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  • View in gallery

    Damage track of the tornado that struck Oklahoma City and suburbs on 3 May 1999. The tornado had a continuous damage path of 61 km (38 mi) and lasted approximately 80 min. Only a portion of the tornado path actually struck densely populated areas (hatched area). Our damage survey covered the path from A to B. Map adapted from Burgess and Magsig (2000)

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    Typical cross section of exterior wall in a Moore, OK, house, showing building nomenclature and fastener type

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    Unanchored rural house (in background) near Newcastle was moved approximately 90 m (295 ft) to the east. The unanchored house was supported on concrete masonry units

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    Aerial view of Country Place subdivision in southern Oklahoma City where F5 damage was found. Wooden-framed houses were attached to concrete slab foundations with tapered cut nails. The tornado continued northeast through the suburbs of Moore, Del City, and Midwest City

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    Leg of steel folding chair penetrated a solid wooden post. The post partially supported a second-story porch on a house; the house sustained only F0 damage

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    Scrape mark and conical spalled area in the surface of concrete slab foundation indicating where the wall bottom plate along with the tapered cut nail connection were moved laterally. The entire house was swept away, resulting in an F5 damage rating

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    Wall bottom plate found in debris still had tapered cut nail attached

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    Tapered cut nail that remained in the foundation. The wall bottom plate simply pulled through the fastener, leaving the nail

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    Bent shot pins remained in concrete foundation. The wall bottom plate had been broken around the fasteners as the house was destroyed by the tornado

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    Wall stud pulled out, leaving only the strapped bottom plate intact. The wall stud was straight nailed, a connection that is inherently weak in uplift/tension

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    Wall studs pulled out, leaving only the anchor-bolted bottom plate. Local building codes unfortunately still allow straight-nailed connections between the wall plates and studs

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    Typical failure of attached garage when exposed to tornadic winds. The garage door blew in, resulting in displacement of the roof from combined loads of aerodynamic pressure and internal pressure

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    A new house being constructed after the tornado had a metal strap bent outside the base of the exterior wall rather than being nailed to the wooden bottom plate. As a result, the wall was not attached to the foundation. The exposed metal strap was likely hidden later when the brick masonry was installed

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    Perimeter wall plate was fastened to the foundation with tapered cut nails. Nails were spaced between 30 and 130 cm (12 and 51 in.) apart

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    Tapered cut nails that secured wall bottom plates extended only 1.3 cm (0.5 in.) into the foundation. Such nails were used in lieu of anchor bolts or metal straps both in new construction and in houses built prior to the tornado

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    Lack of let-in wall brace in new house being built after the tornado. Note that notches were cut in the wall studs to receive the let-in brace; however, exterior insulation board already had been installed

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    Rafters were fastened with a single nail to the wall top plate in this house under construction after the tornado. The nails were installed too close to the edge of the top plate, splitting the wood. Two nails should have been used at each connection

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Tornado Damage Survey at Moore, Oklahoma

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Abstract

On 4 May 1999, the Wind Science and Engineering Research Center at Texas Tech University dispatched three survey teams to the Oklahoma City area to conduct a tornado damage survey. The author was the leader of one of the teams whose purpose was to survey tornado damage in and around the suburb of Moore, Oklahoma. The survey team was given five tasks: 1) to map out the damage path and assign F-scale numbers to damaged buildings, 2) to document the performance of housing, 3) to interview witnesses, 4) to document projectiles, and 5) to assess the performance of any above- or belowground shelters within the damage path. This paper will present the methodology utilized for conducting the tornado damage survey and will summarize the observations and findings of the survey team. Wind speeds necessary to cause the observed damage to residences were found to be significantly lower than the established F-scale wind speeds. The author returned to the disaster area three months later and discovered that, in general, the quality of new home construction had not improved.

Corresponding author address: Timothy P. Marshall, Haag Engineering Co., P.O. Box 814245, Dallas, TX 75381-4285. Email: timpmarshall@cs.com

Abstract

On 4 May 1999, the Wind Science and Engineering Research Center at Texas Tech University dispatched three survey teams to the Oklahoma City area to conduct a tornado damage survey. The author was the leader of one of the teams whose purpose was to survey tornado damage in and around the suburb of Moore, Oklahoma. The survey team was given five tasks: 1) to map out the damage path and assign F-scale numbers to damaged buildings, 2) to document the performance of housing, 3) to interview witnesses, 4) to document projectiles, and 5) to assess the performance of any above- or belowground shelters within the damage path. This paper will present the methodology utilized for conducting the tornado damage survey and will summarize the observations and findings of the survey team. Wind speeds necessary to cause the observed damage to residences were found to be significantly lower than the established F-scale wind speeds. The author returned to the disaster area three months later and discovered that, in general, the quality of new home construction had not improved.

Corresponding author address: Timothy P. Marshall, Haag Engineering Co., P.O. Box 814245, Dallas, TX 75381-4285. Email: timpmarshall@cs.com

1. Introduction

A tornado outbreak occurred in central Oklahoma during the evening of 3 May 1999. One tornado struck densely populated suburbs of Oklahoma City. Burgess and Magsig (2000) indicated this violent tornado began near the rural community of Amber at 2327 UTC and traveled northeast through mostly rural areas paralleling Interstate Highway 44 before reaching southern sections of Oklahoma City around 0020 UTC. The tornado continued through the city of Moore and crossed Interstate Highway 35 (I-35) around 0030 UTC before turning more northerly and striking Del City and Midwest City before dissipating around 0047 UTC (Fig. 1). The Federal Emergency Management Agency (FEMA 1999a) reported that this single tornado killed 42 people, inflicted 800 injuries, and caused over $1 billion in property damage. The tornado lasted approximately 80 min and left a continuous damage path 61 km (38 mi) long, giving an average translational speed of 33 kt (38 mi h–1). The damage path averaged 400 m (1300 ft) wide through suburban areas.

Within 24 h after the tornado, three survey teams were dispatched to the disaster area by the Wind Science and Engineering Research Center at Texas Tech University. The survey teams focused their attention on the densely populated communities where building damage was most severe. The author's team was given the task of conducting a damage survey from rural Newcastle through the city of Moore to Interstate Highway 240 (I-240) on the east side of Oklahoma City. Our team was asked 1) to map out the damage path and assign F-scale numbers to damaged buildings, 2) to document the performance of housing, 3) to interview witnesses, 4) to document projectiles, and 5) to assess the performance of any above- or belowground shelters within the damage path.

