A PREDICTIVE MODEL FOR STRUCTURAL TORNADO DAMAGE TO RESIDENTIAL STRUCTURES USING HOUSING DATA FROM THE 2011 JOPLIN, MO TORNADO
By
ALYSSA C EGNEW
A THESIS PRESENTED TO THE UNDERGRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR SUMMA CUM LAUDE IN ENGINEERING
UNIVERSITY OF FLORIDA
2016
© 2016 Alyssa C Egnew
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To my Lord and Savior, Jesus Christ, in whom I can do all things, and to my parents who keep me sane in the meantime.
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ACKNOWLEDGMENTS
I would like to thank Dr. David O. Prevatt for the opportunity to work under his supervision with the UF Wind Hazard Damage Assessment Group, and David Roueche for his indispensable guidance through this research project. I would like to thank Dr. Kurtis Gurley and Dr. Curtis Taylor for agreeing to be on the panel for the thesis defense. I would also like to thank all of those who helped augment the database, specifically Malcolm Ammons, Austin Bouchard, and Jeandona
Doreste. And a final thank you to the UF Center for Undergraduate Research, which funded this investigation through the University Scholars Program.
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TABLE OF CONTENTS
page
ACKNOWLEDGMENTS ...... 4
LIST OF TABLES ...... 6
LIST OF FIGURES ...... 7
LIST OF ABBREVIATIONS ...... 8
ABSTRACT ...... 9
INTRODUCTION ...... 10
BACKGROUND ...... 14
METHODOLOGY ...... 16
RESULTS ...... 20
CONCLUSION ...... 35
FUTURE WORK ...... 36
REFERENCES ...... 37
APPENDIX
A: Metadata on the 488 Homes Used in this Study ...... 40
B: MATLAB R2015 Script Used to Generate Graphs of Parameters ...... 49
C: MATLAB R2015 Script Used to Generate Multinomial Logistic Regression with Random Input ...... 51
D: MATLAB R2015 Script Used to Generate Multinomial Logistic Regression with Significant Parameters Only ...... 52
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LIST OF TABLES
Table page
1 EF-Scale Wind Speed Ranges ...... 15
2 Degree of Damage Description ...... 16
3 Building Code History of Joplin, Missouri ...... 26
4 Y-Intercepts and Coefficients from the Proportional Odds Model ...... 31
5 Testing the Proportional Odds Likelihood of Damage Output ...... 32
6 Comparison of Findings to Literature ...... 33
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LIST OF FIGURES
Figure page
1 Screenshot of the Beacon GIS Site ...... 17
2 Example of Geo-Tagged Survey Imagery ...... 18
3 Screenshot of Google Street View® imagery ...... 18
4 Square Footage Histogram ...... 21
5 Appraised Value Histogram ...... 22
6 Year Built Histogram ...... 22
7 Number of Homes with One Story Versus Two Stories ...... 23
8 Distance from Tornado Centerline Histogram ...... 23
9 Degree of Damage Bar Graph ...... 24
10 Comparison of Year Built Between the Sample Set and the Entire Population ...... 25
11 Percentage of Homes Built Under Each New Building Code Adoption ...... 27
12 Linear Regression of Degree of Damage and Number of Stories ...... 28
13 Linear Regression of DOD and Wind Speed ...... 28
14 Linear Regression of DOD and Appraised Value ...... 29
15 Linear Regression of DOD and Square Footage ...... 29
16 Linear Regression of DOD and Year Built ...... 30
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LIST OF ABBREVIATIONS
EF-Rating Enhanced Fujita scale rating of a tornado’s severity; used when estimating wind speeds based on damage to a particular Damage Indicator
DOD-rating Degree of Damage rating for a particular structure type relating its level of structural damage to a wind speed range
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Abstract of Thesis Presented to the Undergraduate School of the University of Florida in Partial Fulfillment of the Requirements for Summa Cum Laude in Engineering
A PREDICTIVE MODEL FOR STRUCTURAL TORNADO DAMAGE TO RESIDENTIAL STRUCTURES USING HOUSING DATA FROM THE 2011 JOPLIN, MO TORNADO
By
Alyssa C Egnew
March 2016
Chair: David O. Prevatt Major: Civil Engineering
