In the previous edition of International Fire Protection we learned that a fundamental fire protection strategy is to divide a building into fire-rated compartments intended to contain fire to the compartment of origin. When openings are created in the walls and floors forming the compartments, their fire rating and ability to resist smoke migration is compromised. Often, openings cannot be avoided, in the case of penetrants or when barriers intersect.
This article is Part Two of a three-part series, with Part Three appearing in the next edition of International Fire Protection. In Part One, we defined fire-stopping as: “the process of installing third-party tested and listed materials into openings in fire-rated barriers to restore fire-resistance ratings” and talked about through-penetration fire-stops (mostly mechanical, electrical and plumbing applications). In this article, we will look at construction joints and perimeter fire barriers (curtain walls). In the next article, we will look at fire-stopping data cables (a huge source of life-safety violations due to the frequent changes they require) and healthcare and industrial applications.
Parts of a Fire-stop System
Construction joints are gaps in or between barriers. Sometimes they are the result of connections or intersections; other times, they are purposely introduced to allow movement. Common joint conditions include floor-to-floor, floor-to-wall, wall-to-wall, top-of-wall (the head-of-wall), or bottom-of-wall. Joints must be fire-stopped to prevent the spread of fire, smoke, and hot gasses. Figures 1 & 2 show a properly protected head-of-wall joint.
A joint system consists of both the barriers that form the joint boundaries, and the fire-stop products installed into the joint. The complete assemblage of elements, or system, achieves the rating, not the fire-stop product itself.
Tests for Construction Joint Fire-stop Systems
In areas of the world that have standardised on NFPA or ICC codes, the standards used to evaluate construction joint fire-stop systems are ASTM E 1966, entitled Standard Test Method for Fire-Resistive Joint Systems, or UL 2079, entitled Tests for Fire Resistance of Building Joint Systems. These standards are virtually synonymous and thus interchangeable. In Europe, the standard is EN 1366, although individual countries may have their own legacy standards such as BS 476 in the UK or DIN 4102 in Germany.
ASTM E1966 (UL 2079) exposes the test specimen to a standardised time-temperature curve, which assures all systems are tested in a consistent manner to the same rigorous requirements. This curve is shown in Figure 3.
The fire-rating derived from ASTM E1966 (UL2079) is called an Assembly Rating. Expressed in hours, it is the time period the joint system resisted both flame passage and temperature rise. Temperature rise is measured by thermocouples, and the standards impose a maximum single point rise of 181ºC over the initial starting temperature.
Figure 4 shows a typical head-of-wall joint system between a gypsum wall and steel deck floor being tested. The specimen is positioned on the furnace and exposed to fire. Failure occurs by observing flaming on the unexposed surface, or by excessive temperature rise.
Immediately following fire exposure, the assembly is subjected to the impact, erosion, and cooling effects of a water hose stream test. The blast of high-pressure, cold water distresses the assembly by both the force and the thermal shock of rapid cooling. The pressure and duration varies according to the hourly rating tested. The hose stream is considered an excellent measure of system integrity, and provides an added safety margin to building occupants and fire-fighting personnel.
The fire exposure dictated by EN1366 (and other European standards) is somewhat similar to that of ASTM E1966 (UL2079) although the derived ratings use different terminology. A major difference is that European standards only evaluate fire exposure and do not include a hose stream. Designers, inspectors, and installers should be aware of the differences since systems tested to EN1366 need not be as robust as those tested to ASTM E1966 (UL2079).
Some joint systems are subjected to cyclic movement prior to the fire test to evaluate the ability of the system to accommodate movement due to thermal expansion, wind-sway, or even seismic conditions. Movement testing has the effect of fatiguing the joint sealant prior to fire exposure, and may affect the performance of the system under fire exposure conditions. A movement cycle consists of extending the joint and compressing it at a prescribed rate, as tabulated below.
L & W Ratings
UL2079 includes optional test protocols for evaluating air leakage (L Rating) and short term water-tightness (W Rating).
For air leakage, the specimen is secured to a pressurised chamber that measures the rate of airflow through the system. The L Rating is often used as an indication of smoke resistance. It also helps a designer select the best system for controlling migration of particulate matter, such as tobacco smoke in office buildings or infectious/duct in hospitals.
Water-tightness is evaluated by placing a water vessel on top of the fire-stop system. The vessel is filled to a 914mm water column (or equivalent pressure) and exposed for 72 hours. A Class 1 W Rating requires the fire-stop system to resist the passage of water for the exposure period of 72 hours. W-Rated systems offer performance information for environments that may be subject to transient water exposure, such as some mechanical rooms or wash-down areas.
