How structures are protected from fire: standards and performance levels

The recent tragedy in Switzerland has brought attention to the importance of security measures for the protection of buildings in the event of fire. The risk of fire concerns any closed environment but, in certain contexts, it is addressed through the fire prevention: a specific area of ​​building design. Regulations and technical provisions establish that structural systems – i.e. the load-bearing framework of buildings – guarantee safety margins defined by fire exposure time. Doors, windows and non-load-bearing walls must also comply with sealing and insulation requirements for the time required by the fire prevention project. Let’s see what they consist of.

The regulatory approach

Italian and European technical standards consider fire to be one exceptional design conditionfor which specific load combinations are adopted based on probabilistic criteria. A fire is defined as the self-sustaining and uncontrolled combustion of combustible materials present in a compartment. From a regulatory point of view, the event is represented by a design fire curvewhich describes the temperature trend of the combustion gases over time, in the vicinity of the structural elements. The high temperature reduces the stiffness and resistance of the materials, compromising the load-bearing capacity of the structure. As time passes, the phenomenon worsens until it reaches the point where the resistance of the elements is no longer sufficient to guarantee the static safety of the building. However, the fire’s evolution is limited by the quantity of combustible material present and, over time, temperatures tend to reduce compared to the peak values ​​recorded in the acute phase. For this reason, the resistance requirement is considered met only if guaranteed for a pre-established minimum timevariable depending on the importance of the work and the project objectives.

Performance levels in the event of fire

European regulations define five performance levels:

• Level 1: no requirement (negligible fire risk).
• Level 2: sufficient resistance to allow the exodus of the occupants.
• Level 3: adequate resistance for emergency management only.
• Level 4: resistance even after the end of the fire, with limited damage.
• Level 5: total maintenance of the building’s functionality during and after the fire.

The choice of level depends on the importance of the structure and the activities carried out within it, which determine danger, exposure and vulnerability. Level 2 is used for the design of most civil buildings. Levels 4 and 5 are reserved for strategic buildings, such as hospitals or operations centers.

Fire protection

There fire resistance it is the ability of a construction (or part of it) to maintain for a predetermined time:

• R: bearing capacity, i.e. the resistance of the elements under the operating load;
• AND: resistant to flames, fumes and hot gases;
• THE: thermal insulation, to limit heat transmission to other environments not directly exposed.

A “pure” structural element (such as a column) will only require the requirement Rwhile a fire door will have to guarantee EI (not being load-bearing). The symbol REI instead it identifies those elements that perform all three functions simultaneously. The requirement is therefore expressed with the acronym REIxxwhere “xx” indicates the minutes of resistance. Example: REI60 → resistance, tightness and insulation guaranteed for 60 minutes (compared to nominal fire curves). By introducing these acronyms it is possible to clearly identify what the degree of protection requested to the structure or a part of it.

How to implement it: active and passive protection

Fire protection can be achieved through two main approaches: active and passive. There active protection includes all those systems that are activated automatically, or manually, in the event of fire, and which contribute to limiting its spread and damage. A typical example are the sprinklersdevices installed in false ceilings which, when a fire occurs, release water to cool the environment and contain the flames. These systems do not act on the resistance of the structure, but on the control of the fire itself. There passive protectionon the other hand, concerns construction solutions and materials that have intrinsic fire resistance characteristics. Let’s look at some examples based on the type of structure.

• Reinforced concrete structures: generally, fire resistance is guaranteed by the concrete cover and the plaster, i.e. layers that protect the internal metal reinforcements. If these thicknesses are not sufficient, the project may include an increase in the concrete cover to achieve the required requirements.
• Metallic structures: They are more vulnerable to heat than reinforced concrete, as steel rapidly loses load-bearing capacity as the temperature rises. To protect them, specific coatings are used. The intumescent paints they are a very widespread solution: in contact with heat, they expand, forming an insulating layer which slows down the heating of the steel. Alternatively, they can be used plasterboard coveringscheaper but also more invasive from an aesthetic and spatial point of view.
• Wooden structures: the resistance requirement is achieved by oversizing the sections or by using protective coatings. In the event of a fire, the carbonization phenomenon of the external layers forms a protective barrier that slows the penetration of heat towards the internal core, allowing the load-bearing capacity to be maintained for a significant time.

Another passive protection strategy is the compartmentalization of environmentswhich consists in creating physical and thermal separations between the different spaces, so as to confine the fire to the compartment in which it originated and delay its spread to other areas of the building. This approach reduces the risk of the fire spreading and allows for safer management of the emergency.