DAMAGE SURVEY OF THE MOORE, OKLAHOMA TORNADO(presented at the 20th Conference of Severe Local Storms, Orlando, FL)

Timothy P Marshall*Haag Engineering Co. Dallas, Texas

1. INTRODUCTION

On May 3, 1999, a violent tornado traveled through the southern and eastern suburbs ofOklahoma City. The next morning, the author was asked to lead one of three damage surveyteams sponsored by the Institute for Disaster Research at Texas Tech University in Lubbock,Texas. The teams were given five tasks: 1) to map out the damage path and assign F-scale ratingsto the damaged residences, 2) to document the performance of housing, 3) to interview witnesses,4) to document projectiles, and 5) to determine if there were any above or below ground shelterswithin the damage path and to assess their performance. Both aerial and ground surveys wereconducted. The damage path was divided between the teams for the survey. A reportsummarizing the findings of all three teams was published by Gardner et. al. (2000).

This paper will present some of the findings from the author’s team which surveyed thedamage at Moore, Oklahoma where the most severe damage (F-5) occurred. Various modes ofhouse construction were found including various methods to secure wall bottom plates to theconcrete slab foundations. Thousands of projectiles were launched by the tornadic winds rangingfrom conventional wooden boards to unusual projectiles such as a metal sewer pipe and a metalfolding chair. The author returned to the area three months after the disaster and noted severaldeficiencies in the construction of new housing.

2. GROUND DAMAGE SURVEY

The tornado that went through Moore, Oklahoma initially began just northeast ofChickasha, Oklahoma located about 70km southwest of Oklahoma City. The tornado travelednortheastward paralleling I-44 and had a continuous damage path which at times reached 1.5kmwide. The tornado continued through the city of Moore and into portions of Del City andMidwest City before lifting (Fig 1).

As the tornado entered the west side of Moore, the width of the damage path narrowed toabout 400m wide. The tornado became quite intense inflicting F-4 and F-5 damage to a numberof homes in the Country Place subdivision located just south of 134th Street (Fig 2). Thesewooden-framed homes were built on concrete slab foundations. Most of the wall bottom plateswere attached with tapered concrete nails driven into the concrete slab foundation at variousintervals.

The tornado damage path remained about 400m wide through the Briarhollowsubdivision. These were older tract homes with attached two-car garages. The author found amixture of homes that were either nailed, strapped and nailed, or anchor bolted to their concreteslab foundations. The Westmoore High School was located on the north side of the damage pathand sustained some damage to the roof and metal cladding. An awards ceremony was being heldat the time the tornado struck and hundreds of people were in attendance. The people were

evacuated to interior portions of the school and all survived, however, their vehicles in theparking lot were tossed around. Some of the automobiles ended up in the houses south of theHigh School.

As the tornado crossed Western Ave., it struck the Emerald Springs Apartments. Some ofthe two-story wooden structures were reduced to one-story or less. Several injuries and fatalitieswere reported here. A metal church building was completely destroyed. The tornado continuedthrough the Greenleaf subdivision reducing many wooden-framed townhomes to rubble.Numerous townhomes sustained F-4 damage and one home sustained F-5 damage. The tornadocompletely destroyed a two-story metal church building, then crossed Santa Fe Avenue enteringthe heart of city. The damage path extended diagonally from NW 12th to NW 19th Streetthrough smaller wooden-homes with attached one-car garages inflicting up to F-5 damage.

The tornado traveled right over Kelly Elementary School. The steel frame buckled andmost of the exterior walls collapsed. The tornado continued through another subdivisioninflicting up to F-4 damage before crossing I-35 at Shields Blvd. A fatality and several injuriesoccurred when people tried to seek shelter under the bridge there. However, the bridge was aconcrete slab (no steel girders) and offered no protection. The tornado crossed I-35 and struckan apartment complex and Best Western Hotel. Roofs were removed and some of the two-storybuildings were reduced by one-level. The tornado then traveled through the Ridgewoodsubdivision causing up to F-4 damage before traveling out of town over rural areas. The tornadoturned more northerly at I-240 and continued through portions of Del City and Midwest Citybefore dissipating. In all, the teams rated F-1 damage to 284 homes, F-2 damage to 405 homes,F-3 damage to 558 homes, F-4 damage to 317 homes, and F-5 damage to 17 homes.

