A dashed empty box marked with a question mark stands before the figure "120 min", showing a fire rating with its performance criterion missing. To the right, the three criteria that belong in the box: R for loadbearing, E for integrity, I for insulation.Passive Fire Protection

Rated for 120 minutes of what?

The most common passive fire protection enquiry names a duration and stops there. A number with no performance criterion behind it isn’t a specification.

An enquiry lands: provide 120-minute fire protection to the steel. Sometimes it says R 120. Both get treated as specifications and neither is one. A fire rating is two things bolted together, a duration and a performance criterion, and the enquiries that drop the criterion are asking a question nobody can answer without quietly inventing the rest of it.

Fig 1. The EN 13501 performance criteria. BS 476 uses different words for the same three ideas: loadbearing capacity, integrity, insulation. The minutes say how long; the letters say what.

A duration is not a performance

Fire resistance is classified as a set of letters and a time, for example EI 120 or REI 90. The letters are the performance the element has to deliver.

  • R is loadbearing capacity: the member keeps carrying its load and stays standing.
  • E is integrity: it holds back flames and hot gases from passing through.
  • I is insulation: it limits the temperature rise on the unexposed face so fire can’t spread by conduction to the far side.

A BOQ line that reads "120 minutes fire rating" specifies the duration and omits the performance. It reads like a specification because it has a familiar number in it, but it tells the supplier nothing about what the element is being asked to do for those 120 minutes.

Match the criterion to the element

Different elements have different jobs, and the right letters follow from the job, not from habit. A steel column or beam is a loadbearing element, not a separating one. What it needs is R: stay standing for the period. It is not there to be a fire barrier, so E and I usually don’t apply to the bare member. A compartment wall or floor is a separating element: it needs E and I to keep fire on one side. If it does both jobs, load and separation, it needs all three, REI.

Fig 2. The performance letters are a property of the element’s function. Getting them wrong on the enquiry means specifying the wrong product before any thickness or detail is even discussed.

Here is where the coatings enquiries go wrong. An intumescent coating on steel delivers R, and only R. It keeps the section below its critical temperature so the member holds its load. It does not turn a column into a fire-rated wall, and it gives no integrity or insulation to a space. So an enquiry asking for EI 120 on a bare steel column is a category error: you cannot buy integrity or insulation from a coating on an exposed section. And an enquiry that says only “120 minutes” leaves the supplier guessing whether you meant R (protect the steel) or EI (build a barrier), which are entirely different products.

And even a correct R 120 isn’t a thickness

Once the enquiry properly says R 120 to the steel, you still don’t have a number to apply. What goes on the member is a dry film thickness, and it is not one value across the frame. It depends on the member’s critical temperature (its true limiting temperature under load, not a default), its section factor Hp/A (how fast the section heats), and the specific product’s certified table.

Fig 3. Section factor traces the heated perimeter (ember line). A three-sided beam and a four-sided column of identical section resolve to different thicknesses under the same R 120. Values indicative.

Three points are worth carrying forward without turning this into a coatings manual. The loading table belongs to one product: a thickness read from Product A is not valid for Product B. Most tables assume a critical temperature of 550°C; if the member’s true limiting temperature is different, the number changes. And you may interpolate inside a certified range but never extrapolate beyond it. All of that, though, is second-order. It only starts once the enquiry has said R 120 in the first place.

Before you send a fire-rating enquiry

  • Does the rating state a performance criterion (R / E / I), or just a number of minutes?
  • Do the letters match the element’s job: R for loadbearing steel, EI for separating walls and floors, REI when it does both?
  • Is the coating being asked only for what a coating can give (R), not integrity or insulation it physically cannot?
  • For steel: is a critical temperature stated per member, or is 550°C being assumed by default?
  • Is the section factor given for the correct exposure, three-sided or four-sided?
  • Does the certified table cover your section factors, and is it that specific product’s data?

A rating is a duration and a criterion. Name the criterion, put the right one against the right element, and everything downstream, product, thickness, method, finally has something real to hang on. Send “120 minutes” into the market on its own and the market will answer the question it assumes you asked, which is not always the one your building needs answered.