It was discouraging enough when the national building codes in Canada and the United States began permitting large 4-story wood framed building for residential uses like apartments and hotels/motels. But then another increase in the height of these buildings emerged when the National Building Code of Canada (NBCC) was further modified in the 2015 edition to permit taller buildings of light wood frame up to 6-stories in height. The sawdust has not even settled from the that last code cycle for the NBCC and already new proposals are being placed in the hopper to further increase the height of buildings using mass timber (MT) wood materials. And these strategies by the North American wood industry for bigger wood buildings is not limited to Canada. Similar MT changes were submitted for consideration in 2015 during the last code cycle of the International Building Code (IBC), the model building code for the United States. Fortunately, the US code change was not approved and the next edition of the IBC regarding taller wood buildings remains the same.
Regarding the Canadian code the new wood industry strategy is to permit taller buildings when the structural system uses heavy timber members such as cross-laminated timber panels referred to in the building community as CLTs (See Figure 1). The proposals seek to permit CLT buildings in the range of twelve (12) stories. The present code permitted height for mass timber buildings is six (6) stories. To exceed six (6) stories the code requires the structure to be constructed primarily of noncombustible materials like concrete, masonry and/or steel. This is a significant deviation of the present code to suggest that taller buildings of heavy wood timber materials will perform like similar buildings of noncombustible materials.
For the reader’s information, taller MT wood buildings are not necessarily prohibited in the present Canadian or US building codes. There are no prescriptive provisions in these codes that tell the user what specific requirements must be met to build such buildings above six stories. Thus, designers who wish to provide taller MT wood buildings for their clients instead have several options of design based on the goals and objectives embodied in the provisions of either the NBCC or the IBC. One option is to provide an alternate design that demonstrates equivalent performance to the prescriptive goals and objectives of the code. If a straight-line equivalency cannot be demonstrated the designer may pursue a performance based design approach. With this approach, the project team, comprised of stakeholders establish the performance objectives that the design must meet and the necessary building features required to meet those objectives. Stakeholders commonly include the owner or owner’s representatives, the design team (architect, engineers, etc.), the authority having jurisdiction (building and fire officials) and the contractor.
It is through the performance based approach that most tall MT wood buildings have been built in Canada (e.g. Brocks Common, University of British Columbia, Vancouver, Canada) or are being designed in the US (e.g. Framework Building, Portland, Oregon, USA). In the case of Brocks Common, it was designed as an 18-story dormitory on the UBC campus for student housing using encapsulated mass timber elements. It was completed in late 2016. The Framework Building is a 12-story multi-use building with planned retail on floors 1-3, businesses on floors 4-6 and residential apartments on floors 7-12. The Framework building, still under design, is expected to follow an exposed MT wood concept (See Figure 2 for an example of exposed MT).
The Table below shows a simplified comparison of the Canadian Wood Council (CWC) proposals for the NBCC and the American Wood Council proposal for the IBC. From the table, you can see both proposals will require the structural frame of CLT buildings (i.e. load-bearings walls and floors/roofs) to have a 2-hour fire resistance rating. In addition to this fire resistance rating, the interior surfaces of the CLT panels are required to be covered by fire rated gypsum board. By covering all the CLT wood surfaces with gypsum board the wood industry is introducing a new term in the building codes that is called “encapsulated mass timber (EMT)” (See Figures 3). There are several supposed purposes for encapsulating the wood members. One, by covering the wood surface of the CLT building the potential for the wood to contribute directly to a fire event within a room in the early stages of a compartment fire might be minimized. Second, the effects a fire in a room would have on the CLT members may be reduced.
As these proposals are being discussed in the technical committee meetings for the Canadian codes one theme has been consistently occurring. As questions come up it is apparent there are many more unanswered questions to be addressed before the proposed changes in the building code should be approved. Questions include what are the sets of fire tests performed to-date to document meeting fire resistances of the building code in the same manner as masonry, concrete or steel? Knowing wood will burn, what technical documentation and justification exist to support a position that a combustible material like wood, under fire conditions in a building, has the same performance as traditional, time tested noncombustible materials like masonry? Are the effects of mass wood timber elements on fire intensity and total heat release rate within a room with exposed wood sufficiently known? Are there challenges for the fire service who respond and place themselves at risk on floors high above ground in these wood compartments that are different than the experiences they presently know in noncombustible structures? And what about occupant safety – if the taller wood buildings are allowed, should the exit stairs and elevators continue to be placed in shafts enclosed by noncombustible materials like masonry or concrete to maintain levels of safety consistent with existing provisions for the taller noncombustible buildings?
After disapproval of the previous AWC proposal in 2016, the International Code Council (ICC), the entity responsible for development of the IBC in the US, appointed an ad-hoc committee to review the technical merits of taller MT wood buildings. These same unanswered questions of fire resistance, encapsulation, fire performance within compartments, effects on fire service response and occupant safety are cropping up as the ICC technical committee reviews the similar taller MT wood buildings proposals. At present, answers are still unknown.
From what has been discussed in meetings the author has participated in it appears to be premature to place any provisions in the NBCC to permit taller mass timber buildings without further testing and documentation. In addition, the technical data collected needs to be thoroughly examined and vetted to determine what, if any provisions for taller MT buildings be incorporated in the building code. Hopefully the technical committees for the Canadian Commission on Building and Fire Codes (CCBFC) and the Commission itself will take the time to properly evaluate these be proposals before incorporation.
|
FEATURE |
NBCC | IBC |
| No. of Stories | 12 | 9 |
| Occupancy | Residential and Business | Residential |
| Structural Fire Resistance | 2-hour | 2-hour |
| “Encapsulation”
See explanation |
Two layers
5/8-inch Type X Gypsum Board |
Two layers
5/8-inch Type X Gypsum Board |
| Exposed Wood Surfaces | Limited amount of exposed surface
for walls and ceilings |
Not Permitted |
Encapsulation is a new concept to eliminate exposure between the wood surfaces of the CLT members and the room. The intent is to limit the contribution of the wood to the fire intensity within the room and reduce the effects of the fire exposure to the wood element.