Fire design

Assess fire resistance of concrete sections using heat transfer analysis, the AS 3600 Section 5 tabulated design method, or AS 3600 Section 5 tabular cover requirements.

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 three complementary fire assessments:

1. Heat transfer + fire capacity — a 2D finite element heat transfer analysis followed by a fire-reduced biaxial interaction surface and utilisation check (available for all design codes)
2. AS 3600 Section 5 tabulated design method — a deemed-to-satisfy design gate that looks up minimum section width and axis distance from the Section 5 tables using your actual support condition and computed fire load level μ_fi; outputs a PASS, FAIL, or NOT ASSESSED verdict with demand/supply ratios (AS 3600 only)
3. AS 3600 Section 5 tabular cover — reads axis distances from the code tables using conservative worst-case structural assumptions and converts the axis distance to a minimum clear cover for use in the Cl 4.10.1 cover calculation (AS 3600 only; driven by the Member Type setting in the General tab)

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

Member type (General tab)

The Member Type selector in the General tab drives both the AS 3600 Section 5 tabular cover calculation and the AS 3600 Section 5 tabulated design method. ACS currently supports four member types:

The Governing assumption column above describes the tabular cover resolver only — the cover is always computed on the conservative branch so the cover number is valid for any structural configuration without requiring you to specify end conditions or load factors. The AS 3600 Section 5 tabulated design method uses your actual inputs instead: the support condition you have set for beams and slabs, and the fire load level μ_fi computed from your actual loads for columns and walls (see AS 3600 Section 5 tabulated design method below).

The member type does not affect the heat transfer analysis or the fire-reduced interaction surface — those depend only on the section geometry, fire duration, fire curve, and exposed edges.

Axis distance and clear cover

AS 3600 Cl 5.2.1 defines the axis distance $a_s$ as the distance from the nearest exposed concrete surface to the centre of the reinforcing bar. The minimum clear cover for fire is the axis distance less however far ACS actually places that bar’s centre in from the face — and that depends on where the bar sits.

A bar on a face — a mid-edge bar — sits one fitment plus one bar radius in:

A corner bar does not. It nestles inside the fitment’s bend, so its centre is pushed further in, and the fire cover is:

Where:

• $a_s$ = axis distance from Table 5.4.1(A), 5.5.2(B), 5.6.3, or 5.7.2 (mm)
• $d_{fitment}$ = fitment (stirrup) bar diameter (mm); zero when no fitments are present
• $d_b$ = governing fire-exposed longitudinal bar diameter (mm)
• $r_{eff}$ = the fitment’s effective bend radius, $\max(r_{internal}, d_{b,corner,max}/2)$ (mm) — the largest corner bar on the tie opens the bend for every corner bar on it
• $\theta$ = the interior angle of the corner the bar sits in
• $c_{fire}$ = minimum clear cover for fire (mm)

Both terms exist because ACS derives the cover by inverting its own bar placement: a bar sitting outside a stirrup has its centre that much further from the surface than the cover alone, and a bar seated in a bend further still. Ignoring the fitment overstates the fire cover; using the mid-edge expression for a corner bar understates it, burying the steel deeper than the standard asks. The two expressions coincide when there is no fitment ($r_{eff} = 0$) and when the corner bar is fat enough to open the bend itself ($r_{eff} = d_b/2$).

The distinction matters most for Table 5.4.1 Note 1, whose $a_{sd}$ is a corner-bar requirement by definition — the corner expression always applies there. The cover panel’s Section 5 disclosure names whichever term was subtracted, so the printed arithmetic always describes the placement the canvas draws.

The governing cover for the section is the largest of the fire cover, the durability cover (from the exposure class and concrete grade), and the placement cover (minimum 20 mm, or 30 mm adjacent to formed surfaces per AS 3600 Cl 4.10.1).

Beam axis distances (Table 5.4.1(A))

For simply-supported beams, the required axis distance depends on both the FRL and the minimum web width $b$. ACS reads the section’s actual width and selects the governing row:

Axis distances in mm. A dash means the section is too narrow for that FRL — ACS reports Resolved = false and shows a failure reason rather than returning a silently-substituted value.

