Passive fire-containment system supports the building enclosure for One World Trade Center.
Towering above a city renowned for its skyscrapers, One World Trade Center stands as the tallest building in the Western Hemisphere. Linking New York City’s Tribeca neighborhood to the Financial District and standing 1,776 ft. high, the 104-story, 2.6-million-sq.-ft. building is home to businesses operating in industries ranging from finance to publishing to real estate and international commerce. Its dramatic, shimmering form blends geometric shapes and prismatic glass to create a stunning visual that reflects light in myriad ways throughout the day.
While the building’s design is breathtaking, One World Trade Center also evokes awe due to its location—standing at the very site where the Twin Towers once stood. Given the location’s painful moment in American history, it is not surprising that the building facts posted on its website mention, “life-safety systems far exceed NYC building code.”
Whether specifying building materials for a marquee project like the World Trade Center or a lower-profile building, the building community evaluates three elements when specifying life-safety systems. Detection systems include alarm systems such as smoke and heat detectors. A second element focuses on “active” systems that suppress a fire. Sprinkler systems are the most common suppression systems used in buildings. The effectiveness of active systems is contingent upon the turn-on/turn-off mechanism operating properly. A third life-safety element is “compartmentation,” also referred to as a passive approach to fire containment. Unlike active systems that are dependent upon an “on” mechanism to activate the system, passive fire-containment systems feature no activating device. Therefore, the passive approach promises to always work once the system is correctly installed. Passive systems also play a key role in containing a fire to a room of origin, delaying its spread, and adding precious time for occupants to evacuate or take refuge in a safe zone.
Editorial director Gary L. Parr talks with Angie Ogino, technical services leader at Thermafiber Owens Corning, about the three components of a complete fire-protection system and the roles they play in high-rise buildings, in addition to installing proper fire protection in renovation projects.
As passive fire-containment systems slow the spread of fire, firefighters have an opportunity to enter the building and extinguish the fire while it is still manageable. In conjunction with a building enclosure’s curtain wall, perimeter fire-containment systems are designed to prevent the spread of flame and hot gases at the void created where the exterior curtain wall bypasses the floor assembly. History has borne out that relying on just one or two life-safety elements is associated with more risks than a life-safety system that incorporates detection, active, and passive elements.
Impeding Fire Progress
Passive fire-containment systems hinder a fire’s spread in part by addressing the three key paths that allow fire to spread vertically at the building exterior perimeter. A first path of fire propagation occurs when the void between the floor slab and the exterior curtain wall is left unprotected, permitting flame and hot gasses to spread up through the joint.
A second path of vertical fire spread is not as commonly understood. Typically, the building materials used to construct a building’s curtain wall are made of low-melt and heat-sensitive materials. If the spandrel panel is not properly protected, fire can cause an early failure of the curtain wall, allowing fire to propagate up and through the curtain-wall cavity.
Finally, a third path of fire propagation—although not a requirement of the building codes and ASTM E2307—demands consideration when it comes to delivering the maximum level of safety in a high-rise structure. Referred to as leapfrog, this path of fire spread occurs when fire breaks out the vision glass on the floor of fire origin. When the vision glass is broken, flame and hot gasses can escape outside the building and lick up the exterior face of the curtain wall. In turn, the fire causes the vision glass on the floor above to break, allowing fire to re-enter the building through the window opening and engaging combustibles on that floor. The fire will continue to jump from floor to floor using this path.
Thermafiber Firespan 90 curtain wall and Thermafiber Safing mineral wool insulation from Owens Corning, Toledo, OH, were used in One World Trade Center to deliver fire-containment barriers between a fire source and the building’s floor slab perimeter and curtain wall.
In addition, Thermafiber Impasse mineral-wool-insulation hanger system uses a stepwise technique of overlapping components during installation to lock the Firespan curtain wall insulation materials in place. The Firespan 90 insulation is secured to the aluminum framing of the exterior curtain wall. This protection is an important part of any building’s design because the longer the curtain wall insulation stays in place, the longer it can help provide protection to the building’s aluminum framing, thus maintaining the structural integrity of the system. Additionally, this approach allows the Safing insulation, which is compression-fit between the curtain wall and the perimeter edge of the floor slab, to remain in place and continue providing a barrier to flame and hot gases at this critical intersection.
Materials, Installation Matter
While the precision design and installation of perimeter containment fire systems are critical, the materials comprising these systems are also of paramount importance. Qualities intrinsic to mineral wool make the material a natural choice for high-rise applications. Mineral wool is the only tested and proven curtain-wall insulation material that resists temperatures and maintains integrity within perimeter fire-containment systems where exposure temperatures can exceed 1,800 F. Thermafiber Firespan 90 curtain wall and Thermafiber Safing mineral wool insulation systems are rated to three hours when tested in accordance to ASTM E2307-Standard Test Method for Determining Fire Resistance of Perimeter Fire Barriers Using the Intermediate-Scale Multi-Story Test Apparatus.
An ongoing challenge that must be addressed is the unique design of every building. Architects want to design aesthetically desirable buildings with beautiful facades. But engineering judgments evaluate whether a building’s design will provide perimeter fire containment for the required hourly rating. Issues with mullion and transom spacing, multiple transoms, spandrel height limitations, floor location with respect to sill height, and types of curtain-wall mounting brackets and their locations can all vary widely, creating challenges for architects.
However, in the final building approval process, perimeter fire containment must provide a system that meets the building code requirements. Perimeter fire-containment systems must be tested or determined using an engineering judgment to be in accordance with ASTM E2307. While the code allows alternative materials, design, and methods of construction and equipment as a means to resolve an issue, the key is that engineering judgments should be supported by a tested system that is similar in nature to the construction details. Additionally, sufficient data should support that the proposed system meets the basic principles necessary for perimeter fire-barrier protection.
The World Trade construction conformed to the USGBC’s LEED-Gold standard. Thermafiber mineral-wool insulation was specified to help support not only fire protection but sound control and energy conservation in the structure, and contributes to the overall sustainable building goals as the product contains a minimum 70% recycled content that is non-combustible, inorganic, and does not support mold growth.
One World Trade Center opened in October 2014. Today it stands as a public beacon for urban building design and a tribute to American history. More quietly, it is an example of incorporating passive fire-containment systems into the building enclosure.