Hailstone and freezer iceball mechanics and damage profiles

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2018-12

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ABSTRACT FOR THE COMPRESSIVE STRENGTH OF HAIL AND DIRECT RELATIONSHIP TO FREEZER ICEBALLS Hail damage to building envelopes is clearly on the rise. This is confirmed by insurance companies reporting that 2017/2018 are the first years that hail damage has been the most expensive insured peril for real property claims in the US - for the first time since such records have been kept. The causes for this unprecedent hail damage cost are many but include, more frequent damaging hailstorms, urban growth that puts more structures at risk, bigger structures that have bigger roofs, more expensive repairs due to higher materials and labor cost and increased public awareness of the insureds rights and responsibilities under the terms of their policy.
This paper addresses the issue of damaging hail events and specifically addresses the compressive strength of hail. Hail impacts have typically been related to size of the hailstones. Some investigators compute the kinetic energy of hail impacts and adjusted the hail falling speed by the wind speed occurring during the hail event. ASTM E822-92 provide numerical methods for adjusting hail falling speed based upon the hail’s terminal velocity and wind speed. Many people have the experience of observing hail in the form of soft slush balls to those that are seemingly as hard as a ball bearing. This presentation will review what we do know about hail compressive strength, what we do not know, and what is being done to close the gap on understanding why some hail events are more damaging than others. Some researchers have investigated the amount of hail kinetic energy required to produce a specified damage profile. Most of the time this is related to hail diameter using freezer iceballs. This work is done without regard to projectile hardness (compressive strength) density, or terminal velocity. Marshall, et al (2012), Crenshaw (2001a, 2001b), Koontz (1991), Greenfield (1969) all reported on the size and some the amount of KE required to exceed the damage threshold of several different cladding materials. None of these authors measured or reported the projectiles weight, density or how the computed the terminal velocity, most of them simple reported the diameter of the iceballs used in their demonstration. This procedure produces a bias result that assumes that freezer iceballs possess the same density, terminal velocity and compressive strength as natural hail. More than 875 natural hailstones were analyzed for size, density, compressive strength (Fo), and compressive stress (σc) by the Insurance Institute of Business and Home Safety (IBHS). These data were analyzed with a variety of methods and analysis shows a clear trend in the increase of compressive strength and compressive strength uniformity with increases in hail diameter by size bin. These data were considered by hail size bins beginning with ½” through 2 ½” in ¼” group bins. These data, Fo and σc were regressed with an exponential equation with R2 = 0.9694 and 0.9516 respectively. The data shows that as diameter increases so does Fo and its uniformity; however, σc will decrease with diameter increase, uniformity of σc increases with increased diameter. The hail Fo and σc population distribution was developed for each hail size bin. The Kinetic Energy (KE) was computed for each hail size bin such that the probability of the hail impacts exceeding a damage threshold were computed. Compressive strength population distributions allowed us to perform a Monte Carlo analysis that in turn allows us to compute the probability of a specific event from the hail compressive strength data set.   ABSTRACT FOR NATURAL HAIL AND FREEZER ICEBALL OPACITY A NON-DESTRUCTIVE TEST METHOD Many aspects of hail have been studied for years, with most of the published literature focusing on the meteorological aspects of hail. Many reports are on ice which in part, infers hail information. Schulson (1999), reports “Over the past ten years alone {since 1999} more than 10,000 papers on ice have appeared in the scientific and engineering literature.” Without an understanding of the physical and engineering properties of freezer ice balls, simulated and natural hails, meaningful understanding of hail compressive strength, strain rate response, and stress strain relationships within and between sets remains largely unknown. Hail occurs in many different locations, climates and storm types. Kinetic Energy (KE) is related to velocity by the square of the term, increases in falling velocity have a dramatic effect on KE and damage potential of hail stones. Opacity is the measurement of light that is refracted or adsorbed and not passing through an object. In other words, opacity is the amount of absorption, scattering or reflected light, electromagnetic or other radiation. In this paper we focus on light from the visible spectrum passing through hailstones and freezer iceballs by measuring the light intensity passing through the hailstones and freezer iceballs (I) and comparing it to the initial light intensity (Io). The percentage