Final Report Tornado Damage Investigation Greensburg, Kansas 1699 DR-KS December 2007 U.S. Department of Homeland Security 500 C Street, SW Washington, DC 20472 This document was prepared by URS Group, Inc. 200 Orchard Ridge Drive, Suite 101 Gaithersburg, MD 20878 Section 1 ONE Introduction 1-1 1.1 Description of the Event 1-1 1.2 Purpose of Report 1-1 1.3 Description of the Work 4 Section 2 TWO Study Methodology and Site Visit 2-1 2.1 Study Methodology 2-1 2.2 Field Observations 2-1 Section 3 THREE Results 3-1 3.1 Application of the EF Rating Scale to Observed Damage 3-1 3.2 Structural Analysis Results for Four Buildings 3-1 3.2.1 Elementary School 3-1 3.2.2 Church 3-3 3.2.3 Hospital 3-4 3.2.4 John Deere Building 3-5 3.3 Mapped Results 3-6 Section 4 FOUR Conclusions 4-1 Section 5 FIVE References 5-1 Section 6 SIX Study Team 6-1 Section 7 SEVEN Acronyms 7-1 Tables Table 1. Example of EF Scale Table: DOD Scale for One- and Two-Family Residences 1-2 Table 2. Results of Greensburg Building Classification by DOD 3-1 Figures Figure 1: Example of Residential Structure with DOD 2 – Structure 32 1-3 Figure 2: Example of Residential Structure with DOD 3 – Structure 18 1-3 Figure 3: Example of Residential Structure with DOD 4 – Structure 15 1-3 Figure 4: Example of Residential Structure with DOD 6 – Structure 8 1-3 Figure 5: Greensburg, KS, after May 4, 2007 Tornado Event 2-2 Figure 6: Example of Roof Covering that Survived the Tornadic Winds 2-2 Figure 7: PEMB Elementary School Auditorium 3-2 Figure 8: Bolt Failure 3-2 Figure 9: Laminated Roof Beam Failure 3-3 Figure 10: Glulam Beam from Church 3-3 Figure 11: Hospital with Precast Concrete Twin Tees 3-4 Figure 12: PEMB for the John Deere Building 3-5 Figure 13: Wind Speed Map for Greensburg, KS Tornado 3-7 Appendices Appendix A Data Collected by PBA Architects, May 21, 2007 Site Visit Appendix B Detailed Calculations of Damage Classification Appendix C Greensburg KS Tornado Photographs 1. Section 1 ONE Introduction 1.1 DESCRIPTION OF THE EVENT On the evening of May 4, 2007, “supercell” thunderstorms formed across portions of the Midwestern United States, spawning tornadoes in several States. According to the National Weather Service, an intense supercell developed southwest of Greensburg, KS that evening, resulting in the formation of 12 tornadoes. One of these tornadoes formed in northwest Comanche County at approximately 9:00 pm and moved northeastward through Kiowa County. At approximately 9:45 pm, this tornado reached Greensburg, KS, a small city of approximately 1,400 people. The tornado traversed the city traveling from its southern edge to its northwest border. The tornado had a reported damage path that was 1.7 miles across and the funnel cloud itself was estimated to be 1 mile across. The tornado destroyed or severely damaged many of the buildings in Greensburg and caused the deaths of 10 people. The death toll in Greensburg might have been higher had it not been for a tornado warning issued by the National Weather Service 20 minutes before the tornado reached the city, giving the residents of Greensburg time to take refuge. 1.2 PURPOSE OF REPORT The purpose of this report is two-fold: to provide a preliminary “ground-truth” of the new Enhanced Fujita (EF) Scale wind speed classification system1 and to document the damage caused by the tornado. The tornado that struck Greensburg was the first EF5 tornado occurrence since the EF Scale was implemented and thus provided an opportunity to compare the wind speeds derived through use of the EF Scale (observed damage) with wind speeds calculated through material failure analysis. Comparing the newly implemented EF Scale to a material failure analysis provides a data set that helps to build a body of knowledge about the accuracy of the scale. The EF Scale uses observed Degrees of Damage (DOD) to derive wind speed ranges, which are then used to classify tornados. The report prepared by Texas Tech University on the EF Scale includes tables showing DOD for 23 different building types and provides wind speed ranges for each of the DOD. The ranges list lower-bound, upper-bound, and expected wind speed (see Table 1). All of the wind speeds are presented as 3-second gusts2 to match with the American Society of Civil Engineers (ASCE) 7-05, Minimum Design Loads for Buildings and Other Structures engineering loads standard. To use the EF Scale, the degree of observed damage to buildings is compared to the DOD for the appropriate building type, thereby yielding a probable wind speed range. The table below shows an example of the EF Scale tables. The range of wind speeds is a function of variations in the wind resistance of specific buildings (due to design, construction, and maintenance variations) and uncertainty in the wind speed necessary to cause a specific type of damage. For reference, Figures 1 through 4 show residential structures with damage levels of DOD 2, 3, 4, and 6 (locations