By Canadian Wood Council/Wood Works! BC
Fire Protecting the Eight-Storey Wood Innovation and Design Centre, B.C.Buildings fire protection wood structures
Site specific rules were developed for the design of the Wood Innovation and Design Centre in Prince George, B.C. to ensure the structure would safely withstand fire.
From the August-September 2015 print and digital editions, page 45.
With a height of 29.5 metres, the Wood Innovation and Design Centre (WIDC) is the tallest contemporary wood building in North America. Located in the city of Prince George in northern British Columbia, the WIDC was conceived as a showcase for local wood products and as a demonstration of the province’s growing expertise in the design and construction of large wood buildings. Meanwhile, its striking presence in the heart of the city will assist in the revitalization of the downtown.
The building has eight levels (six storeys, plus a ground floor mezzanine and a rooftop mechanical penthouse). The lower levels will accommodate faculty and students enrolled in the new Master of Engineering in Integrated Wood Design to be launched by the University of Northern British Columbia in January 2016. Academic facilities include a research/teaching lab that will support the design, fabrication and testing of wood products; a 75-seat lecture theatre; classrooms; a student lounge; gathering and meeting areas; and a learning resource centre. The upper floors will provide office space for public and private sector organizations associated with the wood industry.
This article describes the most important innovations that were implemented at WIDC to meet design and safety criteria in what is a new class of buildings for B.C. These innovations included:
– A set of site-specific regulations to ensure life safety and structural integrity;
– The use of vertical cross-laminated timber (CLT) elements (including mechanical, elevator and stair shafts) to provide lateral stability to the structure;
– The use of double layer CLT floors to meet structural requirements and contribute to acoustic isolation;
– The use of superimposed (end grain-to-end grain bearing) columns to control shrinkage over the height of the building; and,
– The use of high strength proprietary connectors to speed construction and improve structural performance.
Tall wood buildings and codes
British Columbia has been a leader in North America in the implementation of building regulations that permit the greater use of wood construction in larger and taller buildings. In 2009, the B.C. Building Code was amended to permit wood construction of up to six storeys for residential occupancies. Elsewhere in North America several other provinces and states, as well as the model National Building Code of Canada, are conducting their own research with the intention of following B.C.’s lead.
The WIDC was designed to the 2012 BC Building Code as amended by the Wood Innovation Design Centre Regulation (the site-specific regulations as noted above). Combined with research and testing, the regulations were developed to provide code-equivalent levels of safety to those required for similar buildings of non-combustible construction. The major criteria established for the WIDC were:
– A building area of not more than 1,125 m2 ;
– A building height of six storeys, and floor areas that together total not more than 4,800 m2;
– Not more than 30 metres in height measured from grade to the highest point of the uppermost roof; and,
– Major occupancy classifications consisting of:
First and second storeys – Group A, Division 2 assembly occupancy or Group D business or personal services occupancies;
Third to sixth storeys – Group D business and personal service occupancies.
Financed by the Province of B.C., the design and construction process was fast-tracked to meet funding criteria, and to make the most of the region’s short construction season. This meant that detailed design of the superstructure was still in progress when work began on site.
Design began in early 2013, and by the end of August 2014, the building was substantially complete and the university and common areas of the building were ready for occupancy. The upper floors were left as open, unfinished areas, to be let out to suitable tenants.
In addition to using a variety of locally manufactured engineered wood products, the WIDC incorporates many other sustainable design strategies and is targeting LEED (Leadership in Energy and Environmental Design) Gold certification.
Glulam and CLT structure
The primary structure is an innovative combination of glulam post-and-beam frame construction, a custom designed CLT floor system, and CLT elevator, stair and mechanical shafts. Concrete was used only for the ground floor slab and for the floor of the penthouse mechanical room. Wherever possible the wood structural members were left exposed. Appearance-grade Douglas fir was used for the bottom lamination of the CLT floors (visible from below), and these were given a clear coat finish.
The building is balloon-framed, meaning that the columns are superimposed one above the other, with end grain-to-end grain bearing. The beams then frame into the sides of the columns, meaning that there is no cross grain in the vertical section of the building. This technique minimizes cumulative vertical shrinkage that could otherwise impact the performance of the structure. The building systems are repeatable and expandable to other building types and sizes.
The building envelope is a combination of glazing installed in vertically laminated LVL mullions and structural insulated panels (SIPs) clad with natural or charred Western red cedar siding.
Design for fire safety was based on the B.C. Building Code and the site-specific regulation created for the Wood Innovation and Design Centre.
Prevention during construction
The risks and hazards on a construction site differ in both nature and potential impact to those of a completed building because they can occur at a time when the safeguards that are designed to be part of the completed building are not yet in place. Building codes focus on protecting the occupants of completed buildings. In addition to meeting any provincial regulations for fire safety during construction, there are best practices that should be applied.
To this end, the Canadian Wood Council has developed best practices (http://cwc.ca/publications/) for buildings under construction. For example, 24-hour security is not usually a code requirement, but is a way of reducing the risk of vandalism, theft or arson, and for detecting problems before they grow out of control. In the case of the WIDC, the contractor, PCL, worked with the Prince George fire department to develop a series of fire safety practices to be implemented during construction. Twenty-four-hour security was provided, and stand-pipes for firefighting were installed as the building height increased. In addition, “hot works” were minimized during construction. For example, crimped pipe connections were used in lieu of soldering to eliminate the possibility of an accidental fire.
