Why does structural steel need fire protection?
Steel is non-combustible but thermally conductive. Its mechanical strength reduces significantly as temperature increases. Without passive fire protection, an unprotected structural steel member can lose sufficient load-bearing capacity to cause collapse within minutes of fire exposure. Building Regulations Approved Document B, BS 9999 and BS 9991 require structural elements to maintain their load-bearing function for a defined resistance period to allow safe evacuation and fire-service intervention. Passive fire protection slows the rate at which the steel heats, extending the time before its critical temperature is reached.
What is structural fire resistance?
Structural fire resistance is the ability of a construction element to maintain its load-bearing capacity (R), insulation (I) and integrity (E) during a standard fire exposure for a stated period. For structural steel, the load-bearing requirement (R) is the primary criterion — the member must not fail structurally for the required duration. The resistance period is not an inherent property of a material or product; it depends on the specific tested or assessed system, the member configuration, the section factor and the critical steel temperature. Each structural element must be assessed individually against the project's structural fire design.
What is the difference between reaction to fire and structural fire resistance?
These are different characteristics measured by different test standards serving different functions. Reaction to fire (EN 13501-1) describes how a product contributes to fire growth, smoke production and flaming droplets if it becomes involved in a fire. A2-s1,d0 is a reaction-to-fire classification for surface linings. It does not provide structural fire resistance. Structural fire resistance (EN 13501-2, BS EN 13381) describes how a construction element maintains its load-bearing capacity, insulation and integrity during a standard fire exposure for a stated period. A product with an A2-s1,d0 surface classification is the correct specification for a wall, ceiling or soffit classification requirement — not for a beam or column requiring structural fire resistance.
What fire-resistance periods can structural-steel protection systems provide?
Available fire-resistance periods depend on the specific tested or assessed system, member configuration, design inputs and installed build-up. Periods of 30, 60, 90 and 120 minutes are achievable in principle for intumescent, board, spray and encasement routes — but the achievable period for a specific member on a specific project must be confirmed from the product or system assessment documentation for the actual section factor, critical steel temperature and member type. No route inherently provides any period without confirmed assessment coverage for the exact member configuration.
How is the correct passive-fire system selected?
Route selection follows from the structural fire design inputs, the practical constraints of the project, and the available assessment evidence for each system. Intumescent coatings are suited to exposed architectural steel where a slim profile and paint-like appearance are required. Board systems are suited to robust environments where impact resistance, moisture tolerance and defined installation tolerances are important. Tested spray-applied systems are suited to large frame areas where speed and area coverage outweigh appearance requirements. Encasement — concrete, masonry or proprietary casing — suits very aggressive environments where long-term durability takes precedence. No single route is universally appropriate, and each must be confirmed against the structural fire design inputs before specification.
What is the difference between intumescent coating and fire-protection boards?
Intumescent coatings are reactive thin-film or thick-film systems that expand under heat to form an insulating char. They provide a slim, paint-like finish and are suited to exposed architectural steel. The required dry-film thickness is calculated per member from the approved product loading schedule using section factor and critical temperature — this varies across every member on the project. Board systems are fixed casings made from calcium silicate, mineral fibre, gypsum-based or proprietary board products. They are applied in defined layers and fixings around the steel member and are suited to robust environments where impact resistance and moisture tolerance are required. Board installation is visually inspectable; intumescent DFT requires measurement. The complete intumescent specification sequence is covered on the Intumescent Coatings page.
What is the difference between intumescent coating and spray-applied fire protection?
Intumescent coatings are reactive materials that expand under heat. They produce a slim, paint-like result suitable for exposed steel and are applied to member-specific thickness requirements confirmed from a loading schedule. Tested spray-applied passive-fire protection — which may be cementitious, fibrous or based on another documented composition depending on the system — is applied at a uniform thickness confirmed from the product assessment for the section factor and resistance period. Spray systems are rapid on large frame areas and can follow complex profiles, but are not suitable for exposed architectural finishes and require thickness measurement before concealment. Neither system should be described as cementitious unless that specific composition has been confirmed for the selected product.
What is a section factor?
Section factor (Hp/A in m⁻¹) relates the heated perimeter of a steel section to its cross-sectional area. More slender sections — with a higher section factor — heat more rapidly under fire conditions and require greater protection to achieve the same resistance period. The section factor changes with member type (beam or column), section profile (universal section, hollow section, cellular or castellated), exposure (three or four sides heated), and connection conditions. It must be calculated for each actual member and exposure condition — not estimated from general tables or assumed from a section catalogue reference. Forti Nova must not calculate or guess the structural fire-design inputs; they must be supplied or confirmed by the competent project design team.
What is critical steel temperature?
Critical steel temperature is the temperature at which a member's load-bearing capacity reduces to the level corresponding to the design load under fire conditions. It is established by the structural fire design (EN 1993-1-2) and depends on the structural utilisation or load ratio of each member. It is not universally fixed at 550°C — this is a commonly cited default, not a universal physical property. A more heavily utilised member has a lower critical temperature and may require greater protection than a lightly loaded member of the same section. The critical temperature for each member must come from the structural fire design and must be used in the approved product loading schedule to determine required protection thickness.
Do beams, columns, braces and connections require different protection?
Yes. Beams, columns, braces, connections and built-up members each have different section geometries, section factors and exposure conditions. Universal beams typically have higher section factors than universal columns of equivalent weight — and therefore may require greater protection for the same resistance period. Connections, column bases and member junctions require specific detailing to maintain the protection at the interface. Hollow sections (CHS, RHS) heat differently from open sections (UB, UC) and require confirmation that the chosen protection system's assessment covers the specific section type. Each member type must be assessed individually.
