Joplin,+Missouri+Tornado+Disaster

//Christina DiPaolo, BAE/MAE, Penn State 2012// toc
 * Joplin, Missouri Tornado Disaster**

Introduction
On May 22, 2011 the 8th worst tornado in US history ripped through Joplin, Missouri taking with it 142 lives and leaving over 750 people injured. Rated an EF5 tornado, with wind speeds upwards of 200 mph, the tornado damaged an estimated 8000 structures costing roughly $2.8 billion. The most destructive tornado since before modern weather record keeping in 1950, structural engineers from around the country are trying to learn if such a loss of life can be prevented. 

Figure 1 shows the path of the tornado through Joplin, which directly impacted an area approximately ¾ of a mile wide and 22 miles long (FEMA 2011). This tornado remained on the ground the entire length of Joplin causing severe damage. The vast majority of the structures damaged were residential as well as an estimated 400 commercial buildings, several schools, two fire stations, and a hospital (CHNI 2011).

Keywords
Joplin tornado, multiple-vortex tornado, wind uplift, tornado resistant buildings, safe rooms

Tornado Background
The United States experiences an average of 1200 tornadoes a year, however tornado tracking methods have changed so much in the past few decades that the records are incomplete. Although there are so many tornadoes each year, the average chances of a tornado hitting a specific plot of land is once in every thousand years. However, it varies throughout the country. The tornado that tore through Joplin, Missouri on May 22 is believed to have been a rare multiple-vortex tornado. A multiple-vortex tornado has "two or more small and intense centers of rotation orbiting the larger funnel" (News Leader 2011). These rare tornadoes are often the most destructive due to these smaller centers of rotation, called subvortices or suction vortices. The multiple vortexes create strong suction areas that often level buildings within seconds. Multiple-vortex tornadoes usually have from two to five subvortices, which last less than a minute. The nature of these multiple-vortex tornadoes creates extremely uneven damage patterns. One area could experience the EF-5 level wind speeds of over 200 mph while the neighboring area within the damage path experienced very weak damage (Edwards 2011). By the short time when the tornado first hit the city of Joplin it was already an EF-3 storm and continued to increase as it crossed the city, ultimately reaching the EF-5 level. The table above shows the breakdown of the wind speeds per the Enhanced Fujita (EF) scale currently in practice in the United States. It is important to note that the wind speeds in the EF scale are not directly measured but rather are 3-second gusts estimated by the damage assessed after the tornado (Edwards 2011). When tornado speeds are measured they are referred to as the "one-minute-mile" speed. Figure 2 below shows a multiple-vortex storm.
 * ~ EF Rating Wind Speeds ||
 * = EF Rating ||= 3 Second Gust (mph) ||
 * = EF-0 ||= 65-85 ||
 * = EF-1 ||= 86-110 ||
 * = EF-2 ||= 111-135 ||
 * = EF-3 ||= 136-165 ||
 * = EF-4 ||= 166-199 ||
 * = EF-5 ||= over 200 ||

Current Codes
Most buildings in the United States are currently designed to withstand a three second gust of 90 mph winds outside of hurricane regions. The most recent ASCE 7 standard (ASCE 7-10) has increased that gust to 115 miles per hour; however this increase would clearly not stand up to 200 plus mph winds as seen in Joplin, Missouri. In hurricane regions, which often experience many tornadoes, the building code requires engineers to design to as high as 140 mph. The difference between the designs of homes in the two regions is the detailing. Basic hurricane ties provide the tie downs necessary to keep the weak points of the structure together. When resisting lateral loads, the structure needs to have a continuous load path. Unfortunately, under today’s building codes a structural engineer is not required to design most residential lightweight wood structures and therefore basic load paths are often overlooked (Prevatt 2011). Due to the extremely low chances of a building being hit by a tornado, the code will most likely not change after this disaster, requiring engineers to design for tornados. However, this hopefully will not stop many engineers in regions like Joplin, MO from designing more durable structures. ASCE plans to draft code recommendations when the studies in Joplin and Tuscaloosa, the site of another deadly EF5 tornado in April of this year are complete.

Summary of Failures
Over 8000 structures were damaged on May 22. The bulk of the structures damaged were residential as well as an estimated 400 commercial buildings, several schools, two fire stations, and a hospital (CNHI 2011). Obviously, the majority of buildings directly in the line of the tornado were flattened, but many buildings were damaged at much lower wind speeds as well. Different damage was found with different construction types. According to the National Institute of Standards and Technology (NIST) technical study being conducted, critical and high-occupancy buildings in Joplin did not perform better than the lower-risk categories for the same construction type (Newman 2011). Most of the buildings impacted by the tornado suffered complete loss of function. Reinforced concrete frame and steel frame performed the best, however still suffered complete loss of function. Most of the other construction types suffered partial or complete collapse, including pre-cast concrete, metal, concrete and brick masonry, and wood-frame. In addition, there were no designated public safe rooms or tornado shelters in Joplin, and most buildings did not have basements (Newman 2011). Here pre-engineered metal buildings and heavy concrete and steel frame construction types will be discussed, because they fared the worst and best respectively. In addition steel frame buildings and light weight wood frame structures, which were the most common structure in Joplin.

//Pre-Engineered Metal Buildings//
Pre-Engineered Metal Buildings performed the worst in the tornado. The stability in these structures is in their cladding. When the tornado ripped off their cladding the structures were destroyed. These structures use such light steel members that they would not be able to stand without the cladding. If heavier steel were used many of these buildings may not have crumpled as they did, however, the light steel members are the reason this building type is chosen. With heavier steel the structures would just get too costly to be pre-engineered.

