Wood Post and Beam Type Foundations

Started by Medeek, August 31, 2014, 06:59:30 PM

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Medeek

I'm interested in everyone'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.

Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer

Don_P

That's quite a bit of wind. Could the exterior perimeter foundation have carried the forces and left the interior foundation conventional if the window walls were steel moment frames or had sections of strongwall panels?


MountainDon

#2
Quote from: Medeek on August 31, 2014, 06:59:30 PM
I'm interested in everyone's experience with wood post and beam type foundations .....

We built on a pier and beam in July 2008; 16 x 30, five 6x6 piers in two rows of 5 piers. Exterior walls all framed to 8 feet tall with 4 x 9 foot 7/16" OSB panels nailed vertically with the bottom of the sheets overlapping the rim joists. Rafter roof, no loft. Note; each pier on its own poured concrete footer, no continuous footer, no code compliance. It was built before I knew better.   :-[

This morning Karen noticed 9  ceramic floor tiles have a hairline crack, in a continuous line. This is about three feet in from a long wall which would seem to line up with the joint line in the hardi cement backerboard that was installed on top of the sub flooring OSB. The crack does not start at an end wall but starts about 4 feet from the end with the entrance door. I'd like to blame it on the lightning strike.   ;D  However, my guess is that it is more likely due to some foundation movement. We think this crack line was not there a month or two ago, but are not certain. The past couple years have been very dry; this last July was the 4th wettest on record and August was pretty wet too. Doors and windows still are as free as before, no cracks in any drywall, so movement has been minor. It doesn't take much movement to cause a ceramic crack though.

I doubt this info will help you but I can't miss an opportunity to bring this out in the open so other future owner-builders can take note.

FWIW, the piers on both ends were retro fitted with shear wall reinforcement between the piers, from the ground to the floor joists.


Other than the tile crack and the lightning strike we have had no problems. 
Just because something has been done and has not failed, doesn't mean it is good design.

Medeek

Thank-you for sharing your experience with a pier and beam foundation.

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 8"x18" continuous footings.  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".

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.
Nathaniel P. Wilkerson, P.E.
Designer, Programmer and Engineer