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Complex Curtain Wall Geometry and Material Selection for Passive Fire Protection
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Complex Curtain Wall Geometry and Material Selection for Passive Fire Protection

Meeting the challenges imposed by todays innovative architectural designs.

By Ajla Aksamija, Ph.D. and Bruce Toman | Fire Protection Engineering

Overview of Passive and Active Fire Protection Systems

Fire protection for buildings relies on both passive and active measures.


Active measures include fire detection and suppression systems, such as smoke detectors, fire alarms, sprinklers and building management systems.

Active measures are used to detect fire occurrence, to notify building occupants of the potential danger, to extinguish fires and minimize their dispersal within a building. Passive measures are used to minimize the potential for fire occurrence and to slow the spread of fire throughout a building.

Passive measures include use of fire-resistant walls, floors and materials, and rely on compartmentalizing the overall building. Organization of a building into smaller compartments, consisting of one or more rooms or floors, slows the spread of fire from the area of origin to other building spaces, thus limiting damage to the building and providing building occupants time for evacuation. Especially important is the separation between floors and protection of the joint system between the exterior wall and slab edge.

Exterior Wall Perimeter Fire Barrier Systems

The mechanisms of floor-to-floor fire spread at the exterior walls have been established by the work of fire researchers and fire engineers dating back to the 1960s.1 Testing efforts of product manufacturers and testing laboratories during the 1990s have indicated that flames emitting from an exterior window can extend higher than 16.5 ft. (5m) above the top of the window; therefore, fire-stops must be included in the design.2




 
In multi-story construction, prevention of fire spreading from floor to floor is achieved by including a fire-stop in the space between the floor slab and a curtain wall. If the void between the floor and the curtain wall is not properly sealed, the fire can spread from the lower to the upper floors. Therefore, perimeter fire barrier systems are used to provide fire resistance and prevent passage of fire from floor to floor within the building.

During the 1970s and 1980s, perimeter fire barrier systems usually consisted of fire-rated insulation, supported by Z-shaped galvanized sheet metal impaling clips, which held the insulation in place between the slab edge and the back of the curtain wall, sealing off any opening between floors and walls. The width of these spaces was limited to 7 in. (180 mm). But, several unfortunate events during this time have shown that a more robust system is required to produce an intact seal between curtain walls and the vertical face of the floor slab.

For example, the fire at the First Interstate Bank Building originated in an open-plan office on the 12th floor of the building and extended to the 16th floor - primarily through the outer wall of the building. Windows broke and flames penetrated behind the spandrel panels around the edges of the floor slab and the exterior curtain wall. The curtain walls, including windows, spandrel panels and mullions, were almost completely destroyed by the fire. This event showed that vertical fire spread can be rapid in buildings without adequate compartmentation, and buildings must be designed to reduce the risk of vertical flame spread.3




 

A similar occurrence at the office building at One Meridian Plaza resulted in fire spread from the 22nd to 30th floors through exterior walls.4

Following the January 1994 earthquake in Northridge, Calif., inspectors reviewing buildings for damage to the superstructure observed that fire-rated insulation shifted during the earthquake, and was on top of the ceiling systems at the perimeter of the buildings. It became apparent that a more robust system was needed to keep the insulation in place.5 Code requirements and testing criteria were revisited and improved systems were developed to be more versatile in their retention and to more effectively resist the passage of heat and smoke.

It should be noted that fire protection material alone will not provide perimeter containment; a joint system is needed to prevent the passage of flame and hot gasses. The perimeter joint must be sealed with a material or system that extends the fire rating of the floor to the exterior wall surface. Secure connection of the insulation material to the spandrel is important in order to keep it in place and to prevent it from moving or falling out, and sealant may be needed at the top of the insulation.