Column axis distances (Table 5.6.3)

For columns at μ_fi = 0.7, the required axis distance depends on the smaller cross-section dimension and the number of exposed sides:

For a multi-side-exposed column at FRL 240, AS 3600 Table 5.6.3 has no entry — ACS refuses and reports a failure reason rather than extrapolating.

The number of exposed sides is determined by the Exposed Elements configuration in the Fire tab.

Slab axis distances (Table 5.5.2(B))

For one-way simply-supported slabs, the required axis distance depends only on the FRL:

Wall axis distances (Table 5.7.2)

For load-bearing walls at N*/φNu = 0.7:

AS 3600 Section 5 tabulated design method

The AS 3600 Section 5 tabulated design method is a deemed-to-satisfy design gate and is an alternative to the EN 1992-1-2 heat transfer and fire capacity calculation methods — the two approaches are never cumulative (AS 3600 §4.1). It is available for AS 3600 designs only.

Unlike the tabular cover resolver (which uses conservative worst-case inputs and outputs a cover number), the tabulated design method uses your actual inputs and outputs a PASS, FAIL, or NOT ASSESSED verdict alongside demand/supply ratios.

Verdicts

Demand/supply ratios

The assessment expresses each check as a demand/supply ratio so they slot into the governing-entry result machinery:

• $b$ — minimum section width: the minimum web width for beams; the smaller cross-sectional dimension (thickness or lesser side) for all other member types
• $a_{s,\text{required}}$ — the axis distance read from the applicable Section 5 table
• $a_m$ — the Cl 5.2.1 average axis distance measured from the section geometry (see below)
• $a_{sd,\text{required}}$ — the Table 5.4.1 Note 1 corner-bar side axis distance, $a_s + 10$ mm (beams only, see Corner-bar side axis distance); $a_{sd,\text{min}}$ is the shallowest corner bar’s measured distance to the side face

The governing ratio is the larger of the applicable ratios. A governing ratio ≤ 1.0 is a PASS; > 1.0 is a FAIL. Slabs have no tabulated minimum width, so ratio_b does not apply to them, and ratio_asd applies only to beams that meet Note 1’s conditions.

:::caution[ratio_asd can fail while ratio_a passes] $a_m$ is an area-weighted average over all the longitudinal steel, so a single shallow corner bar can be masked by deeper or larger bars elsewhere in the section. Note 1 is a per-bar requirement on the corner bars alone. The two are reported as separate ratios for exactly this reason — read ratio_asd on its own, not as a stricter version of ratio_a. :::

:::note[Compliance margin, not capacity margin] The ratios are compliance margins, not capacity margins. The Section 5 tables are a step function calibrated at discrete fire-resistance periods, so ratio_a = 0.9 does not mean 10% spare structural capacity — it means the section is 10% below the next discrete table entry. Small changes in axis distance near a table boundary can shift the verdict without a corresponding change in structural capacity. :::

Average axis distance $a_m$ (Cl 5.2.1)

The average axis distance is an area-weighted mean over the longitudinal steel that has at least one fire-exposed surface within measuring distance:

where $a_i$ is the perpendicular distance from bar $i$ to the nearest fire-exposed surface of the section and $A_{si}$ is the bar’s cross-sectional area. Bars located on unexposed edges are excluded from the sum per Cl 5.2.1.

:::note[Beams average the bottom reinforcement only] Cl 5.2.1 gives the formula; Cl 5.4.1(b)(iii)(B) decides which steel a beam sums it over — “the average axis distance to the longitudinal bottom reinforcement”. Table 5.4.1’s legend scopes the companion quantity the same way (“b = width of the beam at the centroid of the bottom reinforcement”). So for a beam, ACS averages the steel below the section’s mid-height and leaves the top steel out. Columns (Cl 5.6.3), slabs (Cl 5.5.2) and walls (Cl 5.7.2) carry no such scoping and are averaged over all their longitudinal steel.

The difference is not cosmetic. Top bars are usually larger and further from the exposed surfaces — worst of all where the top face is shielded by a slab, the very case Cl 5.4.1(b)(i) addresses, because a top bar’s nearest exposed face is then a side face. On a 300 × 600 beam at FRL 90 with three N20 bottom bars 25 mm from the soffit and two N32 top bars, including the top steel lifts $a_m$ from 25 mm to 47 mm and turns a genuine 1.60 FAIL into a 0.85 PASS.

