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3B.2 the Enhanced Fujita (Ef) Scale

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3B.2 the Enhanced Fujita (Ef) Scale 3B.2 THE ENHANCED FUJITA (EF) SCALE James R. McDonald Gregory S. Forbes Timothy P. Marshall* Texas Tech University The Weather Channel Haag Engineering Co. Lubbock, TX Atlanta, GA Dallas, TX 1. INTRODUCTION Original Fujita (F) Scale Although the Fujita Scale has been in use for 30 No. Wind Speed Damage Description with years, the limitations of the scale are well known to its -1 (mph and ms ) respect to housing users. The primary limitations include a lack of F0 40-72 mph Light damage: Some damage damage indicators, no account of construction quality 18-32 ms-1 to chimneys and variability, and no definitive correlation between F1 73-112 mph Moderate damage: Peel damage and wind speed. These limitations have led 33-50 ms-1 surfaces off roofs; mobile to inconsistent ratings of tornado damage and, in homes pushed off some cases, overestimates of tornado wind speeds. foundations or overturned. F2 113-157 mph Considerable damage: Roofs Thus, there is a need to revisit the concept of the -1 Fujita Scale and to improve and eliminate some of the 51-70 ms torn off framed houses; limitations. mobile homes destroyed. F3 158-206 mph Severe damage: Roofs and Recognizing the need to address these limitations, 71-92 ms-1 some walls torn from well- Texas Tech University (TTU) Wind Science and constructed houses. Engineering (WISE) Center personnel proposed a F4 207-260 mph Devastating damage: Well project to examine the limitations, revise or enhance 93-116 ms-1 constructed houses leveled; the Fujita Scale, and attempt to gain a consensus structure with weak from the meteorological and engineering foundations blown off some communities. A steering committee first was distance. F5 261-318 mph Incredible damage: Strong organized to initiate the project. The next step -1 involved assembling a forum of users to identify the 117-142 ms frame houses lifted from foundation and carried issues and develop strategies to improve the Fujita considerable distances to scale. A panel of wind damage experts met and disintegrate. assigned failure wind speed values to various Table 1. Original Fujita (F) scale with wind speeds degrees of damage (DOD) to buildings and other and damage description with respect to housing (after objects. Through this expert elicitation process, wind Fujita 1971). speeds corresponding to the DOD's were estimated. These estimated wind speeds then determined the EF Fujita also assigned wind speed ranges to the (Enhanced Fujita)-scale category appropriate for the numerical values in the F-scale. Wind speed ranges observed damage. This paper discusses the work to were derived empirically by dividing the gap between date in finalizing the EF-scale. Beaufort 12 (73 mph/33 ms-1) and Mach 1 (738 mph/330 ms-1) into 12 non-linear increments. F-scale 2. THE ORIGINAL F-SCALE wind speeds were defined as the “fastest 1/4-mile speed” being longer in duration than a gust, usually in Fujita (1971, 1973, 1981) developed the F-scale the five to ten second range for most tornadoes. in order to rate tornado damage to buildings. The F- Fujita deemed the assigned wind speeds as scale is a subjective, visual interpretation of damage “experimental” and awaited engineering assessments which simply assigns a numerical value ranging from of tornado damage to help “calibrate” the wind speed 0 to 5 based on increasing severity of damage ranges. Engineering assessments of tornado damage primarily to a “well-constructed” or “strong” wood- by Minor et al. (1977a, 1977b) questioned the framed house. Refer to Table 1. However, as accuracy of the F-scale wind speeds, especially when Grazulis (1993) noted, the single-paragraph they exceeded 125 mph (56 ms-1). However, no descriptions of damage given by Fujita are vague and formal changes to the F-scale have been made prior limited in scope and this can introduce large errors in to the TTU effort. assigning an F-scale number. For example, homes As Doswell and Burgess (1988) pointed out, "swept clean" from their foundations initially were building damage and tornado intensity are related but assigned an F5 damage rating but were downgraded not perfectly correlated. A destroyed building may to F1 as Marshall (2003) noted in the La Plata, MD have been built poorly leading to an overestimate of tornado. tornado intensity. Since tornadoes are rated by the ________ worst damage they cause along their paths, there *Corresponding author address: Timothy P. Marshall, would be a tendency to over rate them unless the 2455 S. McIver Dr., Carrollton, TX 75006, email: strength of the building was known. Additional [email protected] difficulties in rating a tornado occur when it passes over open country and does not cause damage. between degrees of damage, and wind speed and a Schaefer and Galway (1982) found that tornadoes correlation between the original F-scale and