There is still considerable confusion in the timber construction industry regarding the minimum fire resistance requirements that a building must meet.
First and foremost, it is necessary to distinguish between activities subject to the control of the National Fire Brigade Command and those for which no prescriptive regulations exist.
For public buildings in general, such as schools, hotels, hospitals, theaters, etc., clear and precise standards exist that impose fire resistance and reaction classes.
Residential buildings fall under activities subject to Fire Brigade control when they have a fire height exceeding 24 m, and for these buildings Ministerial Decree No. 246 of May 16, 1987 applies; the Decree also provides guidance for buildings with fire heights between 12 and 24 m.
However, a significant portion of professional activity involves the design of residential buildings with fire heights below 12 m, for which the aforementioned regulations do not apply. Therefore, as in all cases involving activities without specific “vertical” standards, it is necessary to refer to DECREE of March 9, 2007 “Fire resistance performance of constructions in activities subject to the control of the National Fire Brigade“.
This decree defines 5 performance levels that ensure, in a gradual manner, the maintenance of the building’s load-bearing capacity, limited propagation of fire and smoke to adjacent activities, the ability of occupants to evacuate the building safely, and the ability of rescue teams to operate under safe conditions.
The class applicable to residential buildings is performance class II, which requires single-story buildings to maintain mechanical resistance under fire for 30 minutes, while two-story buildings require R60.
In my professional practice, I find that this decree is increasingly used as a reference both by banks, when granting mortgages to clients wishing to build timber houses, and by insurance companies, which, when issuing fire risk policies, inquire whether the design has taken the Decree into account in order to determine the annual premium amount.
In these cases, we are required to issue declarations regarding compliance, during the design phase, with these inputs in defining exceptional load actions as per section 3.6.1. of Ministerial Decree of January 14, 2008.
Strategies to ensure required fire resistance requirements
The subject itself is very broad and would deserve far more than a simple post.
Specifically for residential construction (CLT or timber frame buildings), we can apply different strategies to ensure the required resistance requirements:
- refer to the load-bearing capacity of the “bare” structure and calculate its resistance based on the charring rate of the element;
- Clad the load-bearing elements with protective panels (various materials are available, and the choice must be both economic and technical based on evaluations of the Tch time, defined in EC5-2, necessary to satisfy the required performance);
- Rely on the panels used to create the technical counter-wall (usually 2 layers of plasterboard or fiber-gypsum board), being aware, however, that openings for electrical or HVAC systems locally nullify the protection offered. In these cases, it is necessary to take appropriate measures (solutions to ensure fire protection effectiveness in case of openings are often found in panel manufacturers’ manuals) so that these points do not compromise the fire resistance of the structure.
The choice of strategy is of a technical/economic nature but must be able to ensure compliance with the minimum required requirements.
Unfortunately, in my experience to date, I see, in the case of CLT structures, buildings with increasingly slender walls of 95-100 mm, often even 3-layer (therefore with lower fire resistance performance), which certainly allow the contractor to reduce costs, without, however, taking the necessary measures on the other side to ensure the required fire resistance.
Technicians and construction managers are required to be competent and specialized also in this sector which, unlike traditional construction, requires many more precautions and knowledge.
Analysis of ignition sources and fire propagation
In addition to ensuring the maintenance of load-bearing capacity for a predetermined time interval, it is also the designer’s responsibility to take all necessary measures to ensure that, first and foremost, the possibility of ignition and propagation of a fire is minimized.
There are numerous factors that can trigger a fire in a dwelling; among the main ones we cite the following:
- Combustion product evacuation systems (chimneys);
- Electrical systems;
- Photovoltaic systems.
Among the causes that can facilitate fire propagation we find:
- Ventilated walls and roof floors;
- Use of combustible materials in “sensitive” areas.
These topics, with particular attention to the main causes of ignition and design strategies to limit them, will be the subject of the next post.
Eng. Emanuele Fornalè