Fire engineering — Fire characteristics

Introduction

This note frames how structural fire engineering uses fire development concepts and temperature–time relationships. The same vocabulary appears in furnace ratings, simplified member checks, and performance-based studies—always read curves together with the governing standard and project fire strategy.

What this section covers

• Stages — growth, possible flashover, fully developed burning, and decay in compartment fires.
• Curves — standard furnace exposure (e.g. ISO 834) versus natural/parametric representations of compartment gas temperatures.
• Design link — how the chosen curve ties to prescriptive ratings, analytical models, or rational design submissions.

Regulatory context

South African projects rely on the National Building Regulations, SANS 10400, and referenced material standards. Approved fire engineering reports may specify performance-based fire scenarios and thermal inputs; this page does not replace those documents or local authority requirements.

Notation

• t — time (minutes), common axis for standard and parametric curves.
• θ_g — gas temperature in the fire or furnace (°C), often plotted against t.
• ISO 834 — international standard temperature–time curve used for many furnace tests and simplified structural fire models.

Fire development

Real compartment fires move from ignition through growth; under sufficient fuel and ventilation, flashover can occur, after which the compartment is often treated as a post-flashover “fully developed” fire before decay as fuel is consumed.

Temperature–time curves

Standard curves (e.g. ISO 834) provide a common furnace exposure for rating tests and simplified design. Natural or parametric curves attempt to represent real compartment behaviour (ventilation, fuel load, lining materials) for performance-based design.

Design use

Choosing the appropriate curve is a project decision tied to regulations, fire engineering strategy, and whether design is prescriptive or performance-based.

Why it matters for structures

The assumed gas temperature history drives thermal actions on members and connections; inconsistent curve choice undermines comparability between test ratings, hand calculations, and advanced simulation.

When to use which curve

Use standard curves when aligning with tabulated ratings, furnace-tested assemblies, or code tables that assume that exposure. Use natural or parametric curves when a qualified fire engineering study defines compartment-specific scenarios and the regulator accepts performance-based inputs.

Design question

Diagram sources