Building Details
Concrete Wall Basement
We recommend a 10" thick concrete foundation wall, at least 3000 psi. If you want to use thinner walls, we recommend against it. However if you do, all we require is adequate support underneath all of our posts. For this explanation, we will assume the recommended 10" thick wall.
One thing we require in any basement regardless of wall thickness or wall type is load bearing blocking underneath all of our posts. If the post is in the interior of the house, this can be accomplished with nailed and glued 2x material or a steel column. If it is steel, it needs to have a plate at least the same size as the post's footprint. If it is blocking, the blocking needs to be at least the same size as the post's footprint. If the post is on the foundation wall, then the easiest way to provide support is with load bearing blocking underneath the post. Another but more expensive way is to provide concrete pilasters underneath each of the posts on the foundation wall. THIS SUPPORT UNDERNEATH ALL POSTS IS NOT AN OPTION. If the support is not provided, it is not a question of "if" but when the subfloor will begin to sag underneath the weight of the posts.
The 10" thick wall has three main functions. First, resisting lateral pressure applied by backfilling around the foundation. Second, supporting the point loads created by the timber frame. Lastly, supporting the uniform loads of the SIP's and the subfloor. Since the wall SIP's are typically 4" wide (see the concrete wall section below) and they bear on the sill plate, this means the outer 4" of the 10" wall supports the wall panels. The inner 6" of the 10" wall supports the frame with help from the load bearing blocking. (See the concrete wall section below.)
The footing underneath a normal wall is centered on the wall. With our foundations, we specify a 30" wide footing that is 12" thick. This footing is offset so that there is 8" to the outside of the 10" wall and 12" to the inside. This creates a situation in that when the post's point load is transferred to the footing, then the load is centered on the footing. Because the load of the roof and the loft is carried by the timber frame and thus the timber posts, the only load that is on the foundation wall as a uniform load is the subfloor and the SIP's. The footing, while being offset, is still adequate to support the uniform load and the point load from the timber posts.
When we design a foundation plan, we show the details as we typically recommend. Due to all the differences in local building codes, we choose to leave some of the specifics of the foundation plan to the general contractor (i.e. depth below frost). On our plans, we specify the requirements that the foundation needs in order to support our frame. If the local codes dictate that the foundation should be stronger, then we have no problem. If the codes dictate a lower strength or don't say what strength is needed, then we need the foundation built to our specifications.
While a typical 10" concrete wall was assumed for this explanation, we do work with other walls. In those cases, we make sure that the wall style being suggested is sufficently strong enough to support the point loads created by the posts. A common suggestion is the ICF (Insulated Concrete Form) wall which is a great concept. However in practice with a timber frame, we have found that more reinforcing than normal is required for them to support the point loads. Again, this is just what we have found and is not by any means a statement that we will not put a timber frame on an ICF foundation.
Sections
Concrete Wall Section3D Wall Section
