Tag Archives: fire

Designing Stone Wool Ceiling Assemblies

All images courtesy Rockfon

All images courtesy Rockfon

by Cory Nevins

Specifiers have an increasing number of choices for commercial ceiling systems. Among the performance considerations for selecting the most appropriate for a particular application are acoustics, fire performance, humidity resistance, hygienic properties, dimensional stability, indoor air quality (IAQ), and light reflection. Added to these are choices pertaining to design aesthetics, ease of installation, maintenance, durability, sustainability, and cost.

Stone wool ceiling panels and metal suspension systems meet these selection criteria for both new construction and renovation projects throughout North America. The material was discovered on the islands of Hawaii, where it occurs naturally as a by-product of volcanic activity. The primary rock involved is basalt, the earth’s most abundant bedrock. The igneous material forms by the rapid cooling of lava from eruptions on the sea floor. Seismic activity, including the earth’s volcanoes, produces 38,000 times more rock material than is used by the world’s largest producer of stone wool.

The typical production process for stone wool begins with the fusion of this volcanic rock at a temperature of 1500 C (2732 F). Emerging from the furnace, the melt runs out of the bottom and onto a spinning machine, where wool is whipped into thin strands, similar to making cotton candy. The strands form ‘wool,’ held together with minor amounts of organic binders.

Stone wool ceilings offer good sound absorption, high light refl ectance, fi re protection, and humidity resistance. These panels are well-suited to create modular ceiling designs, such as long corridors.

Stone wool ceilings offer good sound absorption, high light reflectance, fire protection, and humidity resistance. These panels are well-suited to create modular ceiling designs, such as long corridors.

Now a fleecy web, the material is gathered and formed; the number of layers varies depending on the final product’s desired structure and density. The layered fibers then move to a curing oven. Once cured, the wool emerges with non-directional fibers that contribute to its multiple performance characteristics of the stone wool products. In addition to ceiling panels, stone wool’s unique combination of thermal, fire, and acoustic properties make it suitable for:

  • blown insulation in cavity walls;
  • rolls of loft insulation;
  • pre-formed and faced pipe sections; and
  • wall slabs.

A mineral fleece and water-based paint are layered on top of the stone wool to produce the finished ceiling panels. The stone wool products proceed to cutting saws, finishing and packing equipment, or are led to off-line equipment for special treatment. The majority of the waste created during the production is fully recyclable.

Use of suspended ceilings
Since the 1950s, drop ceilings have been the preferred method for concealing HVAC vents, electrical wires, plumbing pipes, phone cables, and security lines in interior commercial buildings. These suspended, interconnected ceiling systems consist of a metal grid comprising cross-tees and main runners.

The main runners are suspended by hanger wires from the structure above, and wall channels or angles provide a clean look throughout the perimeter. Panels are used to conceal the plenum—hiding the visible structure, suspension system, HVAC, and other equipment, while providing simple access for future maintenance.

The suspension ceiling system is selected for aesthetics, maintenance, and specialized performance such as fire resistance, seismic mitigation, or limited accessibility in security applications. For all ceiling designs, specifiers should check the suspension systems are manufactured to ASTM International standards. On request, suspension manufacturers may provide reports from the International Code Council (ICC) and third-party seismic performance testing and certification reports.

Corrosion resistance is also a priority for metal suspension systems supporting stone wool and other ceiling panels. The industry standard is 23.8-mm (15/16-in.) galvanized steel for suspended metal ceiling grids; most may be specified with a minimum of 25 percent recycled content.

While the ceiling panel’s size, orientation, color, finish, and edge largely determine the overall aesthetic, changing the size of the grid’s face also changes the appearance. For example:

  • a 14.28-mm (9/16-in.) narrow face diminishes the distinction between grid and panel for a more monolithic look;
  • adding a 3.17-mm (1/8-in.) slender, center regress with a ‘bolt-slot’ design accentuates the shadow between panel and grid;
  • mitered intersections provide crisp, continuous lines for a uniform ceiling plane;
  • wide-face 34.92-mm (1 3/8-in.) ceiling suspension offers bolder expression of the ceiling grid modules, especially at high elevations; and
  • in curved drywall applications, radius systems create concave and convex shapes, including barrel-vaulted ceilings.
When a sound wave hits a surface, part of the energy is refl ected, part of it is absorbed by the material, and the rest is transmitted. Undesired sound from various potential sources can include noise transmitted into the building from the exterior, or coming in from other interior spaces.

When a sound wave hits a surface, part of the energy is reflected, part of it is absorbed by the material, and the rest is transmitted. Undesired sound from various potential sources can include noise transmitted into the building from the exterior, or coming in from other interior spaces.

The noise reduction coeffi cient (NRC) refers to a surface’s ability to reduce noise by absorbing sound. NRC is important in areas where high levels of noise (like a photocopier) are present.

The noise reduction coefficient (NRC) refers to a surface’s ability to reduce noise by absorbing sound. NRC is important in areas where high levels of noise (like a photocopier) are present.

 

 

 

 

 

 

 

 

 

 

 

Specifying acoustic comfort
According to the World Health Organization (WHO):

noise seriously harms human health by causing short- and long-term health problems. Noise interferes with people’s daily activities at school, at work, at home and during leisure time. It can disturb sleep, cause cardiovascular and psychophysiological effects, hinder work and school performance and provoke annoyance responses and changes in social behavior.1

Therefore, it could be argued design professionals have a duty to create acoustic comfort and well-being for the occupants of their buildings. Stone wool can help with two primary components of acoustic comfort: speech intelligibility and noise reduction.

