What the R rating is
The fire resistance of a structural element is expressed with the letter R followed by a time in minutes: an R 90 column maintains its load-bearing capacity for 90 minutes of exposure to the standardised design fire. When the structural element is also a compartment boundary, a floor slab between storeys for example, the requirement is completed with integrity and insulation, and the classification becomes REI, consistent with the vocabulary presented in the article on fire compartments.
It is worth underlining what R is not: it is not the time the structure survives a real fire, whose severity depends on the fire load and ventilation, but a conventional time against a standardised fire that allows homogeneous comparison and requirement. The distinction matters when comparing solutions from different manufacturers.
What values section SI 6 requires
The requirement is modulated by the document's two classic variables: the building's use and the evacuation height, with specific rules for basement floors, always more demanding given the difficulty of intervention. In indicative terms, the structure of an ordinary-use building ranges between R 60 at low evacuation heights and R 120 at large heights and in the basements of demanding uses, with the exact values set in the section's table for each combination of use, height and situation. Lightweight roofs not intended for evacuation enjoy reduced requirements under defined conditions, and secondary elements whose collapse does not compromise the whole may be exempted.
As in the rest of the document, the economic reading of the table lies in the steps: crossing a height threshold can raise the required resistance of the entire structure by one grade, and that grade is paid element by element. It is worth checking the evacuation height before fixing the number of storeys.
How each material resists
The strategy for reaching the required R depends radically on the structural material, and that dependence is one of the serious comparative variables between systems. Concrete reaches it through cover and steel through applied protection.
Reinforced concrete. The best-placed material from the start: its mass and thermal inertia protect the reinforcement, and resistance is achieved with adequate dimensions and cover, verifiable with the tables and methods of the corresponding annex. In most ordinary buildings, a concrete structure reaches the requirement without added protection, with cover as the only, marginal, cost variable.
Steel. The opposite case: steel loses capacity quickly with temperature, and the usual slender profiles heat up fast. Except at low requirements or with favourable checks, steel structures need passive protection: intumescent paints that foam with heat, sprayed perlite or vermiculite mortars, or boards. The choice among them is a trade-off between cost, aesthetics and application conditions: paint respects the exposed profile and costs more per resistance grade, mortar is cheap and coarse, board offers a finish and takes up space.
Timber. A particular and favourable case: timber chars at the surface at a known rate, and the charred layer protects the interior. Justification is done by residual section, sizing the element so that, once the design char depth is consumed, the remaining section still carries. Exposed structural timber, in full growth with cross-laminated systems, thus resolves medium requirements with oversizing instead of added protection.
Masonry. Load-bearing masonry walls generally offer high resistance through their mass, verifiable with the tables of their annex.
The justification routes
The section admits several routes to justify resistance, ordered from the simplest to the most sophisticated: the tables and simplified methods of the document's annexes, test results of specific solutions, and advanced calculation methods, which model the fire and the structural response and allow, in expert hands, requirements to be fine-tuned in singular buildings. For ordinary projects, the first two routes cover practically all cases; the third is an optimisation tool reserved for structures where the potential saving justifies the study.
Budget reading: structural passive protection
The characteristic budget item of section SI 6 is the passive protection of steel structures, and its budgeting has rules of its own worth spelling out. It is measured by profile surface and not by weight of steel.
| Solution | Price logic | Points of attention |
|---|---|---|
| Intumescent paint | By thickness, growing with R and profile massivity | Certificates per profile, thickness control on site |
| Sprayed mortar | Cheap per developed square metre | Coarse finish, adhesion, compatibility with services |
| Board | Per square metre with auxiliary framing | Space take-up, trims, accessibility |
The technical variable governing price is the profile's massivity, the ratio between exposed perimeter and section: slender profiles demand more protection for the same R. This is why passive protection cost is not well estimated with a single ratio per tonne of steel: two structures of the same weight can differ appreciably according to their repertoire of profiles. Correct measurement is done by developed surface and by resistance grade, with the chosen system's certificates on the table.
Within the general framework of the article on the cost of CTE compliance, structural protection belongs to the category of items that exist because of the requirement: without section SI 6 there would be no intumescent paint in the budget. It is also a clear example of cost avoidable by design: the early choice of structural material, or of the structure's exposure, fixes the magnitude of this item in advance.
Frequent errors
The first is budgeting steel protection per tonne with an inherited ratio, ignoring massivities and grades; the usual deviations of this simplification are large in both directions. Correct measurement starts from the schedule of profiles and their massivity.
The second is forgetting boundary elements: the slab separating compartments needs REI, not just R, and the difference can change the solution. Each element's requirement is read from what it separates.
The third is protecting on site what the calculation exempted, or the reverse: coordination between the structural annex and the quantities avoids paying for unnecessary protection and, worse, leaving what is required unprotected. The joint review of annex and quantities is done before tendering.
The fourth is neglecting execution control of intumescent paint, whose thicknesses per profile are the real guarantee of the certified R; a documented thickness control is worth more than any product certificate. Thicknesses are checked profile by profile and recorded.
Note: the values cited are indicative; the resistances required in each case are those of the table of the consolidated DB-SI in force and its annexes, published on the official CTE portal.