On May 22, 2011, an EF-5 tornado struck Joplin, Missouri, leaving 158 fatalities and over 1000 injuries in its aftermath (National Weather Service, 2011, para. 1-2). The damaged residential structures were surveyed by a reconnaissance team under Dr. David O. Prevatt just days later. This investigation used the extracted empirical data from the tornado path to find trends in the damage for future applications. In order to use the collected information, the database of damaged homes first needed to be augmented, one by one, to contain street addresses and parcel IDs that could be cross referenced with the Jasper County Tax Assessor’s database of house characteristics. In this way, the year built, appraised value, square footage, and number of stories for each home was obtained for a final sample size of 488 homes. The maximum wind speed encountered by each home was assigned using each home’s distance from the tornado centerline (from the geo-tagged imagery taken by the reconnaissance team) and the near-surface wind model of the Joplin tornado developed by Lombardo, Roueche, and Prevatt (2015). To see how each of these parameters influences the expected level of damage in a home, a multinomial logistic regression (“proportional odds model”) with an ordinal response was developed for this dataset. A multiple linear regression could not have been used because of the non-continuous categories of degree of damage (as described by McDonald, Mehta, and Mani, 2004). Hence, a proportional odds model best fit the data type and allowed for the varying wind speed with distance from the tornado centerline to be more accurately accounted for. The output of this model is the odds of the expected level of damage being less than or equal to a reference damage level, as compared to greater than that reference level. According to the model, the number of stories in a home was rejected as being a significant predictor of damage level. However, an increase in year built, wind speed, and square footage all result in greater odds of having a higher state of damage. On the other hand, an increase in appraised value results in lower odds of having a higher state of damage. The most influential parameter on damage was the one with the opposite trend, appraised value. These results can be further investigated regarding the underlying correlations to other parameters; perhaps the appraised value of the house is significant because of better maintenance, which allows it to perform better in a tornado than a house with decay. This proportional odds model, when coupled with other related studies, would be useful for city officials to forecast weak residential areas in their cities.
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Introduction
Tornadoes are one of the most potent natural disasters experienced in the United States. From
1993 to 2012, tornadoes accounted for 36% of insured catastrophe losses in the United States, which is more than winter storms, terrorism, geological events, wind, hail, fires, and floods combined (The
Property Claims Services, 2014). "A total of 1,625 tornadoes occurred in the United States in 2011, resulting in economic losses that exceeded $25 billion” (Prevatt et al., 2012, p. 254). Tornadoes in
Tuscaloosa, Alabama, on April 27 and Joplin, Missouri, on May 22 caused more than half of those
2011 losses, with a combined effect of 223 fatalities and more than 13,000 damaged buildings
(Prevatt et al., 2012). Such significant damage and loss of human life merits consideration of whether the current building codes, which do not consider tornadoes in their construction requirements, are appropriate for tornado-prone regions without further strengthening. Regardless of any future advances in building codes, however, there exists a legacy of nearly 132 million homes in the US that are highly vulnerable to tornado damage. It is important for community decision makers to be aware of this vulnerability, and what portions of their community are most vulnerable, so that they can appropriately allocate mitigation and first response resources.
In order to pinpoint vulnerable areas, though, an understanding of tornadoes and the risks associated with them is needed. There has been a significant amount of research dedicated to this; as early as 1897, there have been studies on the wind pressures of tornadoes and the direct need for wind bracing in taller structures (Baier, 1897). The focus in the mid-twentieth century turned to tornado mechanics studies (e.g. wind velocities, pressure differentials, etc.) (Melaragno, 1968). In recent decades, researchers have been trying to model the risk of tornado outbreak during any time of the year for anywhere in the contiguous United States (Brooks, Doswell, & Kay, 2003), what the recurring patterns are in supercell storm tracks (Hocker & Basara, 2008), and what the strengths and
10 weaknesses are of in-home storm shelters (Masters, Gurley, & Prevatt, 2014). Moreover, many studies in this field incorporate mathematical models to increase understanding of trends in tornado outbreaks (Boruff et al., 2003; Hocker & Basara, 2008; Schaefer et al., 1986; Schaefer et al., 2002).