are shown in Figure 13). Table 1. Example of EF Scale Table: DOD Scale for One- and Two-Family Residences Degree of Damage (DOD) Damage Description Expected wind speed (Exp) Lower-bound wind speed (LB) Upper-bound wind speed (UP) Wind speed values in miles per hour (mph) 1 Threshold of visible damage 65 53 80 2 Loss of roof covering material (<20%), gutter and/or awning; loss of vinyl or metal siding 79 63 97 3 Broken glass in doors and windows 96 79 114 4 Uplift of roof deck and loss of significant roof covering material (>20%); collapse of chimney; garage doors collapse inward or outward; failure of porch or carport 97 81 116 5 Entire house shifts off foundation 121 103 141 6 Large sections of roof structure removed; most walls remain standing 122 104 142 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 interior rooms 152 127 178 10 Total destruction of entire building 170 142 198 Source: A Recommendation for an Enhanced Fujita Scale submitted to the National Weather Service by the Wind Science and Engineering Center at Texas Tech University on January 26, 2006 and revised in October 2006. Figure 1: Example of Residential Structure with DOD 2 – Structure 32 Figure 2: Example of Residential Structure with DOD 3 – Structure 18 Figure 3: Example of Residential Structure with DOD 4 – Structure 15 Figure 4: Example of Residential Structure with DOD 6 – Structure 8 1.3 DESCRIPTION OF THE WORK A site visit to Greenburg was conducted on May 10-11, 2007, with FEMA, contractor structural engineers, and consultants. A second site visit took place on May 21, 2007. The site investigations were coordinated with FEMA Mitigation representatives, Jim Donley of FEMA Region VII and Chris Hudson of FEMA Headquarters. The consultants were tasked with assessing the damage in Greensburg and classifying the tornado using the Enhanced Fujita tornado damage scale. The methods used in conducting this work are described in the section titled Study Methodology. No attempt was made to compare the tornado wind speeds experienced in Greensburg with any building code design wind speeds. Most of the buildings in Greensburg were either designed and constructed to older building codes, such as the Uniform Building Code (UBC) or no building codes, although a few newer buildings may have been designed and constructed in accordance with the International Building Code (IBC). Regardless of the code used (IBC or older code such as the UBC), there are currently no design guidelines in the IBC for catastrophic wind events of the magnitude experienced by Greensburg (although there are high wind provisions in the building code for hurricane-prone areas). Consequently, a building code comparison was judged to not provide any useful information. The work also included developing three recovery advisories that the community, or any community that is impacted by a tornado, can use as reconstruction guidance. One tornado risk advisory was developed to provide the community with a better idea of how the risk of tornadoes in Greensburg might compare with other areas of the country and how that risk might affect decision-making about shelter locations, design, and construction. A second tornado risk advisory was written to provide storm shelter design criteria describing the design concepts important for tornado events. The third tornado risk advisory was written to provide residential sheltering guidance to residents who want to create a safe refuge inside their home. These advisories are available on FEMA’s website3. 2. Section 2 TWO Study Methodology and Site Visit 2.1 STUDY METHODOLOGY In order to assess the damage in Greensburg, two site visits were conducted (May 10-11 and May 21). The observations made during the site visits were used for two purposes. First, the observations of 43 residential structures were used to rate the damage according to the EF Scale and thereby derive estimated wind speeds (see Section 3.1). The estimated wind speeds were then mapped (Section 3.3). In addition to these 43 residential structures, additional DOD assessments were made in the field; these assessments included damage to seven non-residential structures and trees in the tornado path. Second, specific damage information was collected at four sites to perform structural analysis and evaluate failure stresses in selected materials. These buildings were selected where all or a portion of a building was damaged or destroyed, where the damage had not been disturbed, and where, in the Study Team’s opinion, there was a possibility of back-calculating the pressures causing the failures to determine the approximate wind speed Detailed information was collected for: the elementary school auditorium (a pre-engineered metal building [PEMB]), the John Deere PEMB, the precast concrete tee roof failure at the hospital, and the glulam beam failures in the First United Methodist Church. Attempts were made to secure building drawings for the PEMBs during the field assessment, but the Study Team was not able to obtain these drawings. After performing a materials analysis to calculate the wind speeds required to cause the observed failures (Section 3.2), the calculated wind speeds were mapped and compared to the wind speeds derived from the EF Scale assessment (Section 3.3). 