o The building is not a dwelling, but its use is associated with domestic purposes.
o The BCA Classification of the building is 10a, which is appropriate for a non-habitable shed, garage, or carport. There are no structural implications of this classification.
o BCA Volume 1 or 2 may be used, but not necessarily AS 4055 for wind classification.
o Full internal pressure should be used for structural design if the doors are open during peak wind events, and therefore wind class cannot be assigned using AS 4055. If the doors and windows are wind rated to protect the building during design wind conditions internal pressure can be reduced.
SITE FACTORS
· Check region with Council.
o The Council should confirm in writing that the allotment on which the proposed garage will be built is located in Region A2, as defined in AS 1170.2.
· The consequences of structural failure are considered to be moderate in terms of human hazard (because the garage is associated with domestic use) and low in terms of impact on the public (because the house and garage are on a large allotment).
o Importance Level 2 is assigned – this is consistent with outbuildings associated with dwellings generally. Importance Level 1 can only be justified if both hazard and impact of failure are low.
o Importance Level 2 requires an annual probability of exceedance for wind events of 1:500.
· The exact orientation of the building is not shown on the site layout drawing. Also, the owner wishes to have the option of facing the roller doors to the west or east.
o Easterly and westerly directions in Region A2 have direction multipliers varying from 0.8 to 1.0. Must assume 1.0 unless building orientation and door position are finally specified on the Development Application.
· The general terrain of the property and its neighbours is observed to be previously cleared open farming land with relatively few trees or buildings. There is no indication of future rezoning and redevelopment plans in available Council documentation.
o The terrain is currently Category 2, and is assumed to be “fully developed” in the absence of future rezoning proposals.
· The immediate vicinity of the proposed garage is partially shielded by the house and an existing garage. The shielding is only on the eastern side, leaving 3 sides basically unshielded.
o Unless both alternative roller door directions are shielded by other buildings, the proposed building must be regarded as unshielded.
· The site and surrounding geography are essentially flat and level.
o There are no obvious reasons to apply a topographic factor higher than 1.0.
· The steps in the calculation of site wind speed are:
o Look up regional wind speed VR for region A2 and 1:500
o Select wind directional multiplier Md = 1.0 for “any direction” wind in region A
o Look up terrain/height multiplier Mz, cat
o Look up shielding multiplier Ms
o Look up topography multiplier Mt
o Calculate Vsit = VR x Md x Mz, cat x Ms x Mt
o Value for this example = 45 x 1.0 x 0.91 x 1.0 x 1.0 = 41 m/s
o In this case, as the building orientation is not known, this is also the design wind speed Vdes
· The calculated design wind speed is then used to calculate the design wind pressures acting on various parts of the structure in accordance with AS/NZS 1170.2 Clause 2.4.
SUMMARY OF WIND-RELEVANT DESIGN INFORMATION – EXAMPLE 1
| LINE | ITEM |
VALUE |
SOURCES & COMMENTS |
|
1 |
Building description |
Supplied |
Owner |
|
2 |
Specification reference & date |
Supplied |
Owner |
|
3 |
Owner's stated intended use |
Vehicle garage |
Owner |
|
4 |
BCA classification |
10a |
BCA |
|
5 |
Length |
6.0 m |
Owner's plans |
|
6 |
Width |
6.0 m |
Owner's plans |
|
7 |
Maximum height |
3.0 m |
Owner's plans |
|
8 |
Roof pitch |
20 deg |
Owner's plans |
|
9 |
Permeability type (IP Code) |
IP 9 |
AS/NZS 1170.2, Tables 5.1A&B & 5.2A&B |
|
10 |
Average Cpe roof |
- 0.7 |
AS/NZS 1170.2, Tables 5.3A, B & C |
|
11 |
Average Cpe walls |
+ 0.7, - 0.5 |
AS/NZS 1170.2, Tables 5.2A, B & C |
|
12 |
Local pressure effects applied? |
Yes |
AS/NZS 1170.2, Table 5.6 |
|
13 |
Site address |
Supplied |
Owner |
|
14 |
Site plan reference & date |
Supplied |
Owner |
|
15 |
Wind region |
A2 |
Council |
|
16 |
Importance level |
2 |
BCA Guide and Volume 1 Table 1.2a |
|
17 |
Annual probability of exceedance for wind |
1:500 |
BCA Volume 1 Table 1.2b |
|
18 |
Cyclonic factor (FC, FD) (if applicable) |
NA |
AS/NZS 1170.2 Clause 3.4 |
|
19 |
Regional wind speed (VR) |
45 m/s |
AS/NZS 1170.2, Table 3.1 |
|
20 |
Wind direction multiplier (Md) |
1.0 |
AS/NZS 1170.2, Table 3.2 |
|
21 |
Terrain category |
2 |
AS/NZS 1170.2, Clause 4.2.1 |
|
22 |
Terrain-height multiplier (Mz,cat) |
0.91 |
AS/NZS 1170.2, Table 4.1(A) |
|
23 |
Shielding multiplier (Ms) |
1.0 |
AS/NZS 1170.2, Clause 4.3 |
|
24 |
Topographic multiplier (Mt) |
1.0 |
AS/NZS 1170.2, Clause 4.4 |
|
25 |
Site wind speed (Vsit) |
41 m/s |
AS/NZS 1170.2, Clause 2.2 |
|
26 |
Design wind speed (Vdes) |
41 m/s |
AS/NZS 1170.2, Clause 2.2 |