The performance requirements of tornado mitigation are not familiar, but over the years there has been a growing interest in the development of glass systems that provide impact protection in tornado-prone areas. Although 77% of tornadoes in the United States are classified as EF1 or below on the enhanced Fujita scale, which means that sustained wind speeds are below 110MPH, it is important to consider the impact of more powerful tornadoes.

This presentation focuses on the current standards and building codes that specify performance requirements, as well as the results of the test plan for various glass structures using ionic plastic interlayers in accordance with the test guidelines determined by ASTM E1886. When tested in accordance with ASTM standards, wooden debris "missiles" are propelled to travel at different speeds, simulating different levels of tornado activity.

After Hurricane Andrew in 1992, the United States adopted building code requirements and standards related to wind-borne debris protection, which led to the widespread use of impact glass systems for 20 years. 17 coastal states are affected by these building code requirements, including Florida, Georgia, North and South Carolina, Virginia, Maryland, Delaware, New Jersey, New York, Connecticut, Rhode Island States, Massachusetts, Mississippi, Alabama, Louisiana, Texas, and Hawaii. States affected by tornadoes clearly do not exist.

Despite voluntary building code requirements, the states most affected by tornadoes have not yet adopted these regulations. Because the wind speed associated with tornadoes is much higher than that of hurricanes, the test requirements for wind-borne debris protection are more stringent. The hurricane impact system that has been developed and tested for hurricanes cannot pass the impact requirements of tornadoes. Although glass systems designed for tornado protection are rare, the desire to provide higher levels of protection in key locations such as hospitals, schools, and community buildings has prompted building owners to demand higher-performance systems.

Tornadoes have been found on all continents except Antarctica. Many countries in Europe have experienced tornadoes, as well as Australia, Japan, South Africa, Brazil and Bangladesh. The United States reports more than 1,200 tornadoes each year, mainly in the so-called "tornado alley"-northwestern Texas through Oklahoma and northeastern Kansas.

Dr. Tetsuya Theodore Fujita introduced the first tornado classification system in 1971. He called his system Fujita Scale (F-Scale), which is divided into six categories. In 2007, F-Scale was replaced by Enhanced Fujita Scale (EF-Scale) and used in the United States. Canada adopted EF-Scale in 2013. EF-Scale wind speed is based on the degree of damage, ranging from minor damage to complete damage.

The TORRO scale (T scale) is mainly used in the UK to measure the intensity of tornadoes from T0 to T11. This scale is based on wind speed and was developed by Terence Meaden of the Tornado and Storm Research Organization (TORRO) as an extension of the Beaufort scale.

The Federal Emergency Management Agency (FEMA) Standard 361 "Guidelines for the Design and Construction of Community Shelters" applies to community shelters (also known as "safe rooms"). The design and construction of community shelters is to protect a large number of people from the impact of natural disasters. These rooms may be independent buildings or internal spaces, where the community safe room or area is structurally independent of the building. Schools, hospitals and other critical facilities occupied by a large number of people are usually the places of community safe rooms. Due to the importance of shelters to life safety, FEMA 361 only cited the EF5 tornado debris protection requirements.

The ICC/NSSA 500 storm shelter design and construction standards have been adopted and used by any jurisdiction in the United States since 2008. This code applies to the design, construction, installation and inspection of storm shelters in hurricanes. Or tornado-prone areas. Alabama is the only state that has adopted this code as a tool for providing school storm shelters. The 2015 IBC discussed storm shelters in Section 423. Storm shelters are required in areas where tornado shelters are designed for wind speeds of 250 mph (according to ICC500) and in most Group E homes with a total occupancy of 50 people or more. Testing at this wind speed requires an impact force of 6804 grams (15 lbs) 2 x 4 inches and a speed of 161 km/h (100 mph) or 45 m/s (147 ft/s).

The ASTM E1996 requirements for hurricane impact systems and missile ratings are classified as follows:

The protection categories include enhanced protection and basic protection, which are further divided into the area closest to the ground (missile C, D or E below) and the part with a height greater than 9.1 meters (missile A below). Most commercial glass systems have undergone basic protection tests; however, some buildings or basic facilities with glass systems must meet the enhanced protection against impact requirements of "E" class missiles.

According to FEMA 361, the standard missile used to determine impact resistance in all wind conditions is 6804 g (15 lb) wood, 2 x 4, and usually 3.6 m (12 feet) long. The missile advances at a speed of 45 m/s (147 f/s). "Passed" means that the missile did not penetrate the glass, the glass is still attached to its frame, and the glass fragments remain in the glass unit. In general, the FEMA test is much stricter than the missile’s Class E requirements in ASTM E1996.

The American Association of Building Manufacturers released its AAMA 512 Voluntary Specification for Tornado Disaster Mitigation Window Openings in 2011. The standard enables window manufacturers to assess the impact of their products, pressure cycling and water penetration (only in hurricane areas).