2.2 FIELD OBSERVATIONS 1. The primary site observations were obtained from an initial study of overhead imagery and initial site inspections conducted on May 10-11, 2007: a. A five to six block swath through the center of the city received the most severe damage. The damage observed was progressively less severe in the two to four blocks on either side of this swath. See Figure 5 for an aerial view of Greensburg after the event and Figure 13 for a map of the damage swath. b. Many of the residential buildings were observed to have basements and some of these buildings were shifted on their basement foundations. c. The roofs of many of the buildings located near the edge of the storm’s path had roof coverings that either completely or partially survived the tornadic winds (see Figure 6). d. Most residential and commercial buildings in the city were older than current model building codes. Exceptions were the John Deere Building built in the mid-1990s and the elementary school auditorium PEMB erected in 2002 (see Figure 13 for locations). The residential and commercial buildings observed during the site visit did not have any specific design features that would have been intended to protect building occupants from the effects of this tornado except for basements or below ground areas and any small interior room, such as a closet or bathroom, used as a best available refuge area. No “tornado shelters,” designed to resist the wind and debris associated with a tornado, were identified during this effort. Figure 5: Greensburg, KS, after May 4, 2007 Tornado Event Figure 6: Example of Roof Covering that Survived the Tornadic Winds 2. Corey Schultz of PBA Architects (a subcontractor located in Wichita, KS) conducted a second site visit on May 21, 2007, specifically to evaluate roof covering damage. The data collected from this site visit is included in Appendix A. a. Forty-six buildings were visited (see Figure 13 for building locations). b. Forty-three photos were taken to document the damage and were used to determine the DOD for those buildings. a. 3. Section 3 THREE Results Forty-three buildings were assessed and plotted using the EF Scale as described in Section 3.1. The four buildings subject to structural analysis are described in detail in Section 3.2 and the results of the calculated wind speed for these buildings are compared with the wind speeds derived by applying the EF Scale. A map showing both sets of wind speeds is included in Section 3.3 as Figure 13. The tornado was rated an EF5 with an estimated wind speed greater than 200 miles per hour (mph). 3.1 APPLICATION OF THE EF RATING SCALE TO OBSERVED DAMAGE Based on a review of the EF Scale and the damage levels associated with the 23 various building types, the Study Team plotted the Greensburg damage according to the EF Scale wind speeds, as close as possible, to delineate wind speeds and associated degree of damage. Information from the field visits, described in Section 2, was developed initially by driving around the city, taking photos of damage, and attempting to classify the damage as minor, moderate, major, or destroyed. The Study Team initially attempted to map this damage by plotting, in block-size elements, the type of damage that occurred by block; however, there were many places where there was a gradation of the primary damage level. The second site visit (May 21, 2007) gave the Study Team another opportunity to classify the damage. Forty-three buildings were classified by DOD so these buildings could be used to help determine wind speeds. The DOD and the associated EF Scale rating are shown in Table 2 below. The analysis and resulting building damage classification by EF Scale is presented on a map of Greensburg (refer to Figure 13 in Section 3.3). Table 2. Results of Greensburg Building Classification by DOD DOD Number of houses Expected Wind Speed (mph) EF Scale Rating 2 15 79 EF0 3 7 96 EF1 4 14 97 EF1 6 7 122 EF2 Note: Three structures inspected did not have an associated photo, so identifying the DOD was not possible. 