Bracing Demand & Capacity, in plain English

Bracing demand is the earthquake and wind load each elevation of a house has to resist, measured in BU (Bracing Units) and calculated per NZS 3604:2011 §5. It matters because your bracing capacity — the BU your linings and bracing elements provide — has to exceed that demand on every elevation, or the job won’t pass the bracing part of consent.

Demand vs capacity — the basics

Two demand figures drive everything: earthquake demand from NZS 3604:2011 Table 5.8, and wind demand from Table 5.10. You read the BU figures for your earthquake and wind zones off your licensed copy of the standard, or let GIB EzyBrace calculate them for you.

• Earthquake bracing demand = floor area × the BU/m² figure for your EQ zone (from NZS 3604:2011 Table 5.8).
• Wind bracing demand = wall length × the BU/m figure for your wind zone (from Table 5.10).
• Bracing capacity = the sum of all bracing elements. In NZ residential work, GIB lining is the main contributor.
• Capacity must exceed demand on every elevation.

The rules to watch

• 50% rule: each elevation needs to carry at least 50% of the total demand on its own.
• There is also a per-metre minimum and a maximum on timber floors — read the exact figures off NZS 3604:2011 §5.

Looking up the demand figures

The BU/m² (earthquake) and BU/m (wind) demand figures vary by zone, roof weight and number of storeys. They’re set out in NZS 3604:2011 Tables 5.8 and 5.10 — consult your licensed copy of the standard, or have GIB EzyBrace or your designer produce them.

Worked example (illustrative figures only)

The numbers below are hypothetical placeholders to show the method — they are not the NZS 3604 table values. Substitute the real demand figures from your copy of the standard for your zones. Take a single-storey house, 8m × 12m, with a light roof:

1. Earthquake demand = 8 × 12 × (assume X BU/m² for your EQ zone, from Table 5.8).
2. Wind demand = wall length × (assume Y BU/m for your wind zone, from Table 5.10) — do this for both axes.
3. Whichever is larger governs — design the bracing capacity to exceed it.
4. Add bracing elements (e.g. lengths of BL1-H at its rated BU/m) until capacity ≥ demand.
5. Check each elevation carries at least 50% of the demand for its axis.

Always cross-check with the EzyBrace software (free from gib.co.nz) — it pulls the demand from your zones and generates a PS3 that satisfies the bracing component of consent.