 

Five basic features are needed for a successful perimeter fire containment system:6

  1. Installation of a reinforcement member at the area behind the spandrel insulation to keep it from bending due to the compression fit of the insulation.
  2. Mechanical attachment of the mineral wool spandrel insulation to hold it in place.
  3. Curtain wall mullion protection with mineral wool mullion covers.
  4. Compressive fitting of the insulation between the slab edge and the interior face of the spandrel insulation.
  5. Application of smoke sealant material at the top of the insulation to provide a barrier. UL lists approved perimeter fire containment systems for standard curtain wall designs, and the above features apply to all of the tested systems.




 

 

 

Building Codes and Testing Procedures

Review of Building Code Requirements for Fire Protection of Curtain Walls


When a curtain wall is constructed on a building, a gap or void is created at the intersection of the floor and the interior face of the curtain wall. The International Building Code7 (IBC) requires the sealing of these voids along the perimeter of the building with an approved system, thus creating perimeter fire containment. The material used to fill the void must be:

  • Approved material, tested in accordance with the ASTM E1198 testing procedure;
  • Securely fastened to the interior face of the curtain wall; and
  • Capable of preventing the passage of flame and hot gasses.

Section 714.4 of the IBC addresses fire protection for curtain wall and floor intersections. It specifies that a curtain wall and fire-resistant floor intersection must be sealed with an approved system to prevent the interior spread of fire, and it must be tested in accordance with a recently adopted two-story fire test ASTM E 23079 procedure, which partly conforms to the ASTM E 119 standard.

 

 

Standard Testing Procedures for Fire Tests
The ASTM E 2307 test method measures the performance of perimeter fire barriers and their ability to prevent fire spread during the deflection and deformation of the exterior wall and floor assemblies. The main objective is to measure the ability of the fire barrier system to maintain its integrity to prevent fire spread. Exposure conditions and temperatures are specified by this standard for the first 30 minutes of exposure, and then conform to the ASTM E 119 time-temperature curve for the remainder of the test. The temperature ranges for the first 30 minutes of the test are slightly higher than the ASTM E 119 requirements. This test method does not provide quantitative information about the perimeter fire barrier relative to the rate of leakage of smoke. It indicates whether the system can last as long as the floors, and whether the protection scheme can adequately protect wall framing and attachments.

 

UL listings are available for some of the standard curtain wall types that indicate fire resistance ratings for tested systems. They are available for curtain walls where the maximum distance between the curtain wall and perimeter floor is 12 in. (300mm), but more unique and complex curtain walls may require testing to ensure that the perimeter fire barrier systems are adequate and conform to building code requirements.

 

 

Complex Geometry and Joint Conditions

Curtain wall systems have evolved to more complex and customized solutions, driven by architectural design aspirations and technical capabilities. Unique faade designs, twisting geometries, curved surfaces and rotated floor plates are now being developed by architects to express innovative building forms, but they also impose new challenges in terms of facade structural stability, fire protection and material selection compared to traditional flat facades and standard curtain walls.

 

A recent residential tall building design illustrates in detail how complex curtain wall geometry influences material selection for perimeter fire containment and fosters development of new design concepts for curtain walls. The twisting tower geometry resulted in irregular spandrel transitions, which would require a flexible perimeter system to adapt to the curvature of the faade. Moreover, the twisting geometry resulted in gaps between the curtain wall and edge of slab that exceed 12 in. (300 mm); therefore, listed systems were not available.

 

Detail Development and Mock-Ups
The detailing was developed in conjunction with a manufacturer who performed preliminary constructability mock-ups, detail and specification review, and performed a small-scale furnace test to verify that the design concept and materials would achieve desired fire rating. Figure 1 shows a detailed section of the perimeter fire containment concept and the location of insulation.

 

An initial wood profile constructability mock-up was built to test how the insulation would be installed and adhered to the back pan of the spandrel, since the twisting conditions and the changing relationship of slab edge to spandrel required that this material be flexible and able to adapt to the geometry. First, the mock-up was constructed using 3-1/2 in. (89 mm) of mineral wool spandrel insulation.

