A-9.15.3.4.(2) Footing Sizes.

The footing sizes in Table 9.15.3.4. are based on typical construction consisting of a roof, not more than 3 storeys, and centre bearing walls or beams. For this reason, Clause [b-9.15.3.3.#^table-91533hplca|9.15.3.3.](b) stipulates a maximum supported joist span of 4.9 m.

It has become common to use flat wood trusses or wood I-joists to span greater distances in floors of small buildings. Where these spans exceed 4.9 m, minimum footing sizes may be based on the following method:

(a)

Determine for each storey the span of joists that will be supported on a given footing. Sum these lengths (sum1).

(b)

Determine the product of the number of storeys times 4.9 m (sum2).

(c)

Determine the ratio of sum1, to sum2.

(d)

Multiply this ratio by the minimum footing sizes in Table 9.15.3.4. to get the required minimum footing size.

Example: A 2-storey house is built using wood I-joists spanning 6 m.

(a)

sum1 = 6 + 6 = 12 m

(b)

sum2 = 4.9 x 2 = 9.8 m

(c)

ratio suml /sum2 = 12/9.8 = 1.22

(d)

required minimum footing size = 1.22 x 350 mm (minimum footing size provided in Table 9.15.3.4.) = 427 mm.