3.2 STRUCTURAL ANALYSIS RESULTS FOR FOUR BUILDINGS For the four sites studied in detail, the Study Team used data on the ultimate strengths of the materials that failed to determine wind pressures, and thus infer the wind speeds that led to the observed damage. Calculated wind speeds for building damage or failures are shown on Figure 13. Detailed calculations are included in Appendix B. 3.2.1 Elementary School Auditorium The elementary school auditorium is a PEMB located on the southeast edge of Greensburg. Based on ground observation, this is the side of the city that received the strongest tornado winds coming from the east. The storm track through the city was from south to northwest and with counterclockwise winds of the storm, the winds at this site must have approached from the east. In addition, other nearby school buildings had brick walls that fell to the west, consistent with winds approaching from the east. Figure 7 shows the school auditorium blown over and leaning to the west, consistent with the wind coming from the east. Materials Analysis A key damage observation was the failure at a column base plate where all four steel bolts sheared off. This observation was judged to be useful in determining the wind speed that caused the building collapse. The bolt failure appeared to be a combination of twisting and shearing/tensile failure similar to that illustrated in Figure 8 (example shown had three bolts sheared off). Figure 7: PEMB Elementary School Auditorium The bolts were assumed to be A307 and are 5/8 inch in diameter (American Institute of Steel Construction, 1998). The ultimate tensile load required to snap the bolt was determined to be 25 kips per square inch (ksi)4 or 7,670 lbs for a 5/8-inch diameter bolt. The required uplift (creating bolt tension) would be 30,680 lbs (4 bolts at 7,670 lbs each) for the column attached to this base plate. The tributary area for this column was 30 feet (height of wall) times 20 feet (column spacing). Figure 8: Bolt Failure The wind pressure (q) required for this force is 30,680 lbs/600 square foot (sf) = 51.1 pounds per square foot (psf). The wind speed (velocity or V) required to develop 51.1 psf of force using the external pressure coefficients5 noted below could have ranged from 134 mph to 330 mph. The Study Team judged the combination of external and internal pressure coefficients that equal 1.0 fairly represent what the physical evidence suggested. The use of external and internal pressure coefficients that sum to 1.0 results in the velocity being approximately 153 mph (3-second gust) when the velocity pressure equation is solved for V (Velocity) and when q = 51.1 psf. Depending on how the building really failed, the roof coefficient (from ASCE 7-05) could be -0.9 and the wall coefficients could be +0.8 to -0.5. The internal pressure coefficients were assumed to be +/- 0.55 for a partially enclosed condition. A gust effect factor of 0.85 was used. There are many things that could have happened to shear four bolts in a column base plate. Based on the evidence, in the most probable scenario, the building could start to collapse due to wind pressure and then the weight of the building would push the building over, pulling the base plate from the concrete and shearing off the four bolts. Comparison of Calculated Wind Speed with EF Scale Wind Speed The calculated wind speed of 153 mph represents a DOD 8 for a metal building system (damage indicator/building type number 21) in the new EF Scale. The DOD 8 is described as total destruction of the building resulting from an expected wind speed of 155 mph. The Greensburg wind speed result for this elementary school auditorium PEMB provides one data point on the DOD 8 scale that corresponds with the field damage, as represented by the four failed column base plate bolts. 3.2.2 Church Figure 9: Laminated Roof Beam Failure The First United Methodist Church is located on the southwest side of Greensburg in an area that appears to have experienced mostly EF0-EF2 wind speeds (a range of 65-137 mph, see Figure 13) based on its ranking using the EF Scale (Section 3.1). The church roof was framed with 3.25-inch wide by 11.5-inch deep laminated timber beams. These beams were bolted into steel saddles on each side of the church sanctuary. The saddles were connected to concrete girders shown in Figure 9. Materials Analysis The failure mode appears to be roof uplift. Uplift forces acting on the roof appear to have separated the laminated beams from the steel saddles as illustrated in Figure 10. As the laminated beam separated from the steel saddles, the original bolt holes became slots at this failure point. The bolt slots that are ripped in the beam are 4¼-inch long. There appeared to be some deterioration in the end of the beam that would likely have reduced the shear strength parallel to the grain of the beam. Figure 10: Glulam Beam from Church The shear design value from the National Design Specification for Wood Construction (American National Standards Institute/American Forest & Paper Association, 1997) is 200-250 pounds per square inch (psi). The failure shear value might be approximately the same as the design value given the condition of the end of the beam. The calculated area of each slot failure is 13.32 square inches (sq in), or 27.62 sq in for the two slots. It is likely the wind started lifting up on the roof and while the roof failure might have been complete and total within seconds, the failure could have