Other research has been dedicated to improving near-surface tornado wind models by incorporating tree-fall data (Lombardo, Roueche, & Prevatt, 2015).
Additionally, there have been efforts to analyze the spatial and temporal trends of tornado hazards using GIS technology (Boruff et al., 2003) and to adjust paradigms on the tornado rating system (Schaefer, Schneider, & Kay, 2002). The Fujita Scale, which became the official tornado rating system in the early 1970s, generally over-estimated wind speeds, and was replaced in 2007 by the Enhanced Fujita (EF) Scale as described by McDonald, Mehta, and Mani (2004), and provides more accurate wind estimations (Doswell, Brooks, & Dotzek, 2009, pp. 555-556). Despite these improvements, though, the tornado rating system is still subjective because of its use of damage to estimate wind speeds. There is no way to account for whether or not the building code of the time was enforced or if the building had preexisting damages that made it susceptible even to lower wind speeds.
In addition to understanding the mechanics behind tornadoes and their path characteristics, an understanding of structural damage patterns in tornadoes is necessary to locate regions of particular vulnerability. For example, wind tunnel tests on finite element models of homes are being used to predict their damage potential and stress distribution, which will reveal the sequence and type of failures within the homes (Kumar, Dayal, & Sarkar, 2012). Roueche and Prevatt identified seven common failure mechanisms due to tornado wind hazards in residential structures and established correlations between them (2013, p. 5). De Silva, Kruse, and Wang have investigated the
“debris effect” that occurs during a tornado outbreak when debris is picked up and turned into a
11 missile, creating further damage (2008, p. 318). Damage surveys after major tornado outbreaks have become increasingly common, as well. One such survey in Joplin, Missouri was conducted by the
National Institute of Standards and Technology ([NIST], 2013). Their report reconstructs the proceedings of the tornado outbreak and responses to advance warnings using a collection of photos, videos, building plans, wind models, and interviews. The report also describes the wind pressures and fields of the tornado, the damage associated with flying debris, the emergency communication systems in place and how people responded, the factors affecting survival and injury, and the need for safety code revisions. In doing so, they present recommendations that range from new methods for tornado-resistant design of buildings, to improved measurement and characterization of tornado hazards, and improved emergency communication systems.
Because tornado strength decreases with distance from the tornado centerline, improved construction techniques could help reduce damage on the peripheries of even an EF-5 tornado
(Sutter, DeSilva, & Kruse, 2009, p. 114). Sutter et al. investigated this application to tornadoes by performing a benefit-cost analysis of retrofitting measures (such as hurricane clips, anchor bolts, and directional nailing), and concluded that it “may be cost effective” in mitigating damage for states with a high-risk of tornadoes (2009, pp. 113, 119). A more recent benefit-cost analysis corroborated these findings; the benefits of adopting wind-resistant building codes was shown to outweigh the costs by a factor of 3 to 1, showing that improved construction and even retrofitting measures can help mitigate tornado damage (Simmons, Kovacs, & Kopp, 2015). It has also been shown by the
Federal Emergency Management Agency that structural retrofitting can help reduce the risk of damage to buildings in hurricane prone regions, which further validates their use in other high wind load situations, such as tornadoes ([FEMA], 2010).
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Despite all of this research on structural damage mitigation, there have not been considerations for tornado wind loads in building codes until recently. Previously, the design wind speed was 90 miles per hour, but in 2014, Moore, Oklahoma adopted the first building code for wind speeds of 135 miles per hour (City of Moore, Oklahoma, 2014, p. 1). This code incorporates mitigation techniques based on structural retrofitting for hurricanes (i.e. hurricane clips) to help reduce the risk of structural damage from tornadoes (p. 1).