Excluded steel is still measured and listed in the Cl 5.2.1 derivation table, marked as not entering $a_m$ (the report’s In a_m column, and ”— not in a_m” in the panel), so an item left out of the average stays distinguishable from one you never placed. A tendon is partitioned the same way: one draped low at the section under assessment is bottom reinforcement, one draped high is not. :::

:::note[Reinforcement and tendons are averaged separately] Cl 5.2.1 permits this average where the steel “consists of either reinforcement or prestressing tendons with the same characteristic strength”. A section holding both therefore gets two averages, each weighed against its own requirement: the reinforcing bars against the tabulated $a_s$, and the tendons against $a_s$ plus the Cl 5.3.3 increase. The panel shows them as separate rows — $a_m$ for the bars, $a_{m,p}$ for the tendons.

This matters because a single blended average lets one family compensate for the other: a deep bar can average a shallow tendon back into apparent compliance even though the tendon carries the higher requirement of the two. :::

If the section has no longitudinal steel with a positive area, or has no fire-exposed surface to measure from, the average axis distance cannot be computed and the assessment returns NOT ASSESSED. A beam whose steel all sits above mid-height returns NOT ASSESSED for the same reason: there is no bottom reinforcement for Cl 5.4.1(b)(iii)(B) to average, and the Section 5 beam tables address the bottom reinforcement of a simply-supported or continuous beam.

Corner-bar side axis distance (Table 5.4.1 Note 1)

Note 1 under Table 5.4.1(A) and Table 5.4.1(B) imposes an extra requirement on beams:

In beams with only one layer of bottom reinforcement, the axis distance to the side of the beam for the corner bars including tendons or wires, shall be increased by 10 mm, except, where the value of b is greater than that given in Combination 4, no increase is required.

The reason is geometric: a corner bar is heated from two faces at once, so it runs hotter than a mid-face bar at the same axis distance, and an area-weighted average cannot see that. Where the beam has a second layer of bottom reinforcement the inner layer shields the corner, and the note does not apply.

ACS applies the increase when all three conditions hold:

The third condition is a strict comparison: a beam exactly as wide as Combination 4’s $b$ still owes the increase. Where a table row has no Combination 4 at all — Table 5.4.1(B) tabulates only two combinations below FRL 120 — there is no width to exceed, so the increase always applies.

:::note[Per-edge cover] The corner test compares each face’s distance past that face’s own cover, not the raw distances. Under a single cover the two are the same question; when you set a different cover on the soffit than on the sides, they are not. A bar seated in the tie’s bend sits the same amount past each of its two covers, so it is recognised as a corner bar whatever the two covers are.

Because the test is measured against cover, it cannot be answered without one. Inside ACS the cover always comes from your section, so this never arises; a caller of the fire_capacity API that omits cover gets the conservative reading — corner bars assumed present — and a disclosure saying so, rather than a silent “this beam has no corner bars”.

When cover is supplied with per-edge values — a different cover on the soffit than on the sides, for instance — those values are now propagated through to the full fire capacity calculation unchanged. Prior to 2026-07, the stateless capacity route collapsed per-edge cover to a single value before the fire check; results for sections with non-uniform cover from that route will differ.

The result states which of the three it reached, as cornerClassification: corner-bars-found, none (a determined absence, measured — the note is switched off and this beam owes no increase), or undetermined (the question could not be asked, so the note was applied on the conservative assumption). It is absent where the corner set is not what decided the note — a non-beam, $b$ greater than Combination 4’s, or more than one layer of bottom reinforcement — so an absent value never means none. Where no $a_{sd}$ ratio was produced, the results panel and the report state the classification in words rather than leaving the check silently absent. :::

The requirement is measured to the side face specifically — not to whichever of the corner’s two faces happens to be nearest, which under unequal cover is the soffit. It raises the required cover for the corner bars, and the Cover panel discloses it as its own line so the +10 mm is traceable back to the note:

Axis distance a_s 65 mm (AS 3600:2018 Table 5.4.1(A), b = 200 mm, FRP 120 min)
Corner-bar side axis distance a_sd 75 mm = a_s 65 + 10 (Note 1 — one layer of
  bottom reinforcement, b not greater than Combination 4)
Fire cover 55 mm = a_sd 75 − fitment Ø10 − ½ × Ø20

Before reinforcement is placed, ACS cannot yet establish whether the section will have corner bars in a single layer, so it applies Note 1 on the conservative assumption that it will and shows a disclosure saying so. The advised cover tightens once the bars exist and the conditions can be checked.