passing (I/Io) was measured for each observation. Prior hailstone research that utilized opacity, did so through the examination of thin sections of hailstones and looked at the component parts of the hailstone (ice, liquid water, and air content). This research focus is on hailstones as a single structure in much the same was as a building is comprised of various structural elements. In a structural engineering sense, opacity measurements of thin sections of ice (hailstones) is analogous to load assessments of individual beams and columns within a structure, whereas opacity measurements of the whole hailstone are analogous to load assessments of the whole structures Main Wind Force Resisting System (MWFRS) as described in the American Society of Civil Engineering (ASCE) reference Minimum Design Guide for Buildings and Other Structures (ASCE 7). Since hailstone impacts occur as whole hailstones, it seems reasonable to assess them as whole units. By understanding the compressive strength (Fo) and compressive stress (σc) the stress strain (ε) relationship, and strain rate behavior we can more accurately describe hails damage potential and relationships between manmade and natural hails, and difference with in types and between types of hailstones. With a solid understanding of hail Fo, σc, and strain rate response, we have developed a system that might allow meteorologist to identify hailstone Fo signatures within storms and allow practicing engineers to reliable describe the amount of energy imparted by hailstone impacts.   ABSTRACT FOR FRACTURE MECHANICS AND FINIATE ELEMENT ANALYSIS OF NATURAL HAILSTONES AND FREEZER ICEBALL COMPRESSIVE STRESS RESPONSE RELATIONSHIP The study of the built environment has traditionally focused on construction, materials, methods of assembly, and functionality. In recent years the topics of sustainability, resilience, and energy conservation have expanded the scope of study for the built environment. In most college curriculum’s the scope of study of the built environment is mostly limited to structural elements, how those elements are clad, installed, and protected are frequently left up to the general contractor or even sub-contractors. Incredibly, non-professionals have taken the lead on most building envelop design and resulting performance issues. Building history is full of ruined structures that are structurally recoverable; however, the damage to the exterior cladding and resulting environmental intrusion has ruined the building such that all remains is its structural shell. The built environment is exposed to all the elements of the local environment. Some environments have commonality’s that are a concern for very large geographic areas. One of these commonalities that effect large areas of the US is hail. Annually, hail effects larger land surface areas than tornados, hurricanes, sinkholes and earthquakes combined. From the period 2010 through 2012 property loss insurance claims resulting from hail increased by 84%. The Insurance Institute for Business and Home Safety (IBHS) reports that in 2017 hail was the most expensive insured peril for real property claims. Costlier than tornados, hurricanes or earth quakes. Hail effects all areas where thunderstorms occur. Many aspects of hail have been studied for years, with most of the published literature focusing on the meteorological aspects of hail. Without an understanding of the physical and engineering properties of freezer ice balls, simulated hail, and natural hails, meaningful understanding of hail hardness, strain rate response, or compressive strength, remain unclear. The relationship between natural and manmade hails cannot be quantified without understanding the engineering properties of freezer iceballs and then applying that knowledge to natural hails. Natural hails must be assessed to validate the comparison between hail types and structures. There are about four basic hail structures with dozens of variables that can produce a multitude of different hailstones of different appearance, structure, hardness, and falling kinematics. Some researchers have investigated the amount of hail kinetic energy required to produce a specified damage profile. Most of the time this is related to hail diameter using freezer ice balls. This work is done without regard to projectile hardness. Some investigators have reported that freezer ice balls are harder than natural hail stones and should be regarded as the “worst case scenario”. In a structural engineering sense, opacity measurements of thin sections of ice (hailstones) is analogous to load assessments of individual beams and columns in a structure, whereas opacity measurements of the whole hailstone is analogous to load assessments of the whole structures Main Wind Force Resisting System (MWFRS) as described in the American Society of Civil Engineering (ASCE) reference Minimum Design Guide for Buildings and Other Structures (ASCE 7). Since hailstone impacts occur as a whole, it seems reasonable to assess them as whole units.

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Hail, Iceball, Compressive Strength, Hardness, Damage, Forensic, Insurance, Test, Impact

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