Protection in service
It has long been recognized that large timber members have an inherent fire resistance because of their slow and predictable rate of charring when they are exposed to fire. This slow rate of char is approximately 40 mm per hour, allowing large timber systems to maintain significant structural capacity for an extended duration during a fire. New engineered wood panel and beam products, such as cross-laminated timber (CLT), parallel strand lumber (PSL) and others can take advantage of this attribute because of their large cross section. Fire performance is further enhanced through the use of concealed connectors, whereby structural or non-structural wood elements provide protection to the vulnerable steel components.
The fire design of the CLT components at WIDC is based on the methodology set out in the Canadian CLT Handbook, which includes a chapter titled “Fire Performance of Cross-laminated Timber Assemblies.” A subsequent U.S. edition provides further guidance on the performance of joints between CLT panels. The calculation methodology is based on the standard fire exposure and is a means for predicting the expected fire resistance that would be determined when testing to CAN/ULC-S101.
The handbook uses the reduced-cross section method to estimate the residual capacity of structural members after some duration of fire exposure. A char depth is calculated based on the fire exposure time, and an additional depth is subtracted to account for the heated wood that has lost some strength, leaving a reduced cross-section. The capacity of the reduced cross-section can then be determined using the full design strength of the member.
In addition to supporting the structural loads in the event of a fire, it was also required that the mass-timber panel assemblies at WIDC resist the passage of flames and hot gases, and limit the temperature rise on the unexposed surface of the assembly in order to prevent fire spread from one compartment to another.
Since wood is an effective insulator, and CLT is manufactured by laminating together individual pieces of lumber, a CLT assembly will always experience either structural failure or integrity failure before thermal failure can occur. Also, as CLT panels tend to be sealed well (through the thickness of the panel), in part because of the use of polyurethane adhesives which foam to fill voids during manufacturing, the main concern with respect to integrity failure is the joints between adjacent CLT panels or between assemblies (e.g. wall-to-floor joints).
While it was the site-specific regulation that allowed for this tall wood building to be constructed, it was the B.C. Building Code that governed the fire separations applicable between spaces and at the building core elements (stair, elevator shaft). In the case of the WIDC, the requirement for these separations was one hour. Engineering judgment, supported by existing fire test data, showed that the structural CLT walls, stringer panels, and landing floors within the stair, were appropriately sized to provide that one-hour separation, without requiring additional fire protection elements such as gypsum drywall.
The design criteria for a typical floor assembly are shown in Table 1 and those for a typical wall assembly in Table 2. In both cases, the CLT assemblies meet the one-hour fire-resistance requirements based on the calculation method.
Since there were no prescribed fire stop systems for penetrations in CLT assemblies, all penetrations and major joint configurations were tested in accordance with CAN/ULC-S115 Fire Tests of Fire Stop Systems as required by the B.C. Building Code.
The site-specific regulation required the provision of direct access for firefighting from the outside of the building at every level less than 25 metres above grade. Specifically, this meant at least one unobstructed window or access panel being provided for each 15 metres of wall, in at least one wall facing a street or lane. Consequently, every level of the WIDC, starting from Level 2, has two fire department access doors facing George Street.
The elevator shafts, exit stair and exit corridor walls, scissor-stair dividing walls, and scissor-stair floor assemblies required a one-hour fire separation. In addition, the scissor-stair walls, floor assemblies and the shafts around the standpipe risers (at Level 1) were constructed to prevent the migration of smoke from one scissor stair to the other. This required careful design and proper sealing of panel-to-panel joints. Joint designs were evaluated by the team for their constructability and effectiveness in creating a smoke seal. The most promising joint designs were then laboratory tested to confirm their effectiveness.
The elevator shaft is composed of CLT panels installed vertically, like other elements of the “core” structure. The inside surfaces were site-treated with a ULC-listed intumescent coating, a treatment that expands in fire to provide a degree of fire protection. The treated CLT surfaces have a flame-spread rating of not more than 25. This rating is based on thin samples of Douglas fir, but more recent testing has shown that the resistance to flame spread is better for intumescent coatings applied to CLT panels due to their mass.
To function effectively, an elevator must be able to cope with the anticipated vertical movement in a tall building. The design precautions taken to minimize shrinkage (as outlined above) appear to have successfully addressed any such problems on the WIDC project. Based on data from sensors placed in the shaft, this movement did not exceed the design tolerance. However, service technicians have had to reduce the rail sensor sensitivity to improve reliability.
Because it is not located adjacent to classroom or office space, the elevator shaft was not required to meet any special acoustic or noise suppression parameters.
Emergency egress from the WIDC is provided by double-scissor stairways. As with the elevator shaft, the CLT walls and ceilings in the exit stairs were treated with a fire-retardant coating to reduce the flame spread rating to 25.
To ensure that smoke could not migrate from one scissor stair to the other, it was necessary to drill a small hole and apply sealant at each board joint in the CLT stair shaft wall, immediately above and below the stringer and landing panels. This sealed the small cracks that typically form between the boards on the outer layer of CLT panels due to shrinkage. cce
This article is an adapted excerpt from “Wood Innovation and Design Technical Case Study,” to be published by the Canadian Wood Council/WoodWORKS! BC in September 2015.