Fire load level $\mu_{fi}$ (columns and walls)

For columns and walls, the Section 5 tables are subdivided by the fire load level $\mu_{fi}$, which is the ratio of the fire combination axial demand to the section’s ambient axial capacity at the design eccentricity:

where $N^*_f$ is the axial force from the fire load combination and $\phi N_u$ is the axial capacity from the ambient interaction surface at the eccentricity of the fire combination — not the squash load, because the squash load overstates capacity whenever a moment is present.

When no fire load combination is defined: ACS falls back to $\mu_{fi} = 0.7$, the most onerous tabulated level. This is conservative and may require a larger axis distance than the actual design needs. A disclosure is shown in the result to flag this. Defining a fire load combination with a compressive axial design action will allow ACS to compute $\mu_{fi}$ from your actual loads.

$\mu_{fi}$ is a table selector for columns and walls only. For beams and slabs the table is keyed on the support condition, not the load level — a missing fire combination does not affect the beam or slab assessment and does not raise the disclosure.

Tendons (Cl 5.3.3)

When the section contains prestressing tendons, AS 3600 Cl 5.3.3 requires the axis distance read from the Section 5 table to be increased for those tendons:

ACS checks this as a separate ratio, alongside the reinforcing bars’ own check:

where $a_{m,p}$ is the Cl 5.2.1 average taken over the tendons alone. It can govern the verdict while the bar ratio sits comfortably below 1.0, so read it in its own right.

:::caution[The increase applies to the tendons, not to the section] Cl 5.3.3 raises the requirement for prestressing tendons. It is not a section-wide uplift of the tabulated value, and it does not enter the code-based cover — a tendon is positioned by its own profile, never off the cover + fitment + ½·d_b chain, so its requirement cannot be inverted into a cover for the reinforcing bars.

A practical consequence: adding or removing a tendon does not change the cover the Cover panel advises. If you are checking a PT section against an earlier ACS release, note that versions before this behaviour landed did raise the advised cover when a tendon was present, and attributed the result to fire when durability governed. :::

Which increase applies

ACS reads the arm from the product form on the tendon’s material — strand, wire, or bar. Strand and wire share arm (a); only bar tendons take arm (b). The applied clause and its arithmetic are named in the result disclosures, so a result always states which of the two it used and how it got there.

A section may hold both products. Cl 5.3.3 reads per tendon, and Cl 5.2.1 will not average across steels that do not share a characteristic strength, so each arm is checked separately — its own $a_{m,p}$ against its own requirement. The governing one is reported as $\text{ratio}_{ap}$, and both carry their own disclosure.

:::caution[An undeclared product form takes the more onerous arm] A tendon with no catalogue material selected — or one saved before ACS recorded the product form — has nothing to read. ACS then applies Cl 5.3.3(a)‘s +15 mm, the more onerous of the two, and adds a disclosure saying the type was assumed. Select a catalogue material on the tendon to have the requirement come from your actual product; a bar-tendon design assessed this way is being asked for 5 mm more axis distance than AS 3600 requires of it. :::

The beam tables assume a protected upper surface (Cl 5.4.1)

Table 5.4.1 as reached through Cl 5.4.1 is conditional. The clause applies to a beam that:

has the upper surface integral with or protected by a slab conforming with Clause 5.5

Three-side exposure is therefore the premise of the beam table, not an incidental configuration. A beam that can be exposed on all four sides is governed by the separate Cl 5.4.2 route, which uses the same tables but adds three further requirements: the total depth $D$ must be no less than the least $b$ tabulated for the fire-resistance period, the concrete area must satisfy $A_c \ge 2b^2$, and $a_m$ must be determined using the beam’s minimum dimension for $b$ and taken over all longitudinal reinforcement and tendons — not the bottom steel alone.

Because a protected upper surface is the ordinary configuration for a beam in a floor system, selecting the AS 3600 Section 5 method for a beam or slab marks the section’s upward-facing outline edges as not exposed, once, at the moment you select it. The change is visible — the canvas repaints and the exposed-element count drops — and a single undo reverses it. It is applied on selection only, never re-imposed afterwards, so a deliberate decision to expose the top face is never silently overwritten.