Most of the structures damaged by the tornado were one- and two-story, wooden-framed houses with one- or two-car attached garages. The vast majority of houses were constructed on concrete slab foundations (Fig. 2). Such a large number of houses of similar construction allowed a large sample size by which to assess and to compare building damage. A report summarizing the findings of all three teams was published by Gardner et al. (2000).

2. Damage survey methodology

a. Logistics

A meeting was held in Oklahoma City the day after the tornado to establish the procedures to be followed during the damage survey. Radar images and newspaper accounts were gathered to define better the locations of the damage path. Team members realized it was essential to begin the damage survey as quickly as possible to determine the extent of building damage. Cleanup operations already had begun and had accelerated as fair weather continued for the next several days. Work crews had cleared all primary roads almost immediately and opened secondary roads within a few days after the tornado. However, police and U.S. Army National Guard personnel had cordoned off the damaged areas, and permission was required to enter. Our team proceeded to walk much of the damage path to talk to witnesses and to examine failed building components closely.

An aerial survey by helicopter also was conducted of the tornado damage path. We quickly determined the overall extent of the tornado damage path and noted interesting areas for later study on the ground. Similar methodologies for conducting damage surveys have been described by McDonald and Marshall (1984) and Bunting and Smith (1990).

b. Equipment

It was important to have proper equipment for conducting the damage survey. Detailed road maps were obtained before the damage survey began. Still cameras with both print and slide film were employed to photograph the damage. A wide-angle lens on one camera captured the overall damage scene, whereas a zoom lens on another camera captured specific details. A second camera also served as a backup in case one of the cameras malfunctioned. A notepad and writing pens were brought along for documentation purposes. A tape recorder was utilized to record the locations of the photographs and to record pertinent observations. A tape measure was helpful to determine the distances between objects and to obtain dimensions of building components and projectiles. Business cards and magnetic signs mounted on the survey vehicle provided identification. Hard hats were worn in the disaster areas. A hand-held global positioning system receiver was available to pinpoint ground locations accurately.

c. Use of the F scale

Fujita (1971, 1973, 1981) developed the F scale for rating the degree of wind damage to buildings. The F scale is a subjective, visual interpretation of wind damage ranging from F0 to F5 based on the increasing severity of damage, primarily to a “well-constructed” or “strong” wooden-framed house. Team members assigned an F-scale rating to each damaged house within our study area based on the damage descriptions presented by Fujita. However, as Grazulis (1993) noted, the single-paragraph descriptions of damage given by Fujita are vague and limited in scope. Thus, the author developed additional damage descriptions to aid in assigning F-scale numbers. A house was rated F0 damage if it had lost a few roof shingles, had a downed television antenna, had broken windows, or had a damaged garage door. A house was rated F1 damage if large areas of the roof covering had been removed, if there had been loss of any roof decking, if the gable end had been blown in or out, or if the garage door had failed, causing uplift of the garage roof or collapse of the garage walls. A house was rated F2 damage if most of the roof structure had been removed but perimeter walls remained intact. A house was rated F3 damage if the roof structure and most perimeter walls had been removed, leaving interior walls standing. A house was rated F4 damage if the house structure had collapsed, leaving a pile of debris on the foundation. A house was rated F5 damage if the majority of the house structure and contents had been displaced downwind from the foundation.

Fujita (1971, 1973, 1981) also assigned wind speed ranges to the numerical values in his F scale. Wind speed ranges were derived empirically by dividing the gap between Beaufort 12 (33 m s–1, 74 mi h–1) and Mach 1 (about 330 m s–1, 738 mi h–1) into 12 nonlinear increments. The F-scale wind speeds were defined as the “fastest 1/4-mile speed” being longer in duration than a gust, usually in the 5–10-s range for most tornadoes. The F scale was deemed “experimental” by Fujita, and he awaited engineering assessments of tornado damage to help to calibrate the wind speed ranges. Engineering assessments of tornado damage by Minor et al. (1977) and Minor (1977) questioned the accuracy of the F-scale wind speeds, especially when they exceeded 56 m s–1 (125 mi h–1). Based on their engineering damage assessments, they revised the F-scale wind speed ranges downward. Marshall (1983) utilized load and resistance statistics to demonstrate how uncertainties in assessing building damage can lead to large errors in assigning F-scale ratings, especially in the upper ranges of the F scale.

There are a number of potential problems in determining tornado intensity based on assessing building damage. Most of the tornado damage paths on 3 May 1999 occurred in rural areas in which buildings were spaced relatively far apart. Thus, it was unknown how long the tornadoes remained at a certain intensity level or what the maximum intensity levels might have been in rural areas. As Doswell and Burgess (1988) pointed out, building damage and tornado intensity are related but are not perfectly correlated. A destroyed building may have been built poorly, leading to an overestimate of tornado intensity. Because tornadoes are rated by the worst damage caused, there would be a tendency to overrate tornado intensities unless the relative strengths of the damaged buildings are known. Additional difficulties in rating tornadoes occur when they are in open country and do not cause building damage. Schaefer and Galway (1982) found that tornadoes that strike populated areas are more likely to be assigned a higher F-scale rating than those tornadoes that remain in open country.

Fujita (1992) realized residences were not constructed homogeneously, and he devised corrections to compensate for those differences in assigning F-scale numbers. For example, Fujita indicated that a strong wooden-framed house may sustain F2 damage whereas the same wind might cause F0 damage to a concrete building or F5 damage to a poorly constructed outbuilding. Thus, Fujita believed that relative strengths of buildings must be considered when assigning F-scale numbers. However, some people who utilize the F scale have yet to consider the relative differences in building resistance. This may be due to a lack of understanding in how buildings are constructed and/or confusion in how to apply corrections to the F scale. The large number of similarly built residential structures within our study area fortunately provided more uniformity in which to assign or compare F-scale ratings. Thus, corrections to the F-scale ratings were not employed given that all but a few homes were constructed on concrete slab foundations.

Reynolds (1971) indicated that flying debris is an important factor in the destruction of buildings and that the part played is not always obvious. Our team members found many homes along the edge of the tornado damage path that had been compromised by debris impact. However, a portion of the building had to remain to allow determination of the extent of debris impact. Doswell and Burgess (1988) indicated that complete failure of a building would yield only a lower-bound estimate of tornado intensity. Therefore, our team members were interested especially in less damaged or undamaged buildings in the tornado path where an upper bound of tornado wind speed could be determined.