Tests for Perimeter Fire Barrier Systems
A perimeter fire barrier system (PFBS) describes the fire-stop system installed into the linear opening between a fire-rated floor and a non-rated exterior wall (curtain wall). A PFBS evaluates the ability of the system to resist interior propagation of fire through the gap between floor and exterior wall for a time period equal to the floor. The correct test standard used to evaluate PFBS is ASTM E2307, entitled, Standard Test Method for Determining Fire Resistance of Perimeter Fire Barrier Systems Using Intermediate-Scale, Multi-Story Test Apparatus.
ASTM E2307, referenced in both NFPA and ICC codes, is the most developed standard for evaluating these critical intersections. There is presently no equivalent to it. In fact, many countries that do not generally use ASTM standards have informally accepted the use of PFBS evaluated according to ASTM E2307. The use of these systems is necessary to ensure that continuity of the floor assembly is maintained from one exterior side of the building to the other. In addition, gaps often are made to accommodate the mounting hardware that secures the exterior wall to the edge of the floor. Left unprotected, the mounting hardware can fail and the building can shed panels, an extremely dangerous condition during occupant egress and firefighter ingress. An effective PFBS will therefore also protect the curtain wall mounting hardware from direct fire exposure.
ASTM E2307 uses a special test structure called the Intermediate-Scale, Multi-Story Test Apparatus. The ISMA structure simulates fire exposure in a high-rise structure where, as the fire intensifies and positive pressure builds, a fire induced window break occurs, thereby allowing oxygen to feed the fire. The flame plume erupts out of the broken windows and begins to attack the exterior of the curtain wall. By subjecting the PFBS to fire from two sides simultaneously, this two-story structure dramatically erodes critical wall framing elements that help keep the system in place for the full duration of fire exposure.
Many exterior walls integrate materials with sub-par fire resistance. Common wall types include glass, aluminium, thin stone veneers, or combustible core insulation panels. Typically, such products will not survive fire exposure. In fire exposure experiments, glass panels break in five to 15 minutes. Aluminium melts at 660ºC, a temperature reached within minutes in a big fire. Panel systems with polystyrene or polyurethane foams burn readily. Even inorganic materials with excellent fire retardant properties such as stone panels will fail prematurely. Trapped moisture can cause explosive spalling, while differential expansion can lead to cracking.
Therefore, a properly designed PFBS per ASTM E2307 must use high-melt point insulations or gypsum board installed in a different manner than the one the industry traditionally used, in order to protect the curtain wall and/or stay intact long enough to allow the fire-stop system in the safing gap to survive for a time period equal to that of the floor – generally two hours.
Properly designed and installed PFBS are pretty consistent. The spandrel area at the floor line is protected with heavy density mineral wool panels or gypsum board. The safing gap is filled with light density but compressed mineral wool batt insulation. A fire-stop coating or sealant is applied on top of the mineral wool safing insulation to complete the system. The fire-stop coating is the glue that holds the system together, reduces temperature transmission through it, and, perhaps more importantly, prevents smoke passage from floor-to-floor.
Smoke passage in these conditions can be exacerbated by the stack effect in high-rise construction, which is the natural flow of air that occurs due to temperature differentials between the conditioned air inside the building and the temperature outside the building. L and W Ratings can also be evaluated for PFBS by laboratories such as UL. Flexible coatings with enhanced water resistance are ideally suited to situations where portions of the exterior wall are yet to be installed, and threat of inclement weather is possible. The use of a steel plate underneath the safing gap is no longer required or desired with such a design.
Joint systems and perimeter fire barrier systems are important elements for designers, installers, and inspectors. Selecting the right products and systems begins by understanding the nuances of the ratings reported on labels and manufacturer’s literature. The key to regulatory compliance and life safety is proper installation. Reading the systems, consulting with the manufacturer’s qualified technical staff, working with experienced designers and contractors, and planning the work ahead of time makes the process easier.
In the next article in International Fire Protection we will be discussing two other items. High traffic openings (typically data cables) are a major headache for IT personnel, as they require constant changes and the number-one source of life safety violations for most buildings as walls are turned into Swiss cheese. The needs of healthcare and industrial facilities have also changed over time, yet many of the solutions they use are antiquated. But there are better ways of addressing both challenges.
For further information, go to www.stifirestop.com
Featured Image. A properly protected head-of-wall joint