1. SECTION 8 - RACKING (BRACING) AND SHEAR FORCES STUDENT HANDOUT
AS SECTION 8 - RACKING AND SHEAR FORCES

2. AS 1684 SECTION 8 - RACKING AND SHEAR FORCES
The wind forces are transmitted to the ceiling diaphragm from the walls and also the roof. They are then transferred through the ceiling diaphragm to the bracing walls that transmit them to the floor structure, foundations and then into the ground.
Without
ceiling
diaphragm
With
ceiling
diaphragm
AS SECTION 8 - RACKING AND SHEAR FORCES

3. AS 1684 SECTION 8 - RACKING AND SHEAR FORCES

4. AS 1684 SECTION 8 - RACKING AND SHEAR FORCES
As indicated by Figures 8.2 (A) and Note 1, the area of an elevation includes only the top half of the wall.
Note: 1 - h = half the height of the wall (half of the floor to ceiling height).
This is the area used to calculate single or upper storey bracing
Ceiling diaphragm
Floor Slab
AS SECTION 8 - RACKING AND SHEAR FORCES

5. AS 1684 SECTION 8 - RACKING AND SHEAR FORCES
Note 3 of Figures 8.2 (A, B & C) pg 113 states
The area of elevation of the triangular portion of eaves overhang up to 1000 mm wide may be ignored in the determination of area of elevation.
Area of Elevation
AS SECTION 8 - RACKING AND SHEAR FORCES

6. AS 1684 SECTION 8 - RACKING AND SHEAR FORCES
When wind flows over a building it applies different pressures (forces) on a flat vertical wall to that on the sloping roof surface.
Pressure on roof
kPa*
Pressure on wall
kPa*
* These values are indicative only and will vary with roof pitch, building height to depth ratio etc.
The tables need to know the ratio between how much roof area the wind ‘sees’ as opposed to how much wall area the wind ‘sees’. The building width and roof pitch will establish this ratio.
AS SECTION 8 - RACKING AND SHEAR FORCES

7. x = 8.3.4 Racking force (pg 116) Projected area of elevation (m2)
The total racking force, in kN, shall be calculated as follows:
Projected area of elevation (m2)
Lateral wind pressure (kPa)
Total racking force x =
The racking force shall be calculated for both directions (long and short sides) of the building
AS SECTION 8 - RACKING AND SHEAR FORCES

8. AS 1684 SECTION 8 - RACKING AND SHEAR FORCES

9. AS 1684 SECTION 8 - RACKING AND SHEAR FORCES
As indicated by Figures 8.2 (B) and Note 1, the area of an elevation
For lower storey of two storey section h = half the height of the lower storey (i.e. lower storey floor to lower storey ceiling)
Ceiling diaphragm
Floor diaphragm
This is the area used to calculate lower storey bracing
AS SECTION 8 - RACKING AND SHEAR FORCES

10. AS 1684 SECTION 8 - RACKING AND SHEAR FORCES
Determine the wind classification
Determine the wind pressure
Determine area of elevation
Calculate racking force
AS SECTION 8 - RACKING AND SHEAR FORCES

11. AS 1684 SECTION 8 - RACKING AND SHEAR FORCES
Design Bracing system for walls
Check even distribution and spacing
Check connection of bracing to roof/ceilings and floors
AS SECTION 8 - RACKING AND SHEAR FORCES