Phan and Simiu (1998) determined that wind speeds of longer duration resulted in greater damage to residences in the Jarrell, Texas, tornado. Residences near the center of the Jarrell tornado were subjected to tornadic winds for about 3 min. By comparison, we calculated that houses near the tornado center at Moore were subjected to tornadic winds for about 30 s. Last, there is the human factor in determining the tornado intensity based on analyzing damage. A person with knowledge of how buildings fail will tend to rate a building differently than a person who does not possess such knowledge.

d. Aerial survey

Fujita and Smith (1993) have demonstrated that near-ground wind fields can be inferred better by viewing damage patterns from the air. Our team employed a helicopter to conduct the aerial survey. The aircraft was flown between 300 m (1000 ft) and 900 m (3000 ft) above the ground in overlapping circles parallel to the tornado damage path. Flight clearance of air space had to be obtained from air traffic control because portions of the disaster area were restricted. Numerous photographs were taken of the damage path. The best perspective was obtained when photographing from almost directly above the damaged buildings. Clear skies provided the best contrast and sharpest images for photography, as opposed to cloudy skies. The aerial survey was conducted in the morning when the air was least turbulent and the sky was not hazy. In many cases, specific buildings such as churches and schools were identified and served as landmarks. In most instances, it was not possible to determine from the air how well a structure was built. This problem demonstrated the importance of conducting a comprehensive ground survey.

3. The damage path

a. General observations

Our team began the damage survey in a rural area 3 mi west of Newcastle, Oklahoma. Rural houses were built on concrete pier and wooden beam foundations or concrete masonry foundations. Houses on pier and beam foundations usually were secured to the concrete beams with anchor bolts whereas houses built on masonry foundations were not anchored. Some unanchored houses along the edge of the tornado damage path were moved intact from their foundations by as much as 90 m (295 ft; Fig. 3). Movement of unanchored homes off their foundations illustrated how F5 damage could occur at F1 or F2 wind speeds.

As the tornado entered southern sections of Oklahoma City, it inflicted F4 and F5 damage to a number of houses in the Country Place subdivision located just south of 134th Street (Fig. 4). These wooden-framed houses had been built recently on concrete slab foundations that averaged 150 m2 (1615 ft2). Houses had wooden bottom plates attached to their concrete slab foundations with tapered cut nails. Little was left of the homes near the center of the damage path other than a swath of concentrated debris. This swath of concentrated debris continued through an open field extending about 1000 m (3280 ft) beyond the subdivision. Most of the debris was composed of wooden boards from house framing along with furniture and a number of automobiles. One vehicle traveled 1000 m (3280 ft) and ended up inside a bridge culvert. It was apparent to team members that houses were a major debris source that contributed to their own destruction. As homes broke apart, their debris impacted neighboring homes, compromising them as well. Damage to houses on the edge of the tornado path varied greatly depending on their orientations. Houses with attached garage doors facing the wind typically sustained more severe damage than houses for which garage doors opposed the wind. Similar observations were made by Marshall and McDonald (1982) in their survey of the Grand Island, Nebraska, tornadoes.

The tornado continued northeast through the Briarhollow subdivision where the damage path remained about 400 m (1312 ft) wide. These were older tract houses with attached two-car garages. Wooden-framed houses had been built on concrete slab foundations that averaged 150 m2 (1615 ft2). Houses were attached to their foundations with either tapered cut nails, shot pins, metal straps, or anchor bolts. Without regard to method of attachment, none of the houses survived within the center of the damage path.

The Westmoore High School was located on the north side of the damage path and sustained damage to the roof covering along with removal of some metal roof decking and metal cladding. An awards ceremony was being held at the time the tornado struck the school, and hundreds of people were in attendance. People were evacuated to interior portions of the school, and all survived; however, many vehicles in the parking lot were displaced. Some vehicles tumbled southward and crashed into houses adjacent to the school.

As the tornado crossed Western Avenue, it struck the Emerald Springs Apartments, reducing some of the two-story wooden structures to one story or less. A pair of traffic signals in front of the school remained intact although the signal lights had been removed by the wind. A nearby metal church building collapsed. The tornado continued through the Greenleaf subdivision, reducing many of the wooden-framed townhouses to rubble. Numerous townhouses sustained F4 damage, and one house sustained F5 damage. The tornado completely destroyed a two-story metal church building, then proceeded across Santa Fe Avenue and entered the heart of the city of Moore. The damage path extended diagonally from NW 12th to NW 19th Streets through a densely populated area of smaller wooden houses that averaged 100 m2 (1076 ft2) and that were typically one story with one-car garages. An occasional interior closet or hallway remained; however, most houses near the center of the damage path had sustained F4 or F5 damage.

The tornado struck Kelly Elementary School directly and severely damaged the building. The school was a steel-framed structure with load-bearing masonry walls constructed on a concrete slab foundation. A concrete bond beam had been constructed atop the masonry walls to secure the ends of open-web steel roof joists. Many of the steel roof joists and pieces of the decking were removed, along with some of the concrete bond beams, and were transported downwind and deposited in a field. The school was the only engineered building in the center of the damage path within our study area. The survey team unfortunately was not allowed inside the school to conduct a more detailed assessment.

On the north edge of the damage path was the Regency Park Baptist Church. The church sanctuary had a high-pitched wooden-framed roof structure supported by glued–laminated (glulam) wooden arches. Tornadic winds removed the roof structure but left the arches intact.

The tornado continued through another subdivision, inflicting up to F4 damage before crossing I-35 at Shields Boulevard. One fatality and several injuries occurred when people tried to seek shelter under the overpass. However, the overpass had a concrete slab deck with sloping abutments and offered no protection. The tornado crossed I-35 and struck a two-story apartment complex and the two-story Best Western Hotel. Roofs of these wooden-framed buildings were removed, and some second-story walls failed. The tornado then traveled through the Ridgewood subdivision, inflicting up to F4 damage before heading out of town over rural areas. The tornado turned more northerly and then crossed I-240, continuing through portions of Del City and Midwest City before dissipating.