ACS identifies the upper surface from each edge’s outward normal, not from its height. On a T-section that selects the flange top while leaving the flange soffits and the flange tips exposed, which is exactly the configuration Cl 5.4.1 describes — the bounding-box top alone would also catch the tips.

Beams exposed on all four sides (Cl 5.4.2)

If you re-expose the upper surface, Cl 5.4.1(b)(i) is no longer satisfied, and ACS moves the assessment to Cl 5.4.2 rather than continuing to report the three-side reading. Three things change, all in the conservative direction:

:::note[Results change in 2026-07] Prior to 2026-07, a four-side-exposed beam was assessed on the three-side Cl 5.4.1 route rather than being moved to Cl 5.4.2. Results for four-side-exposed beam configurations will differ from earlier versions — specifically, the table entry point (minimum section dimension rather than minimum web width), the averaging scope for $a_m$ (all longitudinal steel, not bottom steel only), and the two added proportioning ratios (Cl 5.4.2(i) and (ii)). :::

• The table is entered on the beam’s minimum dimension, not its minimum web width. For a beam that is deeper than it is wide these are the same number and nothing moves. For a wide, shallow beam they differ by the aspect ratio, and the web width is the less onerous of the two: a 900 × 250 beam at FRL 90 reads $a_s = 35$ mm on $b = 900$ and $a_s = 45$ mm on $b = 250$. The advised cover follows, so such a beam is now detailed deeper.
• Two proportioning requirements are added and reported as their own rows: $D \ge$ the least $b$ tabulated for the period (Cl 5.4.2(i)), and $A_c \ge 2b^2$ (Cl 5.4.2(ii)). The area requirement genuinely binds — a near-square beam sitting at its least tabulated $b$ fails it.
• The axis-distance requirement extends to all longitudinal steel. Cl 5.4.2(iii) applies it to all longitudinal reinforcement and tendons, because a beam heated on four sides heats its top steel too. ACS keeps the bottom reinforcement’s $a_m$ as the governing average and weighs the remaining longitudinal steel as its own Cl 5.2.1 average against the same requirement, reported as $a_{m,\text{other}}$. The two are kept apart deliberately: one blended average would let deep bottom bars raise $a_m$ and report shallow top steel as compliant.

The clause substitution is stated on the compliance result, in the report, and beside the advised cover — $b$ in the results table is the minimum dimension on this route, so it will not match a hand lookup on the beam’s width.

:::caution[Two configurations have no tabulated route at all] Cl 5.4.2 covers “a beam of approximately rectangular cross-section”, and it is a beam clause. Two cases therefore fall outside Section 5 entirely, and ACS refuses them rather than borrowing a clause: no compliance ratio, and no code-based cover.

• A non-rectangular beam exposed on all four sides — a T, I or L section, or a thin-walled box. Cl 5.4.1 needs its upper surface protected and Cl 5.4.2 needs it approximately rectangular, so neither reaches it. ACS treats a section filling less than 95 % of its bounding box as not approximately rectangular, and states the measured figure in the refusal.
• A slab with its upper surface exposed. Cl 5.5.2 tabulates a slab heated from beneath, and Cl 5.4.2 does not extend to slabs.

In both cases, mark the upper surface unexposed if it is in fact protected, or assess the member by choosing a different analysis method. :::

Shielded faces and the advised cover

Section 5 tabulates an axis distance to a fire-exposed surface (Cl 5.2.1), so a face you have marked as not exposed carries no fire requirement. The code-based cover follows that: the fire term applies only to fire-exposed faces, while the durability (Cl 4.10.3) and placement (Cl 4.10.2) terms are section-wide and apply to every face.

A section with a shielded face therefore resolves to a per-edge cover — the exposed faces on the fire-governed value, the shielded ones on max(durability, placement). The Cover panel states which faces the fire term reached and the governing term for each edge, and locks the cover mode to Per-edge while that derivation is live so a switch to Uniform cannot discard it. Switching to Manual cover, choosing another fire method, or exposing every face releases the lock.