 

The preliminary mock-up confirmed that the curving and twisting spandrels would not facilitate the installation and retention of insulation. A second mock-up was constructed using 1 in. (25 mm) needled mineral wool blanket insulation, which would be able to twist and adapt to the geometry of the twisting spandrels. It was mechanically fastened to the top of the floor slab and the interior face of the spandrel. Figure 2 shows the comparison between the mock-ups using mineral wool and needled mineral wool blanket insulation.

 

Testing Procedure and Results
A small-scale furnace test of the perimeter fire containment system was performed at Underwriters Laboratories. The purpose of the test was to confirm that the proposed building perimeter fire barrier system offered a concept capable of achieving the specified ratings. The test consisted of placing the spandrel and insulation mock-up of the proposed materials in a small-scale vertical furnace and applying the flame to the interior side of the assembly for a period of two hours following the ASTM E119 time-temperature curve.

 

Two thermocouples were located on the exterior side of the mock-up - one on top of the insulation and one attached to the face of the steel attachment clip. They were used to monitor temperature rise on the non-fire side of the assembly. The furnace frame was adapted and configured with a 30-degree inclined angle on the metal back pan with a perimeter void of 8-3/8 in. (213 mm). The back pan had a 3-1/2 in. (89 mm) thick layer of mineral wool insulation, which was mechanically attached to the exterior face, as can be seen in Figure 3. The interior face that was exposed to the testing equipment was covered with needled mineral wool blanket and mechanically attached.

 

The cavity created at the perimeter void was filled with wool insulation. It successfully sustained a 2-hour rating, indicating that it would be able to with stand temperatures up to 1,800F (982C). This perimeter joint successfully prevented flame and hot gasses from breaching through the perimeter void, which is required by ASTM E 2307 and building code requirements. Figure 4 shows photographs of the testing set-up.

 

This case study shows that a few key elements are necessary for fire protection of innovative and complex facade designs:

  1. Integrated design approach with early involvement of all involved stakeholders, including architects, engineers, manufacturers and fire protection engineers;
  2. Concept design and constructability exploration through mock-ups; and
  3. Development and verification of concepts through testing.

Acknowledgements

Authors would like to acknowledge Ian Bush (Perkins + Will) for providing detailed drawings and resources relating to mock-up development and testing; Thermafiber for the concept development, design and testing of the protection system; Billings Design Associates for the technical design of the wall, and Aon Fire Protection Engineering for fire protection consulting. Special thanks to Jim Shriver (Thermafiber Inc.) for comments on the earlier versions of this article and permissions to use some of the images; and Dan OConnor (Aon Fire Protection) for discussions of the fire protection systems for curtain walls.

 

Ajla Aksamija and Bruce Toman are with Perkins + Will.

 

References:

  1. Yokoi, S., Study on the Prevention of Fire-Spread Caused by Hot Upward Current, Report of the Building Research Institute, Tokyo, 1960.
  2. OConnor, D., Building Faade or Fire Safety Faade?, Proceedings of CTBUH 8th World Congress, Dubai, UAE, March 3-5, Council on Tall Buildings and Urban Habitat, Chicago, IL, 2008.
  3. Routley, G., Interstate Bank Building Fire, Technical Report, USFA-TR-O22/May 1988, US Fire Administration, Washington, DC, 1988.
  4. Routley, G. et al., High-Rise Office Building Fire: One Meridian Plaza, Technical Report, USFA-TR-049, US Fire Administration,Washington, DC, 1991.
  5. Reducing the Risks of Nonstructural Earthquake Damage: A Practical Guide, 3rd ed., FEMA 741, Federal Emergency Management Agency, Washington, DC, 1994.
  6. Schiver, J., Perimeter Fire Containment - The Basics, Thermafiber, Wabash, IN, (undated).
  7. International Building Code, International Code Council, Washington, DC, 2009.
  8. ASTM E119, Standard Test Methods for Fire Tests of Building Construction and Materials, ASTM International, West Conshohocken, PA, 2011.
  9. ASTM E2307, Standard Test Method for Determining Fire Resistance of Perimeter Fire Barrier Systems Using Intermediate-Scale, Multi-story Test Apparatus, ASTM International, West Conshohocken, PA, 2010.

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