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Fire characteristics of building materials and structures

26.05.2023

Let’s find out what fire resistance is, the spread of flames through the structure and roof, the class of reaction to fire and flammability.

The fire hazard indicators that certain building materials and structures must satisfy are defined by state regulations and are binding on manufacturers and consumers. Understanding the difference and characteristics of fire performance is important for the safe operation of buildings and the safety of property and life in the event of a fire.

 

Ruukki SP2D W sandwich panels after a fire

Ruukki SP2D W sandwich panels after a fire

 

Depending on the specifics of operation, building structures may have different performance requirements in terms of fire resistance. DBN B.1.1-7:2016 establishes eight levels of fire resistance of a building depending on its purpose, fire hazard category, conditional height (number of floors) and floor area within the fire compartment. Depending on the level of fire resistance of the building, requirements for fire resistance classes of building structures and fire spread groups are determined.

 

What is fire resistance and REI limit states?

 

Fire resistance is the ability of a structure to resist the effects of fire and high temperatures and to perform load-bearing, protective and insulating functions for a specified period.

 

A quantitative characteristic of the fire resistance of a structure is the fire resistance limit, which is measured in minutes from the beginning of fire exposure during fire tests to the occurrence of one of the limit states of the structure.

 

Limit states of building structures

Basic according to DBN B.1.1-7:2016Additional according to DBN B.1.2-7:2021

R – loss of load-bearing capacity

E – loss of integrity

I – loss of thermal insulation capacity

W – excess heat flux (radiation)

M – loss of ability to resist mechanical impact

 

Depending on the purpose, building structures may have different fire resistance classes, the limit states of which are indicated by letters (R, E, I) and a number indicating the fire resistance limit in minutes. For external non-load-bearing walls and internal partitions, the EI limit states are used, and for load-bearing and self-supporting structures, REI and RE are used. For example, Ruukki sandwich panels with PIR filling can have a fire resistance class of up to EI60, and with mineral wool filling – up to EI240. For fire partitions made of sandwich panels, tests are additionally carried out to determine the loss of the structure’s ability to resist mechanical impact M.

 

The common method of determining the fire resistance limit is by conducting fire tests, but there is also a calculation method that is mostly used in specific cases.

 

Fire test of Ruukki sandwich panels

Fire test of Ruukki sandwich panels

 

Fire test results are only applicable to certain conditions and specific product samples. For example, the ability of sandwich panels to withstand fire is influenced by many factors, including the configuration of the locking joint, the design of the components, the density of the filler, the thickness of the cladding, etc. Therefore, the use of sandwich panels in construction projects should be in the design that was submitted for fire tests and specified in certificates and/or protocols with the appropriate fire resistance class.

 

It should be noted that European fire test methods consider a greater number of structural parameters, including the direction of fire spread (outside, inside, between rooms), the purpose of the structure, the direction of installation (vertical or horizontal), span dimensions, load, heat flow, presence of cladding and sealing elements, etc. EU methods exclude the possibility of manipulation to improve the results, for example, by using additional fasteners for locked joints or other tricks, so they are more objective. Ukrainian suppliers of sandwich panels sometimes go to tricks when they submit for certification a sandwich panel structure with the highest possible characteristics (filler density, cover thickness, etc.), which is tested on minimum spans with additional sealing and locking, making it possible to obtain overestimated results. As a result of such manipulative tests, the documents obtained do not meet the actual conditions of use and this increases the risk of fire spreading throughout the premises and, as a result, premature destruction of the building. Products used in construction projects should have characteristics no worse than those obtained during fire tests.

 

Fastening panel locks to artificially increase fire resistance

Fastening panel locks to artificially increase fire resistance

 

Additional confirmation of the high fire safety of sandwich panels is the presence of a certificate from the American company FM, which evaluates products by the highest global safety standards, which allows significant savings on insurance. All types of Ruukki sandwich panels are certified according to FM standards for spans up to 9 m.

 

Fire tests according to the FM standard

Fire tests according to the FM standard

 

What is the flame spread group according to the M design?

 

In Ukraine, it is mandatory to determine an indicator that characterizes the ability of a building structure to resist the spread of flames. The test methodology is to measure the extent of damage to the construction outside the fire zone. During the test, a small fragment of a 2×2 m structure is exposed to continuous fire for 15 minutes, after which the damage to the inner layer of the sample is measured. As a result, the structure is assigned to one of the groups:

  • M0 – the limit of flame spread is 0 m
  • M1 – the limit of flame spread does not exceed 0.25 m for horizontal structures and 0.40 m for vertical and inclined structures, with charring, melting and burning of materials to a depth of 2 mm
  • M2 – everything that has higher values than for M1

 

Sandwich panels with non-combustible mineral wool filling are assigned flame spread group M0 without testing. All Ruukki sandwich panels with mineral wool and polyisocyanurate foam F-PIR and X-PIR have a flame spread group M0 and with E-PIR – M1.

 

Construction of a fire partition made of Ruukki sandwich panels

Construction of a fire partition made of Ruukki sandwich panels

 

For sandwich panels and ventilated facades, the possibility of flame spreading through the insulation or smouldering under the cladding/facade is especially dangerous, as these areas are difficult to access when extinguishing a fire. Therefore, the correct selection of the type of sandwich panels and ventilated facades is essential for the safe and durable operation of the building.