The Cl 4.10.1 fire term is attributed face-by-face: each edge’s fire cover is derived from the axis distance applicable to that specific exposed face. Results for sections with mixed exposure — some faces fire-exposed, some shielded — may differ from versions prior to 2026-07, where the term was not attributed to the specific face of the section.

With no face exposed, Section 5 asks nothing of the member: no fire term is applied, and the cover resolves on durability and placement alone.

Fire exposure configuration

Open the Fire tab in the right panel to configure the heat transfer and fire capacity analysis:

Fire duration

Set the required fire exposure duration in minutes. Quick buttons provide common values (30, 60, 90, 120, 180, 240 min); the picker opens at 60 minutes — the first standard FRL value and a valid entry for both the tabular cover and heat transfer + capacity analysis paths. The maximum supported duration for the heat transfer + fire capacity analysis is 360 minutes.

AS 3600 Section 5 tabular cover is limited to FRLs up to 240 minutes. When the fire duration exceeds 240 minutes, the Cover panel shows Resolved = false together with a failure reason identifying which required FRL falls outside the tabulated range. The tabular method cannot produce a compliant value for an out-of-range FRL — ACS refuses rather than silently using the 240-minute axis distance, which would understate the actual cover requirement. The heat transfer and fire capacity analysis is unaffected: it continues to run for any duration from 1 to 360 minutes regardless of whether the tabular cover resolves. For members requiring fire resistance beyond 240 minutes, use the heat transfer + fire capacity analysis to demonstrate structural adequacy by calculation.

Fire curve

Select the time-temperature relationship:

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 exposed-edge configuration affects both the heat transfer analysis and — for columns and walls — the number of exposed sides used to look up the Section 5 axis distance.

Aggregate type

Select the aggregate type used in the concrete mix. This input is active when the design code is AS 3600 or EN 1992-1-2; it has no effect under ACI 318-19. AS 3600 Cl 5.3.1(b) Note delegates elevated-temperature material properties to EN 1992-1-2 Tables 3.1, 3.2a and 3.3, so AS 3600 fire calculations use the same siliceous/calcareous distinction as EN 1992-1-2.

The aggregate type affects the concrete strength reduction factor $k_c(\theta)$ used in the fire capacity calculation. Calcareous aggregate concrete generally provides better fire performance.

Steel class

Select the steel class for the reinforcement. This input is active when the design code is AS 3600 or EN 1992-1-2; it has no effect under ACI 318-19. AS 3600 Cl 5.3.1(b) Note adopts the EN 1992-1-2 material tables for elevated-temperature steel properties, including the distinction between hot-rolled and cold-worked reinforcing bar.

For most reinforced concrete sections, use hot-rolled. Select cold-worked only when the reinforcement was specified as cold-worked deformed bar. Cold-worked steel generally results in a lower (more conservative) fire capacity.

Analysis method

Select the method for computing fire-reduced capacity:

:::note The 500°C isotherm method is the default because its conservatism is calibrated: the “full ambient strength below 500°C” overestimate is deliberately paired against the “zero strength above 500°C” underestimate, and EN validated the pairing. The advanced-fibre method is more rigorous but returns the higher capacity of the two and is substantially slower, so it is a deliberate opt-in rather than the default.

Annex B.1 carries an applicability limit on minimum section width — on very thin sections the 500°C isotherm crowds the reinforcement and the calibration degrades. Prefer the advanced-fibre method there. :::

:::caution[Removed in 2026-07] A third mean-temperature tier was previously offered and was the default. It applied a single area-weighted mean $k_c(\bar{\theta})$ over the full, unclipped section — it was not the EN 1992-1-2 Annex B.2 zone method and did not divide the section into zones, despite once being documented that way. Because it retained fire-damaged perimeter concrete at mean strength its conservatism was not guaranteed: measured on a 400 × 600 mm column at 90 min four-sided ISO 834 exposure ($f'_c$ = 32 MPa) it overstated the isotherm squash load by 12.5 %. It has been removed, and designs that used it now resolve to the 500°C isotherm. :::

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

:::tip[New: the Fire Heatmap can now be zoomed] The Fire Heatmap previously rendered at a fixed fit-to-window scale with no way to get closer. It is now a full viewport: scroll to zoom, middle-drag or Ctrl (⌘) + left-drag to pan, and the house icon in the bottom-left corner to reframe the whole section.

