New fire prevention and sealing technology for interlayer gaps in high-rise building curtain walls

　　(I) Introduction

　　Globally, research on the spread of fire in high-rise buildings has a long history. Over the past few decades, there have been excellent engineering practices on how to control the spread of fire. Many countries and regions have adopted standardized tests for fire-blocking methods of curtain wall inter-layer gaps. Relevant building and safety codes have begun to regulate high-rise buildings with curtain walls to ensure life and property safety.

　　To better understand how fire spreads in high-rise buildings with glass curtain walls, we need to imagine a scenario: If a fire occurs on a certain floor, as the fire spreads, the indoor pressure of that floor gradually increases. The glass in the curtain wall breaks under the engulfment of flames, allowing more oxygen to enter the room, fueling the fire to further increase and spread to the bottom of the upper floor and the inside of the curtain wall structure. Moreover, a certain flame may burst out from the broken window and begin to attack the outside of the upper curtain wall structure. If there is a gap between the floor slab and the curtain wall structure, the flames will quickly rush up from this inter-layer gap to the upper floor (see Figure 1). For this challenge, it is necessary to reasonably design and install an inter-layer gap fire protection system that can remain stable throughout the fire and meet the requirements of double-sided fire exposure to prevent the vertical spread of fire.

　　Because many curtain wall structures are not made of fire-resistant materials, this further exacerbates the spread of fire in high-rise buildings. Materials such as glass or aluminum profiles are often integrated into modern curtain wall structures. These materials are usually made to be relatively thin, weakening their fire resistance. Generally, glass will break after 5-15 minutes of exposure to fire, and aluminum profiles will melt when they reach 660°C. Although materials considered to have better fire resistance are sometimes used, these materials often fail in actual fires. For example, granite, marble, travertine, or sandstone stone partitions or infill partitions are good fire-resistant materials and will not burn. However, these materials will crack when heated in a fire, and may even explode due to different degrees of expansion in various directions, or because the stone contains a certain amount of moisture. Does this mean that these materials cannot be used? Absolutely not.

　　The most feasible approach is to improve the fire resistance of these components by using a simple system with high-temperature insulation materials, and at the same time, correctly installing a fire-blocking system. This article discusses related issues and shares relevant engineering application experience.

　　(II) Introduction to Inter-layer Gap Fire-blocking System

　　The term "inter-layer gap fire-blocking system" has quickly become synonymous with preventing the spread of fire in high-rise buildings. An inter-layer gap fire-blocking system includes fire-resistant floor components, exterior wall components, and materials installed in the gap between the exterior wall and the floor, preventing the formation of a fire spread path between floors.

　　Some countries use test standards to evaluate the performance of inter-layer gap fire protection systems. One standard is the American "Test Standard for Inter-layer Gap Fire Resistance (ISMA)" ASTM E2307-10, which uses a two-layer medium-scale structural model to evaluate the fire resistance time of inter-layer gap fire-blocking systems used in high-rise buildings (see Figures 2 and 3). The front of this model is a glass curtain wall structure; the upper layer is an observation room, and the lower layer is a laboratory (furnace), with a reinforced concrete floor slab between the upper and lower layers. The inter-layer gap fire protection system is installed in the horizontal gap between the floor slab and the curtain wall separation strip (see Figure 4).

　　After the test structure is installed, the furnace in the laboratory is ignited (see Figure 5). After 5 minutes, another fire source is ignited outside the glass curtain wall to simulate the upward spread of flames after the window breaks (see Figure 6). That is, while the external components of the curtain wall structure are directly exposed to fire, the internal fire source has already attacked the internal components of the curtain wall structure, the bottom of the floor slab, and the related materials of the inter-layer gap fire-blocking system. The test is terminated when the test failure phenomenon occurs or after the expected fire resistance time is reached. Test failure means that flames have passed through the fire-blocking system in the observation room. In addition, the appearance of any hot spots is also considered a test failure. A cotton mattress is placed on the inter-layer gap fire protection system in the observation room and kept for 30 seconds to observe whether there are any cracks or holes in the fire-blocking system. If the cotton mattress catches fire, the fire-blocking system is also considered to have failed. If the blocking system cannot effectively reduce the heat from the overheated glass, the cotton mattress will burn prematurely (see Figures 7 and 8).

　　During the test, the heat diffusion is analyzed by monitoring the temperature of the backfire surface. The tested fire resistance time and heat resistance time are obtained according to the relevant steps of ASTM E2307-10. Fire resistance time is the time the fire-blocking system prevents flames from passing through; heat resistance time measures the effect of heat transfer in the system. Most curtain wall fire-blocking systems have a heat resistance time of no more than 15 to 30 minutes and a fire resistance time of usually 2 hours or more due to the use of aluminum profiles or anchorages with good thermal conductivity.

　　(III) Structure of the Inter-layer Gap Fire-blocking System

　　2.1 Installation of Mineral Wool Insulation Board

　　The most common aluminum frame curtain wall structures with traditional (combustible materials such as glass, aluminum, or stone) separation strips use high-temperature resistant mineral wool insulation boards. Mineral wool insulation boards are made of mineral slag wool or rock wool and can withstand temperatures as high as 100 degrees. Usually, mineral wool insulation boards (usually with a density of 96～128kg/m3) wrap the vertical and horizontal components of the curtain wall frame around the separation strip. (See Figure 9) There are various methods for installing mineral wool boards. Sometimes steel plates are used, but usually, the role of steel plates is only to beautify the shape of the separation strip; sometimes a series of horizontal steel components are installed on the separation strip between the vertical components; or mineral wool boards are installed and fixed through some steel clamps and steel nails. In addition to fixing the mineral wool boards with steel anchor nails and protective covers, steel bolts with large steel washers or cup-shaped rivets can also be used to fix the mineral wool boards to angle steel, steel plates, or channel steel at the joints. This curtain wall mineral wool insulation board is a crucial component. After the external insulation layer of the curtain wall falls off, even though the surrounding aluminum profiles and partitions are heated and melted and fall off, the mineral wool insulation board can still maintain its integrity throughout the entire process of the fire-blocking system being exposed to fire.

