How a bracing calculation works: a worked example

Bracing keeps your build standing up when the wind blows and the ground shakes — and under NZS 3604:2011 §5 you have to prove the walls carry enough bracing units (BU) on every axis. This is a worked example of how that calculation runs on a typical NZ residential floor, so you can see the method end to end before you feed real numbers in.

Important: the BU/m² and BU/m figures below are hypothetical placeholders to show the method, not the NZS 3604 table values. Substitute the real demand figures from your licensed copy of the standard for your earthquake and wind zones.

The example house

The walk-through uses one worked scenario:

• 8m × 12m single-storey timber house
• Light steel roof
• Wellington — EQ Zone 4, Wind High

The five steps

A bracing calculation works through demand, then capacity, then the detail checks that keep it valid on site. Here is the flow for the example house:

1. Earthquake bracing demand (NZS 3604:2011 Table 5.8). Floor area = 8 × 12 = 96 m². Read the BU/m² demand for your EQ zone, roof weight and soil class off Table 5.8 — assume X BU/m² for this example. EQ demand = 96 × X = (96·X) BU total. Distribute it so at least 50% sits on each axis, and each elevation needs proportional capacity — not just two axes.
2. Wind bracing demand (NZS 3604:2011 Table 5.10). Read the BU/m-of-wall demand for your wind zone off Table 5.10 — assume Y BU/m for this example. Long axis (12m): demand = 12 × Y. Short axis (8m): demand = 8 × Y. Compare the wind and earthquake demands on each axis — the larger one governs.
3. Bracing capacity (GIB EzyBrace selection). For each axis, add bracing elements until the total capacity exceeds the governing demand. Choose a GIB EzyBrace system at its rated BU/m (from the GIB BRANZ appraisal) and work out how many metres of wall you need. A higher-rated system (e.g. Braceline) needs fewer metres of wall than a Standard system, at higher cost — trade that off accordingly. Aim for some margin over the demand rather than exactly meeting it.
4. Element check (NZS 3604:2011 §5.5). There is a minimum bracing-element length, a per-metre minimum on every wall, and a maximum BU/m on timber floors — read the exact figures off §5.5. Each elevation needs at least 50% of total demand independently.
5. Hold-down requirements. A BL1-H element requires hold-downs at each end. Simpson HD2A or Pryda HD500 — 2× M16 bolts to slab, within 150mm of the plate ends. Mid-plate fixings sit at 900mm centres meeting the kN demand.

In practice

You don’t do this by hand. Use the free EzyBrace software at gib.co.nz — it does this calculation automatically and generates a PS3 (Producer Statement — Construction) for council. The PS3 is what makes the bracing component consent-ready.

What can go wrong

A sound design on paper can still fail on site if the detail isn’t built as drawn. Common ways a bracing element loses its value:

• The designer specs a great bracing layout but the builder doesn’t install the hold-downs — the BU value drops to the GS2-NOM equivalent (no hold-down).
• Bracing elements get shortened below 400mm during the build (a door reposition, plumbing) — the element no longer counts.
• Internal walls used for bracing get plumbing chases cut into them — the element is invalidated.