About 100 survivors were interviewed during our damage survey. All of the people interviewed had known the tornado was approaching. Most people said they received the tornado warning via local television, and others received the warning from relatives or friends via telephone. Hammer and Schmidlin (2000) conducted detailed interviews of the survivors and found that several people actually drove away from the tornado. Although driving away from the tornado was a successful strategy, fleeing a tornado in lieu of seeking shelter in one's home is generally not recommended. We found that most survivors who remained at home sought shelter in a bathtub, closet, or interior hallway because none of the houses had basements. We also found that the majority of people who sought shelter inside their homes survived the tornado without serious injury. In all, our team rated F1 damage to 284 houses, F2 damage to 405 houses, F3 damage to 558 houses, F4 damage to 317 houses, and F5 damage to 17 houses.

b. Wind-borne projectiles

A tremendous amount of debris was generated by the tornado as it traveled through the residential areas. Debris ingested into the tornado became high-speed projectiles that penetrated building roofs and walls and vehicles. Most projectiles were broken pieces of wood from houses, furniture, and trees. Larger wooden projectiles included 2.4-m (8 ft) long, wooden, 5 cm × 10 cm (2 in. × 4 in.) or 5 cm × 15 cm (2 in. × 6 in.) lumber from residences. One 5 cm × 15 cm (2 in. × 6 in.) board had entered through the roof of a house and penetrated the refrigerator freezer. In another house, we found a 2.4-m (8 ft) long, wooden fence post that had gone through a window and lodged in an interior wall. A number of wooden boards also were found penetrating brick veneer. Projectiles were found on both the north and south sides of the damage path, usually in areas where F0–F2 damage had occurred to residences.

The largest projectile found was a 3.7-m (12 ft) diameter, 4.3-m (14 ft) tall steel oil tank that had tumbled end over end for 276 m (905 ft), leaving gouge marks in the ground just west of the Newcastle overpass. The tank had not been anchored. The longest projectile found was an 11-m (36 ft) long, steel beam from a mobile home that had traveled 300 m (984 ft). The beam was twisted but remained on the ground surface. A number of vehicles had been rolled or tossed up to 1000 m (3280 ft) and were barely recognizable. In some instances, debris filled the passenger space of the vehicles or penetrated body metal. A barbed wire fence was rolled up into a ball measuring 1 m (3.28 ft) in diameter.

Our survey team found a few unusual projectiles. The leg of a steel lawn chair had penetrated a solid wooden post that measured 13 cm × 13 cm (5 in. × 5 in.; Fig. 5). The post had supported a second-story balcony on a rural house that sustained only F0 damage. A 1.8-m (6 ft) long section of steel sewer pipe, weighing about 23 kg (50 lb), went through the front door of a residence and came to rest in an interior hallway.

4. Performance of housing

a. Foundation connections

There were four methods of anchoring stud walls to the concrete slab foundations. Most houses had wall bottom plates attached to their foundations with 5-cm (2 in.) long, tapered cut nails. Tapered cut nails had been driven through the top sides of wall bottom plates at intervals ranging from 30 to 130 cm (12–51 in.). Wall bottom plates were 5 cm × 10 cm (2 in. × 4 in.) lumber with actual dimensions of 3.8 cm × 8.9 cm (1.5 in. × 3.5 in.). Thus, the tapered cut nails extended a maximum of only 1.3 cm (0.5 in.) into the concrete slab foundation. In destroyed structures, scrape marks in the surfaces of concrete slab foundations extended from the points where the tapered cut nails had been installed, indicating the walls moved laterally (Fig. 6). Bottom plates were found in the debris with tapered cut nails still attached to them (Fig. 7). Such foundation attachments were inherently weak, and, in many instances, conical spalled areas or divots were found in the slab surface where the tapered cut nails had been driven. These divots resulted when the tapered cut nails were installed or when the connection failed during the tornado. In either case, failure of the nailed connection at the foundation indicated to us it was weaker than the nailed connection between the wall stud and bottom plate. In other instances, the bottom plates pulled through the fasteners, leaving only nails attached to the concrete foundation (Fig. 8).

Some homes had perimeter walls attached to concrete foundations with metal shot pins. These pins were 7.6 cm (3 in.) long and had square metal washers. Pins were driven vertically at 40–45-cm (16–18 in.) intervals through the top sides of the bottom plates with a powder-actuated tool. The pins extended up to 3.8 cm (1.5 in.) into the concrete slab, or about 3 times the depth of tapered cut nails. Failure of houses with pin attachments occurred when the wooden bottom plates broke around the fasteners or when the wall studs pulled away from the bottom plates (Fig. 9).

FEMA (1999a) indicated that most residential construction in Oklahoma, including the city of Moore, was required to meet the design requirements of the one- and two-family dwelling building code as published by Council of American Building Officials (CABO 1995). However, it should be noted that houses built prior to 1995 were governed by a less restrictive building code. The CABO (1995) building code specified that floor systems be anchored to concrete foundations with 1.3-cm (0.5 in.) diameter, steel anchor bolts spaced a maximum of 1.8 m (6 ft) apart and not more than 30 cm (1 ft) from the wall corners. Anchor bolts also should extend a minimum of 18 cm (7 in.) into the concrete. Nailing wooden bottom plates to concrete slab foundations would not meet the requirements set forth in the CABO (1995) building code. Local building officials indicated that shot pins could be utilized to anchor bottom plates around the perimeter of concrete slab foundations whereas tapered cut nails were allowed to attach bottom plates of interior walls within the city of Moore. Such variances in the building code probably were allowed because straps, cut nails, or shot pins would be adequate to keep the bottom plates of walls in place for normal gravity loads. However, these variances did not adequately consider how these connections would work when wind forces were applied.