The material’s airflow resistance and density contribute to its high noise absorption properties. The fibers’ size and non-directional orientation lead to stone wool’s inherent sound-absorbing qualities. The measures and concepts discussed in this article provide a foundation for understanding the relationship between stone wool’s characteristics as a material and achieving acoustic comfort.

Speech intelligibility
One important component of acoustic comfort and sustainability, speech intelligibility refers to a listener’s ability to hear and understand a speaker in a room or space. It is measured as a signal-to-noise ratio, expressed in decibels (dB). For this application, the signal typically is speech and the noise usually is everything else in the background.

Reverberation time
An important factor for creating speech intelligibility, it is defined as the time it takes for the sound pressure level to decrease 60 dB below its original level. In most situations (excluding unamplified music performance), a lower reverberation time improves speech intelligibility and acoustic comfort. For most rooms requiring speech intelligibility, mid-frequency reverberation time should be between 0.50 and 1.00 seconds when the room is unoccupied.

Noise reduction coefficient
The noise reduction coefficient (NRC) indicates a surface’s ability to reduce noise by absorbing sound. It is calculated by averaging the absorption coefficients from the 250-Hz, 500-Hz, 1-kHz, and 2-kHz octave bands. It varies between 0.0 (i.e. absorbs very little sound) and 1.0 (i.e. absorbs a lot of sound). NRC is one of two important variables in determining reverberation time (the other being room volume). A higher NRC indicates more noise reduction (or sound absorption) and leads to lower reverberation times and greater speech intelligibility. Stone wool ceiling products typically have an NRC of 0.85 or higher.

Background noise
Undesired sound from various potential sources can include noise transmitted into the building from the exterior, or coming in from other interior spaces. It can also include sounds generated by the building’s systems or even those reverberating too long inside the room.

Speech intelligibility
Factors influencing speech intelligibility include:

  • speech signal’s strength and clarity;
  • sound source’s direction;
  • level of background noise;
  • room’s reverberation time and shape; and
  • listeners’ hearing acuity and attention span.

Reverberation time depends on two main variables: the volume of the room and the amount of sound-absorbing materials. As volume decreases or as the amount of sound-absorbing materials increases, reverberation time decreases and speech intelligibility generally increases. Since the volume of the room often depends on functional and aesthetic criteria, reverberation time is often solely dependent on the amount and efficacy of sound-absorbing materials.

In many cases, placing sound-absorbing materials on the walls is not desirable due to its tendency to get damaged, dirty, or worn because of occupant contact. As a result, whether speech intelligibility is poor, fair, or good can highly depend on the ceiling specified. This is why acoustic standards and guidelines for schools, hospitals, offices, and other types of facilities have minimum NRCs of 0.70 and up to 0.90. Stone wool ceiling panels, more than other panels made of less-absorbing materials, help ensure projects comply with acoustic performance requirements in these standards and guidelines.

Even if reverberation time is appropriate, speech intelligibility can be low if the background noise in the room is too loud. Speech intelligibility equates to a high signal to noise ratio. Consequently, it is also important to ensure noise from the exterior, other interior spaces, and from the building’s systems is controlled.

Noise reduction
In other rooms or spaces like open offices, cafeterias, libraries, and gymnasia, speech intelligibility is not the primary acoustic goal; rather, the push is for overall noise reduction for stress relief and concentration. Noise reduction equates to an overall decrease in sound pressure level from loud continuous noise (e.g. traffic noise transmitting into the building), as well as event-specific noise (e.g. a crying baby). The sound pressure level in a room depends on the strength of the sound source, the room’s size, and the quantity and quality of sound-absorbing surfaces.

Just 30 decibels of periodic noise can be disturbing to sleep or concentration. Conversational speech is generally between 50 to 70 dB. Noise with sound levels of 35 decibels or more can interfere with speech intelligibility in smaller rooms. This is demonstrated by a phenomena known as the ‘cocktail party effect,’ whereby as noise levels get louder and louder, people try to talk louder and louder to be understood. Despite their efforts, speech intelligibility decreases and acoustic stress increases. It is not until someone leaves the ‘party’ that they realize just how agitated they were as their muscles begin to relax, heart rate slows, and respiration deepens. Stone wool, because of its high noise-absorbing characteristics, also helps achieve the overall noise reduction goals.

Whether sound reduction is needed for speech intelligibility or overall acoustic comfort, blocking noise that could be in the plenum above the ceiling can also be important in some instances. As more acoustics standards and guidelines place minimum noise control criteria on wall constructions (i.e. sound transmission class [STC]), the need for ceilings to block noise from adjacent spaces traveling via the overhead plenum is becoming less frequent. This is because achieving the minimum STC wall requirements necessitates the walls be extended up to, and sealed against, the underside of the deck above them. However, in the cases where the walls do not extend full height, or where there may be noisy mechanical equipment in the plenum, the ceiling also may need to block noise from transmitting into the space below them.

Ceiling attenuation class (CAC) indicates the ceiling’s ability to prevent airborne sound from traveling between adjacent rooms when the demising walls do not intersect with the structural deck above. CAC is also a good measure to judge how much protection is offered against noisy mechanical equipment in the plenum. The higher the CAC value, the greater the ceiling’s blocking capacity. A CAC value of 35 dB is considered to be moderately high and may be specified for stone wool ceiling panels. When even higher sound-blocking capacity is required, stone wool ceiling panels can be specified with a CAC value up to 43 dB in combination with a high NRC of 0.85.

Insulation infl uences the sound level in the receiving space, helping provide more privacy between rooms and better concentration in the adjacent room.

Insulation influences the sound level in the receiving space, helping provide more privacy between rooms and better  concentration in the adjacent room.