 

What is the fire reaction class or flammability?

 

Flammability is the property of a material to resist the effects of flame. Combustible materials are those that, when exposed to fire or high temperature, ignite, smoulder, char and smoke, and continue to burn after the ignition source disappears.

 

According to DBN B.1.1-7:2016, construction materials are classified as non-combustible (NC) or combustible (CC). In turn, combustible materials are divided into groups according to their flammability (F), flammability (F), flame spread over the surface (F), smoke-forming ability (S) and toxicity of combustion products (T). At the same time, non-combustible materials are not divided by fire safety indicators (G, B, FP, D, T).

 

Today, test methods for flammability indicators (G, V, RP, D, T) are not in force in Ukraine. Therefore, to determine and classify flammability indicators, it is necessary to use the harmonized standard DSTU EN 13501-1:2016, according to which fire performance is determined by the fire reaction class, which is as follows:

 

[Main classification] – [smoke formation], [droplet formation]

 

The main classification – A (1,2), B, C, D, E, F – determines the flammability of products, where A is non-combustible materials, and F is the most highly combustible. Non-combustible, such as mineral wool and steel, are classified as A1, Ruukki sandwich panels with mineral wool due to the glue and polymer coating are classified as A2 and sandwich panels with PIR filling are classified as B.

 

An additional smoke classification – s1, s2, s3 – determines the ability of a product to produce smoke when exposed to flame or high temperature. The lower the number, the lower the smoke generation. Ruukki sandwich panels with mineral wool and F-PIR and X-PIR polyisocyanurate foam belong to the s1 smoke class, and those with E-PIR foam – to the s2 class.

 

An additional droplet classification – d0, d1, d2 – defines the ability of a product to form burning droplets and particles when exposed to flame or high temperature. The lower the number, the lower the droplet formation. All Ruukki sandwich panels belong to the d0 class, i.e. they do not form droplets when burning.

 

An approximate comparison of fire classification for wall and ceiling cladding according to the old and new standards in accordance with DBN B.1.1-7:2016.

 

IndicatorOld classificationNew classification
Non-combustibleNGA1,A2-s1,d0
FlammabilityLow flammabilityG1A2-s1,d1; A2-s1,d2; A2-s2,d0; A2-s2,d1; A2-s2,d2; A2-s3,d0; A2-s3,d1; A2-s3,d2; B-s1,d0; B-s1,d1; B-s1,d2; B-s2,d0; B-s2,d1; B-s2,d3; B-s3,d0; B-s3,d1; B-s3,d2
Moderate flammabilityG2C-s1,d0; C-s1,d1; C-s1,d2; C-s2,d0; C-s2,d1; C-s2,d2; C-s3,d0; C-s3,d1; C-s3,d2
Medium flammabilityG3D-s1,d0; D-s1,d1; D-s1,d2; D-s2,d0; D-s2,d1; D-s2,d2; D-s3,d0; D-s3,d1; D-s3,d2
High flammabilityG4E; E-d2; F
Smoke-forming abilityLow smoke-forming capacityD1s1
With moderate smoke-forming capacityD2s2
With a high smoke-forming capacityD3s3

 

What is the flame spread over the roof (BROOF – FROOF)?

 

The resistance of a roof covering to external fire is an important characteristic, the definition of which is regulated by the DSTU EN 13501-5:2016 standard. Tests are carried out by one of four tests that assess the ability of a structure to resist flame spread under various conditions of fire exposure to burning elements, wind and thermal radiation in different roof designs.

 

In EU countries, the indication of flame spread is mandatory for all roofing structures, but in Ukraine, it is still voluntary and used mainly by leading manufacturers who care about consumers and can guarantee the high quality of their products. The indicator can have the values BROOF (tX), CROOF (tX), DROOF (tX), EROOF (tX), FROOF (tX), where X is a number from 1 to 4, which indicates the test number for which the test was performed. The structure with the lowest flame spread is classified as BROOF, and the structure with the highest flame spread is classified as FROOF. All Ruukki roof sandwich panels are tested using the first test methodology and belong to the BROOF (t1) class.

 

Model of a real building fire

 

At the request of the manufacturer or the customer, fire tests of facade structures with thermal insulation can also be carried out in the conditions of a real fire. Large-scale fire tests are carried out to DSTU B V.1.1-21-2009, and medium-scale fire tests – to DSTU B V.1.1-22-2009.

 

Large-scale fire tests

Large-scale fire tests

 

The large-scale fire test method involves three fire scenarios for the corner joint of facades:

  1. Flames escaping from the interior through a window
  2. External flames from materials burning against the wall
  3. Heat radiation from a fire in a neighbouring building

 

The real fire model helps to identify weaknesses in the building structure and consider the impact of environmental factors, thereby increasing the ability of constructions to resist the spread of fire.

 

Fire hazard indicators of building materials and structures are among the most important since they affect property and human life safety. Therefore, before purchasing sandwich panels, ventilated facades or other building structures, it is important to understand the essence of these indicators and the specifics of their determination for different products. The fire performance of building materials and structures supplied by Rauta is specified in the declarations of conformity, available in the public domain on the company’s website. All declared parameters are confirmed by European and Ukrainian test reports and can be provided to the customer upon request.

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