When drawing up the plans for my timberframe, I asked the framer what the foundation connection should look like. Basically, I needed to know how the panels would be supported – there’s a 6-1/2″ thick panel and an 8×8 timber needing to sit on an 8″ thick concrete wall. He said that the entire thickness of the 8×8 timber should be supported by the wall (on a PT sill plate) and that the panels should overhang the foundation 100%, and basically be “hung” entirely by lag screws on the outside of the frame.
I have somewhat of a problem with this – I think the sill plate should be a 2×12, and that part of it should support the panels and part should support the frame.
Any opinions on this, or does it matter? If no difference, I’m going to install wider sill plates. FWIW, the first floor deck will be floor trusses and 7/8″ T&G plywood, but the frame will extend down to the sill plate and be secured (somehow).
Replies
Our timberframer usually asks for a 10" wall and an 11 1/4 sill. We provide what he requests. It is hard to believe that the posts need to be fully supported at the bottom to handle the load of the house. I know that many of the posts are sized by the number of joints that need to fit together at the most crucial points. Nonetheless, we do as we are told. Why not?
You ask why not? Well, even though he's an experienced framer, I am a licensed engineer, and I know a little bit about shear loads. I don't believe that the weight of a SIP, with fiber cement siding on the outside, screwed to white pine timbers, will NOT begin to sag or fail over time. Besides, I have since searched other responses to similar topics here and on other forums, and the consensus is that at the minimum, the near edge (closest to the frame) should be supported. If only to make the attachment to the frame easier by not having the panel hanging in midair. He's basically deferring to me with regard to design - he'll build what I tell him to. In this case, I want to get it right, but I need to know more, or at least get some heads nodding "yep, that'll do it".
I do believe that it's not necessary to have the entire cross section of the post resting on the sill. The weight of the house isn't enough to compress or shear each post by any significant amount. So, what I picture, is a 2x12 plate (11.25") with 6" overhanging the wall and 5-1/4" on the wall. That leaves 2-3/4" of each post unsupported, and there's no reason that "space" can't be filled with a short piece of PT 2x4 attached to the top of the wall. If I go with a 4-1/2" panel, I can avoid any fillers entirely, and that will leave more sill plate supported by the wall.
And if the 2x12's have a poor grain configuration or a checking flaw or are of poor grade and grain then the load will split the 2x12 in half down the length.
You are applying shear direction in the weakest dimension of the 2x12 timber.
Maybe a cantilevered steel plate under the sill plate/bolts/tmbers, or an anchored steel angle, or a corbelled brick ledge in the pour?
............Iron Helix
Edited 5/15/2004 7:29 am ET by Iron Helix
On my timberframe I built a 12 inch wall for exactly that reason.. The rule of thumb for any cantalever is to extend over something no more than 1/3 of the distance it's set back over. Anotherwords if you're gonna stick out 6 inches you'd better have it stuck in 12.....
He's counting on what amounts to a 1/4 inch diameter long screw to hold a panel tightly against a beam for hundreds of years.. The onlyreal strength in the panel is whatever there is in OSB and foam.. not a very convincing arguement for durability.
SIPs while lite for their size still weigh hundreds of pounds(depending on size)
what you can do is what I did where there wasn't room (for set back reasons) to put the full width foundation in place. I put Granite panels along side the foundation. they were two inches thick and secured with anchor bolts.. Granite looks far more elegant than a concrete foundation and it was surprisingly easy to do while not costing me an arm or a leg!
Another solution would be to cantalever the post over the interior of the foundation.. In other words put the SIP on the wall and then run another beam on the interior of the basement down to the floor..
For what it's worth, I dislike pressure treated wood. what I did was rather than use pressure treated (which is expensive and poses issues of health) was to have the local saw mill saw me up some tamarack planks instead. (you could use white oak or almost any decay resistant wood) and used those instead of pressure treated.
ps if you'd like to see my timberframe go to 3944.1