In practice, there is a strong link between sound absorption and room-to-room sound insulation. This link may not be accurately refl ected in laboratory testing. In practice, two ceilings with the same ceiling attenuation class (CAC), but different NRCs, produce different levels of perceived sound insulation. The ceiling with the highest NRC will do a better job of lowering the sound pressure in both the sending and the receiving room.

In practice, there is a strong link between sound absorption and
room-to-room sound insulation. This link may not be accurately
refl ected in laboratory testing. In practice, two ceilings with the same ceiling attenuation class (CAC), but different NRCs, produce different levels of perceived sound insulation. The ceiling with the highest NRC will do a better job of lowering the sound pressure in both the sending and the receiving room.

Total sound insulation is the ability of a total construction (e.g. partitions, ceiling, fl oor and all connections) to prevent sound from traveling through the ceiling void and through building elements. Sound insulation of ceilings is measured using CAC, while walls are measured using the sound transmission class value (STC).

Total sound insulation is the ability of a total construction (e.g. partitions, ceiling, floor and all connections) to prevent sound from traveling through the ceiling void and through building elements. Sound insulation of ceilings is measured using CAC, while walls are measured using the sound transmission class  value (STC).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Fire performance
Every second counts once a fire has started. Specifiers know choosing the right building materials can delay the spread of fire and provide the vital extra minutes needed to save the occupants and limit the damage.

Given its volcanic origins, stone wool can withstand temperatures up to 1177 C (2150 F). It is non-combustible, will not develop toxic smoke, and does not contribute to the development and spread of fire even when directly exposed to fire.

Ceiling panel products are required to be tested for surface burning characteristics to Underwriters Laboratories (UL) 723/ASTM E84, Standard Test Method for Surface Burning Characteristics of Building Materials. Testing requires 7.31 m (24 ft) of material to be exposed to a flame ignition source in a Steiner Tunnel Test to determine how far the fire will spread during 10 minutes, and how much smoke is developed during this period.

The test was developed by Al Steiner of UL and has been incorporated as a reference into North American standards for materials testing. The progress of the flame front across the test material is measured by visual observation, while the smoke emitted from the end of the test assembly is measured as a factor of optical density. A Flame Spread Index and a Smoke Developed Index are calculated from these results. Both indices use an arbitrary scale in which asbestos-cement board has a value of 0, and red oak wood has a value of 100.

Many commercial applications require a Flame Spread Index of 25 or less and a Smoke Developed Index of 50 or less. Products labeled “FHC 25/50” (Fire Hazard Classification 25/50) or “Class A” (ASTM E1264, Standard Classification for Acoustical Ceiling Products) fulfill these requirements. Stone wool ceiling panels may be specified to meet the most stringent requirements with a maximum Flame Spread Index of 0 and a maximum Smoke Developed Index of 5.

Humidity and hygienic attributes
Humidity can weaken the structure of certain ceiling materials, causing them to sag and, in extreme cases, even fall out of the suspension system. This often happens in buildings under construction where the building is not yet temperature- and humidity-controlled, or materials have not yet dried. Additionally, humidity levels are naturally high in wet rooms like kitchens and sanitary areas, and moisture problems may occur.

The stone-wool core in acoustic ceiling panels can be specified as hydrophobic, which means it neither absorbs water nor holds moisture. This makes the ceiling panels ‘sag-resistant,’ even up to 100 percent relative humidity (RH) and in temperatures ranging from 0 to 40 C (32 to 104 F). The material is dimensionally stable and does not warp, curl, or cup. It also neither rots nor corrodes. Further, its characteristics remain unaltered over time, maintaining its dimensions and physical characteristics throughout a building’s lifecycle.

Since stone wool is inorganic, it also does not promote the growth of mold or bacteria. North American studies show a relationship between mold and damp conditions, and an increase in allergic reactions, along with eye, nose, and throat irritation.2 They have also been associated with litigious concerns that some commercial building owners have termed ‘sick building syndrome.’

Twenty-three percent of office workers experience frequent symptoms of respiratory ailments, allergies, and asthma. The impact has been an increased number of sick days, lower productivity, and increased medical costs. The economic impact is enormous, with an estimated decrease in productivity around two percent nationwide, at a cost of $60 billion annually.3

Helping maintain cleanliness, stone wool ceiling panels may be specified with a smooth, non-textured finish that can be vacuumed with a soft brush attachment. Specially treated hygienic and medical surface finishes allow cleaning with water and some diluted disinfectants, such as chlorine, ammonia, and quaternary ammonium. In some cases, specially treated surface finishes on stone wool ceiling panels allow for more intensive cleaning, such as steam cleaning twice a year following a defined protocol.

Sustainability
In addition to being composed from the earth’s most abundant bedrock, stone wool ceiling panels can contain up to 42 percent recycled content. When removed, undamaged stone wool products may be reused or recycled for other projects. However, if recycling, one should be observant of recycling plant locations.

Stone wool is an excellent thermal insulator and contributes to energy-efficient buildings. Stone wool ceilings’ reflective, smooth surface also can play a significant role in enhancing energy efficiency through better light distribution. The health benefits of natural light include a more positive mood, improved productivity, and lower absenteeism.4 Maximizing use of natural daylight may allow a reduction in the number of lighting fixtures. The subsequent lowering of electric loads may reduce cooling costs.

Further contributing to sustainable goals, stone wool ceiling panels may be specified with UL Environment’s Greenguard Gold Certification for low-emitting products. Certification is only given to products compliant with the associated requirements, which among others include stringent limits on emissions of more than 360 volatile organic compounds (VOCs).

