Modern codes and mass timber buildings
- Examine the historic construction of Notre-Dame and compare its construction to the International Building Code requirements for modern cathedrals.
- Examine the fire-resistive properties of modern mass timber members.
- Review upcoming changes to the IBC that will allow taller mass timber structures.
The recent fire at the iconic Notre-Dame de Paris Cathedral left many people around the world feeling a sense of loss that’s not commonly felt for an inanimate structure. While steel and concrete have been the catalyst for taller buildings, wood-framed structures like Notre-Dame have started to make a comeback.
Advancements in fire science have given the design and construction industry a better understanding of how timber structures perform in fire conditions. These advancements, coupled with rising steel costs and sustainability benefits, have led to a recent resurgence in the use of one of the world’s earliest building materials.
By better understanding the capabilities of mass timber construction, modern building codes are able to specify additional safety measures to maintain the fire safety of mass timber buildings. As a result, Notre-Dame is not only unique because of its historical significance, it’s also unique because a cathedral built under modern building codes would look significantly different from the historic landmark.
The construction of Notre-Dame
Notre-Dame is a religious, historical and architectural landmark that’s been brought to life through 850 years of history. According to the cathedral’s website, construction of the iconic building began in 1163 and continued until the mid-1200s. The cathedral has undergone several reconstructions and renovations, including the renovations that were underway when the recent fire started.
As illustrated by Vox and the New York Times, the majority of the outer structure of the building is constructed of stone, while solid wood beams were used for the framing of the roof, towers and iconic spire. The wood framing for the roof contained so many beams that that the structure was nicknamed “the forest.” The framing also supported a lead roof weighing approximately 210 metric tons.
According to Encyclopaedia Britannica, the footprint of the cathedral is approximately 67,000 square feet. As reported by CNN and Vox, the building was equipped with a fire alarm system, which activated during the fire. While the cathedral did not have a sprinkler system, the activation of the fire alarm system alerted the occupants of the building, allowing them to evacuate safely.
Although the fire alarm system was successful at giving occupants time to escape the building, it ultimately was not sufficient to protect the iconic structure from fire. Reporting by the New York Times detailed how the system alerts caused confusion among security personnel. As a result, personnel weren’t able to locate the fire until 30 minutes after the initial alarm. This delay allowed the fire to spread unabated in the cathedral’s attic.
The detection system that was installed in the cathedral is known as an air-sampling or aspirating, smoke detection system. As further detail in NFPA’s Fire Protection Handbook, these types of systems draw air from a protected space, filter the air to remove dust particles and transport the air to smoke detector. Air-sampling smoke detectors are capable of achieving a very high level of sensitivity and are often used in high-value spaces that require early detection of a fire.
However, the system was only capable of detecting a fire and alerting personnel of the condition. Since Notre-Dame was not protected by an automatic fire suppression system (like a sprinkler system), upon detection of a fire, successful suppression was entirely dependent on human intervention. Heroic intervention by firefighters saved the cathedral’s north and south towers and prevented total collapse of the building. However, the initial delay in response to the fire meant the cathedral’s “forest” and spire were beyond saving.
Modern building code requirements
In the 850-years or so since Notre-Dame was built, construction methods and building codes have evolved to better protect buildings from fire. The International Building Code is published by the International Code Council and used as a model code throughout the United States and other countries around the world. The IBC contains design and construction requirements for various types of buildings, including places of religious worship. The latest edition of the IBC is the 2018 edition; the 2021 edition is under development.
While in general, there are specific exemptions from certain code requirements for religious buildings, such buildings must still comply with several general construction requirements. For example, based on the construction materials used for Notre-Dame and the lack of an automatic sprinkler system in the building, a similar building constructed under the IBC would be limited to 15,000 square feet (IBC Table 506.2). Notre-Dame cathedral is more than four times that size. In addition, IBC 903.2.1.3 requires sprinklers to be installed in assembly spaces (e.g., religious buildings) where the “fire area” exceeds 12,000 square feet. Since Notre-Dame did not contain fire-rated separations, the “fire area” for the building is the building footprint (IBC 202). Since the footprint exceeds 12,000 square feet, a sprinkler system would have been required in Notre-Dame, had it been built under the current edition of the IBC.
There are exemptions in the IBC that allow religious buildings to be unlimited in area; however, there are several safety measures required to use such exemptions (see Figure 1). Most notably, regardless of the type of construction materials, the building would need to be fully sprinklered under the IBC to take advantage of the unlimited area allowance (IBC 507.6 and 507.7).
