Disaster Investigation Report: Tornado and Severe Storm Damage in Tennessee, November 10, 2002 Behavior of Brick Veneer Structures

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1 Disaster Investigation Report: and Severe Storm Damage in Tennessee, November 10, 2002 Behavior of Brick Veneer Structures Jim Bryja 1 and Richard M. Bennett 2 On Sunday, November 10, 2002, a huge system of devastating tornados and severe storms swept through the Mid-South and Eastern United States including Alabama, Tennessee, and Ohio. These storms resulted in wide spread damage of property and the deaths of at least 35 people. This report focuses on masonry structure behavior and damage in east Tennessee as a result of these storms. A historical perspective is given, and the results of the inspection of two brick veneer residential structures in Mossy Grove, TN are documented. Additional information is provided on the behavior of brick veneer homes in Coffee County, TN. Due to the satisfactory performance of many brick veneer structures, the authors do not feel that any code change is required. DESCRIPTION OF STORM In Tennessee, two waves of storms and tornadoes caused deaths, injuries, and damage affecting virtually the entire state from western Tipton County to Eastern Anderson County. The first wave of storms and tornadoes struck in the evening of Saturday, November 9, 2002 and the morning of Sunday, November 10, The storms affected areas from west to middle Tennessee including Tipton, Madison, Crockett, Gibson, Carroll, Montgomery, Robertson, and Sumner counties. The second wave of storms on Sunday, November 10, 2002 occurred between approximately 8 to 11 PM and affected areas from middle to east Tennessee, including Coffee, Warren, Van Buren, Cumberland, DeKalb, Fentress, Morgan, and Anderson counties. A total of at least 16 deaths were reported in the storms and tornadoes that affected the state of Tennessee. The National Weather Service has rated the tornadoes that touched down in Tennessee as F1, F2, and F3 rankings on the Fujita Damage Scale (Table 1). In general, wind speeds of over 140 mph (225 km/h) were reported throughout the state. The insured damage in the state was approximately $170 million. A total of seven tornadoes were reported across the northern Cumberland Plateau region, with six being in east Tennessee. The tornadoes were the result of strong winds that increased and changed direction with height as well as the strong difference between the unseasonably warm temperatures at the surface and colder temperatures aloft. A summary of the six East Tennessee tornadoes is given in Table 2, with their paths being shown in Figure 1. The seventh tornado in the Cumberland Plateau region was an F3 tornado in Cumberland County which was responsible for killing four people, two in a modular home Table 1. Fujita Intensity Scale (1 mph = 1.61km/h) F scale Description Wind Speeds Expected Damage F0 F1 F2 F3 F4 F5 F6 Gale Moderate Significant Severe Devastating Incredible Inconceivable 1 General Shale Brick 2 The University of Tennessee mph Light damage. Broken braches and some damage to chimneys. Shallow-rooted trees uprooted mph Moderate damage. Hurricane-speed winds peel off roofs. Cars, garages and mobile homes overturned mph Considerable damage. Roofs torn from frame houses; mobile homes destroyed; large trees snapped or uprooted mph Severe damage. Roofs and some walls torn off well-constructed houses; trains overturned; most trees uprooted; heavy cars thrown mph Devastating damage. Well-constructed houses leveled; structures with weak foundations blown some distance; cars thrown. Incredible damage. Strong frame houses carried considerable distances mph to disintegrate; trees debarked; steel-reinforced concrete structures badly damaged mph Unlikely to occur. Winds leave small area of unrecognizable landscape. TMS Journal September