UL Environment states indoor air can be two to five times more polluted than outdoor air. Greenguard Gold criteria incorporate health-based emissions requirements as denoted by the U.S. Environmental Protection Agency (EPA), the State of California Department of Public Health’s Section 01350, and others.

More than 400 green building codes, standards, guidelines, procurements policies, and rating systems give credit for Greenguard products. Certification also fulfills the low emission requirements of the U.S. Green Building Council’s (USGBC’s) Leadership in Energy and Environmental Design (LEED) v4 program, and the Collaborative for High Performance Schools’ Criteria (CHPS) for low-emitting materials.

Stone wool, the core material of stone wool ceiling products, can withstand temperatures up to 1177 C (2150 F). It is made from basalt rock and is non-combustible; it will not contribute to the development and spread of fi re.

Stone wool, the core material of stone wool ceiling products, can withstand temperatures up to 1177 C (2150 F). It is made from basalt rock and is non-combustible; it will not contribute to the development and spread of fire.

Stone wool acoustic ceiling products that have been certifi ed to GreenGuard Gold certifi cation standards for low chemical emissions into indoor air during product usage are suitable for environments such as schools and healthcare facilities.

Stone wool acoustic ceiling products that have been certified to GreenGuard Gold certification standards for low chemical emissions into indoor air during product usage are suitable for environments such as schools and healthcare facilities.

Aesthetic design
Beyond sustainability and performance, there are numerous aesthetic considerations in selecting the best stone wool ceiling panels to achieve the desired architectural expression.

Shape
The shape of a stone wool panel’s edge significantly contributes to the ceiling’s overall appearance. Demountable options include:

  • square lay-in—cost-effective, provides easy access to the plenum, and mounts in standard suspension systems;
  • tegular—square or angled, hangs on a visible and recessed suspension system that creates a shadow between the tiles, and mounts in standard suspension systems;
  • semi-concealed—appears to float under the suspension system, the profiled edge and deeply recessed grid profiles presents an elegant shadow (an effect emphasized by specifying the suspension system in black); and
  • concealed—hides the suspension system to create a monolithic appearance, but only some concealed panels are demountable.

Another option is the direct-mount assembly, where ceiling panels are directly bonded to the structural soffit or an existing ceiling surface. These systems are for areas where ceiling heights do not permit the use of the suspension setup.

Panels are not limited to two dimensions of squares and rectangles; they may be formed into three-dimensional cubes. Baffles and clouds provide an alternative solution for rooms where contiguous ceilings are unsuitable. They are suited to thermal mass applications where the soffit needs to be left exposed. They can be used as part of a retrofit or to create a design feature.

A wide range of sizes contributes to the design freedom and flexibility offered with stone wool ceiling panels. By combining different module sizes, even small rooms may seem larger and long corridors can seem less distant. The line of a ceiling impacts the perception of a space and creates focal points that may show direction, outline an object, or divide a large space into more comfortable zones.

Horizontal lines convey stability, grounding, and direction. Vertical lines, on the other hand, also communicate stability, as well as pillar-like attributes of strength and balance. Diagonal lines are perceived as dynamic and transformational with overtones of freedom, while curves are considered playful, organic, and soothing.

Texture and color
Based on today’s design styles, stone wool ceiling panels are preferred in smooth and lightly textured surface finishes. This gives the impression the ceiling is lighter in texture, weight, and color. White and neutral tones are the most popular color choices for interior ceilings. For more vibrant spaces, stone wool ceiling panels can be specified in a breadth of other hues.

A viewer’s perception and relation to a color changes depending on whether it stands alone, is dominating a space, or if it is in play with other colors. It also is influenced by the quality and quantity of light hitting it.

Colors evoke physical and psychological reactions, and the brightness or color temperature creates different moods and ambiance: Warm colors—such as red, orange, and yellow—are considered stimulating. Cool colors—like blue, purple, and light green—generally have a calming effect.

Spatial perception is also affected by color. Lighter hues tend to make spaces seem bigger, while darker ones can make spaces feel more intimate. A dark ceiling will seem lower than it really is, or—when installed high enough above—simply disappears.

Color schemes also can indicate the purpose and usage of a space with boundaries and transitions. Consideration should be given to how the visual stimulation in a space will be perceived by the brain to evoke a desired response. This is of utmost importance in environments where varied spaces have different tasks and functions, to avoid any confusion that can cause stress in the occupants.

Segment-specific demands
Color certainly has a place in educational settings, but aesthetics may need to be secondary to performance requirements. Fire performance and indoor air quality are top-of-mind, and acoustics also need to be of primary importance. Classrooms in the U.S. typically have speech intelligibility ratings of 75 percent or less, meaning every fourth spoken word is not understood.5 Loud or reverberant classrooms may cause teachers to raise their voices, leading to increased teacher stress and fatigue.6

In school activity areas, stone wool ceiling panels may be specified with both a high acoustic performance and impact-resistance. The panel’s reinforced surface withstands tougher-than-average wear and tear, as well as frequent mounting and demounting.

Along with durability and flexibility for future redesign, health care facilities seek products with easy-to-clean surfaces to support infection control. Most Methicillin-resistant Staphylococcus Aureus (MRSA) infections occur in people who have been in hospitals or other health care settings and are resistant to the antibiotics commonly used to treat ordinary staph infections.7

Stone wool ceiling panels designed for medical use have been classified Class 5, or better, in accordance with International Organization for Standardization (ISO) 14644-1, Cleanrooms and Associated Controlled Environments−Part 1: Classification of Air Cleanliness. Those that have specially treated medical and hygienic surface finishes also help mitigate:

  • MRSA bacteria resistant to antibiotics and responsible for post-surgery infections and septicaemias;
  • Candida Albicans, which is yeast responsible for skin infections and pneumonias; and
  • Aspergillus Niger, which is mold responsible for pneumonias.