The remaining houses in our survey had wall bottom plates attached to the concrete slab foundation with metal straps or anchor bolts. These connections were stronger than the tapered cut nails because they usually held the wooden bottom plates in place. Failure of these houses occurred when the wall studs pulled away from the bottom plates. Wall studs were end-nailed with pairs of 16-penny (8.9 cm or 3.5 in.) nails driven from the bottom side of the wooden bottom plate, which is the minimum requirement under the CABO (1995) building code. The nailed connection frequently had pulled apart, leaving pairs of nail shanks pointing skyward from the strapped or bolted wooden bottom plates (Figs. 10 and 11). Straight-nailed connections were weak in tension. Marshall (1983) tested 30 pairs of 16-penny straight-nailed connections and found an average pullout strength was 980 N (220 lb), with a standard deviation of 167 N (37 lb). In F4- and F5-damaged areas, some of the wooden bottom plates had split around the anchor bolts and had been pulled apart, leaving only the anchor bolt attached to the foundation. In no instance did we find anchor bolts that had failed on residences.

b. Wall connections

Stud walls were framed using 5 cm × 10 cm (2 in. × 4 in.) lumber and stiffened with 2.5 cm × 10 cm (1 in. × 4 in.) let-in bracing as called for in the CABO (1995) building code. Let-in braces were installed diagonally at wall corners extending from the tops of the wall plates to the bottom plates at angles between 45° and 60°. Notches were cut into the wall studs to receive the let-in braces. The purpose of the let-in braces was to transfer lateral wind loads to the foundation applied from the perpendicular wall. Nominal dimensions of the let-in wall brace were 1.9 cm × 9 cm (0.75 in. × 3.5 in.). Team members noted that let-in wall bracing worked adequately as long as the perpendicular wall was subjected to positive external pressure. However, such wall bracing was ineffective when walls were subjected to internal pressures. The CABO (1995) building code permits the use of wall sheathing as an alternative to let-in braces. Such sheathing is usually 1.2 m × 2.4 m (4 ft × 8 ft) plywood or oriented strand board (OSB) placed on its short side so that it can be nailed to the top plates, wall studs, and bottom plates. Sheathing nailed to the plates and studs help stiffen the wall frame to resist racking (side to side) forces. However, a sheet of plywood costs more than a 2.5 cm × 10 cm (1 in. × 4 in.) lumber; thus, contractors were more apt to install a let-in brace. A problem with using let-in braces at wall corners occurred when the intersecting wall was interrupted by a large opening such as a garage door or window. Thus, walls containing large openings usually were not braced. In such instances, sheathing would have been a better alternative. Another problem arose when notches were cut too deep into the wall studs, leaving let-in braces elevated by their nail shanks. It is important that let-in braces fit snugly into the notches and be fastened correctly to transfer lateral loads.

Wooden-framed walls were capped by a doubled top plate. The top plates were doubled in order to support the roof framing and to connect intersecting wall corners. Wooden top plates were straight-nailed into the tops of the wall studs using pairs of 16-penny nails. Such connections were inherently weak when uplifted; however, they would meet the minimum requirements set forth by the CABO (1995) building code. Team members did find a few instances in which top plates had separated from the wall studs or where both boards composing the top plates had pulled apart.

Many houses within the damage path had brick masonry exterior walls. Such walls were erected around the perimeters of the concrete slab foundations and were nonstructural; that is, they did not support the roof. According to the CABO (1995) building code, brick masonry walls must be secured to the wooden stud walls by metal wire or ties about every 60 cm (2 ft) vertically and horizontally. Team members found many instances in which wall ties had not been installed or in which ties had not been bent to engage the masonry. Such masonry walls were free standing and could be pushed back and forth by hand.

As mentioned previously, most houses had attached garages. Team members identified numerous garage door failures along the edge of the tornado path. Garage doors in newer homes typically were aluminum panels. Most two-car garages had continuous doors that were 4.9 m (16 ft) wide and 2.1 m (7 ft) tall. Garage door failures initiated when the aluminum door panels buckled and the metal or plastic rollers pulled out from the door cleats. Garage door failures resulted from external pressures as well as internal pressures. Loss of the garage door on the windward side of the house allowed wind to enter the garage, increasing internal pressures on the roof and walls. If winds were sufficiently strong, the garage lost its roof and/or walls (Fig. 12).

c. Roof connections

Team members found that roof structures frequently pulled apart from the top plates. Rafters were toenailed to the top plates, usually with two fasteners. In some instances, the fasteners were pulled out along with the rafters; in other instances, the rafters split around the fasteners, leaving the nails in the top plates. Rafters secured with pairs of 16-penny nails toenailed into the top plates met the minimum requirements as called for by the CABO (1995) building code. However, toenailed connections were relatively weak in tension. Marshall (1983) also conducted tests on 30 pairs of 16-penny toenailed connections and found an average pullout strength of about 1313 N (295 lb) with a standard deviation of 304 N (68 lb). Canfield et al. (1991) conducted similar pull tests and found similar results. However, they could increase the average pullout strength to 1842 N (414 lb) if the wood did not split when nailed. By comparison, Canfield et al. (1991) also conducted pull tests on metal “hurricane” clips and found an average strength of 5341 N (1200 lb) depending on the type of clip used. Thus, a metal clip can be about 3 times as strong as a 16-penny toenailed connection.

Wood roof decking was attached to the rafters with staples, usually at 15-cm (6 in.) intervals. However, some houses had lost large portions of the roof decking, leaving the roof structure exposed to the weather. Close examination revealed that deck staples partially or totally had missed the underlying rafters. Therefore, a poorly installed roof deck was very vulnerable to being uplifted and removed by the wind.

d. Sequence of house failure

Wind was forced to go over and around a house in its path. As a result, aerodynamic pressures were applied to the building exterior, which included (inward acting) positive pressures on the windward walls, uplift pressures on the roof, and (outward acting) negative pressures on the leeward walls. Windward walls literally were pushed inward when they could not adequately transfer the wind load to the foundation or to intersecting walls. Uplift on the roof resulted in the removal of the roof covering, roof decking, or roof structure. If the building was breached on its windward side from a broken window or door, internal pressures added to the aerodynamic uplift pressures on the roof. This frequently resulted in forcing the roof upward and the exterior walls outward. Once the roof structure had been removed, it was relatively easy for the winds to topple or push out the perimeter walls because these walls no longer were braced at the top. Liu et al. (1989) indicated that inadequate tie-down of roofs was the most serious common problem with wooden-framed houses. In our study, we found a fairly even split between the numbers of houses that sustained wall–foundation connection failures and those that sustained roof–wall connection failures.

Buildings are designed with a concept of a continuous load path by which applied loads are transferred from the roof down through the walls and into the foundation. The “dead weight” of a building normally keeps it in place on its foundation. However, the load path direction reverses when wind lifts the roof. Connections adequate under gravity-load situations actually may pull apart when the load-path direction is reversed. Thus, connections must be designed to accept loads from opposite directions. Each connection between structural members can be thought of as a link in a chain, with a building only as strong as its weakest link.