2 Table 2. Summary of November 10, 2002 es in East Tennessee (1 mile = 1.61 km, 1 yd = m) Location Time (EST) Path Length Path Width mile (km) yd (m) Rating 1 Northern Morgan County, SW of Rugby 6:35 PM 1.9 (3.1) 50 (46) F0 2 Just north of Elgin in Scott County 6:40 PM 4.1 (6.6) 100 (91) F1 3 Highway 63 near Winona 6:50 PM 4.2 (6.8) 100 (91) F1 4 Morgan County; from Mossy Grove to :30 PM 8.3 (13.4) Joyner and Petros ( ) F3 5 Briceville, Anderson County 9:00 PM 5.5 (8.8) 200 (180) F2 6 North of Bellview; Bledsoe County 9:30 PM 6.2 (10.0) 100 (91) F1 Source: National Weather Service, Morristown, TN Station Figure 1 Map of November 10 th Tracks Across East Tennessee (Source: National Weather Service, Morristown Station) and another couple in a mobile home. The tornado touched down about 5 miles south of Crossville, TN, and had a path length of 8.2 miles (13.2 km) and a path width of a half-mile (0.8 km). The National Weather Service (Nashville office) reported a brick home on Highway 127 lost a roof and other well constructed homes in the same area sustained roof damage. This investigation concentrated on damage in areas of Morgan County, Tennessee, which was one of the hardest affected areas in the state. Morgan County is approximately 40 miles (64 km) west of Knoxville, Tennessee. A total of 7 people died and 28 were injured in Morgan County. The communities affected in Morgan County included Wartburg, Mossy Grove, Petros, and Joyner. A summary of causalities and damage in Morgan County is given in Table 3. An initial tornado warning was issued for Morgan County at 6:26 PM. A weak tornado touched down in northern Morgan County just southwest of Rugby. The tornado was rated as an F0. The tornado downed a number of trees but no building damage was reported. A second tornado warning was issued for Morgan County at 8:12 PM. The tornado was reported to have touched down just west of Mossy Grove around 8:30 PM. The tornado continued on an east-northeast path to Joyner and Petros, hitting them about 8:40 PM. A detailed path of the Morgan County tornado is shown in Figure 2. TORNADO RISK IN EAST TENNESSEE Summary data on tornadoes in the state of Tennessee as recorded by the National Weather Service Storm Prediction Center are given in Table 4 for the years TMS Journal September 2004

3 Table 3. Summary of Casualities and Property Damage in Morgan County, TN Due to November 10, 2002 es Description Number Causalities Confirmed Dead 7 Injuries (23 hospitalized, 4 critical) 28 Damage Houses Destroyed 24 Houses with Major Damage 17 Houses with Moderate Damage 20 Houses with Minor Damage 29 Mobile Homes Destroyed 12 Mobile Homes with Major Damage 3 Mobile Homes with Moderate Damage 5 Mobile Homes with Minor Damage 10 Public Buildings Destroyed 1 Public Buildings Damaged 1 Businesses Damaged 4 Motor Vehicles Damaged or Destroyed 100 Outbuildings Destroyed 40 Barns Destroyed 4 Source: Tennessee Emergency Management Agency For the years , the Storm Prediction Center reports a higher average of 20 tornadoes in Tennessee per year. Many of the recorded tornadoes in Tennessee are in the western portion of the state, where the topography is quite flat. Few tornadoes occur in the eastern valley and ridge province. The following paragraphs give a summary of the three previous recorded severe tornado outbreaks that have hit East Tennessee based on reports in the Knoxville News-Sentinel. Category Per Year State Rank Number of es Fatalities Injuries Damage (CPI adjusted) $5.1 million 25 Source: National Weather Service Storm Prediction Center Table 4. Historical Data for Tennessee, A tornado struck southwest Knox County at about 7:30 PM on April 15, 1965, cutting a swath through the Blue Grass-Concord area. One person was hospitalized in critical condition. Damage was estimated to be $1 million. The paper reported a concrete-block house was destroyed. A church which appeared to be constructed of unreinforced masonry was also destroyed. Five sections of metal roof were blown off of a brick masonry bearing wall elementary school. The masonry was undamaged, and the school is still in use. There was a severe outbreak of tornadoes on April 3, 1974 that caused damage in ten southern and midwestern states. There were 46 tornadoes reported in Tennessee and a total of 52 deaths in the state. In Knox County, two infant twins were killed when a mobile home was destroyed. At least 21 people were injured, and there were millions of dollars of property damage. A newspaper map showed a tornado touching down in Morgan County, but there was no indication of the damage. In counties west of Morgan County, there were 10 fatalities in Putnam County and 3 fatalities in DeKalb County, while in Fentress County to the northwest there were 7 fatalities. Figure 2 Detailed Map of November 10 th Track Across Morgan County (Source: Tennessee Emergency Management Agency) TMS Journal September