Noise also contributes to patients’ slower recovery times. Studies show high levels of sound have negative physical and psychological effects on patients by disrupting sleep and increasing stress.8

With respect to auditory privacy, acoustic performance not only is relevant to patient decency and respect, but also to the protection of corporate intellectual property, and to increased concentration levels in working environments. After surveying 65,000 people over the past decade in North America, Europe, Africa, and Australia, researchers at the University of California-Berkeley reported more than half of office workers are dissatisfied with the level of speech privacy, making it the leading complaint in offices everywhere.9

Conclusion
From acoustics to fire performance and aesthetics to sustainability, stone wool ceiling systems provide the versatility and attributes to meet the varied requirements of commercial and institutional buildings’ new construction and renovation projects.

Notes
1 Visit www.euro.who.int/en/health-topics/environment-and-health/noise. (back to top)
2 Visit www.hc-sc.gc.ca/ewh-semt/air/in/poll/mould-moisissure/effects-effets-eng.php(back to top)
3 See William J. Fisk’s “Health and Productivity Gains from Better Indoor Environments,” from the 2000 edition of Annual Review of Energy and the Environment. Visit www2.bren.ucsb.edu/~modular/private/Articles/Fisk%20HealthandProductivity%202000.pdf(back to top)
4 For more, see Vanessa Loder’s article, “Maybe Money Really Does Grow on Trees,” in the May 4, 2014 edition of Forbes. Visit www.forbes.com/sites/vanessaloder/2014/05/04/maybe-money-really-does-grow-on-trees/2(back to top)
5 See Classroom Acoustics, by Seep et al, published in 2000 by the Acoustical Society of America (ASA).(back to top
6 See Tiesler & Oberdörster’s 2008 article, “Noise: A Stressor? Acoustic Ergonomics of Schools,” in Building Acoustics (15 [3]). (back to top)
7 Visit www.mayoclinic.org/mrsa(back to top)
8 See “Sound Practices: Noise Control in the Healthcare Environment?” published by HermanMiller Healthcare in 2009, and “Sound Control for Improved Outcomes in Healthcare Settings,” by Joseph Ulrich, published in 2004 by the Center for Health Design. (back to top)
9 See John Tierney’s article, “From Cubicles, Cry for Quiet Pierces Office Buzz,” in the May 19, 2012 edition of the New York Times. Visit www.nytimes.com/2012/05/20/science/when-buzz-at-your-cubicle-is-too-loud-for-work.html. Also, visit www.cbe.berkeley.edu/research/index.htm(back to top)

Cory Nevins is Rockfon’s director of marketing, leading the company’s continuing education and training programs to keep commercial building team members updated on acoustic stone wool ceiling panels, specialty metal ceiling panels, and ceiling suspension systems. He has nearly 20 years of experience in the building products industry, the majority of which has focused on ceiling systems, and a bachelor’s degree in marketing from Miami University in Oxford, Ohio. Nevins can be contacted at cory.nevins@rockfon.com.

Design of Fire-resistive Exposed Wood Members

Photo courtesy Structurlam

Photo courtesy Structurlam

by Bradford Douglas, PE, and Jason Smart, PE

Designing for life safety is complex and multifaceted, and fire-related issues comprise a large portion of model codes. For nearly 15 years, a mechanics-based design method has been used in the United States to estimate the capacity of exposed wood members using basic wood engineering mechanics.

This mechanics-based design method first appeared in the 1999 version of Technical Report (TR) 10, Calculating the Fire Resistance of Exposed Wood Members (TR 10). This method was then incorporated into new design procedures of the 2001 National Design Specification (NDS) for Wood Construction. These procedures were later adopted into the model building codes through reference to the NDS for calculating fire resistance of wood members. The procedures have been used extensively for design of large, exposed wood members, but are now also beginning to be employed for estimating structural fire resistance of smaller exposed wood members.

Excerpt from Design for Code Acceptance (DCA) 2, Design of Fire-resistive Exposed Wood Members, Table 1 (one-hr). This shows tabulated one-hour design load ratios, Rs, for flexural members exposed on three sides. [CREDIT] Data courtesy American Wood Council

Excerpt from Design for Code Acceptance (DCA) 2, Design of Fire-resistive Exposed Wood Members, Table 1 (one-hr). This shows tabulated one-hour design load ratios, Rs, for flexural members exposed on three sides. Data courtesy American Wood Council

Wood buildings can be designed to meet rigorous standards for performance, which is why the International Building Code (IBC) allows the material’s use in a wide range of building types—including structures that are taller and have more area than some designers realize. Table 601 of the IBC shows the required fire resistance of building elements (i.e. structural frame, walls, floors, and roofs) for each construction type. Ratings are given in hours. The exception is Type IV, where the wood structural elements are assumed to have inherent fire resistance due to their required minimum dimensions—in other words, no fire resistance rating is necessary except for exterior walls.

Fire resistance is a measure of the length of time an assembly or structural element can sustain a given load when subjected to a standardized fire exposure condition. Fire resistance of wood members and assemblies may be established by any one of five means listed in IBC Section 703.3, “Alternative methods for determining fire resistance.” The most common methods are tested assemblies and calculated fire resistance.

Excerpt from DCA 2, Table 2 (two-hr), showing tabulated two-hour design load ratios, Rs, for compression members exposed on four sides.

Excerpt from DCA 2, Table 2 (two-hr), showing tabulated two-hour design load ratios, Rs, for compression members exposed on four sides.