3. PERFORMANCE OF HOUSING

Houses surveyed were conventionally constructed. Wooden bottom plates were eithernailed, strapped, or bolted to their concrete foundations with the majority being nailed. Nailswere 5cm long, concrete type, with tapered shanks, and penetrated into the concrete foundationabout 1.3cm (Fig. 3). In many instances, the nailed connection failed. The wooden bottom plateeither pulled through the nail leaving the nail in the concrete foundation or the wooden bottomplate and nail were removed (Fig. 4). Stronger straps and anchor bolts held the wooden bottomplates in place. In these cases, failure occurred where the wall studs were attached to the bottomplate. Most wall studs were straight-nailed (two nails) to the wooden bottom plate and toe-nailing was observed in a few instances. The straight and toe-nailed connections were weakerthan the straps or anchor bolts. Thus, the nailed connections failed first. Similar observationswere made by the Building Performance Assessment Team (1999).

Typical house failure initiated as the garage doors buckled inward and windows brokeallowing the wind to enter and pressurize the buildings. Roofs were then uplifted and wallsremoved. Houses with attached garages facing the wind sustained more damage. Similarobservations were made by Marshall (1982). As expected, none of the houses were built towithstand an F-5 tornado. Failure of most homes initiated at wind speeds closer to code valueswhich are 40 m/s (3-sec gust) for the Oklahoma City area. It should be noted that such buildingcodes represent the minimum requirements.

4. F-SCALE RATINGS

The survey team assigned F-scale numbers to each damaged house (Fujita, 1971). The F-scale is a subjective, visual interpretation of damage ranging from 0 to 5 based on increasingseverity of damage to a structure. Homes with minor roof damage or toppled chimneys wererated F-0. Homes which lost their roof covering or had attached garages that were destroyedwere rated F-1. Homes which lost their roof structures were rated F-2. Homes which lost theirroofs and most exterior walls were rated F-3. Well-constructed homes that were leveled with apile of debris remaining were rated F-4, and homes that were completely removed from theirfoundations and no debris remained were rated F-5. Sample maps are shown in Figures 5.

Wind speed ranges associated with the F-scale have been known to be inaccurateespecially in the higher F-scale numbers. The wind speeds were originally derived when Fujitadivided the gap between the top wind velocity in the Beaufort-scale and Mach 1 into 12 equalincrements. Minor et. al (1993) suggested that the wind velocity ranges be lowered for F-2 orgreater damage based on engineered assessments (See Table 1).

Other shortcomings in using the F-scale wind speed ranges are noted. The F-scaleassumes each structure is homogeneously constructed. Thus, a weak building may fail when anadjacent building that is better built remains. Residences are usually non-engineered, so there willbe greater uncertainty in an F-scale rating. Minor et. al (1993) indicated an accuracy of plus orminus one F-scale rating for buildings rated F-3 or above especially if inherent strengths orweaknesses in buildings are not considered.

There are also problems with rating tornado intensity based on the degree of damage to astructure. A house may fail at a lower wind velocity than the maximum wind speed of the tornadoor the house may not be in the direct path of the tornado and sustain minimal damage in a strongtornado. Thus, the F-scale rating of damage to a structure does not necessarily reflect thestrength or intensity of the tornado. Also, the translational speed of the tornado may affect theseverity of residential damage as noted by Phan and Simiu (1998) in the Jarrell, Texas tornado in1997. Wind speeds of longer duration generally causes more severe damage to residences.

5. PROJECTILES

The Moore, Oklahoma tornado generated thousands of projectiles. These projectilesimpacted and/or went through houses and vehicles. Most of the projectiles were broken woodenpieces from houses and trees. The largest projectile found was a 3.7m diameter, 4.3m tall steel oiltank which tumbled end-over-end for 276m just west of the Newcastle bridge. The tank was notanchored. Several vehicles traveled up to 300m and ended up in ravines or culverts. Parts ofmobile homes (i.e. steel beams up to 11m long) traveled similar distances.