4 Eight tornadoes struck East Tennessee on the evening of February 21, One of the tornadoes hit Lenoir City in Loudon County (about 20 miles (32 km) west of Knoxville). The National Weather Service estimated this tornado as an F3 tornado. There was one fatality and 55 injuries, with damage covering a 25 block area of Lenoir City. Another tornado struck the Powell area in north Knoxville. Eleven houses in the Powell area were destroyed and 75 homes were damaged. It was estimated that there was $1.1 million worth of damage to residential property and $300 thousand damage to commercial property. GENERAL OVERVIEW OF DAMAGE IN MORGAN COUNTY As expected with tornadoes the damage observed was consistent with very violent forces, Figure 3. Many structures were completely destroyed down to the foundation level, Figure 4. Some of these structures appeared to be conventional stick framed masonry veneer residences, Figure 5. The path of damage was also very erratic. In some instances homes with little or no apparent damage were standing within yards of homes that were virtually leveled, Figure 6. At one location a mobile home standing on concrete blocks was left unscathed within 30 yards of a stick frame built home which was leveled down to the slab foundation, Figure 7. In some cases the major structural damage was not directly the result of wind pressures, but was due to large hardwood trees falling onto the building. In other cases significant damage and shearing of the uppermost limbs and branches of large hardwood trees was observed, yet stick framed homes standing next to the trees were left standing with relatively minor damage. Again the path of destruction was very erratic with discontinuities and changes in the path noted both vertically and horizontally. Figure 4 Overview of Damage in Morgan County, TN Figure 5 Damaged Brick Veneer House In Morgan County, TN Figure 3 Violent Nature of Forces (Picture from Morgan County, TN) Figure 6 An Undamaged House Just Up The Hill From A House That Was Completely Destroyed In Morgan County, Tn 114 TMS Journal September 2004

5 Figure 7 Undamaged Mobile Home Supported On Concrete Blocks Near Completely Destroyed Stick Frame House In Morgan County, TN Structural damage was not always the major cause of loss. Homes that appeared to suffer minor structural damage were deemed to be unsalvageable by insurance companies because of water damage. BRICK VENEER HOMES We had the opportunity to observe some failures of anchorage of brick masonry veneer to wood framed residential structures. In one instance the failure appeared to be due to wind pressure on the leeward side of the building. In another home the main cause of structural failure was due to a large hardwood tree falling onto the structure. The homes are discussed in detail in the following. We also observed a brick veneer home that was heavily damaged, but the veneer appeared to be intact, Figure 8. Brick Veneer Home 1 This brick veneer home was located in Mossy Grove and constructed with Norman bricks. For the most part, there was no damage to the brick veneer, Figure 9. A Figure 8 Heavily Damaged Home In Mossy Grove, TN With Undamaged Brick Veneer corner had fallen off and remained largely intact on the ground, Figure 10. There were only six 28 gauge corrugated ties that were visible in the corner section. The ties were concentrated near the top of one face, were all in the same bed joint, and were attached to the backup with 1 1 /4 inch (32 mm) roofing nails. Four of the ties were in one wall and failed through nail pullout. The two ties in the perpendicular wall failed in shear at the point of attachment to the backing. Clearly the house was subjected to a severe wind. However, for the most part the veneer remained attached. The failed portion obviously did not have the number or distribution of ties that would be required by current codes. The failed corner covered an area of approximately 40 ft 2 (3.72 mm 2 ). Each wall tie was thus supporting an area of approximately twice the International Residential Code [IRC (2000)] allowable tributary area of 3 1 / 4 sq. ft (0.302 mm 2 ). This, compounded with all the ties being concentrated in one location, led to very little support of the veneer. Since most of the veneer remained attached, we were unable to determine the number and spacing of the ties for the rest of the wall. However, since the inadequately attached Ties Figure 9 Damaged Brick Veneer House with Failure in One Front Corner Figure 10 Failed Corner of Brick Veneer TMS Journal September

6 Tree limbs Figure 11 Brick Veneer Home Damaged From Large Oak Tree Falling on Front Right Corner Figure 12 Damaged Corner of Brick Veneer Home (Note Portion of Tree Still on Roof) corner was the only area of significant veneer failure, it is our opinion that the veneer would have remained intact if there had been proper nails and correct spacing of the ties. Also of interest in this structure was the condition of the building envelope. East Tennessee is a comparatively humid climate with the mean annual relative humidity being approximately 70%. It is estimated that this house was at least 30 years old. There were no signs, though, of any corrosion or deterioration of the ties. The building envelope inside the cavity was also in excellent condition with no signs of any related moisture problems or mold. It appears that the typical building envelope and durability requirements used in residential brick veneer construction are very adequate. The cavity is functioning as designed and kept moisture out of the structure. Brick Veneer Home 2 The second home was also located in Mossy Grove, TN and was primarily damaged from a large, 2-3 ft ( m) diameter, oak tree falling on the corner of the house, Figure 11 and 12. Some of the brick veneer was knocked off the house, Figure 13. The brick veneer had also pulled away from other sections of the front wall, Figure 14 and 15. Like the previous house, corrugated 28 gauge ties were used and attached to the backup with 1 1 /4 inch roofing nails. The ties were doubled over and both ends embedded in the bed joint. The failure was always due to pull-out of the roofing nail. The tie anchorage into the masonry did not appear to fail. A close-up of a failed tie is shown in Figure 16. A small portion of the wall with an embedded tie, Figure 17, was brought back and tested in The University of Tennessee structural engineering laboratory. The brick was clamped to a Tinius-Olsen universal testing machine and the nail was pulled in direct tension. The nail pulled through the tie at a load of 250 lb (1,110 N). Figure 18 Ties Figure 13 Failed Brick Veneer Figure 14 Brick Veneer Pulling Away From Front Wall 116 TMS Journal September 2004