Fire design of exposed wood members
The fire resistance of exposed wood members, including lumber, glued-laminated (glulam) timber, and structural composite lumber (SCL), may be calculated using provisions of NDS. This allowable stress design approach is referenced in IBC Section 722, “Calculated Fire Resistance.” The design procedure allows calculation of the capacity of exposed wood members using basic engineering mechanics.

Actual mechanical and physical properties of the wood are used, with member capacity directly calculated for a given period—up to two hours. Section properties are computed assuming an effective char rate (i.e. βeff) at a given time (i.e. t). Reductions of strength and stiffness of wood directly adjacent to the char layer are addressed by accelerating the char rate by 20 percent. Average member strength properties are approximated from existing accepted procedures used to calculate design properties. Finally, wood members are designed using accepted engineering procedures found in NDS for allowable stress design. (The design procedures presented in NDS Chapter 16 are not intended to be used for the design and retrofit of structures after a fire.)

For sawn lumber, glulam, and SCL, the nominal char rate (i.e. βn) is typically assumed to be 38 mm/hour (1.5 in./hour). For a given time in hours, the effective char rate is then:

β_(eff=) 1.8/t^0.187 in./hour

To calculate section properties of wood members, the effective char layer thickness (i.e. aeff), for structural calculations is computed as:

a_(eff=) 1.8t^0.813 in.

This photo showcases glued-laminated lumber (glulam). Photos courtesy Structurlam

This photo showcases glued-laminated lumber (glulam). Photos courtesy Structurlam

The 2012 and earlier versions of IBC have also contained an empirical calculation method for estimating the structural fire resistance of wood beams and columns exposed to a standard fire exposure for up to one hour. However, this empirical method has been removed from the 2015 IBC in favor of provisions contained within NDS Chapter 16 that are much broader in application and leave less room for design error.

Basis for NDS chapter 16 approach
AWC’s TR 10 contains full details of the NDS method as well as design examples.1 TR 10 was recently revised to incorporate a new section that supports the use of the design method with smaller dimension sizes associated with lumber joist floor assemblies. It also has revised design examples to match the 2012 NDS.

The revised design tables in Appendix A allow more accurate calculation of fire resistance of columns with any slenderness ratio. They also eliminate tabulation of special cases that can be misapplied, such as:

  • deleted beams exposed on four sides that are assumed to be fully braced throughout the fire rating;
  • columns only exposed on three sides, but assumed to be unbraced; and
  • tension members that do not resist flexure due to member dead load.

Finally, a new Appendix B that calculates the fire resistance of single-span lumber joists for any design stress ratio when joists are exposed on three sides and braced on the top edge was incorporated.

Simplified approach
AWC’s Design for Code Acceptance (DCA) 2, Design of Fire-resistive Exposed Wood Members has been revised to replace the empirical design equations currently in the 2012 IBC with simplified design information developed in accordance with the code-approved NDS fire design procedure for exposed wood members. The tables and examples have been rewritten for consistency with the approach outlined in the 2012 NDS and TR 10.

Supporting arches were used at the Boy Scouts of America Camp Fife project.

Supporting arches were used at the Boy Scouts of America Camp Fife project.

For beams and columns stressed in one principal direction, simplifications can be made to allow creation of load ratio tables. These tables can then be used to determine the structural design load ratio at which the member has sufficient capacity for a given fire resistance time. Tables in DCA 2 give load ratios corresponding to one-hour, 1.5-hour, and two-hour fire resistance ratings for specified member dimensions.

All tabulated load ratios apply to standard reference conditions where the load duration factor, wet service factor, and temperature factor equal 1.0 (CD=1.0; CM=1.0; Ct=1.0). For more complex calculations where stress interactions must be considered, or where standard reference conditions do not apply, designers should use the provisions outlined in TR 10, along with the appropriate NDS provisions.

Flexural members
Design load ratios (i.e. Rs) for fire design of flexural members for various beam sizes are tabulated in DCA 2 tables for one-hour, 1.5-hour, and two-hour fire resistance ratings. The Rs values given in these tables apply to three-sided exposure in which the beam’s top edge is protected from fire exposure (e.g. protected by the underside of a floor or roof). These tabulated values apply to flexural members loaded in bending about one axis only, and are continuously laterally supported along the compression edge. The dimension ‘d’ is the actual cross-sectional dimension measured in the direction normal to the axis about which bending occurs, and is not necessarily greater than ‘b’ (Figure 1).

To use the DCA 2 tables, the designer only needs to know the required fire resistance rating (FRR), the structural load ratio (Rs), and the beam dimensions. For example, when a beam is exposed on three sides and is protected by a floor deck bracing the compression edge, it is required to have a one-hour FRR. When the design professional wants to use a 152.4 x 330.2-mm (6 ¾ x 13 ½-in.) beam being loaded in bending about the strong axis, DCA 2 Table 1 (one-hour) shows a 152.4 x 330.2-mm beam would calculate to have a one-hour fire resistance rating if loaded to its full design load (Rs=1).

The entrance of the Rocky Mountain Elk Foundation building. [CREDIT] Photo courtesy Structurlam/OZ Architects

The entrance of the Rocky Mountain Elk Foundation building. Photos courtesy Structurlam/OZ Architects

OZ Architects used structural composite lumber (SCL) for the Rocky Mountain Elk Foundation building in Missoula, Montana.

OZ Architects used structural composite lumber (SCL) for the Rocky Mountain Elk Foundation building in Missoula, Montana.