In 2014, Kuraray sponsored a test program to evaluate the performance of laminated glass installed in insulating glass units. The goal is to test laminates according to EF-3 requirements, because 95% of all tornadoes that occur in North America fall into this category or less. The first series of tests were done on heavy-duty laminated structures. The laminate not only passed the first impact, but was intact, and then broke when the corners were impacted. However, when the test sample was impacted first at the corner and then at the center of the lite, the glass filler could not resist the second impact. These first series of tests were conducted using a missile speed of 45 meters per second.

The second series of tests were carried out about a year later. The composition of the glass and the resulting properties are as follows:

The third round of testing was conducted using three window opening systems. These systems were glazed with the above-mentioned glass components and passed the EF3 impact requirements. The main purpose of the third round of testing is to analyze what effect the system design may have on the performance of the glass filler. These systems are a dry glass pressure plate curtain wall system, a fixed window system with a removable glass block, wet glass, and a four-sided structural glass curtain wall. The glass composition is an insulating glass unit with an 18 mm laminate (6.84 mm ionic plastic) on the outside and a 14 mm laminate (4.56 mm ionic plastic) on the inside.

The dry-glazed pressboard curtain wall system failed due to corner impact pull; it must be pointed out that this particular system has approximately 12 mm of glass edges joined into the curtain wall frame. When the internal glass stopped detaching and allowed damage to the building envelope, the window unit suffered a catastrophic failure. However, the four-sided structural glass curtain wall system easily passed the impact, but the glass would peel off to the inside. One of the test samples had a PET film attached to the inner laminate, but this did not prevent peeling.

It should be pointed out that in this series of tests, the main goal is to prevent missiles from penetrating the glass filler; glass shard control is not the main test target. It is worth noting that the application film with pressure sensitive adhesive does not seem to provide enough movement difference to allow the inner glass plane to deflect upon impact and control peeling. The rupture of the applied PET film resulted in uncontrolled glass fragmentation on the test sample where the film was mounted on a protected surface.

Future tests will examine the use of a thicker inner glass layer to reduce breakage, and the application of PVB/PET composite materials on ships to help eliminate spalling. Obviously, the ability of the window system to hold the glass in place is the main factor in resisting missile impact.

Ionoplast interlayer was first developed twenty years ago for impact-resistant glass used in Florida. The hardness of these sandwich products is 100 times that of standard PVB, and the tear resistance is 5 times that of standard PVB. These two characteristics make the mezzanine an ideal choice for protection from severe storms.

The increased stiffness of the intermediate layer results in a harder overall finished laminate. Through effective thickness calculation, it can be shown that with ionoplast interlayer, the effective thickness is closer to the total thickness of the laminate. This additional stiffness increases the ability of the laminate to resist the initial impact of debris.

Once the laminate is impacted and the glass breaks, the next problem is the tearing of the middle layer under pressure cycling. When the laminate is cycled from positive pressure to negative pressure, small cracks in the interlayer will expand into larger cracks, allowing debris to enter the structure. Due to the high shear modulus and tear resistance of ionoplast interlayers, cracks are more difficult to grow under pressure cycles, thereby reducing the probability of failure under such loading conditions.

The wind-borne debris protection building code for hurricane-prone areas requires the development and use of impact systems to protect occupants and buildings from storm-related damage. ASTM test system standards guide manufacturers and serve as the basis for certification systems. Project specifications in tornado-prone areas usually refer to these standards, but the FEMA 361 standard requires a higher level of performance testing and can lead to different system designs. Testing with larger wood at higher speeds will result in heavier laminated glass structures. It has been proven that the hard ionic plastic interlayer can effectively reduce the impact of larger wood. Additional testing is required to eliminate peeling after impact.

[1] AAMA, AAMA 512-11, Voluntary Specification for Tornado Disaster Mitigation Window Opening Products, which can be ordered at http://pubstore.aamanet.org/pubstore/ProductResults.asp?cat=0&src=512 [2] ASTM International, ASTM E1886 -13a, the standard test method for the performance of external windows, curtain walls, doors and impact protection systems affected by missiles and exposed to cyclic differential pressure, can be ordered from http://www.astm.org/ standard/E1886.htm [3] ASTM International, ASTM E1996-14a, Standard Specification for Performance of Exterior Windows, Curtain Walls, Doors, and Impact Protection Systems Affected by Wind-borne Debris in Hurricane, can be ordered from http://www.astm.org/Standards/E1996.htm [4 ] Condon, Pat, and Valerie Block, "Fighting Against Tornadoes", American Glass, Metal and Glass Windows, July 2011. [5] FEMA, Community Refuge Design and Construction Guidelines, July 2000, http://www.rhinovault.com/fema361.htm [6] International Code Committee, IBC 2015: International Bui lding Code, available from http: //shop.iccsafe.org/2015-international-buildingcoder.html [7] International Code Committee, ICC 500-2014: ICC/NSSA storm shelter design and construction standards, available from http://shop.iccsafe.org/ icc-500-2014-icc-nssa-standard-for-thedesign-and-construction-of-storm-shelters-42931.html order

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