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Minimum Footing Sizes by Code (Canada)

Minimum Footing Sizes by Code (Canada)

Minimum Footing Sizes by Code (Canada)

(OP)
Table 9.15.3.4 of the National Building Code of Canada lists the minimum footing area for columns supporting 1 floor and spaced at 3 m o.c. as 0.4 m2. Those numbers are to be adjusted in proportion to the actual distance between columns.

My question is if a soil bearing is confirmed on site that allows for a footing with a smaller area than prescribed by the code, are you allowed to break it? The title of the table is stated as a "minimum". Of course, the footing would also need to pass checks for one-way and two-way shear and bending obviously along with bearing.

RE: Minimum Footing Sizes by Code (Canada)

The code prescribed ones are based off of the presumed bearing pressure outlined in the code. If you have a geotechnical report, and are obtaining an engineer's seal, then it can be whatever size is necessary.

RE: Minimum Footing Sizes by Code (Canada)

For the most part, Part 9 is less conservative than Part 4. I generally don't use a footing less than 16" wide which is common practice where I live. Most houses are usually no problem with a ftg of 16" wide x 8"Dp. If you have a cantilever fdn wall (with a grade difference of 4' or less), I usually use a ftg that is is 24" wide but Part 9 of the code permits less. Most soil reports don't allow a strip ftg with a width less than 16" regardless of the pressure.

In most modern houses, the minimum Part 9 ftg widths are exceeded. Unless you have extremely high bearing capacities and high loads, an 8" deep ftg that is 16" wide is rarely ever a problem to justify.

If you have concrete floors or long span steel then part 9 is not permitted and you will likely need bigger ftgs. but for typical wood framed houses, 16" wide is a common practice (at least where I am from). If you are on very soft soil, you may end up with 20" or 24" footing (a soil report is usually required in areas with unusually soft soils). In rare circumstances, you may need a raft foundation or piles but these can not be design to part 9.

RE: Minimum Footing Sizes by Code (Canada)

(OP)
So you are allowed to not follow code if you can show, through engineering calculations, that the design is sufficient? Is that true in all jurisdictions? I am in Alberta.

RE: Minimum Footing Sizes by Code (Canada)

I dont have my NBCC handy, but off the top of my head,

is table 9.15.3.4 found in part 9 of the code? this is prescriptive structural design. This section only applies to a limited range of residential wood frame construction. I spent some time in that world, and never actually referenced part 9, instead I stayed in part 4 of the NBCC.

Engineers are free to design out of part 4, which is specific design.

So essentially, to "follow the code", you can either:
reference part 9 with no engineering input, so long as the construction conforms to the limitations of part 9
have an engineered solution that conforms to part 4 of the NBCC

both are "to code"

RE: Minimum Footing Sizes by Code (Canada)

Technically you are still following the code. Part 9 specifically allows you to use Part 4 for Part 9 buildings. The code minimum is also just a minimum. It is conceivable that if you make a footing wider (and not thicker), you may over stress the footing in shear and bending even though the bearing pressure is less. From a practical perspective, you usually don't have an issue (especially if you also thicken the footing proportionately to the width). However, from a more legal perspective, you are utilizing Part 4. Part 4 then references A23.3 which can be used for the concrete design of the footing. A23.3 also has chapter 22 which permits the use of plain concrete. So you are still following the letter of the law.

RE: Minimum Footing Sizes by Code (Canada)

Quote (mas745)

Table 9.15.3.4 of the National Building Code of Canada lists the minimum footing area for columns supporting 1 floor and spaced at 3 m o.c. as 0.4 m2. Those numbers are to be adjusted in proportion to the actual distance between columns.

Didn't know that was in the code. Perhaps it was added after I retired. But 0.4 m2 would be a square footing of side dimension 0.635 m or approximately 2'-1" square. I can't imagine wanting to use a footing much smaller than that under a column, but to each his own.

Quote (mas745)

My question is if a soil bearing is confirmed on site that allows for a footing with a smaller area than prescribed by the code, are you allowed to break it?

You used to be able to depart from code minimums on the basis of engineering judgment, but I don't know whether or not that stipulation still resides in Part 9. If not, it should.

BA

RE: Minimum Footing Sizes by Code (Canada)

What situation are you in where you want to use a smaller sized footing than that

RE: Minimum Footing Sizes by Code (Canada)

I agree with the responses above, Part 9 is prescriptive and if engineered/sealed under Part 4 you can deviate from Part 9.

However, concrete is cheap. Virtually nobody in my area will use less than a 20" wide strip footing for a residential house. Yes, you could calculate and refine the loads and shave off the width, but why??? Concrete is cheap and foundation repairs can be extremely expensive. You have the same forming costs for a 12" wide or 16" wide or 20" wide footing. For spread footings, you rarely see less than 24" x 24", no matter how lightly loaded, unless it is on competent bedrock. The above assumes you have the minimum assumed Part 9 soil bearing capacity and there is no water table close to footing depth.

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