This matters for reading a fire result rather than just looking at it — the 500 °C isotherm at a re-entrant corner, or the temperature at one individual bar, is often only legible zoomed in. Isotherms, bar markers and labels are drawn as vectors, so they stay sharp at any zoom; the temperature field itself is redrawn at full resolution a moment after you stop zooming. :::

The full control set, which is shared with every other canvas in the platform, is in Canvas navigation.

Fire capacity check

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

:::note[Scope] The fire capacity check covers axial-force and biaxial-moment (N–M) interaction only. Shear capacity, torsional resistance, and anchorage at the fire limit state (AS 3600 Cl 5.3.1(b)) are outside the scope of this analysis and must be verified separately using the applicable code provisions. :::

Partial factors at fire limit state

EN 1992-1-2 §2.4.2 sets the material partial factors for the fire limit state to unity:

ACS therefore applies characteristic (nominal) strengths — $f'_c$ and $f_{yk}$ — as the base values before temperature reduction. The resulting fire interaction surface is a born-nominal surface: fire load combination demands are compared against it at the fire combination level (typically $\phi = 1.0$ per EN 1990).

Concrete strength reduction

$k_c(\theta)$ follows EN 1992-1-2 Table 3.1 for the selected aggregate type. The same curve is applied for both analysis methods. For $f'_c > 50$ MPa, EN 1992-1-2 §6 notes that the standard $k_c(\theta)$ curve is non-conservative for high-strength concrete — ACS surfaces a material warning for sections above this threshold regardless of the chosen method.

Steel reduction factors

For the 500°C isotherm method:

Both $k_y$ (yield strength) and $k_{Es}$ (elastic modulus) are applied and tracked; for these simplified methods the stress-strain model is elastic-perfectly-plastic so $k_y$ governs the ultimate-strength result.

For the advanced-fibre method, a third factor is additionally applied:

where $k_{sp}(\theta)$ is the proportional-limit reduction factor — the stress below which the stress-strain curve remains linear. All three factors follow EN 1992-1-2 Table 3.2a.

Advanced-fibre steel law (EN 1992-1-2 §3.2.3 / Figure 3.3)

The advanced-fibre method evaluates a five-region steel stress-strain law at each bar’s local temperature:

Figure 3.3 strain limits (temperature-independent): $\varepsilon_{sy,\theta} = 0.02$, $\varepsilon_{st,\theta} = 0.15$, $\varepsilon_{su,\theta} = 0.20$ (Class B/C ductility). The law applies with odd symmetry in compression (§3.2.3(4)). At 20 °C, $f_{sp} = f_{sy}$ collapses the elliptic region and the law degenerates exactly to the ambient elastic-perfectly-plastic model.

Prestressed concrete (PT)

All three fire analysis tiers carry post-tensioned and pre-tensioned tendons through the fire capacity calculation. The fundamental approach is to hold the ambient-basis locked-in prestrain fixed and reduce only the tendon’s constitutive law with temperature:

• The locked-in tensile strain ($\varepsilon_{pe} + \varepsilon_{decomp}$) is computed on the ambient (20 °C) basis and carried unchanged into the fire state.
• $k_p(\theta)$ and $k_{Ep}(\theta)$ (EN 1992-1-2 Table 3.3, Class A strand/wire) scale the characteristic yield and ultimate strengths and the elastic modulus — they do not inflate the locked-in strain.

Thermal relaxation of the prestressing force is not modelled as a separate time-dependent loss term — it is embedded in the $k_p(\theta)$/$k_{Ep}(\theta)$ reduction factors per EN 1992-1-2 Table 3.3.

When prestress is omitted: if the effective prestress $\sigma_{p.ef}$ is zero (or cannot be resolved), ACS runs the fire capacity check for the reinforcement only and shows a warning. This is not a refusal — the capacity result is valid for the non-prestressed state.

Axis distance (AS 3600 Cl 5.3.3): For tendons, AS 3600 Cl 5.3.3 requires an additional axis distance on top of the base value from Table 5.4.1(A) or 5.5.2(B):

Under the AS 3600 Section 5 tabulated method this is checked for you, as its own ratio against the tendons’ own average axis distance. The Cover panel reports the tabulated axis distance for reinforcement bars only, and the tendon increment never enters the advised cover — a tendon is placed by its profile, not off the cover chain.

