Agricultural Post Framing
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Post framing differs from stud framing in that the vertical members extend from the footing right to the trusses, are typically large dimension pressure treated posts and are spaced much further apart. Post frames have a splash rail instead of a bottom plate to hold the frame rigid at ground level, and instead of a top plate for trusses to sit on, a plate beam is used. Post framed buildings are best suited to low-cost, non-insulated applications
If a building is to be built under section 9 and supplementary standard SB-11 of the Ontario Building Code (meaning it is under 600m2, of low human occupancy and not more than three stories in height) then there are requirements that must be followed to avoid needing a professional engineer to design the building.
Under all of the tables “Post Sizes for … Farm Buildings of Low Human Occupancy” whether the barn is knee braced or diaphragm braced, the same seven notes apply.
- Designs are based on load combinations of total roof load and wind load acting at the same time on a closed building.
- Posts shall be oriented with the long dimension parallel to the building width.
- Bracing systems shall be specified by a competent designer.
- Posts shall be situated on footing and shall be anchored to prevent wind uplift.
- Posts shall be constrained against lateral movement at ground level and at the footing. Concrete floor, splash-rail and uplift anchor help to meet this condition.
- Post designs are based on partial fixity condition due to embedment in soil.
- Footing excavations should be backfilled with parent material unless otherwise specified by a competent designer.
The first note is important because it states that designs are based on load combinations on a closed building. If a building being constructed is to have open sides, such as is often the case of a hay shed or manure storage, it can be interpreted that the building will require competent design to be compliant to the code.
Post frame buildings are almost only used in the agricultural setting, and their method of framing is quite different than typical stud framing, as a result, section 9 of the building code does not address post framing at all. Therefore, the MMAH Supplementary Standard SB-11, “Construction of Farm Buildings” exists. The issue is that SB-11 leaves a lot room for interpretation in what it lacks to say.
One of these grey areas is for post footings. It merely says: “Posts shall be situated on footings and anchored to prevent uplift”. The size of said footing or anchor is not to be found anywhere in the building code so it can be presumed that this is up to the discretion of the builder and the Chief Building Official (CBO). When drawing your building you might want to first go to your CBO to see if they have any specifications for footing design. One option is to reference OMAFRA Publication 809 “Farm Building Standards: For use in Sizing Common Building Components”. Though this OMAFRA publication is not a legal document, most CBO’s will accept its recommendations. It is only offered in print and can be found on the Publications Ontario Website by searching “Farm Building Standards”.
If this is not accepted, then you may be required to get professional engineering for the footings, or you could make the case that a column is like a post and so the details for columns in section 9 would be as follows:
Unless the footing is situated on gravel, sand or silt in which the water table level is less than the width of the footing below the bearing surface the footing area shall be not less than twice the area required by the table.
Table 22.214.171.124 Minimum Footing Sizes
|Item||Column 1||Column 4|
|Number of Floors Supported||Minimum Footing Area for Columns Spaced 3m o.c., m2|
126.96.36.199. Adjustments to Footing Area for Columns
(1) The footing area for column spacings other than shown in Table 188.8.131.52. shall be adjusted in proportion to the distance between columns.
Taking this to mean linearly proportional, then the following table provides interpretation for the post spacings for single story farm buildings described in Supplementary Standard SB-11.
|Interpreted Footing Area for Single Storey Pole Barn Framing Falling Under OBC SB-11|
|Multiplying Factor||Post spacing||Footing Area||Diameter of Auger|
|0.1333 m2/m||2.4 m (8ft)||0.32 m2 (3.5 ft2)||0.64 m (25 in.)|
|3.6 m (12ft)||0.48 m2 (5.2 ft2)||0.78 m (31 in.)|
|4.8 m (16 ft)||0.64 m2 (6.9 ft2)||0.90 m (36 in.)|
184.108.40.206. Footing Thickness
(1) Footing thickness shall be not less than the greater of,
(a) 100mm, or
(b) the width of the projection of the footing beyond the supported element.
Presuming that footings will be augured (circular area) and that rectangular posts will be centered on the footing, then the maximum projection of the footing beyond the supported element is the radius of the footing minus half the smallest dimension of the pole. The resulting interpreted footing thicknesses are shown in the following table:
|Interpreted Minimum Footing Thickness|
|Interpreted Footing Area Based on Post Spacing||Smallest Dimension of Post|
|89 mm (4 in.)||140 mm (6 in.)||184 mm (8 in.)|
|0.32 m2||0.275 mm (11 in.) (Footing Thickness)||0.249 mm (10 in.)||0.227 mm (9 in.)|
|0.48 m2||0.346 mm (14 in.)||0.321 mm (13 in.)||0.299 mm (12 in.)|
|0.64 m2||0.407 mm (16 in.)||0.381 mm (15 in.)||0.359 mm (14 in.)|
Columns in Contact with Concrete,
Wood columns shall be separated from concrete in contact with the ground by 0.05mm polyethylene film or Type S roll roofing.
Posts and Roof Plate Beams
SB-11 specifies dimensions of these members. It is noted under Table 1.3.3.S. “Roof Plate-Beams and Lintels for farm Buildings of Low Human Occupancy” that specifications for anchorage at supporting posts must be specified by a competent designer, and that trusses are to be at 1.2m (4ft) on centre.
After posts are set on their footing and leveled, the plate beam level can be laid out on the posts with a string line. The plate beam can either be notched into the posts or have a scab supporting it on the outside of the pole. The plate beam can either be nailed or through bolted to the post, OMAFRA’s publication 809 gives some specifications for attachment of the plate beams. Again, this is an area that is not fully addressed by the code and very much up to the discretion of the CBO.
Bracing systems shall be specified by a competent designer. This is yet another very large grey area with agricultural buildings as there is no definition of a competent designer. Your local CBO should be able to clarify with you what requirements they have for the bracing designer. It may be required that you seek professional engineering if there is no alternative. Some CBO’s will require that the ‘competent designer’ have proven experience, which would qualify most good carpenters. Others might accept the farmers design if they point to a comparable existing structure.
SB-11 says: “Posts must be restrained against lateral movement at ground level and at the footing. Concrete floor, splash-rail and uplift anchor help to meet this condition.”
A splash rail typically consists of three 2”x6” tongue and groove pressure treated spruce boards that are attached to posts at the ground level. Typically, two of the boards are above grade and one board is back filled so that it stabilizes the wall. If a concrete floor is to be poured, the splash rail can act as the form. If Steel cladding is to be used, the splash rail also helps improve the longevity of the steel by giving it separation from the ground and from contact with tall grasses.
In SB-11 it is specified: “Posts must be situated on footings and shall be anchored to prevent wind uplift”
There is nowhere in the code that specifies the requirements for this anchorage, it is general practice however to use two pieces of pressure treated 2×4, scabbed to either side of the bottom of the post. The length of the 2×4 is typically cut to be 6 inches less than the diameter of the hole.
Open sided buildings are subject to substantially more uplift than closed buildings and so it may be specified by an engineer that the footing depth or anchor size may need to be increased to resist uplift.