occurred as a result of a series of events. As the roof lifted, the bolt slots are likely to have failed some, giving the bolts room to move and thus creating further stress on the wood, eventually allowing the wood beam to pull away from the bolts that secured the wood beam to the steel saddles. If the bolt slot was lengthened to only 1 inch, instead of the full 4.25 inches, and this constituted failure, the force required to pull the wood beam up 1 inch is 1 inch x 3.25 inches x 200 psi = 650 pounds (lbs) x 2 bolts = 1,300 lbs. The uplift force had to overcome the weight of the wood beam, which was approximately 227 lbs, so the total uplift force required was 1527 lbs. The beams were spaced 4 feet on center and spanned 20 feet. There was a 3-foot overhang on each edge of the roof. The tributary area for the beam receiving the uplift pressure was approximately 64 sf. The uplift pressure was 1,527 lbs/64 sf = 23.2 psf. Assuming roof external pressure coefficients of -0.9 and a partially enclosed condition, the wind speed calculated from these conditions is 91 mph. The roof cover material was not investigated, but the weight of normal asphalt shingle roof surfaces is usually 5-10 lbs/sf. The uplift wind pressure for the roof system including this roof covering weight would require a wind speed that is in the range of an EF0 to EF2 tornado. Comparison of Calculated Wind Speed with EF Scale Wind Speed There is no building type in the EF Scale that represents a large, high ceiling timber structure. Using the EF Scale, the 91 mph wind speed lies within the range of wind speeds for an EF1 tornadic wind. The church is located within an EF1 wind swath according to the analysis described in Section 3.1. Figure 11: Hospital with Precast Concrete Twin Tees 3.2.3 Hospital The hospital is located north of Garfield Street and between Grove and Walnut Streets. The hospital roof was flat and framed with precast concrete double tees 40 inches wide and spanning 34 feet with 3-foot overhangs on each end (Figure 11). The precast concrete tee was calculated to weigh 9,979 lbs. There was an additional 15 psf of roof covering (roof felts and gravel ballast) on the roof at the time of the tornado strike. The precast concrete tee was not attached to the wall; it was only resting on the wall and held in place by gravity. The failure mode was the precast tee lifted off the hospital walls and was moved 80 feet from its original location on the roof. It is uncertain how the concrete tee reached this location. It could have tumbled across the roof or been picked up and thrown. Materials Analysis The wind pressure required just to move the concrete roof tee (with its roof covering) was 90 psf. If the pressure coefficients are assumed to be those used for Components & Cladding and the edge pressures are increased for the flat roof system, a 147-mph wind speed would be required to create the suction forces large enough to lift the tee off the roof. As a result of this HMTAP task order work, data was collected and presented as a case study in a new FEMA publication, FEMA 577- Design Guide for Improving Hospital Safety in Earthquakes, Floods and High Winds: Providing Protection to People and Buildings (2007). Comparison of Calculated Wind Speed with EF Scale Wind Speed The EF Scale damage indicator/building type for institutional uses, such as a hospital, is 20. The calculated wind speed of 147 mph required to lift off the precast concrete twin roof tee corresponds with DOD 8, the DOD representing uplift of the slabs. A DOD 8 has an expected wind speed of 142 mph, with a lower bound of 119 mph and an upper bound of 163 mph. These wind speeds represent a tornadic wind rated EF3. There was a new PEMB at the hospital that was also damaged (described in a case study in FEMA 577). The DOD for the damage observed at that PEMB building indicated an expected wind speed of 155 mph. These results are consistent with the other wind speed determinations for the hospital. 3.2.4 John Deere Building The John Deere Building is located on the northwest side of Greensburg near the western edge of the tornado path. The building is a PEMB and was damaged by winds that came from the west. The primary damage was to the steel roof purlins. Most of the structural framing remained intact as illustrated in Figure 12. The failure mode was the purlins failed by buckling as the roof sheathing was torn off. This failure mode was a good determinant of the wind pressure as a DOD damage state is roof purlin buckling on PEMBs. Materials Analysis Figure 12: PEMB for the John Deere Building The purlins were 8-inch Z channels spaced 5 feet on center and 16 feet long. The Moment of Inertia (I) about the y-axis for the purlins was calculated to be 0.281 in4 and a critical buckling load of 2,181 lbs was determined from the buckling formula (Pcr = ?2EI/L2). The slenderness ratio of this purlin does not appear to qualify it as either a compact or non-compact section in conformance with AISC specifications; the buckling formula above considers slenderness with the inclusion of Moment of Inertia about the axis that buckles and the unbraced length L of the purlin. The wind pressure required to create a 2,181-lb buckling load is 27.3 psf. The wind speed required to develop this pressure is 122 mph. Comparison of Calculated Wind Speed with EF Scale Wind Speed From the EF Scale for metal building systems (damage indicator/building type number 21) in the new EF Scale, DOD 5 is for buckling of roof purlins with an expected speed of 118 mph. The calculated speed of 122 mph agrees with the EF Scale DOD value. The 122 mph wind speed represents a tornadic wind rated an EF2. 