Pole barn foundations: treated posts, laminated columns and concrete piers

A post-frame foundation handles three kinds of force. Downward force comes from snow, wind and the weight of the building and its contents. Uplift, as University of Wisconsin engineer David Bohnhoff summarizes it in a Frame Building News article distributed by the National Frame Building Association (NFBA), generally comes only from wind. Lateral force comes from wind, earthquakes and stored material pushing on walls. Because embedded posts concentrate those loads on a small area, foundation calculations are part of routine post-frame design.

Footing size follows from simple arithmetic: the downward design load divided by the allowable soil bearing pressure. Where the soil has not been tested, the Ohio Building Code (Table 1806.2) lists presumptive values of 1,500 pounds per square foot for clays and silts, 2,000 for sands and 3,000 for sandy gravel and gravel. ASABE EP486, the engineering practice for post and pier foundation design, does not require soil testing, but it lets designers use lower safety factors where tests have been done.

For wood in soil, the governing standard is the American Wood Protection Association's U1 Use Category System. AWPA places sawn building structural support posts and sawn agricultural posts and poles in UC4B, Ground Contact Heavy Duty. UC4A, the general ground-contact category, covers items such as fence and deck posts. The building code points the same way: Ohio Building Code Section 1807.3.1 requires wood poles to be treated to AWPA U1, Use Category 4B. The use category, preservative and retention are printed on the end tag, so read the tags before the posts go in the hole.

Two details decide how long a post lasts. Preservative only penetrates so far. Bohnhoff notes that 0.75 inch of penetration fully treats a post built from 1.5-inch-thick lumber but leaves roughly a 4 by 4-inch untreated core in a solid-sawn 6x6, so cutting or drilling a solid post near or below grade exposes wood the treatment never reached. Metal below grade matters too: he recommends silicon bronze or type 304 or 316 stainless steel for the nails and screws that attach collars and cleats to treated posts underground.

The NFBA describes laminated columns as typically three or more plies of 2x dimension lumber. It calls them more stable because of the laminating, and notes that the thin plies generally hold more preservative than a solid-sawn post of the same size. Columns may be nail-laminated (mechanically fastened) or glue-laminated, and AWPA U1 lists glued-laminated timber treated after gluing at UC4B for important structural uses in ground contact. When you compare quotes, ask whether each column is solid-sawn, nail-laminated or glue-laminated, which part of it is treated and to what category, and how it connects to whatever is below grade.

The other route keeps wood out of the soil. A concrete pier, precast or poured in place, goes in the ground, and a steel bracket at its top receives the wood column. Writing in Frame Building News, Bohnhoff called piers slightly more costly than embedded wood posts but listed real advantages: less treated lumber, wood columns roughly 4 to 7 feet shorter, and easier disassembly later. The drawbacks are heavier pieces to haul and set, and harder attachment of other parts to concrete. Cast-in-place piers conform to the soil around them; precast piers bring plant quality control but need a flat, level hole bottom.

One design point is easy to miss. Most steel brackets on cast-in-place piers are treated as pinned connections because they have little bending stiffness, so the roof and walls must brace the building instead. Wet-set brackets are pushed into fresh concrete; one manufacturer's installation guide calls for temporary bracing until the concrete sets, 7 days of curing before posts are attached and 30 days before maximum design loading. Ask which bracket is specified and for its engineering documentation.

Under the post sits a footing: a precast concrete pad, a poured footing or, in light designs, treated wood cleats. Many designs also need an uplift anchor, and an anchor that encircles the post is called a collar. Uplift resistance comes from the cone of soil the anchor would have to drag up with it, so a wider, deeper anchor holds far more. In tests Bohnhoff cites, 4.5 by 5.5-inch posts embedded 50 inches with no anchor averaged 1,400 pounds of withdrawal resistance; two 10-inch wood cleats raised that to 5,630 pounds, and 19-inch-diameter concrete collars exceeded 22,000 pounds.

Backfill governs sideways resistance. Concrete backfill gives the most, but it costs more and is more prone to frost heave in poorly drained silt and clay. Plastic post sleeves can cut uplift resistance unless the anchor is fastened through them. EP486 also sets installation rules: disturbed soil at the bottom of each hole must be compacted to match the bearing value assumed in design, and a post or pier must go in at no less than 90 percent of its specified depth.

Ohio Building Code Section 1809.5 requires foundations to be protected from frost by extending below the local frost line, by following ASCE 32 frost-protected design, or by bearing on solid rock. The exception is narrow: free-standing Risk Category I buildings of 600 square feet or less in light-frame construction with eaves 10 feet or lower. Local design tables set the depth. The City of Cleveland's published residential design criteria list 42 inches, Mahoning County's building regulations list a 38-inch frost line, and the City of Erie lists 48 inches. Other counties and townships publish their own values, and the department issuing your permit has the final word.

Depth alone does not stop heave. Bohnhoff's list: get the base of the foundation below the frost line, grade so water runs away from the building, fill depressions that form as backfill settles, avoid building on or backfilling with clay and silt, and watch for the sump effect, where a hole drilled into but not through a tight soil layer collects water. Concrete poured against rough hole walls, or in holes that narrow toward the bottom, gives frost more to grab. Drainage is covered in our site preparation guide.

• What use category and retention are on the post or column tags?
• What soil bearing value did the design assume, and was the soil tested?
• What resists uplift at each post: cleats, a collar, a poured footing or the pier itself?
• How deep do the footings go compared with the frost depth your building department uses?
• If piers are used, which bracket, and does the design treat it as pinned?

Those answers belong on the drawings. Permits are covered in our Ohio and Pennsylvania permits guide, and slabs in concrete floors.