The CABO (1995) building code is not without its problems. It still allows top and bottom plates to be straight-nailed to the wall studs. Marshall (1983, 1992) has shown that such nailed connections are weak in tension and can be pulled apart easily. Fasteners must be placed in shear, not tension, to provide greater resistance to wind uplift. Solid plywood sheathing also can provide more formidable protection and help to stiffen the frame if it is fastened to the top plates, studs, and bottom plates in lieu of let-in braces.

e. Building codes

Houses are generally nonengineered structures. Thus, the details of house construction are left to the discretion of the building contractor. Larger towns and cities have adopted building codes, but most rural areas and smaller towns do not have building codes or inspectors to enforce the codes. However, building codes serve only as a minimum design requirement. It has been the author's experience that builders whose goal it is to achieve a minimum design invariably fall short of this goal because of variations in such things as workmanship and material qualities. Thus, a “beyond-code” approach should be made in an effort to surpass the minimum design requirements. Although it is true there are no provisions in building codes to construct houses to resist tornadoes, building codes are still important in mitigating tornado damage. A better-built house would yield less debris in a tornado, and occupants would have a better chance of surviving in such a house. In addition, structural improvements could reduce the level of damage to a house that experiences weaker tornadoes or straight-line thunderstorm winds that are just above code-required design levels and hence could mitigate economic loss and improve safety.

The CABO (1995) building code lists the design wind speed for the Oklahoma City area at 31 m s–1 (70 mi h–1) for a fastest-mile wind at 10 m (33 ft) above the ground in open, unobstructed terrain, which equates to about a 40 m s–1 (90 mi h–1) 3-s wind gust. McDonald (2001) indicated that gust wind speeds are utilized to calculate tornado wind pressures on structures. So, it should not be surprising that houses would begin to sustain some damage when winds approach the code design wind speeds. Safety factors are built into the code design wind speeds such that buildings have a reserve strength. According to Gardner et al. (2000), wind gusts in the range of 58–72 m s–1 (130–160 mi h–1) could overcome the reserve strength and completely destroy a structure. Therefore, houses that sustain F5 damage rating actually could fail in wind gusts in the original F2 range. Phan and Simiu (1998) reached a similar conclusion in their analysis of damage in the Jarrell, Texas, tornado.

Table 1 presents the author's adjustments to wind speed ranges of the F scale based on results of engineering assessments of residential damage in Moore, Oklahoma. For engineered buildings, the wind speed ranges would be higher. Likewise, for more poorly built houses, the wind speed ranges would be lower. It is emphasized that failure wind speeds represent the lower bound of tornado intensity. The upper bound of tornado intensity could not be determined in this study because no homes survived in the center of the tornado damage path. Even if we had gained access to Kelly Elementary School and determined the upper-bound wind speeds for those building components that did not fail, this would have represented only one sampled location within the tornado path.

f. Shelter performance

Two belowground shelters were found within our damage survey area. One shelter was located adjacent to a rural house near Newcastle where the team began the damage survey, and the other shelter was located in east Moore. Both shelters were adjacent to houses that sustained F3 damage. Shelters were constructed of steel-reinforced concrete and had wooden doors lined with sheet steel. The shelters were not damaged during the tornado and occupants who used the shelters during the storm were not injured.

Two aboveground shelters were located by other team members in Bridge Creek and Del City, Oklahoma. According to Gardner et al. (2000), one aboveground shelter was located in rural Bridge Creek estates southwest of Oklahoma City and the other shelter was located in Del City. The Bridge Creek house sustained F1 damage; the Del City house sustained F3 damage. Both aboveground shelters survived without damage.

5. Calculation of failure wind speeds

Mehta et al. (1981) showed there was a good correlation between wind speed and building damage. Mehta et al. (1976) and Minor et al. (1977) have shown how to calculate failure wind speeds from buildings damaged by tornadoes and hurricanes. Weights of various building components and strengths of critical connections must be determined. The following wind speed–pressure formula from ASCE (1988) has been utilized to calculate failure wind speeds:
pfCpV2
where p is the wind pressure, f is a density constant, Cp is the pressure coefficient, and V is the wind speed. Reynolds (1971) noted that one of the greatest uncertainties in estimating a failure wind speed involves determining the value of the pressure coefficients. This variable depends on certain building characteristics and its surroundings, including the size and shape of the building and its proximity to neighboring buildings. Such pressure coefficients are derived from wind-tunnel studies under constant wind velocities and can only be estimated for tornadoes. Mehta (1976) acknowledges these uncertainties but states that pressure coefficient values obtained from wind-tunnel studies appear to work reasonably well in calculating failure wind speeds on buildings in tornadoes.

As noted from our damage survey, numerous housing failures initiated with destruction of attached garages. In a typical case, garage doors blew in, allowing internal pressures to act in combination with external aerodynamic uplift pressures to remove the garage roof structure. Uplift on the roof structure caused toenailed connections in the wooden top plates to pull apart. Destruction of the attached garage frequently led to damage or the removal of the remaining roof structure on the residence.

In an effort to determine a range of failure wind speeds, calculations were made for the typical attached garage that measured 6 m × 6 m (20 ft × 20 ft) and had a gable roof covered with 1.3 cm (0.5 in.)-thick plywood and asphalt composition shingles. Rafters were assumed to be conventionally spaced 61 cm (24 in.) apart. Using standard building data, the dead load of the garage roof would be about 59 kg m–2 (12 lb ft2). Given the size of this garage in this example, there would be 22 toenailed connections between the rafters and wall top plates, resulting in about 78 kg m–2 (16 lb ft2) of uplift resistance. Failure of the roof would occur when the wind uplift force is equal to the weight of the garage roof plus the resistance of the connections. Pressure coefficients were utilized from ASCE (1988). If the wind blew directly into the garage and caused the garage door to fail, internal pressures would combine with external aerodynamic pressures, resulting in a failure wind speed of only 38 m s–1 (85 mi h–1). However, if there was no internal pressure contribution, the resulting failure wind speed would be 56 m s–1 (125 mi h–1). However, internal pressure is rarely nil, because houses usually have some natural ventilation. Thus, conventionally constructed wood-framed roofs would likely fail at wind speeds around 50 m s–1 (112 mi h–1) resulting in F2 damage. Once the roofs have been removed, it would not take much additional wind to destroy the structure. Mehta and Carter (1999) calculated failure wind speeds in the Jefferson County, Alabama, tornado and concluded that complete destruction of houses occurred in the wind speed range of 51–67 m s–1 (114–150 mi h–1).

