Wood Post and Beam Foundation
Wood Post and Beam Foundation
(OP)
I'm interested in anyone's experience with wood post and beam type foundations and specifically how they perform with shearwalls on top of them.
I just finished up a house with an exterior stemwall foundation. The internal supports were a fairly standard wood post and beam foundation resting on continuous concrete footers. The house was 72' by 40' by 25.5' high in a 135mph wind zone so the lateral forces were reasonably large. The designer had so many windows that I had to resort to interior shearwalls to resist some of the lateral loads, as well as the large size of the structure dictated it. These shearwalls with chord forces as high as 6000 lbs and base shear as high as 10,000 lbs had me quite nervous about this foundation type. In the end I opted to replace the foundation under the interior shear walls with a stemwall foundation.
I just finished up a house with an exterior stemwall foundation. The internal supports were a fairly standard wood post and beam foundation resting on continuous concrete footers. The house was 72' by 40' by 25.5' high in a 135mph wind zone so the lateral forces were reasonably large. The designer had so many windows that I had to resort to interior shearwalls to resist some of the lateral loads, as well as the large size of the structure dictated it. These shearwalls with chord forces as high as 6000 lbs and base shear as high as 10,000 lbs had me quite nervous about this foundation type. In the end I opted to replace the foundation under the interior shear walls with a stemwall foundation.






RE: Wood Post and Beam Foundation
RE: Wood Post and Beam Foundation
Mike McCann, PE, SE (WA)
RE: Wood Post and Beam Foundation
RE: Wood Post and Beam Foundation
However, it sounds more like a timber frame lower story with pseudo moment resisting connections.
I will leave that to the OP to clarify.
Regardless, my original comment stands.
Mike McCann, PE, SE (WA)
RE: Wood Post and Beam Foundation
RE: Wood Post and Beam Foundation
However, without the stem wall I only had 8" of footing to embed my anchor bolts in for some fairly hefty holdowns (HDU11 with SB1X30 anchor bolts) for a couple of the more highly stressed shearwalls. The uplift alone would probably crack the footings or not have enough developed strength and pull loose in a large windstorm or seismic event. Even if I were to thicken the footing to 12" that would still not be enough, the min. embedment for this anchor bolt is 24". To make the matter even worse if the holdown anchor bolts were at the footing level I would need to use a long extension bolt to reach the holdown up above which increases deflections due to the stretch in this longer bolt.
On top of that my calculated number of anchor bolt to handle the shear at one of these walls required a spacing of 15" o/c. With this type of foundation there is nothing to anchor into except to utilize the post base connections to pickup all of the load.
I guess one could make this work but you would have to turn the post/beam assembly into its own little shear wall underneath the main shearwall. This seemed liked a lot of trouble for nothing so I suggested we change the design to a stemwall foundation under these particular interior partitions.
RE: Wood Post and Beam Foundation
RE: Wood Post and Beam Foundation
If you can make yourself comfortable with the load path then I can't see it not working.
RE: Wood Post and Beam Foundation
Mike McCann, PE, SE (WA)