7 shows the tie after nail pull through. One side of the head of the nail had bent as a result of the test. The tie was then gripped in the machine and pulled in direct tension. The tie pulled out of the brick at a load of 270 lb (1,200 N). Figure 19 shows a section through the wall, and the bed joint crack that had formed as a result of the tie pullout. Figures 20 and 21 respectively show the tie and the brick joint surfaces after testing. Inspection revealed that the tie had been embedded about 2 in. (51 mm) ( into the bed joint, leaving about 1.5 in. (38 mm) of cover. As it is unknown how much the brick and tie were damaged prior to testing, the results from this limited testing should be used with caution. Figure 15 Close-Up of Brick Veneer that Had Pulled Away From Front Wall. Note Close Spacing of Ties Around Door Opening This brick veneer wall for the most part appeared to be well constructed. There were full bed joints, and the head joints were about 75% full. The ties were properly embedded into the masonry, exceeding the Masonry Standards Joint Committee (2002) code required Tie Figure 16 Failed Tie in Brick Veneer Wall Figure 18 Tie After Nail Pull Through Bed Joint Crack Figure 17 Portion of Failed Brick Wall Used for Testing Figure 19 End of Brick After Testing (Note Bed Joint Crack That is Wider on Inside Face) TMS Journal September

8 Location of tie Figure 20 Brick Veneer Tie After Testing embedment depth of 1.5 in. (38 mm). There were extra ties around the door, which exceeded the International Residential Code (2000) and Masonry Standards Joint Committee (2002) spacing requirement of 3 ft (0.9 m). for ties around openings. There were not enough intact failed portions to exactly determine the tie spacing in the wall. It appeared the ties were in every 8 th bed joint, or about every 21 in. (530 mm) vertically. Horizontal spacing could not be determined. The tie spacing thus may not have quite met the Masonry Standards Joint Committee (2002) code requirement of a vertical spacing of 18 inches. The fastenings were 1 1 /4 in. (32 mm) roofing nails instead of the code required 8d common nail. The calculated tensile withdrawal resistance of a nail using the Load and Resistance Factor wood design code [AF&PA/ASCE (1995)] is proportional to the nail diameter and nail length. An 8d nail is twice as long as a 1 1 /4 roofing nail and 9% greater in diameter. Thus, the 1 1 /4 roofing nail had less than half the calculated withdrawal resistance of an 8d common nail, which undoubtedly contributed to the pullout. Figure 21 Brick Joint Surfaces After Testing Figure 22 Brick Veneer Wall On Heavily Damaged Structure In Coffee County, TN The age of this house was estimated to be between 30 and 50 years old. As with the first house, the ties and building envelope were in excellent condition. The approximate 1.5 in. (38 mm) of mortar cover on the ties was adequate to prevent corrosion of the ties. BRICK VENEER STRUCTURES IN COFFEE COUNTY, TENNESSEE An F2 tornado struck Coffee County, TN, which is about halfway between Nashville and Chattanooga. The path length of the tornado was about 5 miles (8 km) and the path width was one-quarter mile (0.4 km). The authors did not visit this site but obtained access to a number of photographs of some of the damage. Figure 22 shows a commercial structure and Figure 23 shows a residential structure in Coffee County that had substantial structural damage, but little damage to the brick veneer. Figure 24 shows a residential structure in which the front overhanging porch roof was blown off. This caused the loss of the top few courses of brick veneer in this area, but the rest of the brick veneer remained intact. Figure 25 shows a brick veneer residential structure constructed in 1999 that lost the carport roof. Figure 26 shows the shifting of the porch columns for this structure. Figure 23 Heavily Damaged Brick Veneer Residential Structure In Coffee County, TN Figure 24 Heavily Damaged Brick Veneer Residential Structure in Coffee County, TN 118 TMS Journal September 2004