In addition to the basic size and construction requirements for buildings similar to Notre-Dame, current building codes require additional safety features such as emergency lighting, exit signage and minimum number and size of exits. The IBC also places a significant emphasis on automatic sprinkler systems in modern buildings. Based on a report from NFPA, in structure fires in the United States between 2010 and 2014, automatic sprinklers were able to control 96% of the fires in which they activated. Automatic sprinkler systems have shown significant effectiveness in limiting the property damage, loss of life and injuries sustained during a building fire. As a result, such systems have become a staple in modern building design.
The future of mass timber
Timber construction similar to that used in Notre-Dame is still permitted in modern buildings (designated as heavy timber construction). In addition, designers are starting to turn to even larger timber members, referred to as mass timber. While steel and concrete have historically been the standard building materials for large-scale buildings, mass timber has started to emerge as a more favorable option. According to Bloomberg News, due to the rising cost of steel, a lower carbon footprint and faster construction schedule, mass timber has become increasingly popular in modern buildings. However, modern-day mass timber is distinctly different from traditional heavy timber.
IBC Table 2304.11 currently prescribes minimum dimensions for heavy timber members, to ensure the members have a certain amount of inherent fire resistance. Although wood is combustible, larger timber members have greater fire resistance due to the char layer that forms on the member when exposed to fire. The Society of Fire Protection Engineers’ Handbook of Fire Protection Engineering details the science behind char layers.
As the outer layer of a wood member degrades during a fire, the wood is converted into a layer of char. This char layer insulates the rest of the wood and slows the degradation of the member. As a result of this inherent fire resistance, members with the minimum dimensions specified in Table 2304.11 are recognized by the IBC as generally equivalent to one-hour fire-resistance rated construction (IBC Table 601). However, even if the wood structure in Notre-Dame was consistent with the dimensions in Table 2304.11, the building is still more than four times larger than the current IBC allows.
Modern construction techniques also have introduced composite mass timber members such as cross-laminated timber. CLT consists of at least three layers of solid-sawn or structural composite lumber that are bonded by structural adhesive. CLT and other composite mass timber members are considered heavy timber construction and can be used under the IBC.
The development of CLT and other composite timber members has led to the ability to construct larger structural members out of wood. As a result of the insulating properties of char layers, these mass timber members can provide greater fire resistance and structural capacity. Coupling larger timber members with noncombustible components such as gypsum and concrete can achieve an even greater level of fire-resistance than exposed timber members.
As a result of the advancements in timber construction and recent fire testing of modern timber assemblies, approved changes to the 2021 IBC will allow larger timber structures than previous editions. The existing timber construction classification (Type IV) will remain unchanged, but the 2021 IBC will introduce three new timber construction types, each with an increased level of fire-resistance and decreased amount of exposed wood (see Figure 2). Type IV-C construction is similar to the existing heavy timber construction classification; however, a greater level of fire-resistance is provided by using larger timber members. Type IV-B construction has similar fire-resistance to Type IV-C construction; however, less exposed wood is permitted in Type IV-B construction. Finally Type IV-A construction provides the greatest level of fire-resistance and does not allow any exposed timber.
The ability to create larger timber members with greater fire-resistance now allows timber buildings to be larger than previously permitted by the IBC. Advancements in construction techniques and fire science have given the construction industry a better understanding of the allowances and limitations of wood construction. These advancements now allow designers to safely use wood construction in larger buildings.
Old versus new
A modern Notre-Dame would have several noticeable differences from the historic landmark. Under today’s building codes, a religious building of similar size requires either an automatic sprinkler system, noncombustible construction, larger timber members and/or less exposed wood. Such safety measures are now required to reduce the fire hazard in modern buildings. While Notre-Dame and other historic buildings are not built to modern codes, many historic buildings are retrofitted to include modern fire safety technology (including Notre-Dame). While the damage to the structure is tragic, the fire could have been much more tragic had occupants not been given the advanced warning provided by the fire alarm system.
Ultimately, it isn’t fair to compare an 850-year–old structure to the requirements and advancements of modern building codes. However, the building can be used to reflect on how far such codes have come. Since the construction of Notre-Dame, architects and engineers have continued to push the boundaries of construction. With today’s advancements in construction techniques, modern buildings are able to harness the benefits of wood construction, while still achieving fire safety. It’s buildings like Notre-Dame that have helped to advance the world’s collective understanding of what can be achieved through architecture and engineering. As a result, buildings have continued to be built stronger and safer than those before.