This mitigation effort can be further narrowed by targeting more specific trends in structural damage. De Silva, Kruse, and Wang’s general spatial model (GSM) identified house characteristics that tended to cause greater loss when present in a structure; specifically, when a house is larger, is single-story (versus multi-story), has a hip-only roof, has a slab foundation (versus a conventional foundation), has any other exterior besides frame hardboard or stucco, or is newer, it is at a greater risk of economic loss (2008, p. 328). They corroborate the claim that newer houses are at a greater risk of loss with Fronstin and Holtmann’s (1994) explanation of lack of code enforcement and deterioration of construction quality in recent decades. Thus, for a tornado outbreak, it has been shown that certain house characteristics are associated with greater economic loss, and that structural damage is also caused by air-born debris, not just direct tornado impact. There is no quantized correlation between house characteristics and structural damage, though, particularly with respect to distance from the tornado centerline. This would certainly be a useful tool for city officials to identify vulnerable areas in their region.
Hence, this research was conducted to determine the extent to which building construction characteristics correlate to the damage in the residential structures struck by the 2011 Joplin,
Missouri tornado. In this way, future mitigation efforts, building codes, and construction methods can target these features of tornado damage, and awareness can be raised about which existing
13 homes are most at risk. Furthermore, any government official can use the findings from this investigation to pinpoint vulnerable homes in their jurisdiction, since the basic parameters (year built, square footage, number of stories, and appraised value) are readily accessible through any tax assessor’s database. Other house characteristics, such as home orientation with respect to the tornado path and roof construction, are useful to engineers, but are not readily available for city officials; these other characteristics are therefore not considered in this investigation, since their use would require additional time and money on the part of city officials. However, additional data regarding wind speed was readily available for this set of 488 homes, and is included in this investigation.
Thus, the scope of this research is limited to how year built, square footage, appraised value, maximum encountered wind speed, and number of stories influence the expected level of tornado damage.
Background
The tornado that struck Joplin, Missouri on May 22, 2011 was rated as an EF-5 tornado, and caused 158 fatalities and over 1000 injuries; the damage path of this tornado was approximately six miles long and three-quarters of a mile in width within Joplin (NWS, 2011, para. 1-2). Of the 7500 residential structures that were damaged, about 4000 were destroyed, which left an estimated 9200 people displaced (Onstot, 2013, p.2).
From May 30 to June 1, 2011, a reconnaissance team headed by Dr. David O. Prevatt from the University of Florida, and funded by the American Society of Civil Engineers and the Structural
Engineering Institute, set out to survey the damage (Prevatt et al., 2011). The survey team documented the damage to 1,394 structures using 8,750 geo-tagged photographs (p. 12). A database of these damaged structures was created with reference to their geo-tagged imagery, the coordinates of which were used to determine each home’s distance from the centerline of the tornado. The
14 maximum wind speed was determined for each home using its distance from the tornado centerline in conjunction with the near-surface wind model of the Joplin tornado from Lombardo, Roueche, and Prevatt (2015). The number of stories each home had was also previously retrieved from the Tax
Assessor database (Jasper County Tax Assessor Office, n.d.).
The database also includes the manually-assigned damage level for each home. In this investigation, the damage levels are based on the Enhanced Fujita (EF) scale and Degree of Damage
(DOD) scale for Damage Indicator 2, “One- or Two-Family Residences” (FR12), as described by
McDonald, Mehta, and Mani (2004, pp. 7, 8, 11), and are shown in Tables 1 and 2 below.
Table 1: EF classification scale used when estimating wind speeds based on damage to a particular Damage Indicator EF-Scale Wind Speed Ranges EF Classes 3-Second Gust Speed, mph EF0 65-85 EF1 86-110 EF2 111-135 EF3 136-165 EF4 166-200 EF5 >200
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Table 2: Degrees of Damage (DOD) descriptions for one- or two-family residences, with expected (EXP), lower bound (LB), and upper bound (UB) wind speeds (in mph) for each DOD
DOD* Damage description EXP LB UB 1 Threshold of visible damage 65 53 80 2 Loss of roof covering material (<20%), gutters 79 63 97 and/or awning; loss of vinyl or metal siding 3 Broken glass in doors and windows 96 79 114 4 Uplift of roof deck and loss of significant roof 97 81 116 covering material (>20%); collapse of chimney; garage doors collapse inward or outward; failure of porch or carport 5 Entire house shifts off foundation 121 103 141 6 Large sections of roof structure removed; most 122 104 142 walls remain standing 7 Top floor exterior walls collapsed 132 113 153 8 Most interior walls of top story collapsed 148 128 173 9 Most walls collapsed in bottom floor, except small 152 127 178 interior rooms 10 Total destruction of entire building 170 142 198 *DOD is degree of damage
Methodology
Once the database of damaged structures included EF-ratings and DODs, other parameters were needed to describe them. The database from the ASCE damage survey had to be augmented to contain specific house characteristics that could be analyzed for trends with respect to the damage.
