Page 30 - Build 153

MINIMUM DIMENSIONS AND COVER FOR CONCRETE FOOTINGS WITH REINFORCING STEEL
BUILD
Reinforcing footings cover
Do you know the minimum bending diameter for reinforcing steel and its cover requirements in concrete footings? This determines the minimum width you need to make your concrete footings.
RIGHT
BY TREVOR PRINGLE,
ANZIA, BRANZ PRINCIPAL WRITER
SOME DIMENSIONS GIVEN in NZS 3604:2011 Timber-framed buildings for reinforced concrete foundation footings may be insufficient. This
is due to the bending diameters and minimum concrete cover required for steel reinforcing in concrete footings.
Footing widths in NZS 3604:2011
NZS 3604:2011 section 7.5 sets out require- ments for reinforced concrete foundation walls and footings for concrete slabs on ground.
Figures 7.13(B), 7.14(B and C) and 7.16 (B and C) give minimum footing width dimensions. However, when the thickness of the steel and the bending diameter are considered, the required concrete cover cannot be achieved at the footing widths shown.
Reinforcing steel needs 75 mm cover
When footings and foundations are poured against ground (paragraph 6.11.3) or constructed from unprotected masonry blocks, the minimum cover to the steel must be 75 mm.
Bending diameters 5 × bar diameter
For foundation walls to concrete slabs, the horizontal bars are typically D12 and the vertical starter bars are R10.
Minimum bending diameters for reinforcing steel are given in Tables 8.1 and 8.2 of NZS 3101.1&2:2006 Concrete structures standard.
According to Table 8.1, the bending diameter for 6–20 mm diameter deformed and plain steel reinforcing bars must be at least five times the bar diameter. Therefore, the reinforcing bar minimum bending diameter for:
● R10is5×10=atleast50mm ● D12is5×12=atleast60mm ● D16is5×16=atleast80mm.
Minimum foundation width
To calculate the minimum width of a foundation or footing to meet the cover requirements, you need to know:
● the cover on each side of the bent bar
● thebardiameter
● thebarbenddiameter.
Two D12s stacked
For a foundation non-cantilevered wall using two D12s horizontally with R10s vertically, the minimum width (see Figure 1) will be:
● coveroneachside:2×75mm =150mm
● vertical bar diameter: 2 × 10 mm = 20 mm ● minimum bend diameter: 5 × 10 mm =
50 mm.
This gives a minimum foundation width of
220 mm, which is 20 mm more than the details in NZS 3604:2011 specify.
Two D12s side by side
If the bars in the base were installed side by side, bars should be spaced apart by 50 mm
to allow them to be fully encapsulated by the concrete. The footing would then need to be at least 244 mm or 280 mm wide (see Figures 2 and 3).
ground level
as required
Figure 1
R10
2 × D12
75 mm
R10
ground level
as required
Figure 2
2 × D12 with 50 mm gap
75 mm
ground level
as required
Figure 3
R10
2 × D12
75 mm
75 50 75 10 10 220 mm
75
50 75
75 50 50 75
10 10 10 280 mm
Alternate footing with two stacked D12s.
Foundation footing with two D12s side by side.
Foundation footing with alternate R10 starters.
10121210
244 mm
28 — April/May 2016 — Build 153