Compression members
Design load ratios, Rs, for fire design of columns are calculated as the product of two ratios, Rs1 and Rs2. Values of Rs1 and Rs2 for various column sizes are tabulated in DCA 2 for one-hour, 1.5-hour, and two-hour fire resistance ratings. For cases in which the product of Rs1 and Rs2 is greater than 1.0, Rs should be taken as 1.0. These tables apply to cases in which all four sides are exposed to the fire; however, values calculated using these design load ratios may be conservatively applied when one or more sides of the column are protected. Both‘d’ and ‘b’ represent the dry dressed cross-sectional column dimensions. The dimension ‘d’ is the actual dimension measured in the direction perpendicular to the axis about which buckling is being considered, and is not necessarily greater than ‘b’ (Figure 2). The designer should consider buckling about both axes and use the lesser design value.

The tabulated Rs1 values are calculated based on a square column cross-section having dimensions ‘d’ by ‘d,’ and therefore must be multiplied by Rs2 for any column that is not square. Where ‘b’ is less than ‘d,’ Rs2 will be less than 1.0; and where ‘b’ is greater than ‘d,’ Rs2 will be greater than 1.0. The Rs1 values are derived using the more conservative value of the parameter ‘c’ from equation 3.7-1 of the NDS (for long columns, c=0.9 results in lower Rs1 values; for short columns, c=0.8 results in lower Rs1 values). This allows the design load ratios to be used with sawn lumber, structural glulam, or SCL. The Rs1 values are also based on the assumption that Emin’/Fc*= 350. As a result, the design load ratios (Rs) may conservatively be used for all species and grades where the ratio of Emin’ to Fc* is greater than or equal to 350.

CS_July_2014.inddTo use the tables, the designer only needs to know:

  • required fire resistance rating;
  • structural load ratio;
  • column dimensions;
  • unbraced length of the column; and
  • end support conditions.

For example, in a case where an exposed column is required to have a two-hour FRR, the designer may want to use a nominal 14×16 column with actual dry dimensions of 330 x 381 mm (13 ¼ x 15 in.). The column will have an unbraced length of 3.35 m (11 ft) in both directions, and can conservatively be assumed to have pinned end conditions on each end of the column (Ke=1.0).

Since the unbraced length is the same in both directions, buckling would tend to be about the 13¼-in. dimension. The effective length (i.e. Le) would be 3352 mm (132 in.) and the Le/d ratio would be 10. From DCA 2 Table 2 (two-hour), a square column that is 13¼ x 13¼ in. with Le/d=10 would have an Rs1=0.39. A 13¼ x 15-in. column buckling about the 13¼ in. dimension would have an Rs2=1.11. Therefore, the 13¼ x 15-in. column would calculate to have a two-hour fire resistance rating when loaded to 43 percent or less of its full design load (Rs = [Rs1][Rs2] = [0.39][1.11] = 0.43).

Timber decking
DCA 2 also provides tabulated design load ratios (Rs) for various decking types and thicknesses, corresponding to one-hour, 1.5-hour, and two-hour fire resistance ratings. The dimensions ‘b’ and ‘d’ given in these tables are the actual dry dressed dimensions of each individual member. The Rs values given in DCA 2 Table 3.1 are applicable to butt-joint timber decking fully exposed on one face and partially exposed on the sides, in accordance with NDS Section 16.2.5.

The Rsvalues given in DCA 2 Table 3.2 are applicable to double and single tongue-and-groove decking exposed only on one face. These calculation procedures do not address thermal separation.

Constr-Specfr-Douglas-Smart-Fire-140506_Page_3To use the tables, the designer only needs to know the required fire-resistance rating, the structural load ratio (Rs) and the decking type and dimensions. For example, a timber deck is required to have a 1.5-hour FRR, and the designer wants to use 4x tongue-and-groove decking. From DCA 2 Table 3.2, the nominal 4x tongue and groove decking (76.2 mm [3½ in.] actual thickness) would calculate to have a 1.5-hour fire resistance rating if loaded to 23 percent or less of its full design load (Rs=0.23).

Connections
Where a specified fire resistance rating is required, Section 16.3 of the NDS requires connectors and fasteners be protected from fire exposure. This protection can be achieved by any of the following:

  • wood members having dimensions sufficient to prevent the char front from reaching the connectors and fasteners for the duration of the required fire-resistance rating time—the char front can be calculated as: a=1.5t^0.813 in.;
  • fire-rated gypsum board having a finish rating greater than or equal to the required fire-resistance rating; and
  • any approved coating having a fire rating greater than or equal to the required fire resistance rating time.

Conclusion
Structural fire design provisions have been incorporated in Chapter 16 of NDS, which is referenced in Section 722.1 of the 2012 IBC as a method of calculating fire resistance of exposed wood members. A comprehensive discussion of this mechanics-based design procedure can be found in Technical Report No. 10, while DCA 2 provides a simplified design aid of this code-approved fire design procedure for several typical applications with exposed wood members. 2

Notes
1 For more information, TR 10, Calculating the Fire Resistance of Exposed Wood Members, is available at www.awc.org/pdf/TR10.pdf.
2 Additional information on building code requirements for wood can be found in the American Wood Council’s Code Conforming Wood Design documents, available at www.awc.org/codes/ccwdindex.php.

Brad Douglas, PE, joined the American Wood Council (AWC) in 1986, and serves as its vice president of engineering. Douglas directs a program aimed at developing state-of-the-art engineering data, technology, and standards on structural wood products, systems, and assemblies for use by design professionals and building officials to assure safe and efficient design and use of wood. He is a graduate of Virginia Tech. Douglas can be contacted at bdouglas@awc.org.

Jason Smart, PE, joined AWC in 2013 and is manager of engineering technology. Smart focuses on development and support for new and emerging technologies and related changes to design and model building code standards. He is a graduate of Virginia Tech with degrees in civil engineering, wood science and forest products, and timber engineering. He can be reached at jsmart@awc.org.

