Understanding fire risk in tall timber buildings
By Robert Gerard
February 10, 2014
The development of highly sophisticated new wood products, combined with the design industry’s growing commitment to sustainability, has led to increasing interest in tall timber structures. However, building codes and regulations designed to address concerns about fire safety still place strict restrictions on timber construction across much of the world.
Many in the architecture, engineering, and construction industry believe that developing a broader understanding of new technologies, design tools, and research will allay these fears and enable the construction of more and taller timber buildings. In an effort to benchmark progress on this front, the Fire Protection Research Foundation commissioned Arup to investigate the fire safety challenges of tall wood buildings.
My colleagues David Barber and Armin Wolski and I recently published the first report in a multi-phase study to come out of the initiative. One of the main goals during this stage was to identify gaps in existing knowledge and suggest research priorities to address fire safety concerns in tall timber buildings.
Timber in room fires
One of our top recommendations was to invest in greater understanding of exactly how exposed timber influences fire dynamics inside rooms. More needs to be known about how exposed timber materials impact both the development of fires and conditions within affected rooms throughout a fire’s duration, as these can impact occupant safety and structural stability.
For example, when exposed cross-laminated timber (CLT) is used for walls or the undersides of floors, layers of char can form and fall off. This process, known as delamination, may raise temperatures within the room and contribute to the fire’s fuel load. In turn, the increased exposure to heat may result in more severe fire conditions and speed the collapse of structural timber elements.
However, while the general outlines of this process are understood, the specifics are unclear. More needs to be known about the performance of exposed CLT and other timber materials in room fires. This is vital not only to enable designers to create safer timber buildings, but also to overcome negative stereotypes about wood’s safety during fires.
Testing of steel connections such as nails, bolts, plates, and rods has shown that exposed elements quickly lose strength when subjected to fire. However, common means of protecting them — among others, providing a protective layer of gypsum board or embedding them within the structure — can be less than ideal in terms of aesthetics, cost, structural efficiency, and constructability.
More research is needed to allow designers to understand and predict the fire performance of connections in structural systems and develop safe, efficient, and architecturally pleasing connections for new timber technologies. A high level of safety in structural design will enable greater confidence in structural stability during building fires.
Service penetration impact
Builders cut holes into floors, walls, and ceilings to provide for mechanical, plumbing, air conditioning, electrical services, and communications infrastructure. These penetrations must be lined with materials known as fire stopping that contain fires and prevent smoke from spreading to adjacent areas.
In most buildings, the areas surrounding these penetrations are made of noncombustible materials. In timber buildings, however, potential charring where structure meets fire stopping may compromise performance. More research needs to be conducted to determine the best design for fire stopping in wooden buildings.
Expanding the knowledge base
These examples represent some of the most important gaps in knowledge about fire safety in timber buildings. More research and fire testing of these and related topics is critical to developing a better understanding of timber building fire performance and enabling the design and approval of tall timber structures.