6. New housing

The author revisited the disaster area three months after the tornado to check the quality of new house construction. A total of 40 houses were examined in Moore and southern Oklahoma City on sites at which houses previously had been destroyed. The author found that the quality of new home construction generally was no better than homes built prior to the tornado. Most newly built homes were attached to their concrete foundations with tapered cut nails or shot pins as had been noted in homes destroyed by the tornado. Of the 40 new houses inspected, 5 houses had bottom plates bolted to their foundations, 6 houses had bottom plates strapped with metal to their foundations, and 29 houses had bottom plates attached to their foundations with tapered cut nails and/or shot pins.

Homes anchor bolted to their foundations had bolts properly spaced with nuts tightened over washers. Homes strapped to their foundations had most straps fastened to the bottom plates. However, a few straps were found bent outside the walls and extending from beneath the bottom plates (Fig. 13). Such straps were not secured to the bottom plates and exterior insulation board already had been installed on the perimeter walls. These straps evidently were going to be hidden behind the brick veneer. Tapered cut nails utilized to secure bottom plates to newly built homes were the same type as in homes destroyed by the tornado (Fig. 14). These nails only extended 1.3 cm (0.5 in.) into the concrete foundation (Fig. 15). It remains unknown why four different types of foundation anchor systems were being employed in the same general area, especially given that it was apparent that anchor bolts were superior in strength.

There was no change in house-framing techniques from original construction except in one instance in which a house had larger 5 cm × 15 cm (2 in. × 6 in.) perimeter walls with additional fasteners. Some wall corners were covered with OSB sheathing. However, this better-built house was in the middle of a subdivision of conventionally built houses. Building beyond the code is commendable, but its effectiveness is limited when a better-built home is situated in a neighborhood of houses with less wind resistance. Weaker neighboring houses could become debris sources that could damage or destroy better-built houses. In essence, a house is only as tornado resistant as its neighbors. However, this finding should not discourage homeowners from insisting on structural improvements to their home.

None of the houses inspected had hurricane clips or other wind-resistant connections. In one house, cutouts had been made in wall framing for let-in braces, but the braces had not been installed (Fig. 16). Insulation board already had been fastened to the exterior wall, precluding installation of the let-in braces. The same house only had one nail securing each rafter to the top plate of walls. Nails also had been installed too close to the inside edge of the top plate, splitting the wood. (Fig. 17).

Six of the 40 new houses contained “safe rooms” (FEMA 1999b). These closet-sized rooms were constructed with steel-reinforced concrete walls and ceilings. Entry into each safe room was through a heavy, metal door.

7. Summary and conclusions

A violent tornado struck the Oklahoma City metropolitan area on 3 May 1999, providing an opportunity to assess building performance. Our survey team studied building damage in and around the city of Moore. The majority of buildings damaged by the tornado were one- and two-story residences that were constructed conventionally on concrete slab foundations. Many homes were completely destroyed, and no building managed to survive in the center of the tornado path.

A methodology was developed as to how to conduct the damage survey and how to assign F-scale numbers to damaged houses. Additional damage descriptions were defined by the author to aid in assigning F-scale numbers. It was concluded that tornado damage to houses occurred at significantly lower wind speeds than those established by the original F scale. Houses with F4 or F5 damage likely failed when wind gusts reached F2 on the original F scale. Thus, failure wind speed ranges were adjusted by the author to match better the F-scale number. It should be noted that such failure wind speeds represent lower-bound estimates of this tornado.

Approximately one-half of the houses surveyed failed where the walls were fastened to their foundations. Wall bottom plates had been attached with either tapered cut nails, shot pins, metal straps, or anchor bolts, with tapered cut nails being predominant. Tapered cut nails usually pulled out of the foundations, leading to the destruction of the houses. Shot pins remained in place; however, some bottom plates broke around the pins. Metal straps and anchor bolts tended to secure the wooden bottom plates; however, failure then occurred where the wall studs were straight-nailed to the bottom plates. The remaining one-half of the houses surveyed failed where the roof structure was toenailed to the wall top plates. These failures were found predominantly on the edge of the tornado damage path and depended highly on orientation of the attached garage to the wind. Houses with attached garage doors facing the wind typically sustained more severe damage than houses for which garage doors opposed the wind.

A consistent damage sequence was found for residences. Houses typically failed as the attached garage doors buckled inward and/or windows broke, allowing the wind to enter and to pressurize the buildings. Roofs then were uplifted by the combination of internal pressure and aerodynamic uplift. Last, perimeter walls fell or were pushed outward because they no longer were braced at their top ends by the roof structure.

Numerous projectiles were generated by the tornado. Most of these projectiles were broken wooden pieces from house structures, furniture, and trees. Many projectiles were found in areas in which residences had sustained only F0–F2 damage. Only a few tornado shelters were found in our study area. Shelters performed well, and occupants who sought refuge in them were not injured. About 100 survivors were interviewed, and all had known the tornado was approaching. Most people who stayed in their houses sought shelter in an interior bathroom, closet, or hallway.

The author returned to the disaster area three months later and inspected 40 houses that were under construction in replacement of destroyed houses. The author found that, in general, the quality of the new construction was no better than the quality of construction of destroyed homes. One exception was a house that had thicker perimeter walls. Building contractors still were constructing homes to reach the minimum design requirements set forth by the CABO building code and in several instances fell short. Building beyond code was rarely done. In essence, the tornado that struck Moore, Oklahoma, was too intense to have significant impact on the way residential housing was rebuilt. If anything, this event actually bolstered public perception that no building could survive a tornado.

The author found that F-scale wind speeds were too high in relation to the extent of damage to residences, especially for houses that sustained F2 damage or greater. When wind speeds are overstated in the F scale, the general public, designers, and builders may conclude that it is economically unreasonable to design for these higher wind speeds and associated loads on the building. In fact, most residential housing can be made considerably more resistant to damages from most tornadoes by using construction techniques described in this paper. When the author visited the post-tornado reconstruction area, 6 of the 40 new houses contained tornado shelters or safe rooms; however, these homes generally were not built any better than prior to the tornado. Thus, homeowners appeared to be making the decision to provide for personal safety (i.e., building a safe room) instead of to increase housing strength (to provide property protection and increased safety). This decision may, in part, be based on the assumption that it is unreasonable or uneconomical to try to construct houses for the high wind speeds described on the F scale.