the EF- that strike populated areas were more likely to receive scale so the existing tornado database could be a higher F-scale rating than those tornadoes that preserved. At the close of the meeting, each remain in open country. participant was invited to submit written comments Fujita (1992) recognized that residences were not and suggestions. These comments were published in homogeneously constructed and he devised the forum summary report (McDonald and Mehta, corrections to compensate for assigning an F-scale 2001). rating (Table 2). He indicated that a strong-framed The Texas Tech project team was directed to house may receive F2 damage whereas the same develop an Enhanced F-scale that addressed the wind might not damage a concrete building or might limitations and issues identified by the forum totally destroy a poorly built outbuilding. Thus, he participants. They were to explore opportunities for realized that the relative strength of the building must workshops and symposiums to involve a more be considered when assigning an F-scale rating. extensive audience with the goal of obtaining a general consensus. The strategies pursued by the Texas Tech team included the following steps: • Identify additional damage indicators. • Define varying degrees of damage (DOD) for each damage indicator. • Correlate degrees of damage with wind speeds expected to cause the damage. • Propose an EF-scale and relate it to the original F-scale. • Keep both meteorological and engineering communities apprised of the progress. 4. BUILDING DAMAGE INDICATORS (DI's) Buildings and other objects, including towers, light Table 2. The original F-scale shown with corrections poles, trees and crops make up the damage for building strength (after Fujita 1992). indicators (DI's). With each damage indicator, there are increasing degrees of damage (DOD's) caused by Phan and Simiu (1998) found that high-speed higher wind speeds. winds of longer duration resulted in greater damage to The DI's and DOD's are selected to be residences during their damage survey of the Jarrell, recognizable by persons, who have little or no Texas tornado. Residences near the center of the engineering training. Thus, specific uses of buildings, Jarrell tornado were subjected to tornadic winds for structures, and trees are selected for the initial set of about three minutes. By comparison, Marshall (2002) DI's. Additional ones such as crops and missiles can calculated that homes near the tornado center at be added to the DI list later as more information Moore, Oklahoma were subjected to tornadic winds becomes available. for about 30 seconds. Finally, there is the human Twenty-eight DI's with various DOD's are initially factor in determining the tornado intensity based on defined. A few examples include one and two-story analyzing damage. A person with knowledge of how residences, single-wide manufactured homes, small buildings fail will likely rate a building differently than a professional buildings, elementary schools, large person without such knowledge. isolated retail buildings, institutional buildings, free- standing towers, and hardwood trees. A list of all 28 3. ENHANCED (EF) FUJITA SCALE DI's is shown in Table 8 in the appendix. The type of construction for each building is carefully described. A forum of users and interested parties was For example, typical construction for masonry organized by TTU to develop strategies for an apartments or motels is described as: Enhanced Fujita Scale. The forum convened in • Less than or equal to four stories. Grapevine, Texas on March 7-8, 2001. The steering • Facility made up of one or more multi-story committee invited 26 persons; 22 attended the one rectangular buildings. and one-half day meeting. Objectives of the forum • Flat, gable, hip, or mansard roofs. were to identify key issues, make recommendations • for a new or enhanced Fujita Scale, and develop a Asphalt shingles, tile, slate, or BUR roofs. • strategy for reaching a consensus from a broad cross Light steel roof framing with metal deck and section of users. A summary of the initial work on this lightweight insulation. • project was published by McDonald et al. (2003). CMU load bearing and non-load bearing Key issues included the need for additional and walls. • more specific damage indicators, a correlation Stucco, EIFS, or brick veneer wall cladding. • Exterior walkways or balconies. study, it would be simply the team's opinion, just like The number of DOD's for each DI depends on the Fujita's original estimate. A more definitive solution complexity of construction. DOD's range from no was needed. visible damage to total destruction of the entire The concept of expert elicitation has been used building. Table 3 shows an example of the DOD's for successfully to estimate certain unknown parameters masonry apartments or motels. Wind speeds in the related to seismic hazard analysis. The Senior table are discussed in Section 5.
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