The Society of Fire Protection Engineers (SFPE) provided a case study specification for a performance-based design of a high-rise building using cross-laminated timber (CLT), which will be presented in the 12th International Conference on Performance-Based Codes and Fire Safety Design Methods in Honolulu, Oahu, Hawaii on April 23-27, 2018.
The performance-based analysis include the building description, performance criteria, floor plans, trial designs, and final design document. The prescriptive building and fire codes governing this project include the 2015 International Building Code (IBC) and 2015 National Fire Protection Association (NFPA) 101 Life Safety Code. The structural design for the CLT wood framing members is in accordance with the American Wood Council (AWC) 2018 National Design Specification for Wood Construction (NDS). The SFPE Engineering Guide to Performance-Based Fire Protection was also used for the framework of the final report.
The 32-story, 93-m (305-ft.) tall building, located in a financial district of Charlotte, NC, is residential with retail incorporated on the ground floor and two carpark levels below ground. The target market for the building is members of the gig economy and maximizes transient use. The building owner has requested the highest possible environmental standard of sustainability for the project and Occupant Evacuation Elevators (OEE) for emergency situations. The structural behavior of the CLT during a fire event is addressed, and there is an architectural design feature exposing parts of the CLT.
The Tower is designed as a mixed-use structure, including the following occupancy classifications: Assembly, Business, Mercantile, Storage, and Residential. The target market for the Tower is members from the gig economy – transient workers with short-term positions. In addition, the Tower incorporates a green (sustainable) design. The Leadership in Energy and Environmental Design (LEED) 2009 Green Building Rating System for New Construction is used to evaluate the environmental performance, from a whole building perspective, of the Tower.
CLT has numerous advantages over other typical construction materials; yet, public perception and current code restrictions in the United States limits the construction of buildings higher than 6 stories (75 ft.). Thus, building is designed the way that meets the fire safety goals and objectives, including:
Protecting occupants (transient, permanent, and staff) against fire incidents
Protecting firefighters while fire suppression and rescue operation
Maintaining the structure’s integrity in a fire event
Preventing fire spread to adjacent buildings in a fire event
In addition, the performance-based design for the Tower addresses the following building features:
Tall building with a structural timber system
Exposed timber features
Transient occupants
Occupant evacuation elevators
Sustainable design
A semi-quantitative fire hazard assessment for the green building initiatives is proposed for the Tower, assigning values to selected variables, based on professional judgment and experience. Parameters associated with the fire hazards of the above listed LEED categories are identified as performance concerns for the Tower and the façade. The semi-quantitative approach uses a widely recognized index method, establishing an order of magnitude, with relative rankings.
Some parts of the CLT Tower are exposed as an architectural feature. Walls are equipped with total encapsulation which provides a 3-hr. fire resistance rating. The tower is also equipped with a 2-hr. fire resistance rated stairwell, and a1-hr. fire-resistance rated smoke barrier elevator lobby. It is also equipped with a mechanical ventilation to the roof as the main smoke management strategy.
Performance Group III was determined for the CLT tower based on ICCPC, and the following tenability criteria was developed and analyzed in the trial designs:
Radiant Flux Max. 2.5-kW/m2 (793-Btu/ft.2/hr.) at Head Height (1.8-m
or 5.9-ft.)
Elevated Air Temperature 60 ºC (140 ºF) for 30-Minutes Exposure to Air
Saturated with Water Vapor
Elevated Smoke Temperature 60 ºC (140 ºF) for 30-Minutes Exposure
Visibility Distance Max. 10-m (33-ft.) to Doors and Walls
Smoke Toxicity (CO Concentration) Average of 1000-ppm for the First 20-Minutes of Exposure
Smoke Toxicity (Hydrogen Cyanide) Max. 100-ppm for the 20-Minutes of Exposure
To ensure the proper life safety in a fire event, three refuge floors (L9, L17, and L25) were design in the CLT Tower. The areas of refuge are complete encapsulated, and provide a 3-hr. fire resistance rating. Additionally, areas of refuge are designed at all of the stairway’s connection to floors with a 1-hr. fire resistance rating. The CLT Tower design prioritized the life safety of firefighters and occupants with mobility impairment.
The main egress strategy for the tower is elevator evacuation. Thus, egress analysis was performed based on two strategies: zone and total evacuation. Computer modeling using Elevator Evacuation (ELVAC) was performed to evaluate the performance criteria. The results were also compared to hand calculations of the Occupant Evacuation Elevators (OEE) strategy.
8 design fire scenarios were developed, and 4 were evaluated using the Fire Dynamics Simulator (FDS) and PyroSim. Trial designs, egress analysis, and design fire scenarios were then combined and compared to fire safety goals and objectives. Findings based on these evaluations demonstrated an adequate level of life safety for both transient and permanent occupants.