Standards and Terminologies

In the May 2014 issue of The Construction Specifier, we published the article, “Passive Fire Protection and Interior Wall Assemblies,” by Gregg Stahl. Soon after, a reader contacted us regarding what he considered inaccuracies. We reached out to the author and, in the interest of continuing the discourse about this important topic, excerpts from both sides are included below.

Reader: The first issue is the reference to ASTM E603. The author mentions this is one of two standards that rates assemblies. Actually, ASTM E603 is a “guide” standard, and is used to explain the various types of fire tests, whether they are ASTM, NFPA, UL, or FM, and how they can be compared and contrasted. This standard is not a test method.
Author: The reader brings up several good points in regard to the article on passive fire protection. It should be noted, however, this piece was intended to provide a general overview on the basic principles of passive fire protection. As to the first point, the reader is technically correct. E603 is in fact an ASTM “Guide,” not an ASTM “Standard.” In the “Scope” section of this guide, it does state one of the purposes is to “allow(s) users to obtain fire-test-response characteristics of materials, products, or assemblies, which are useful data for describing or appraising their fire performance under actual fire conditions.” In the subsequent paragraphs, I go on to describe how A603 is used as well as differentiating it from the E119 fire test, which is testing the effectiveness of a particular assembly.

Reader: The second issue is the article states ASTM E119 tests the effectiveness of an assembly as a “fire barrier.” Although not untrue, the use of “fire barrier” seems to limit the type of fire-rated assembly that is tested, since a “fire barrier” is a specific type of fire-rated assembly used by the IBC and NFPA. ASTM E119 is used to test any type of assembly for fire-resistance, whether it is a wall, roof system, floor system, column, beam, etc.
Author: I should have been more precise in the selection of the terminology used. The intent of the term was to use a dictionary meaning, not a fire test assembly meaning. A Google search for the term will produce numerous definitions, such as the one below:

fire barrier: a continuous vertical or horizontal assembly, such as a wall or floor, that is designed and constructed with a specified fire resistance rating to limit the spread of fire and that also will restrict the movement of smoke. Such barriers might have protected openings.

Reader: The third issue is mentioning the hose stream test is used to “measure an assembly’s resistance to water pressure.” This is misleading. The hose stream test is not really a measure of an assembly’s resistance to water pressure, but to test the system’s integrity. As the commentary to the standard states, the hose stream tests the “ability of the construction to resist disintegration under adverse conditions.” In other words, it is a way of testing, from a distance (it is very hot) the assembly’s integrity from falling debris.
Author: The reader references “the standard,” but I do not know to which standard he is referring. ASTM E2226, Standard Practice for Application of Hose Stream, states:

1.3 – The result derived from this practice is one factor in assessing the integrity of building elements after fire exposure. The practice prescribes a standard hose stream exposure for comparing performance of building elements after fire exposure and evaluates various materials and construction techniques under common conditions.

The application of the hose stream does exert pressure on the assembly after it has completed either the full cycle of an E119 fire test or 50 percent of the time of the rated wall assembly. I agree the single word “pressure” does not go far enough to explain—the intent was to determine the integrity of the remaining assembly.

Reader: The fourth and final issue is the use of “area separation firewalls” in the article, and its associated endnote. The use of “area separation” walls was dropped when the IBC was published in 2000, and is not a term used by NFPA’s standards. The correct term used by both the IBC and NFPA is “fire wall” (not a single word). The endnote (no. 3) gives the impression these “area separation firewalls” are used to separate residential units or commercial tenants. This is incorrect. A fire wall divides a building—residential or commercial—into separate buildings so they can be considered independently when applying the code. “Fire partitions” are used for residential unit and commercial tenant separations within a single building and do not require the type of requirements described in the article.
Author: I respectfully disagree with the reader, who seems to be making the reference to area separation walls fit his use without recognizing the term can have more than one use or intent. It was employed here with no reference to NFPA or IBC, and was not intended as the reader interpreted it.
The term “area separation wall”—or “ASW” as it is commonly abbreviated—is used for a particular type of fire-rated wall assembly with a two-hour fire resistance rating, which is typically intended to permit controlled collapse of one unit in a multifamily residence, while still remaining intact and able to protect the adjacent unit in a fire situation. This is a common term in the construction industry. The reader can check the literature of various manufacturers and find this type of assembly. There are also various UL assemblies for this type of construction.

Sprayed Fire-resistive Materials, Bond Strength, and the IBC

by John Dalton

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Photo © BigStockPhoto/Tim Markley

Sprayed fire-resistive materials (SFRMs) are passive fire-protection materials intended for direct application to structural building members. They are predominantly cementitious or mineral-fiber-based, with the fire-resistive qualities and physical characteristics varying widely between the respective types. A recent code change pertaining to these materials is important for design/construction professionals to understand. Continue reading

Specifying NFPA 285 Testing

By Joseph Berchenko AIA, CSI, CCS

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Flames can blast through windows to climb along exterior walls—a fact replicated by a National Fire Protection Association (NFPA) test.
Photo © BigStockPhoto/Anne-Louise Quarfoth

For testing certain wall assemblies, the 2012 International Building Code (IBC) requires National Fire Protection Association (NFPA) 285, Standard Fire Test Method for Evaluation of Fire Propagation Characteristics of Exterior Non-load-bearing Wall Assemblies Containing Combustible Components. (That standard’s title notwithstanding, the test is performed on both load-bearing and non-load-bearing wall assemblies.) Its successful completion demonstrates the ability of an exterior wall mockup to contain vertical and horizontal flame spread across the face and through the wall’s core. Continue reading