Post Frame Building with 3'-6" Retained Soil on Sidewall

[TroyD]

I have designed many post-frame structures using the Post Frame Building Design Manual criteria, but never one with 42" of retained soil against the sidewall! The client is working with an architect who detailed a wall section with sheet waterproofing along the exterior, and additional treated wall girts (in weak axis) to hold back the soil. He proposed encasing the post entirely in concrete below grade, but I don't see much benefit to that. At a minimum, the post spacing will likely need to be decreased and densely spaced bookshelf girts added to hold back the soil. I worry about long term creep and deflection of the posts from the lateral soil load. (Hard to find a timber landscape retaining wall that isn't tilting after a 10 years of use).

My research online suggests the alternative is a concrete retaining wall extending 6" above grade with Perma-Column embedded brackets supporting the posts. Has anyone else ever encountered this?

 

[CCox]

No I haven't encountered this. But I would use a concrete retaining wall between posts. Especially if it is only at one sidewall. What the architect wants is like a crappy soldier pile. If you do go this route, specify very permeable, well draining backfill, design for a conservative lateral pressure and maybe you could rely on the slab on grade as a brace at the top of footing. Using the slab depends on slab depth, building dimensions, and reinforcement.

 

[oldestguy]

The concrete surround "columns" will add to the overturning resistance of the wall, as compared to wood posts alone. Lacking soil data, I'd assume you are holding back liquid material weighing (lateral push effect) 80 pounds/ c.f. (conservative typical static load if not saturated). The stone surrounding a footing drain will plug up and malfunction unless the gravel is surrounded with a geotech filter fabric. Sizing the concrete column surround zone you might use a soil "liquid" weighing say 200 #/CF as a conservative figure on the faces tending to have lateral pressure. The footings may help in resisting the overturning. Of course if you involve a local geotech and he knows the soils there, different design numbers may result. With planning the floor slab also may be made to help resist the overturning.

 

[BridgeSmith]

"With planning the floor slab also may be made to help resist the overturning."

Using the floor slab as the 'heel' for the retaining wall is an elegant solution, if you can connect the retaining wall to the slab. Some reinforcing bars in the outside face of the wall, bent and extended into the bottom of the floor slab, and you'll have all the support you need, without involving the columns or footings to resist the soil load.