What is structural fire safety?

A brief definition of structural fire safety

Structural fire safety can be well defined by its goals. Basically, structural fire safety pursues these four goals:

• Goal 1: A fire should first be prevented.
• Goal 2: If a fire does break out, the aim is to prevent the fire from spreading uncontrollably.
• Goal 3: It must be possible to save people and animals.
• Goal 4: It should be possible to extinguish a fire as effectively as possible.

Structural fire safety makes an important contribution to achieving these goals. The measures are supplemented by contributions from organisational, technical and defensive fire safety. If you would like to get an overview of the different fire protection measures, please click here. This article focuses on the partial aspect of structural fire protection.

How is structural fire safety achieved in buildings?

Goal 1: how can a fire be prevented?

The risk of fire is of course greatly reduced if there are as few combustible materials (so-called fire loads) in a building available. Of course, "empty" (i.e. fire load-free) buildings are the dream of every fire officer, but the idea is certainly unrealistic in most cases. As an example, we can look at a warehouse with stored goods. In order to reduce the fire loads, the fire safety planner will choose non-combustible or flame-retardant building materials for the supporting structure of the hall. Since the fire protection expert here optimizes the construction of a building in terms of fire protection, one also speaks of constructive fire protection.

Specifically, this means that the fire protection planner provides, for example, a steel support structure (non-combustible) and non-flammable or hardly flammable sandwich elements for the roof and wall cladding. The fire protection planner has thus achieved the first goal by reducing the fire load, since this makes a fire less likely overall. And even if there were a fire (see goal 2 below), the fire would be more manageable due to the lower fire load.

Goal 2: how can an uncontrolled fire be avoided?

The first measure is very well suited to reducing the risk of fire. That doesn't mean it can't burn though! In the case of the exemplary steel hall given above, the stored goods would still be present as a significant fire load. The fire protection planner always assumes the worst case scenario, namely that a fire will break out.

Now the next question inevitably arises. How can the fire be prevented from spreading uncontrollably? Here we can first consider two different situations. In the case of neighboring buildings, the first aim is to prevent the fire from spreading from one building to the next. The macroscopic isolation principle is also used here. As you will learn from the listing, there is also microscopic isolation when units are structurally separated. Structural fire protection offers the following options, among others, to prevent the uncontrolled spread of fire:

• If the buildings are newly built, they keep certain distances from each other to prevent the fire from spreading. The so-called German model building code (pdf file) defines these distances in §6. This type of isolation between buildings is also referred to as macroscopic isolation.
• The walls between different buildings are classified as fireproof walls (so-called fire walls) or even as complex fire wall executed. Since the buildings are allowed to stand next to each other here, this measure is also referred to as microscopic isolation.
• So-called hard roofing is used that is non-combustible (e.g. roof tiles). This is also a microscopic isolation.
• The load-bearing structure of a building, as well as the walls and ceilings, have a so-called fire resistance. This ensures that the support structure can withstand a defined fire duration (e.g. 90 minutes) and thus seals off. The fire protection requirements then also apply to doors and ventilation ducts, for example.
• So-called fire compartments can also be created within a building. These are areas that are allowed to burn out in a fire. Structural measures, however, prevent the fire from spreading to a neighboring fire compartment. The fire safety planner can implement fire compartments, for example, by defining a fire resistance for the room-enclosing components of a compartment.

The sealing principle is one of the most effective structural fire protection measures. History teaches us the consequences of neglecting this principle. Fires in ancient times and the Middle Ages destroyed large parts of cities because the houses were so close together built and made of combustible materials (straw and wood). The city fire in Rome in 64 under Nero is certainly one of the most famous fires, the over 70% of Rome's urban area was devastated. For this reason, roads also make an important contribution to the isolation principle, as they divide cities into blocks. Thus, a fire can "only" take place in one block, but not easily spread to the entire city.

Since the development is becoming increasingly dense, especially in inner-city locations, it is becoming increasingly difficult or impossible to comply with the required boundary distances (macroscopic isolation). In such cases, the separation of units through structural measures (microscopic isolation) becomes more important.

So that the costs for fire protection do not get out of hand, the legislator defines, for example, the building class different requirements for the fire resistance of components. This means that the microscopic containment only has to work for a certain period of time for the occupants of a building to be able to save themselves or be saved. It is clear that in a ground-level industrial hall there are generally much lower requirements for the components than in a high-rise building, since self-rescue in the event of a fire is becoming increasingly difficult.

The current standards show possible solutions so that components can achieve a specific fire resistance period. If the planner decides on different solutions, it should be ensured that these have a general building authority test certificate as proof of usability. This confirms that a certain fire resistance has been demonstrated for this type of construction in a fire test.

Goal 3: how can people and animals be saved in the event of a fire?

With the safe formation of escape and rescue routes, structural fire protection makes an important contribution to being able to save people and animals. One of the most effective structural measures results from §33 of the German model building code (pdf file). According to this, buildings to which the model building regulations apply must always have two escape routes that are independent of one another. In the event of a fire, building users can use the first escape route to exit the building safely. The second escape route is an additional security in case the first escape route is blocked as a result of the fire.

Similar to the microscopic isolation principle (see Objective 2), the structural fire protection for the escape and rescue routes adapts to the existing building type, which the building code with the so-called building classes describes. It is therefore clear that the building regulations place higher demands on the escape and rescue routes for a multi-storey office building than for a single-storey office building. The primary concern is the length and width of the escape routes and their fire resistance. According to the Technical Rule ASR A2.3 "Escape routes and emergency exits, escape and rescue plan" result in further requirements. Overall, structural fire protection takes the following relationships into account, which are immediately obvious:

• The higher a building is (higher building class), the longer the escape and rescue routes have to withstand a fire (higher fire resistance).
• The more people plan to use a building, the wider the escape route must be.
• The greater the risk of fire, the shorter the escape route must be. In buildings with explosive substances, the escape route must therefore tend to be short. In buildings with sprinklers, however, it can be longer.
• For people with disabilities, there are higher requirements for escape and rescue routes, since it cannot be assumed that these people will be able to save themselves. The escape routes must therefore be usable longer so that these people can be rescued.

Goal 4: how can a fire be extinguished as effectively as possible?

How can the fire brigade extinguish a fire as effectively as possible? The fourth goal of structural fire protection revolves around this question. Even if it may seem trivial, the fire brigade must of course be able to get there first. When you think of dense inner-city locations, this is not a matter of course. In this respect, the fire protection planner ensures that the fire brigade can reach the building unhindered via public transport routes and that they have sufficiently large parking spaces. Of course, the fire brigade needs sufficient extinguishing water connections, which the fire protection planner also takes care of in coordination with the fire brigade.

The building regulations also require that the escape routes must be designed in such a way that they also serve as attack routes for the fire brigade. So they must not collapse, which requires a certain level of fire resistance. In order for the fire brigade to be able to extinguish a fire effectively, it must not spread uncontrollably. In this respect, there is a strong interaction here with the third goal discussed above, the principle of isolation.

Incidentally, it is not undisputed how the building code requirement for "effective" deletion is to be interpreted. If you are interested in this point, you can get more information in a principle paper of the Expert Committee on Building Supervision (this is a pdf file).