3.3 MAPPED RESULTS Figure 13 presents the wind speed map for the Greensburg, KS tornado. This map represents the combined result of the many observations involved in deriving wind speeds using the EF Scale and the wind speeds calculated from the materials analysis performed on four buildings. The width of the actual swaths of various wind speeds may have varied from those represented. The data collected and the variability of a tornado make a more precise determination difficult and of little value. This report validates the estimated wind speeds resulting from use of the new EF Scale DODs. This is the first effort the Study Team is aware of in which the DODs have been validated since the implementation of the new EF Scale. The 43 buildings surveyed for damage and used to estimate the wind speeds based on DOD are shown on the map (Figure 13). Where possible, the specific DOD for the building is shown. The results of the calculated wind speeds for the four sites where detailed structural analysis was performed are also shown on Figure 13. The wind speeds calculated from structural analysis were consistent with the wind speed swaths derived from the DOD information. During the site visits, damage to other infrastructure (i.e., power station, courthouse, high school, fire station, etc.) was observed and these facilities are identified on Figure 13. Calculations of estimated wind speeds that could have caused the damage were not performed for these buildings or vegetation. Figure 13: Wind Speed Map for Greensburg, KS Tornado 4. Section 4 FOUR Conclusions From the observed damage and the building-specific failures noted in the report, the EF Scale DOD determinations correspond well with the associated calculated wind speeds. The mapping of the approximate wind speeds can illustrate wide fluctuations for tornados because of the severity of and rapidly changing wind speeds within the storm. There have been few opportunities to gather this type of information about tornado damage since the revision of the Fujita Scale, and these data points help build the body of information required to verify the EF study findings. While the examples of failures from this event provide limited specific data points for calibrating the new EF Scale, similar comparison studies should continue to build on these results. These studies would serve to validate the speeds associated with the DODs, or would serve to suggest that some of the DOD speeds need to be adjusted. Specific recommendations for the EF Scale based on this study: * Consideration should be given to adding two additional damage indicators/building type numbers to the EF Scale. One would be old load-bearing masonry buildings (of the type built in the early part of the 20th century). There were many of these types of buildings in Greensburg and there are many throughout the Midwest. The other damage indicator/building type to add would be timber frame buildings similar to the First United Methodist Church sanctuary. Timber is very different from other materials and subject to wide variability in response to extreme loads due to the condition and age of the timber. DOD scales for both of these building types would help fill in gaps in the development of probable wind speeds after tornadoes. * Testing of the methodology used in this study is encouraged. There are many ways building failures can occur and the failure modes selected and studied from this event for the four specific cases are only some of several ways the failures could have occurred. Different hypotheses can be developed for each failure case. These different hypotheses and failure modes should be debated and studied in order to further assess the DOD and EF Scale ratings. Other findings arising from this study: * This study supports the findings (discussed after the May 1999 tornado outbreak in Oklahoma and Kansas) that there are wind speed bands on the outer edges of even the deadliest tornadoes in which buildings and occupants can survive if those buildings are built to current high wind design building codes. * This event confirmed that the tornado debris field is filled with many types of missiles (wood studs, steel stairs, pieces of steel tanks, bricks, tree