Acknowledgments

I thank Drs. Ernst Kiesling and Kishor Mehta of Texas Tech University for inviting me to be part of the damage survey team and my teammates, Mr. Mark Conder and Dr. Zhongsan Zhou, for enduring long hours of survey work. Doctor Charles Doswell, Stoney Kirkpatrick, Scott Morrison, Jim Weithorn, and Susie Meyer provided helpful comments and suggestions to improve the manuscript.

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Fig. 1.
Fig. 1.

Damage track of the tornado that struck Oklahoma City and suburbs on 3 May 1999. The tornado had a continuous damage path of 61 km (38 mi) and lasted approximately 80 min. Only a portion of the tornado path actually struck densely populated areas (hatched area). Our damage survey covered the path from A to B. Map adapted from Burgess and Magsig (2000)

Citation: Weather and Forecasting 17, 3; 10.1175/1520-0434(2002)017<0582:TDSAMO>2.0.CO;2

Fig. 2.
Fig. 2.

Typical cross section of exterior wall in a Moore, OK, house, showing building nomenclature and fastener type

Citation: Weather and Forecasting 17, 3; 10.1175/1520-0434(2002)017<0582:TDSAMO>2.0.CO;2

Fig. 3.
Fig. 3.

Unanchored rural house (in background) near Newcastle was moved approximately 90 m (295 ft) to the east. The unanchored house was supported on concrete masonry units

Citation: Weather and Forecasting 17, 3; 10.1175/1520-0434(2002)017<0582:TDSAMO>2.0.CO;2

Fig. 4.
Fig. 4.

Aerial view of Country Place subdivision in southern Oklahoma City where F5 damage was found. Wooden-framed houses were attached to concrete slab foundations with tapered cut nails. The tornado continued northeast through the suburbs of Moore, Del City, and Midwest City

Citation: Weather and Forecasting 17, 3; 10.1175/1520-0434(2002)017<0582:TDSAMO>2.0.CO;2

Fig. 5.
Fig. 5.

Leg of steel folding chair penetrated a solid wooden post. The post partially supported a second-story porch on a house; the house sustained only F0 damage

Citation: Weather and Forecasting 17, 3; 10.1175/1520-0434(2002)017<0582:TDSAMO>2.0.CO;2

Fig. 6.
Fig. 6.

Scrape mark and conical spalled area in the surface of concrete slab foundation indicating where the wall bottom plate along with the tapered cut nail connection were moved laterally. The entire house was swept away, resulting in an F5 damage rating

Citation: Weather and Forecasting 17, 3; 10.1175/1520-0434(2002)017<0582:TDSAMO>2.0.CO;2

Fig. 7.
Fig. 7.

Wall bottom plate found in debris still had tapered cut nail attached

Citation: Weather and Forecasting 17, 3; 10.1175/1520-0434(2002)017<0582:TDSAMO>2.0.CO;2

Fig. 8.
Fig. 8.

Tapered cut nail that remained in the foundation. The wall bottom plate simply pulled through the fastener, leaving the nail

Citation: Weather and Forecasting 17, 3; 10.1175/1520-0434(2002)017<0582:TDSAMO>2.0.CO;2

Fig. 9.
Fig. 9.

Bent shot pins remained in concrete foundation. The wall bottom plate had been broken around the fasteners as the house was destroyed by the tornado

Citation: Weather and Forecasting 17, 3; 10.1175/1520-0434(2002)017<0582:TDSAMO>2.0.CO;2

Fig. 10.
Fig. 10.

Wall stud pulled out, leaving only the strapped bottom plate intact. The wall stud was straight nailed, a connection that is inherently weak in uplift/tension

Citation: Weather and Forecasting 17, 3; 10.1175/1520-0434(2002)017<0582:TDSAMO>2.0.CO;2

Fig. 11.
Fig. 11.

Wall studs pulled out, leaving only the anchor-bolted bottom plate. Local building codes unfortunately still allow straight-nailed connections between the wall plates and studs

Citation: Weather and Forecasting 17, 3; 10.1175/1520-0434(2002)017<0582:TDSAMO>2.0.CO;2

Fig. 12.
Fig. 12.

Typical failure of attached garage when exposed to tornadic winds. The garage door blew in, resulting in displacement of the roof from combined loads of aerodynamic pressure and internal pressure

Citation: Weather and Forecasting 17, 3; 10.1175/1520-0434(2002)017<0582:TDSAMO>2.0.CO;2

Fig. 13.
Fig. 13.

A new house being constructed after the tornado had a metal strap bent outside the base of the exterior wall rather than being nailed to the wooden bottom plate. As a result, the wall was not attached to the foundation. The exposed metal strap was likely hidden later when the brick masonry was installed

Citation: Weather and Forecasting 17, 3; 10.1175/1520-0434(2002)017<0582:TDSAMO>2.0.CO;2

Fig. 14.
Fig. 14.

Perimeter wall plate was fastened to the foundation with tapered cut nails. Nails were spaced between 30 and 130 cm (12 and 51 in.) apart

Citation: Weather and Forecasting 17, 3; 10.1175/1520-0434(2002)017<0582:TDSAMO>2.0.CO;2

Fig. 15.
Fig. 15.

Tapered cut nails that secured wall bottom plates extended only 1.3 cm (0.5 in.) into the foundation. Such nails were used in lieu of anchor bolts or metal straps both in new construction and in houses built prior to the tornado

Citation: Weather and Forecasting 17, 3; 10.1175/1520-0434(2002)017<0582:TDSAMO>2.0.CO;2

Fig. 16.
Fig. 16.

Lack of let-in wall brace in new house being built after the tornado. Note that notches were cut in the wall studs to receive the let-in brace; however, exterior insulation board already had been installed

Citation: Weather and Forecasting 17, 3; 10.1175/1520-0434(2002)017<0582:TDSAMO>2.0.CO;2

Fig. 17.
Fig. 17.

Rafters were fastened with a single nail to the wall top plate in this house under construction after the tornado. The nails were installed too close to the edge of the top plate, splitting the wood. Two nails should have been used at each connection

Citation: Weather and Forecasting 17, 3; 10.1175/1520-0434(2002)017<0582:TDSAMO>2.0.CO;2

Table 1. 

Estimated failure wind velocities in comparison with F scale, based on the degree of damage to wooden-framed residences on concrete slab foundations

Table 1. 
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