Subfloor bracing

By Tom Edhouse - 1 October 2012, Build 132

In Build 131 (pages 29–30), we explained the information needed before starting bracing calculations for a building. This time, we work through a subfloor example.

THE HOUSE BEING USED in this example has a second storey on part of the house (see Figures 1–2).

Data for this example

Refer to Build 131 for how to establish these values.

Wind zone: Medium

Earthquake zone: 2

Floor plan area

This example has a mixture of single and double storeys. Because these have different wind and earthquake demands, two calculations are required – one for the subfloor area of the 2-storey portion and one for the subfloor area of the single-storey (shown in Figure 3). The slab floor in the garage has no subfloor so does not form part of the calculation.

Gross floor plan area for:

2-storey = 10.6 × 5 = 53 m²

1-storey = 8.1 × 9.3 = 75.3 m² (for simplicity, the area has not been reduced for the entry porch).

Once the demand is established, the overlap of the 2-storey will be deducted from the 1-storey.

Soil type: Rock

Weight of claddings: Light subfloor, lower storey, upper storey and roof

Roof pitch: 30 degrees, so choose 25–45 degrees Building shape: Subfloor has no wings or blocks

Heights for building

2-storey to apex H = 7.1 m, roof height above eaves h = 1.8 m.

Note: Where heights don’t exactly match the table, use the next highest bracing unit (BU). For example, in the subfloor structure (using Table 5.5), H = 7.1 m, so round up to 8 m, and h = 1.8 (round down to 1 m, this is a higher BU requirement).

Single-storey to apex H = 4.8 m, h = 1.9 m.

Roof type and building dimension

The 2-storey has a gable roof with 300 mm soffit/verge.

As the roof is over 25°, when considering wind on the 2-storey part of the building, use the overall dimensions of the roof for the width and length.

So, 2-storey section building dimensions are:

Length = 10.6 + 0.300 + 0.300 = 11.2 m

Width = 5.0 + 0.300 + 0.300 = 5.6 m.

Single-storey dimensions are:

Length = 9.3 m (no soffit to lower level)

Width = 8.1 m (no soffit to lower level).

Transfer these values to the calculation sheets (Figures 4 and 6).

Note that, because this is a hip roof shape, wind demand in both the along and across directions is the same, so choice of length and width is not critical.

Bracing calculation sheets

The above data is then entered into bracing calculation sheets to obtain the bracing demand (see Figures 4 and 6). Sheets can be downloaded from the Toolbox on the BRANZ website www.branz.co.nz.

2-storey section

Using the calculation sheets (see Figures 4), bracing demand for the 2-storey section is:

1176 BUs for wind across the ridge
627 BUs for wind along the ridge
636 BUs for earthquake.

Use 1176 BUs for wind across and 636 for both wind along and earthquake.

Single-storey section

Bracing demand results for the single-storey area (see Figure 6) are:

521 BUs for wind across
454 BUs for wind along
603 BUs for earthquake.

Use 603 BUs for along and across as it is the higher value in both directions.

Choose bracing element

The subfloor is 600 mm or less high. Anchor piles have been chosen as the subfloor bracing element as they are rated as 160 BUs for wind and 120 BUs for earthquake.

Moving to the bracing lines

For this example, the exterior walls will be used as bracing lines in each direction along with the common wall between the garage and the house. These are within the 5 m rule and provide an even distribution of bracing throughout the building.

We now need to calculate the minimum bracing needed in each line and check the bracing distribution complies with the requirements of NZS 3604:2011 clause 5.5:

maximum spacing of bracing lines in the subfloor = 5 m
minimum capacity of subfloor bracing lines is the greater of:
• 100 BUs
• 15 BU/m of bracing line
• 50% of the total bracing demand, divided by the number of bracing lines in the direction being considered.

Figure 4 Calculation sheet for demand – 2-storey section of subfloor.

Figure 5 Calculation sheet for bracing achieved – 2-storey section of subfloor.

See Table 1 where this has been worked through.

Minimum bracing for 2-storey section

Using the calculation sheet (see Figure 5) gives:

1280 BUs for wind across
960 BUs for earthquake and along.

This meets the minimum demand requirements from the calculation sheet (see Figures 4) and NZS 3604:2011 clause 5.5.2.

Minimum bracing for single-storey section

Using the calculation sheet (see Figure 7) gives:

1080 BUs for earthquake bracing across
1080 BUs for earthquake bracing along.

This meets the minimum demand requirements from the calculation sheet (see Figure 6) and NZS 3604 clause 5.5.2.

The piles in brace line N are staggered to comply with the requirement that braced or load-bearing walls are within 200 mm of the pile line.

More to check

Buildings where the height exceeds 1.7 times the width must be on a continuous foundation wall (NZS 3604:2011 clause 5.4.3.2). Height is measured from the underside of the bottom plate on the lowest floor to the top of the roof). In this example, width 5 m × 1.7 = 8.5 m, so this design is OK as the height is 6.5 m from underside of bottom plate to top of roof.

There is also a minimum number of subfloor braces (NZS 3604:2011 clause 5.5.6) – a minimum of four braced or anchor piles placed in each direction symmetrically around the perimeter. Wherever practical, they should be placed near a corner. This design has five piles in the across direction and nine in the along direction so is OK.

Figure 6 Calculation sheet for demand – single-storey section of subfloor.

Figure 7 Calculation sheet for bracing achieved – single-storey section of subfloor.

Table 1
MINIMUM BRACING NEEDED IN EACH LINE
	2-STOREY SECTION	SINGLE-STOREY SECTION
WIND ACROSS RIDGE
Bracing lines	B, C, D and E = 5 m long	A, B, C, D = 8.1 m long
Bracing demand per line (greatest value)	100 BUs or 75 BUs (5.0 x 15 BUs) or 147 BUs (1176 BUs divided by 2 = 588 divided by 4 lines)	100 BUs or 122 BUs (8.1 x 15) or 76 BUs (603 BUs divided by 2 = 301.5 divided by 4 lines)
Minimum BUs per line	147 BUs	122 BUs
Minimum anchor piles per line	1 anchor pile = 160 BUs (wind)	2 anchor piles = 240 BUs (120 each for earthquake)
WIND ALONG RIDGE
Bracing lines	M and N = 10.6 m long	M, N, O = 9.3 m long
Bracing demand per line (greater value)	100 BUs or 159 BUs (10.6 x 15) or 159 BUs (636 BUs (for earthquake) divided by 2 = 318 divided by 2 lines)	100 BUs or 140 BUs (9.3 x 15) or 100 BUs (603 BUs divided by 2 = 301.5 divided by 3 lines)
Minimum BUs per line	159 BUs	140 BUs
Minimum piles per line	2 anchor piles = 240 BUs (120 each for earthquake)	2 anchor piles = 240 BUs (120 each for earthquake)