9 Figure 25 Brick Veneer Structure with Failed Carport in Coffee County, TN Although there was this localized damage, the brick veneer on the house itself was undamaged. Based on typical construction at that time, the veneer ties were probably attached to the backing with 8d nails. Figure 27 shows a brick veneer house constructed in the 1970 s that was damaged by the tornado. What was interesting about this house is that it appears that a tie failed by pulling out of the brick, Figure 28. The ties probably did not meet current spacing requirements. LESSONS LEARNED We believe that three major lessons can be learned from the tornadoes that struck East Tennessee. These lessons relate to natural hazards risk, wall envelope construction, and wall ties. As a casual observation it appears it would be economically impractical to design a residential structure to withstand the forces of a tornado event. A more practical solution for protection against tornados may be the use of safe rooms in residential construction. Natural Hazards Risk East Tennessee does not experience many tornadoes, with the recent Morgan County tornado being one of the most devastating tornadoes to strike the area. The area is not a part of tornado alley and the valleys and ridges tend to minimize the tornado risk. However, there have Figure 26 Shifting of Porch Columns in Brick Veneer Residential Structure been a few previous tornados that have caused damage, injuries, and fatalities. East Tennessee is considered to be in a moderate seismic region, with the International Residential Code placing much of East Tennessee is Seismic Design Category D 1. However, in over 100 years of recorded history in Knox County, there has been no structural damage or deaths from an earthquake. Reported injuries have been limited to two minor injuries. Some chimneys have fallen over, windows have been broken, things have fallen off of shelves, pictures have fallen off of walls, but the earthquakes have really caused more panic than actual damage. Thus tornadoes, even though considered a small risk, have caused much more damage in East Tennessee than earthquakes. This does not include damage from wind other than tornadoes. For example, a tornado that struck Knox County on May 2, 1953 did not cause any injuries or death, and was not even reported in the newspaper. Two days later, a wind outburst hit the municipal airport Failed Tie Figure 27 Damaged Brick Veneer Structure in Coffee County, TN Figure 28 Tie Failure In Brick Veneer Residential Structure TMS Journal September

10 and made the front-page news. About 15 planes were damaged and part of a block wall of a hangar was blown over. As building codes continue to be refined, certainly seismic risks need to be considered and to be a part of the design process. However, it must be remembered that there are other natural hazards that the structure will be exposed to. The development of seismic regulations needs to consider these other risks, and the impact any code requirement would have on the vulnerability of the structure to other natural and man-made hazards. The mass of brick veneer that is often considered detrimental in earthquakes aids in providing wind and fire resistance. A multi-hazard design approach is the only logical approach. Wall Envelope Construction The wall envelope of the brick veneer construction was shown to perform exceptionally well with regard to durability. The damage enabled us to see the wall ties and the air space cavity after many years of weather exposure. No deterioration or corrosion was found. There were also no signs of any other moisture related problems. A tie that was pulled out of the bed joint showed the approximate 1.5 in. (38 mm) cover was adequate to prevent corrosion of the tie. These structures verified the excellent longterm performance of properly constructed brick veneer systems. Wall Tie Performance For the most part the wall ties performed adequately and held the veneer intact under extreme wind loadings. There were examples of heavily damaged structures with satisfactorily performing brick veneer in both Morgan and Coffee Counties. One of the failures in Morgan County could be attributed to both an inadequate number, inadequate distribution of the ties, and use of inadequate nails. All tie failures that were observed in Morgan County were due to nail pullout, although it appeared that at least one tie failed in Coffee County by pulling out of the brick. The ties observed in Morgan County were attached with 1 1 /4 in. (32 mm) roofing nails. In talking with locals, many felt that this was the proper construction, and that the brick veneer was well built. Thus, there is a need for education as to proper fastening of brick veneer ties to the backup in residential construction. Due to the satisfactory performance of many brick veneer structures, the authors do not feel that any code change is required. ACKNOWLEDGEMENTS The authors wish to extend their regrets to the families who suffered losses in the storms. They would also like to thank the people who allowed them access to their property and those who took the time out from their cleanup efforts to talk with them. Donnie Redmond of General Shale Brick provided photos of the damage in Coffee County that the authors were unable to visit. Jerry Williams of Signode provided some additional pictures of Morgan County and helped with the historical background of veneer construction in that area. The Council for Masonry Research (CMR) and The Masonry Society (TMS) provided help in coordinating the inspections. The authors appreciate the interest and encouragement of Phillip Samblanet, Executive Director of The Masonry Society, and Arturo Schultz, TMS Investigating Disasters Project Team Leader. REFERENCES AF&PA/ASCE, Standard for Load and Resistance Factor Design (LRFD) for Engineered Wood Construction. American Society of Civil Engineers Standard 16-95, IRC, International Residential Code for One- and Two- Family Dwellings. International Code Council, Masonry Standards Joint Committee, Building Code Requirements for Masonry Structures, ACI /ASCE 5-02/TMS , American Concrete Institute (ACI), Farmington Hills, MI, American Society of Civil Engineers (ASCE), Reston, VA, The Masonry Society, Boulder, CO, TMS Journal September 2004