To get the house characteristics, the addresses of the houses first needed to be assigned. To do so, the GPS coordinates for a house from the damage survey would be searched for in Google Maps® to find the general area in which the house is located. Then, the street that the house is on would be pinpointed in the “Beacon” GIS site for Jasper County, Missouri, as demonstrated in Figure 1. The
“Beacon” account created for the research team granted access to aerial imagery of the tornado damage and a layer on the map showing the location where each geo-tagged photo of a damaged home was taken during the ASCE survey.
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Figure 1: Screenshot of the Beacon GIS site used to find the address of a home surveyed by the ASCE damage assessment team; each geo-tagged photo location is demarked by a yellow triangle as shown
The GPS coordinates do not always line up directly with a specific home, so Pictometry
Online®, Zillow.com, and Google Street View® were used to confirm the address of the home by cross referencing the landmarks from the survey imagery with the online imagery (Figures 2 and 3).
The address and parcel ID of the home would then be obtained from the “Beacon” GIS site and cross-referenced with the Jasper County Tax Assessor’s database of construction characteristics
(Jasper County Tax Assessor Office, n.d.). The four house characteristics that are readily available in any tax assessor’s database are number of stories, year built, square footage, and appraised value.
These were assigned to each house in our set to determine if they are reliable predictors of tornado damage. This entire process is lengthy, though, so due to time constraints, only 488 houses were assigned street addresses and were cross-referenced with the county’s database to obtain their construction characteristics.
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Figure 2: The geo-tagged, survey imagery of a home that was taken by the ASCE damage assessment team
Figure 3: Screenshot of the Google Street View® imagery used to confirm the address of the home in Figure 2
To analyze these parameters’ effects on damage, linear regressions were created for each parameter independently (for example, Area vs DOD) using Minitab17. However, a more rigorous statistical approach was needed to take into account the varying wind speeds with distance from the tornado centerline and the non-continuous nature of DOD categories.
A multinomial logistic regression with an ordinal response was therefore developed to more adequately test whether or not these parameters influence the damage level and in what way. Linear regressions could not account for the categorical nature of DOD, but multinomial logistic regressions
18 can. This method takes into account the change in wind speed with distance from the tornado centerline (something difficult to achieve when just grouping wind speeds into broad ranges based on distance from the centerline). The ordinal response type takes into account the DOD not being a continuous scale, but having a clear progression to its categories (a DOD of 9 has more damage than a DOD of 8) (The MathWorks, Inc., 2016). This multinomial logistic regression is different than a simple linear regression, where the output is the actual DOD and shows how much the parameter influences the damage directly. Here, the multinomial logistic regression gives an output that is a number. This number is the odds of the DOD for a given home being less than or equal to a given reference DOD; the output number is not the DOD itself, but rather the odds of it being less than or equal to that reference DOD.
Using MATLAB R2015, the model was created using mnrfit (see Appendices B and C for the scripts). In addition to providing p-values to test the significance of each parameter, this mnrfit gives an equation for each DOD-rating that is a “proportional odds model” for likelihood of damage.
It is considered a “proportional odds model,” in that the equations have the same slope, but are just offset by different y-intercepts (The MathWorks, Inc., 2016). The model produces one less number of intercepts than number of input categories, because there is no category beyond the greatest DOD- value; hence, the 9 categories of DOD require only 8 intercept values (and accordingly only 8 equations), since you cannot have odds comparing the highest category to nonexistent higher categories of DOD.
The form of the proportional odds model equation based off of the generated parameters is as follows: