The Opening Ratio (R) is an important parameter used to determine the internal pressure coefficient Cp,i according to AS/NZS 1170.2:2011, Table 5.1(B).
First, identify the building surface containing the relevant largest opening. This may be the windward wall, leeward wall, either side wall, or roof. The calculator selects the windward wall by default, but users can change this selection depending on the wind direction and building configuration.
Next, enter the opening areas for all building surfaces, including relevant doors, windows, vents, and other potential openings.
The calculator automatically determines the opening ratio:
R = Opening area on selected surface ÷ Total opening area on all other surfaces
Example: A warehouse has a 12 m² roller door on its windward wall. The combined opening area on the leeward wall, side walls, and roof is 4 m².
Therefore:
R = 12 ÷ 4 = 3.00
The calculator displays this ratio and uses it to determine the applicable internal pressure coefficient.
Important: Different wind directions may require different opening scenarios. For applicable ULS designs in Wind Regions C and D below 25 m, a minimum design opening ratio of 2 generally applies unless an exception under Clause 5.3.1.3 is demonstrated.
Internal Pressure Coefficient — Cp,i
AS/NZS 1170.2:2011 + Amendment 4 | Clause 5.3 | Tables 5.1(A) and 5.1(B)
Enter actual and potential openings, including leakage, on each surface. Calculate separate scenarios for different wind directions, door failures and opening arrangements.
1. Design conditions
Clauses 5.3.1.2–5.3.1.3: For ULS design of parts below 25 m in Regions C/D, use Table 5.1(B) and generally R ≥ 2, unless a specified debris-resistance or permanent-ventilation exception is demonstrated.
What does “Region C/D minimum ratio exception” mean?
For ultimate-limit-state design of building parts below 25 m in cyclonic Regions C and D, Clause 5.3.1.3 requires Table 5.1(B). Normally the ratio R (open area on one surface divided by the combined open area on other walls and roof) must not be taken as less than 2, even if the calculated opening ratio is smaller. This accounts for severe internal pressures from potential accidental openings.
Select “Not demonstrated” (default) unless the designer has documented evidence that at least one of these exceptions applies:
(a) Debris resistance: Demonstrate that windborne debris defined in Clause 2.5.8 will not create an opening in the building envelope.
(b) Permanent roof ventilation: Provide a permanently open roof ventilator with equivalent total free area at least as large as the largest potential accidental wall-opening area, considering each wall in turn.
(c) Permanent wall ventilation: Provide permanently open ventilators on at least two walls, with the equivalent free area on each wall at least as large as the largest potential accidental wall-opening area, considering each wall in turn.
Example: If the calculated opening ratio is R = 1.0 in Region C, ULS, for a part below 25 m, and no exception is demonstrated, the calculator uses a minimum R = 2.0 for the Table 5.1(B) selection. If a qualifying exception is demonstrated, the calculated ratio may be used, subject to the standard.
Important: Selecting the exception does not verify debris resistance, door/window strength or ventilation capacity. Keep supporting engineering documentation. This special minimum-ratio rule does not apply to Regions A, B or W, or to SLS design.
2. Wall and roof opening areas
Clause 5.3.1.1: Open area includes openings and permeability/leakage. Enter total gross surface area and total effective open area in m². Each side wall is separate.
| Surface | Gross area (m²) | Open area (m²) | Open % | Class |
|---|
Table 5.1(A) Note 1: Impermeable < 0.1%; permeable 0.1% to 0.5%; large opening > 0.5%. Table 5.1(A) assumes an impermeable roof.
3. Table 5.1(B) opening scenario
Purpose: This section identifies the opening scenario that may control the internal pressure. Table 5.1(B) applies to buildings with significant actual or potential openings, including openings that could result from door, window or cladding failure. Consider the wind direction and assess other credible opening scenarios separately (Clauses 5.3.1–5.3.2).
Step A — Choose the opening surfaceFor the wind direction being analysed, identify the wall or roof containing the relevant largest opening. “Windward” faces the approaching wind; “leeward” is opposite; the other two walls are side walls. A wall can change category when the wind direction changes.
Step B — Determine the opening ratio RUse the combined open area on the selected surface divided by the combined open area of all other walls and the roof. Include relevant gaps, vents, leakage and potential openings, not just the main door.
Step C — Enter external Cp,eObtain Cp,e for the location of the opening from the appropriate external-pressure provisions. Use the correct sign: positive for pressure and negative for suction. This is not automatically the Cp,e of the beam, wall or roof panel being checked.
Opening ratio: R = (open area on selected surface) ÷ (total open area on all other surfaces)
Worked example — windward roller door: A windward wall has 12 m² of effective openings. The leeward wall has 1 m², side wall 1 has 1 m², side wall 2 has 1 m², and the roof has 1 m². Therefore the other surfaces total 4 m² and R = 12 ÷ 4 = 3. If the external coefficient at the windward opening is Cp,e = +0.70, Table 5.1(B) gives Cp,i = 0.85 × 0.70 = +0.595 for this example. For a leeward, side-wall or roof opening, use the corresponding table column and external coefficient at that opening.
Important distinction: “Auto: largest open area” compares the total open areas entered for the five surfaces. It is a convenience, not a substitute for identifying the largest individual opening or checking all governing potential-opening cases. When several openings exist, select the relevant surface manually and verify the opening arrangement. Never assume one wind direction or one opening failure governs all building elements.
Opening ratio (R) for the selected surface
—
Select an opening surface and enter the opening areas in Section 2.
How to choose the inputs: Select the opening surface based on the wind direction and the potential failure scenario. The entered Cp,e should be taken at that opening location. Automatic exact table rows evaluates the explicitly coded table values; Manual verified Cp,i lets the designer enter a separately established coefficient when engineering interpretation is needed.
Intermediate ratios and multiple load cases: For ratios between specified rows, the calculator does not automatically interpolate where distinct positive/negative load cases may be involved. Verify the applicable standard procedure and assess all required cases. In Regions C/D, the special minimum ratio of 2 for applicable ULS design is applied unless the engineer verifies a Clause 5.3.1.3 exception.
4. Calculate internal pressure coefficient
5. Reference — Table 5.1(A)
| Condition (impermeable roof) | Cpi load cases |
|---|---|
| One permeable wall: windward | Windward Cp,e |
| One permeable wall: windward impermeable | −0.3 |
| Two or three permeable walls: windward permeable | −0.1 and +0.2 |
| Two or three permeable walls: windward impermeable | −0.3 |
| All walls permeable | −0.3 and 0.0 |
| Effectively sealed, non-opening windows | −0.2 and 0.0 |
Where two values are given, assess the relevant alternative load cases with external pressure.
6. Reference — Table 5.1(B)
| Ratio R | Windward opening | Leeward / side / roof opening |
|---|---|---|
| ≤ 0.5 | −0.3 and 0.0 | −0.3 and 0.0 |
| 1 | −0.1 and +0.2 | −0.3 and 0.0 |
| 2 | 0.7 × Cp,e | Cp,e |
| 3 | 0.85 × Cp,e | Cp,e |
| ≥ 6 | Cp,e | Cp,e |
Clause 5.3.2: Check potential openings due to door/window failure and debris impact. Clause 5.3.4: Internal partitions and ceilings have separate differential-pressure requirements. For windward openings in buildings taller than 25 m, use design wind speed at the opening height (Clause 5.3.1.1).