　　Another essential component is the horizontal clamping groove. It is placed between the vertical frames and fixed to the perimeter angle steel, vertical frames, or spot-welded or seam-welded to the steel backing plate. The position of the clamping groove is directly in line with the location of the fire-blocking system to be installed, so it should be installed at half the thickness of the floor slab. The clamping groove can be constructed using channel steel, steel reinforcing bars, or two angle steels assembled into a 'T' shape. The clamping groove is installed simultaneously with other mineral wool support grooves (angle steel or backing plate) and is installed behind the curtain wall insulation layer. Its main function is to ensure that the fire-blocking system (including the mineral wool) remains compressed when exposed to fire.

　　Fire blocking of 2.2 horizontal seams

　　After introducing the installation of the curtain wall mineral wool partition and related support systems, it is necessary to introduce how to seal the horizontal interlayer seams. (See Figure 9) The correct method for sealing this horizontal seam is to insert lightweight (usually 64kg/m3 density) mineral wool strips into the gap. Cut the mineral wool board into mineral wool strips of a certain width, and several mineral wool strips are pieced together, approximately 25%~33% wider than the interlayer seam, so as to obtain a compression rate of 20%~25%. During installation, the mineral wool strips should be kept flat, and usually the mineral wool surface is flush with the floor slab. Generally, the mineral wool only needs to be compressed tightly, and no additional support components are needed. After installing the mineral wool, apply a layer of fire-blocking layer or fire-blocking glue to the surface of the mineral wool. These products are usually latex-based, and most can be brushed or sprayed using a spray gun. This layer of fire-blocking layer is crucial because, on the one hand, it can bond the various previous components (several mineral wool strips, curtain wall frames, and floor slab edges) together to form a system; on the other hand, it is a sealing layer that ensures the integrity of the mineral wool and isolates high-temperature smoke. In some cases, silicone-based materials can also be used for waterproofing.

　　For some designs, the final step in the interlayer seam fire-blocking system is the protection or wrapping of the curtain wall vertical frames. This is a small part for curtain wall insulation layers made of the same mineral wool. By wrapping the upper and lower vertical frames of the floor slab with mineral wool boards, both the insulation performance can be improved, and the curtain wall insulation layer can be connected to form a continuous fire-rated barrier.

　　(Four) Preventing the spread of smoke

　　In a fire scene, smoke is a deadly factor. A properly designed interlayer seam fire-blocking system can help reduce the spread of smoke. Typically, the curtain wall insulation layer has aluminum foil at the barrier. In addition, the fire-blocking layer or fire-blocking glue acts as a continuous component to block the spread of smoke in the gaps. Many relevant laboratories have specialized laboratories for testing the airtightness of fire-blocking layers or fire-blocking glue. The best fire-blocking system can achieve a release rate of less than 1 liter per second per unit length.

　　When a fire occurs, the pressure in the fire area gradually increases, and smoke and combustion byproducts, which are lighter than air, begin to accumulate at the ceiling. If the pressure continues to increase, the smoke and combustion byproducts will be squeezed to some unblocked holes. In addition, in high-rise buildings, the pressure difference caused by the temperature difference between indoors and outdoors causes airflow movement, forming a chimney effect. The chimney effect is considered the main reason for the vertical movement of smoke, and the higher the building, the more obvious the chimney effect. Using products with better airtightness helps to deal with the vertical spread of smoke.

　　(Five) Engineering Cases

　　On February 9, 2009, a fire broke out during the renovation of a commercial and residential building at 510 Madison Avenue in the United States (see Figure 10). The fire was effectively contained to the floor where it started (2nd floor, see Figure 11). This building used a fire-blocking system provided by STI Specialty Technologies, Inc. It is worth noting that on the same day, a fire also broke out in the north substation of the CCTV building in China, which had just been completed, but the fire spread extensively, engulfing almost the entire building.

　　(Six) Conclusion

　　The fire protection scheme introduced in this article is mainly to limit the spread of fire, smoke, and combustion byproducts in the interlayer seams. Due to the vertical structural design of the curtain wall itself, a relatively large gap naturally exists between the related connecting components of the curtain wall structure and the floor slab, sometimes for the installation of water pipes or the leveling of independent partitions. The floor slab effectively ensures the fire-rated partition function through the interlayer seam fire-blocking system, which can be extended to the exterior wall of the building. Modern fire-blocking systems aim to design a safe and simple interlayer seam fire system that can also meet other needs. Of course, in some regions, such as China, a complete set of testing schemes for interlayer seam fire-blocking systems has not been established. However, through relevant engineering guidance provided by qualified engineers, people in these regions can adopt more suitable interlayer seam fire-blocking systems to build safer buildings for production and living. Typical fire-blocking products usually (before they are put on the market) need to undergo thousands or tens of thousands of tests under different working conditions according to relevant regulations, and there are test reports of relevant performance to confirm that they can be used in various specific situations. Using these products, in addition to fire prevention and flame retardancy, insulation, energy saving, moisture-proof, fog-proof, and sound insulation effects can also be achieved without increasing construction costs.