Fire design

Assess fire resistance of concrete sections using heat transfer analysis and reduced material properties at elevated temperatures.

Overview

Fire design verifies that a concrete member retains adequate structural capacity after exposure to fire for a specified duration. Concrete has inherently good fire resistance, but elevated temperatures reduce both concrete compressive strength and steel yield strength. The depth of temperature penetration and the resulting capacity reduction depend on the member geometry, exposure conditions, and fire duration.

ACS performs a two-stage fire analysis:

Heat transfer — determine the temperature distribution through the cross-section at the specified fire duration
Fire capacity — compute the reduced structural capacity using temperature-dependent material properties

When to use fire design

Check fire resistance when:

The member must achieve a specified Fire Resistance Level (FRL), such as 60/60/60 or 120/120/120 (structural adequacy / integrity / insulation)
Building regulations require fire-rated construction for the occupancy class
The member is in a fire compartment boundary or supports fire-rated construction above

Fire exposure configuration

Open the Fire tab in the right panel to configure:

Fire duration

Set the required fire exposure duration in minutes. Quick buttons provide common values (30, 60, 90, 120, 180, 240 min). The maximum supported duration is 360 minutes.

Fire curve

Select the time-temperature relationship:

Curve	Description	Typical use
ISO 834	International standard fire curve	Buildings (default)
ASTM E119	American standard fire curve	Buildings (US practice)
Hydrocarbon	Rapid temperature rise	Petrochemical facilities, tunnels

Exposed elements

By default, all external edges of the section are exposed to fire. Toggle individual edges on or off to model:

Three-sided exposure: typical for beams with a slab on top (top edge unexposed)
One-sided exposure: typical for walls or slabs exposed on one face only
Void exposure: internal void edges can be marked as exposed (e.g., for a duct carrying hot gases)

The number of exposed edges affects the temperature penetration pattern and hence the capacity reduction.

Heat transfer analysis

When the Fire Heatmap tab is selected on the canvas, ACS runs a 2D finite element heat transfer analysis to compute the temperature field through the cross-section at the specified fire duration.

The heatmap shows temperature contours using a colour gradient from ambient (blue) to the fire temperature (red). You can observe:

Temperature penetration depth from exposed surfaces
Corner effects (corners heat faster due to two-sided exposure)
The temperature at each reinforcement bar location

The heat transfer model uses:

Thermal conductivity, specific heat, and density of concrete as temperature-dependent properties (per EN 1992-1-2 Annex A)
Convective and radiative boundary conditions on exposed surfaces
Adiabatic boundaries on unexposed surfaces

Fire capacity check

The fire capacity analysis uses the temperature at each concrete fibre and reinforcement bar to compute reduced material strengths:

f'_{c,\theta} = k_{c}(\theta) \cdot f'_c

f_{y,\theta} = k_{s}(\theta) \cdot f_y

Where:

$k_c(\theta)$ = concrete strength reduction factor at temperature $\theta$ (from code tables)
$k_s(\theta)$ = steel strength reduction factor at temperature $\theta$

The section capacity is then recalculated using these reduced properties and compared against the fire load combination demands.

Fire interaction diagram

ACS generates a fire-rated interaction diagram overlay on the ambient diagram. The fire curve sits inside the ambient curve, showing the reduced capacity envelope. Your fire load combination point must fall inside the fire curve for adequacy.

Interpreting results

The fire capacity check reports:

Result	Description
Fire capacity ( $\phi_{fire} M_u$ )	Moment capacity at elevated temperature
Ambient capacity ( $\phi M_u$ )	Moment capacity at ambient temperature
Capacity ratio	Fire / ambient (shows the proportional loss)
Fire utilisation	$M^*_{fire} / \phi_{fire} M_u$
Status	Pass if fire utilisation $\leq 1.0$

Tips and best practices

Larger sections have better fire resistance due to the thermal mass effect — the core remains cool even after extended exposure
Increasing cover improves fire resistance by insulating the reinforcement from heat
The code-based cover method in the Materials panel automatically accounts for fire rating requirements
For critical members, compare the fire interaction curve with the ambient curve to understand the capacity margin
Check both the slab-supported case (three-sided exposure) and the free-standing case (four-sided exposure) for beams

Fire design theory — heat transfer and material degradation background
Section analysis — overview of all analysis types
Design standards — fire design code references