The most unusual projectiles were: 1) the leg of a steel lawn chair which penetrated a solidwooden post that measured 13cm x 13cm, 2) a 2.4m long wooden board which entered a hole inthe roof of a house and penetrated a refrigerator freezer, 3) a 2.4m long wooden fence post whichwent through a window and lodged in an interior wall, 4) a 1.8m long steel pipe, weighing about280kg, which went through the front door and came to rest in an interior hallway. Also, a bathtubwas moved 100m in which two people were seeking shelter.

6. REBUILDING DEFICIENCIES

The author revisited the disaster area three months after the tornado to monitor therebuilding efforts. A total of 40 buildings were examined in Moore and southern Oklahoma Cityto see if building practices have changed in the wake of the tornado. Out of the 40 new homesinspected, five homes had bottom plates bolted to their foundations, six homes had bottom platesstrapped to their foundations, and 29 had bottom plates nailed to their foundations. Taperedconcrete nails were spaced from 30 to 130cm apart. Nails had a maximum of 1.3cm penetrationinto the concrete. In general, we found that the new construction was no better in quality thanbefore the tornado. However, six of the 40 new homes did have concrete safe rooms constructedon their foundations. Information on constructing safe rooms has been published by the FederalEmergency Management Agency (1999) or can be obtained on their web site athttp://www.fema.gov

7. SUMMARY

The tornado disaster at Moore, Oklahoma provided an opportunity to learn about buildingresponse. As expected, there was significant variability in house construction and performance.Houses with F-4 and F-5 damage provided little shelter to the occupants. Portions of houses needto be “hardened” to provide occupant safety. This is especially true for schools. A number ofprojectiles were documented including a few never before seen items.

8. ACKNOWLEDGMENTS

The author would like to thank Dr. Ernest Kiesling and Dr. Kishor Mehta at Texas TechUniversity for being invited to serve as a member of the disaster team.

9. REFERENCES

Building Performance Assessment Report, 1999: Midwest Tornadoes of May 3, 1999, FederalEmergency Management Agency, Denton, Tx, 216pp.

Federal Emergency Management Agency, 1999: Taking Shelter from the Storm: Building a SafeRoom Inside Your House, 28pp.

Fujita, T.T., 1971: Proposed characterization of tornadoes and hurricanes by area and intensity,SMRP Research Report 91, University of Chicago, Chicago, IL, 15pp.

Gardner, A. et. al., 2000: The Tornadoes of Oklahoma City of May 3, 1999, Wind Science andEngineering Center, Texas Tech University, Lubbock, Tx, 38pp.

Marshall, T.P., and J.R. McDonald, 1982: An Engineering Analysis of the Grand IslandTornadoes, 12th Conf. on Severe Local Storms, San Antonio, Tx, p. 293-296.

Minor, J.E., J.R. McDonald, and K.C. Mehta, 1993: The Tornado: An Engineering-OrientedPerspective, NOAA Technical Memorandum, NWS SR-147, 196pp.

Phan, L.T., and E. Simiu, 1998: The Fujita Tornado Intensity Scale: A Critique Based onObservations of the Jarrell Tornado of May 27, 1997, NIST Technical Note 1426.

TABLE 1 MODIFIED F-SCALE (after Minor et. al 1993)

Original* Adjusted**Wind Speed Wind Speed(mph) (mph)

F0 40-72 40-75F1 73-112 75-110F2 113-157 110-140F3 158-206 140-170F4 207-260 170-200F5 261-318 200-240*after Fujita (1971)**after Minor (1993)

Figure 1. Map showing the pathof the tornado that traveled throughMoore, OK. F-scale damage ratingsare shown. Courtesy of NWS Norman.

Figure 2. One of several areas where F-5 damage was foundin Moore, OK. Photograph by Tim Marshall.

Figure 3. Most homes in Moore, Oklahoma had woodenbase plates attached to their foundations with concretenails. Note how little of the nail would penetrate into theconcrete slab.

Figure 4. Scrape mark in the concrete slab of a residencewhere concrete nail (that had secured the base of the wall)was pushed outward. Many homes failed where their wallswere nailed to their foundations.

Figure 5. F-scale damage ratings to residences in one section of Moore,Oklahoma overlaid on an aerial photograph. Courtesy of Greg Stumpf.