a dynamic situation one or the other posts driving down hard and delivering that load. I like the triangle in there for that reason but do not think it is necessary, it is bracing and can be a nice redundant load path, which is very good.

[glenn kangiser]

Thanks for the info, Don.  I looked up a link to the D/L - maybe I missed it earlier as I can't keep up with everything... [ouch]

http://www.tfguild.org/joinery/joinery.html

[Don_P]

That's the one Glenn, good stuff in there.
There I was studying in the "library" this morning, and saw something very similar to your graph on rammed earth in a book I had. The info is from the same source, just a different way of graphing it.

[Ernest T. Bass]

Sorry about waiting so long to respond; got a little distracted and needed some time to get the gears rolling in this direction again...

I'm definitely thinking of running this by an engineer, but the closer I can get to a finished product that I feel good about the less work he'll have to do.. I also trust your judgment very much, Don, and if you are content with the design it would certainly give me greater peace of mind..

Just a couple other thoughts here..

As far as the joist connections go, I think connecting the 4x8's over the girders with steel plates and lag bolts would be ideal from a structural standpoint, as you have said.. I won't know exactly what option will be most practical until we get a little further into the project, but it looks to me like the 8x8's would be strong enough even if the joists were dovetailed in, as long as they weren't notched much deeper than an inch.

I have no problem adding additional posts to the perimeter to reduce the spans down to 6'.. In fact, according to the calc, couldn't the outer/lower beam be reduced to an 8x10, even if the roof was clearspanned?

What if I were to consider the roof as clearspanned (more exterior posts to support the beam), but still had the interior 2nd floor posts and purlins to make the roof and dormer framing simpler, as well as add bracing to the roof structure? Couldn't the upper purlin be reduced in size, as it isn't attracting roof load, only wind loads and over-dormer-roof-loads? (Hope that makes sense.. )

I was also wondering about the earlier plan of the load-bearing upper purlins; my biggest contention being the installation of the 8x12x12' beams 20' in the air... This might be irrelevant at this point, but IF we went this route, couldn't we just pair up two 4x12s with bolts? The seams could be staggered over the posts too, for a nice strong connection..

[Don_P]

The mill called yesterday, my beams were out of the kiln. Just unloaded the trailer for my next project, a pair of greatroom trusses. 2 of the beams were dense #1 eastern white pine 8x10x20'. It's all KD to about 19%, green is about twice as heavy. I did get it all up on horses solo. One of the beams is heading off to the sign shop for some ecclesiastic carving before I begin notching it. The fun is about to start  . Did get some 4x10x20', no sweat, I could hold them up standing on a stepladder while you bolt.

Without going back through it, yes I think you are on the right track with everything but the joist notching...and I've led you to water there  . The engineer is working on part of my roof where I was optimistic. I made the numbers work but he wasn't thrilled, so don't assume that I am a tough audience.