Under the calculation methods (500 °C isotherm, advanced-fibre) there is no tabulated axis distance to increase: the heat-transfer solve models each tendon’s temperature directly, so Cl 5.3.3 does not apply.

Bar detail factors

The Bar Details table in the Fire results panel shows the temperature-driven reduction factors applied to each bar:

Concrete reduction display

The section capacity is 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

Cover panel (AS 3600)

The Cover panel in the Materials section shows the governing cover term and its source.

Exposure Class requirement: the Cover panel results are disabled until an Exposure Class is selected. Without an Exposure Class ACS cannot compute the durability cover, which is required to determine the governing cover term. Select an Exposure Class in the General tab to activate the panel.

Conservative structural assumptions: the cover resolver always uses the most conservative branch of each Section 5 table (simply-supported beams, μ_fi = 0.7 columns, one-way simply-supported slabs, N*/φNu = 0.7 walls). The tabular cover is therefore valid for any structural configuration within the member type without requiring you to specify end conditions or load ratios.

These pinned worst cases apply to the cover number only. The AS 3600 Section 5 tabulated design method selects its table from your real inputs instead — the support condition you set for beams and slabs, and the Cl 5.6.3(1) load level μ_fi computed from N*_f/φN_u for columns and walls — so a lightly-loaded or continuous member is not failed by an assumption it does not carry. If you know your actual load level is significantly lower than the conservative assumption, either the tabulated design method or the heat transfer + fire capacity analysis will give a less conservative result than the cover figure.

Section width for beams: the width $b$ used to look up Table 5.4.1(A) is the minimum horizontal solid width of the web — not the bounding-box width. For T-sections, I-sections, and L-sections the flange is not part of the beam web; ACS scans the actual section geometry to find the narrowest solid horizontal slice. This means a T-beam with a 600 mm flange and a 200 mm web is looked up at $b = 200$ mm, which gives a higher required axis distance than a 600 mm rectangular section at the same FRL. For rectangular sections the web width and the bounding box are identical.

Narrow-section refusal: when the section’s controlling dimension falls below the narrowest tabulated minimum for a particular FRL at the selected member type, ACS sets Resolved = false and reports a failure reason rather than returning the most-onerous axis distance. For walls that controlling dimension is the tabulated wall thickness $t_w$ of Table 5.7.2, which earlier releases did not check — a wall thinner than the table permits was previously issued the axis distance for its FRL despite satisfying no tabulated combination. This is consistent with the §874 principle — a silently-substituted value would understate the actual cover requirement for an undersized section. Widen the section or reduce the required FRL to obtain a resolved result.

When the fire term governs, increasing the section width (for beams), using a heavier bar schedule with larger diameter bars, or reducing the required FRL will all change the required cover.

Fire capacity results

Enter the fire limit-state action effects in the Fire Demands panel — N*f, Mx*f, and My*f: the applied axial force and biaxial moments at the fire combination level. These are the accidental combination demands per AS/NZS 1170.0 §4.2.4 (typically G + ψ_l Q, no concurrent wind or earthquake). For EN 1990, use the accidental fire combination; for ACI 216.1, use the gravity combination per §3.4.

PDF report output

The fire section of the PDF report includes the following disclosure blocks:

The Bar Details table in the report also includes Aggregate Type and Steel Class rows alongside the temperature and reduction factors for each bar.

Tips and best practices

• Set the Member Type in the General tab before configuring the fire duration — the tabular cover depends on both
• 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 Section 5 tables give the minimum axis distance, not the minimum cover — the clear cover is the axis distance minus however far in ACS places the bar’s centre ($c_{fire} = a_s - d_{fitment} - d_b/2$ for a mid-edge bar; less the deeper tie-bend seat for a corner bar); when fitments are present their diameter adds to the required cover
• The code-based cover method automatically accounts for fire rating requirements; for a given FRL the fire term may or may not govern over the durability and placement covers
• For FRLs above 240 minutes, Section 5 tables do not apply — verify fire resistance by the heat transfer + fire capacity analysis instead
• Check both the slab-supported case (three-sided exposure) and the free-standing case (four-sided exposure) for beams

Related pages

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