Subfloor, wall and roof bracing are an integral part of building design and documentation. In this section we are focusing on Wall Bracing.
Your Building Designer will document, on drawings to be submitted for Building Approval, the type and location of bracing units required to withstand racking forces from wind loads on the building.
All sites need to be classified for a 'design wind gust speed' prior to the designer being able to calculate framing member sizes and the like as the tables in the 'Timber Framing Manual' differentiate between wind classifications.
Classifications as determined under AS4055 or AS1170.2 are nominated as -
In non cyclone areas N1 (28 meters/second), N2 (33m/s), N3 (41m/s) and N4 (50m/s)
In cyclone ares C1 (41m/s), C2 (50m/s), and C3 (60m/s).
To give an example of wall bracing requirements, the Timber Framing Manual AS1684 lists various bracing unit types together with their capacity to resist racking forces measured in kilonewtons per meter of length (kN/m).
Hang in there . . .
The total racking force exerted on a building is calculated by multiplying the area of elevation, measured upwards from a point midway up the wall frame, (one side of the building) by the 'Lateral Wind Pressure' (kPa).
For example a 7 meter wide building with a frame height of 2.7 would calculate to 9.45 m2 (plus the roof area).
Tables in AS1684 list the kPa for various roof pitches, hip or gable end etc. For example N3 classification for a single story gable end roof (wind direction to the flat surface of the gable) is 1.40kPa.
If the m2 area of that end of the building was 25m2 then the total racking force needed to be resisted would be 35.0kN.
Back to the tables . . .
If we take the example of a diagonal metal brace, as is commonly seen in wall framing, the bracing capacity is listed as 0.8 per meter of wall length. Therefore (typically) this brace with a 2.7 meter frame height would cover 2.7 in frame length = 2.7x0.8 = 2.16kN.
Therefore you would need 17 braces (16.2) at right angles to the subject wall to comply.
The same scenario with a N1 classification, designated at 0.67kPa,(as opposed to N3 at 1.4kPa) would require 8 braces. - 25m2 multiplied by 0.67 divided by 2.16 equals 7.75.
With a 2 storey dwelling, the lower wall frame bracing is calculated on the total area from half way up the lower storey frame to the roof top and the upper storey the total from half way up the upper storey frame to the roof top.
The Designer has the scope to employ various bracing types to enable compliance with the resistance required.
A double diagonal metal strap bracing has a capacity up to 3.0kN/m, a plywood sheet brace, typically used on the exterior wall frame of a brick veneer dwelling can have up to 8.7kN/m depending upon stress grade of the ply.
To add to the intricacies of bracing design there are maximum distances allowed between braced walls depending upon the wind classification. This can have a bearing on designs that employ 'open living' with the need to juggle between Clients requirements and fulfilling the framing code requirements.
If you have any questions on this topic feel free to email me.