Can hollow steel sections be protected?
Yes. Hollow sections — circular hollow sections (CHS) and rectangular hollow sections (RHS) — can be protected by intumescent coatings, boards and spray-applied systems. However, not all intumescent systems are assessed for all hollow section types, and the section factor for a hollow section is calculated differently from an open section. The protection system assessment must confirm coverage of the specific hollow section profile, dimensions, section factor range, and exposure category. Where a hollow section is filled with water or concrete as a protection strategy, the design must be confirmed by the structural fire engineer as a specific tested or assessed construction.
How are cellular or castellated beams assessed?
Cellular and castellated beams — fabricated from universal beam sections with openings cut into the web — have complex geometries that differ from the parent solid section. The section factor, critical temperature, web-post behaviour and opening configuration each affect the fire-resistance requirement and the protection build-up. Not all standard assessment documents cover cellular or castellated sections directly. Specialist fire engineering analysis may be required. Protection applied to the solid portions of the section must be confirmed as covering the web-post zones and opening edges adequately. Assessment coverage for the specific beam configuration and opening arrangement must be confirmed before specification.
Can fire protection be applied over corroded steel?
No — not without first addressing the corrosion. Surface corrosion must be removed to achieve the required surface cleanliness and anchor profile before any primer or protection system is applied. Where corrosion has caused section loss, the structural engineer must assess the remaining structural capacity — no protection coating restores lost steel section or structural capacity. Applying protection over corroded steel conceals active deterioration without arresting it, and may cause premature adhesion failure or delamination under fire conditions.
How is existing structural fire protection identified?
Existing protection should be identified from the original specification documents, contractor records, Building Control file, or the building owner's golden thread records. Visual appearance alone cannot establish the product identity, applied thickness, or remaining fire performance of an existing system. Where original documents are unavailable, sampling and laboratory analysis of the existing material may be required to establish composition. An existing system that cannot be identified cannot be specified as 'remaining compliant' — the fire strategy must be re-established against the current occupancy and building regulations before any repair or continuation is confirmed.
Can existing protection be locally repaired?
Local damage can be repaired if the existing system has been identified, the original loading schedule and required protection thickness are known for the affected members, and a compatible repair material is confirmed from the system documentation. Repair materials must be compatible with the existing system — applying a different product over an identified existing protection introduces an untested interface. Where the existing protection cannot be identified, widespread damage, water ingress, detachment, cracking or corrosion beneath is found, or the required DFT cannot be verified, complete removal and replacement is the appropriate route.
How are coating or spray thicknesses inspected?
For intumescent coatings: dry-film thickness (DFT) is measured using calibrated electronic film-thickness gauges on the cured film, at a number and distribution of positions per member confirmed in the inspection plan. Readings must be recorded against individual member references in the member schedule. Wet-film checks during application assist process control but do not substitute for DFT measurement. For tested spray-applied systems: applied thickness is measured using pins or gauges before areas are concealed or enclosed. Once the surface is enclosed, measurement is no longer possible. All readings and member references must be documented and retained for the building safety case.
How are board systems inspected?
Board systems are inspected visually at installation and at defined maintenance intervals. Installation inspection should confirm: product identity and batch; board thickness and number of layers; fixing type, size and centres; framing arrangements where required; joint and junction details at connections, corners and penetrations; and that no unprotected gaps or access openings have been left undetailed. Concealed-work photographs must be taken before enclosure. Photographs must be traceable to individual members. During the building's operational life, inspection should confirm: boards are intact and not dislodged; no moisture damage, staining or deformation; no penetrations or modifications have compromised the protection.
Can structural fire-protection work be undertaken in occupied buildings?
In some cases, subject to the method statement, zone management, containment, dust and VOC controls, occupancy controls and the applicable safety assessment. Work involving preparation — abrasive blasting, grinding, shot blasting — requires strict containment and is generally not compatible with occupied areas. Application of solvent-based intumescent systems in occupied zones requires ventilation and occupancy exclusion during application and cure periods. Water-based systems may offer lower solvent emissions. Spray-applied systems require dust containment. The method statement and risk assessment must address re-occupation conditions. Programme and access must be agreed with the building manager before work begins in any occupied or partially occupied building.
Who determines the system and the required performance?
The fire engineer or structural engineer determines the required fire-resistance period, structural fire design inputs — section factors, critical temperatures, load ratios — and the fire strategy for each element. The protection system and required thickness for each member are then specified against those inputs using the product or system assessment documentation. Building Control or an approved inspector confirms compliance with Building Regulations under the project's approval route. Forti Nova must not calculate or guess the structural fire-design inputs; they must be supplied or confirmed by the competent project design team. Forti Nova provides specification support and system documentation — not structural fire engineering.
When is removal, replacement or another construction solution more appropriate?
Removal and replacement rather than repair is appropriate where: the existing protection cannot be identified or its original assessment documentation cannot be located; widespread damage, moisture ingress, corrosion beneath, or adhesion failure is found on inspection; the original DFT cannot be verified for the majority of members; the system has been incompatibly overcoated; or the fire strategy or occupancy has changed and the existing protection was not designed for the current requirement. Another construction solution — such as structural redesign, the use of fire-resistant steel, or alternative building layout to reduce the required fire-resistance period — may be appropriate where the protection route is unworkable due to programme, access, environment or cost constraints. These decisions must be made by the competent project design team and fire engineer, not by the protection contractor alone.
