Wood Post and Beam Foundation

[medeek]

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.

 

[jdgengineer]

I have personally never and would never try and use a post and beam system below a shear wall to resist lateral forces. I'm located in the bay area so we have fairly high seismic forces so maybe other regions are different. Contractors we work with wouldn't complain too much about needing to cut in a new footings. Its fairly common place.

 

[msquared48]

Talk about a soft story situation... Why would you do this outside of a structure sitting on elevated piling?



Mike McCann, PE, SE (WA)

 

[hokie66]

I am confused by the terminology here. A foundation, whether a shallow footing or a deep foundation, is the element of a building which is in contact with the ground and supports the superstructure. If a post sits on a footing, the post is not foundation, but rather superstructure. And a beam supported by the post is not a foundation element. How did this terminology go astray? And why do some persist in calling footings "footers"?

 

[msquared48]

Well... if the columns are embedded in drilled concrete footings, then that system is a "foundation" system.

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)

 

[medeek]

I'm not sure I follow your comments msquared48. I guess I'm a little loose with the terminology. I stand corrected, from now on I'll only use the word footing.

 

[medeek]

The problem I have with this type of foundation is that even though you may have an incredibly strong shear wall sitting on top of a pier and beam foundation how does it effectively transfer that shear to the footing below unless it is just as braced as the shearwall above. The architect had 4x8 beams supported 48" o/c with 4x6 posts that were anchored to an 8"x18" continuous footing. I do give him credit for going with continuous footings instead of separate little pier footings, much less chance of differential settlement and lateral drifting.

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.

 

[medeek]

I just recently started a similar job with almost the same situation. Interior transverse shearwall on top of post and beam foundation resting on pads. I did find a detail (FEMA) that showed a shearwall with holdown on top of this type of foundation so maybe I'm too conservative in my thinking.

 

[jayrod12]

You can do this. Your beams and posts need to be designed for the loads they are going to see. As for the base shear again the key is in the details. How does the load get from the wall to the beam and the beam to cross bracing?

If you can make yourself comfortable with the load path then I can't see it not working.

 

[msquared48]

The figure is wrong in that the lower dashed hold-down will not work in that orientation. It would be better to extend an A36 threaded hold-down rod through the beam below and install a bearing plate and nut to develop the uplift. The floor support beam would also have to be attached to the pile to develop the tension.

Mike McCann, PE, SE (WA)