Demonstration Study: CFD for Bushfire Tenability in a School Hall (NCC Spec 43C9)

Introduction

Buildings in bushfire-prone areas must ensure that internal environments remain safe and tenable during bushfire exposure, especially when used as temporary refuges. This is particularly critical for class 9 buildings with vulnerable occupancies such as schools, childcare centres, and aged care facilities.

This study demonstrates how Computational Fluid Dynamics (CFD) can be used to evaluate compliance with the National Construction Code (NCC) Specification 43C9, which sets internal temperature limits for bushfire exposure.

3D model of a single-zone school hall

Objective

To verify that during a bushfire assuming total HVAC system failure:

• Internal air temperature remains below 39°C

• Internal wall surfaces remain below 60°C

These thresholds align with Clause C9 of NCC Spec 43, which addresses internal tenability in buildings exposed to bushfire radiant heat.

Methodology

Using Computational Fluid Dynamics (CFD), we modelled a simplified representation of the childcare centre as a thermally enclosed rectangular volume (single-zone room). The simulation included:

• Building: Single-zone school hall

• Wall construction: 200 mm thick with assigned thermal resistance (R-value) based on realistic wall build-ups.

• Bushfire Exposure: 40 kW/m² radiant heat flux applied to the external walls, replicating bushfire flame front radiation.

• Assumptions:

  • HVAC system is non-operational

  • No fresh air or mechanical cooling

  • Doors and windows are shut

• Air Volume: Fully enclosed

• Simulation Duration: 20 minutes (transient analysis)

• Monitor Points: Placed at 0.5 m, 1.0 m, and 1.8 m heights to track air temperature relevant to children and adults

A Computational Fluid Dynamics (CFD) model represents a fluid domain surrounding an environment to analyse bushfire and air flow.

Results

The CFD simulation showed that:

• Internal air temperature remained below 39°C for the full 20-minute exposure.

• Internal wall surfaces remained below 60°C, demonstrating effective insulation and heat resistance.

• The space achieved compliance with Spec 43C9 under worst-case external radiant loading.

Internal surface temperature distribution on the fire exposed wall

Temperature and velocity vector distribution, showing airflow

Video clip shows the gradual increase in indoor air temperature from 25°C to 35°C over a period of 1200 seconds.

Graph illustrating the gradual increase in indoor air temperature from 25°C to 35°C over a period of 1200 seconds.

Conclusion

This study demonstrates that buildings like school halls can meet NCC Specification 43C9 using passive design alone, even during total HVAC failure. By testing different envelope performances, designers can ensure tenability is maintained without relying on mechanical systems.

CFD enables this performance-based approach, helping project teams:

• Make informed material decisions

• Document compliance for approval

• Reduce risk during bushfire events

Why It Matters

With increasing emphasis on climate resilience and bushfire readiness, simulation tools like CFD offer a fast, safe, and repeatable method to assess building envelope performance — especially for schools, aged care, and childcare centres in bushfire zones.

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Need to assess your project’s bushfire tenability? Deratec offers fast, simulation-driven reports to support NCC compliance and early-stage design validation. Let’s talk.