limbs, automobiles, farm equipment, etc.). The current shelter test missile is a 15-lb, 2-foot x 4-foot wood stud. The Study Team recommends additional study on how debris fields in this suburban-like environment affect building performance since multiple missiles in the momentum range of the current test missile could strike a building, causing a breach in the envelope and thus potentially injuring or killing building occupants. * Future studies may be able to make use of DOD and failure analysis information to develop ways to improve specific building element performance in a tornadic event or to study other types of building materials or connections that might improve building performance. This type of study might lead to better utilization of existing building space for ‘area of last resort’ situations. Wind designs developed for building code wind speeds in hurricane-prone areas (for most of the country this is a Category 3 hurricane on the Saffir-Simpson scale) would provide protection for nearly all tornadoes classified EF3 and lower. In tornado-prone areas, the high wind design needs to be accompanied by windborne debris protection because of the numerous high speed missiles generated by a tornado event. Adoption of these measures would provide protection for most tornado events in the country. The primary protection from tornado events, however, should be a shelter?either in the residence or a nearby community shelter. Buildings built specifically to withstand the extreme winds of violent tornadoes and provide protection from wind-borne missiles will provide the most protection. Given that Greensburg, KS is located in ‘Tornado Alley’, and that the history of the area is replete with many examples of tornadoes, shelters built specifically for tornado events should be a significant part of rebuilding this community. There was a surprising lack of designated shelter space in this community, especially in light of the fact they are in a very active tornado region of the country. 5. Section 5 FIVE References American Institute of Steel Construction Manual of Steel Construction, Load & Resistance Factor Design, Second Edition, 1998 American National Standards Institute/American Forest & Paper Association, National Design Specification for Wood Construction, 1997 ASCE 7-05, Minimum Design Loads for Buildings and Other Structures, American Society of Civil Engineers, 2005 FEMA 320, Taking Shelter From a Storm: Building a Safe Room Inside Your House, Federal Emergency Management Agency, 1998 FEMA 361, Design and Construction Guidance for Community Shelters, Federal emergency Management Agency, 2000 FEMA 577, Design Guide for Improving Hospital Safety in Earthquakes, Floods and High Winds: Providing Protection to People and Buildings, 2007 6. Section 6 SIX Study Team Adrienne Sheldon, P.E. URS Thomas Smith, AIA,TLSmith Consulting Corey Schultz, AIA, PBA Architects Bill Coulbourne, P.E.URS Report review provided by: Scott Tezak, P.E., URS 7. Section 7 SEVEN Acronyms ASCE American Society of Civil Engineers DOD Degree of Damage E Modulus of Elasticity (psi) EF Enhanced Fujita I Moment of Inertia (in4) IBC International Building Code kip kilopound ksi kips per square inch lbs pounds mph miles per hour MWFRS Main Wind Force Resisting System PEMB pre-engineered metal building psf pounds per square foot psi pounds per square inch q velocity wind pressure (psf) sf square foot sq in square inch UBC Uniform Building Code V Velocity Appendix A Data Collected by PBA Architects, May 21, 2007 Site Visit Appendix B Detailed Calculations of Damage Classification Appendix C Greensburg KS Tornado Photographs 1 See the Enhanced Fujita Scale paper developed by the Wind Engineering Research Center of Texas Tech University, in cooperation with the U.S. Department of Commerce and the National Institute of Standards and Technology (NIST), and submitted to the National Weather Service in October 2006. This rating scale was the basis for the EF Scale rating that was implemented by the NWS as of February 2007 for classifying tornadoes and replaces the existing Fujita Scale. 2 Unless otherwise noted, all wind speeds stated in this report are 3-second gust wind speeds at 33 feet above grade for Exposure C. This design wind exposure category is drawn from Minimum Design Loads for Buildings and Other Structures, ASCE/SEI 7-05. Areas in the Exposure C category have open terrain with scattered obstructions generally less than 30 feet high. 3 Documents are available from FEMA’s online library (http://www.fema.gov/library/) as Tornado Risks and Hazards in the Midwest United States, FEMA DR-1699-RA1; Storm Shelters: Selecting Design Criteria, FEMA DR-1699-RA2; and Residential Sheltering: In-Residence and Stand-Alone Shelters, FEMA DR-1699-RA3. 4 1 kip = 1000 lbs 5 Coefficients are used to modify a calculated wind and pressure to adjust for roof shape and height response (refer to ASCE 7-05, Chapter 6)