**// Steel Frame Structures //**
Structures that performed badly were steel box frame structures. These buildings are built quickly and with a short lifespan. The long spans and light loads make this the preferred structure for large warehouses, like Home Depot or Walmart. Both a Home Depot and Walmart collapsed during this tragedy crushing shoppers inside. This construction type pose an incredible risk to the public in a tragedy such as the EF-5 tornado due to the high occupancy. These structures depend mostly on their exterior cladding to be their lateral resistance and their roof is the only horizontal diaphragm. When a tornado rips through however, the cladding and roof are the first chucks of the building it rips off. Without its lateral resistance the building simply crumbles. Some engineers feel that these structures should be built with moment-resistant connections in the future to give it more lateral stability ( Gregerson 2011). This would act as the backup lateral system that could keep the building standing long enough for occupants to get to a safe area.

**// Light Weight Wood Frame //**
The majority of the buildings impacted by the tornado were light weight wood frame construction. Most of the light frame construction was lacking basic continuous load paths (Prevatt Oct 2011). Unfortunately, single family residential structures are often not designed by structural engineers leaving the structural design up to the builders. Under gravity loads, these building are extremely repetitive. This allows loads to redistribute as necessary in the event of a problem. Due to the pin-pin connections in wood framing, lateral systems depend on shear walls to transfer the loads. The shear walls are constructed by applying sheathing to the exterior walls. Without continuous load paths from walls to the diaphragms to the ground, any one of these connections could easily fail and the loads would not be redirected. If builders are not careful with how they frame the home, continuous load paths will be interrupted. Figure 3 shows an image of a house that did not have a continuous load path. The roof section stayed intact with minimal structural damage but the entire assembly was lifted off the rest of the house. Tim Reinhold (the chief engineer at the Insurance Institute for Business and Home Safety) tested tornado winds on houses. He found that at high winds light weight wood frame homes simply pop. When remarking how quickly the homes got destroyed Reinhold state he was surprised, "The door popped, and four seconds later, there's nothing." If metal straps were used the helped the buildings at lessor loads (Keen 2011). Reinhold observed that when metal straps were used the homes usually lasted longer. In lower wind speeds as were found Unfortunately, the exact same continuous load path issue was observed just a month earlier at the Tuscaloosa, AL tornado on April 27, 2011. The engineers who studied that tornado that ripped through the University of Alabama also found that many of the structures on the outskirts of the tornado could have been saved with basic hurricane detailing and continuous load paths. Dr. David Prevatt of the University of Florida studied both the tornado damage and Tuscaloosa, AL and Joplin, MO. Dr. Prevatt performed case studies on select buildings throughout Joplin. Prevatt found that most homes that were on the outskirts of the damage had this continuous load path problem. He suggests using hurricane detailing in homes with risk to tornadoes (Prevatt July 2011).

// Heavy Frame Construction //
Heavy frame construction fared much better. St. John’s Regional Medical Center had two towers that were structurally intact after the tornado. One was reinforced concrete and the other was steel framed with concrete deck. Most of the damage to the hospital was due windows shattering and parts of the exterior wall and upper floors. Figure 4 shows the hospital right after the tornado. The top image confirms that most of the structural systems stayed intact but the bottom image shows the damage in the ICU ward from windows and debris. Clearly, the hospital had to be evacuated and temporary facilities started in tents outside shortly after the tornado. Most people were sent to other hospitals. The structure has since been deemed structurally unsound, however it did not collapse ( Gregerson 2011). Although hospitals have more stringent design criteria than most buildings, it also had to be evacuated and four people lost their lives from debris and windows breaking. In addition, NIST has concluded in its intial technical study that buildings with higher occupancy did not perform any better than buildings of low occupancy in the same construction type. Ideally, hospitals would not need to close and could provide care immediately following the tragedy. Consideration of the facade and protection of windows would need to be addressed. "Tornado-proofing" a structure is extremely costly. These facades would need to be designed for drastic changes in atmospheric conditions and withstand debris acting a a pojectile missile. However, much can be learned from the St. John's Regional Medical Center as its structure, both the steel and concrete towers, stayed intact long enough for people to evacuate.

Recommendations
Designing for tornadoes is extremely costly. In addition to extremely high wind loads, the building must be designed for dramatic changes in atmospheric conditions and be able to withstand debris acting as projectile missiles (Spann 2011). Most engineers asked to consider tornadoes recommend creating a safe-zone where people can go to safely wait out the storm. FEMA has two standards to address design criteria for safe rooms FEMA 361 and FEMA 320. Safe rooms or areas designed to this criteria meet the FEMA goal of near absolute protection of the occupants. FEMA 320 even has specifications and plans for homeowners and business owners eager to build a safe place with protection from tornadoes. These public safe rooms are required to have 5sf per person who can stand and 10sf for people in wheelchairs. This approach is much more cost effective. Most safe zones in new construction cost between $6,500 and $8,500 but can go as high as $11,500 to $13,500 for larger rooms with more comfort (FEMA 2011). In residences, homeowners are also encouraged to build a safe room. The criteria for a single family home is slightly different. Safe rooms are only required to have 3sf per person. National Storm Shelter Association (NSSA) provides product listings in order for owners and designers to ensure the correct life safety components are used (FEMA 2011). In addition to the safe zone, many homes in tornado regions could be saved if basic hurricane detailing were designed and provided continuous load paths. The homes in the direct line of the tornado could still be destroyed, but structures further from the eye of the storm could be salvaged.

Conclusions
The tornado that destroyed buildings, homes and lives in Joplin, Missouri left the country searching for answers. 8000 structures collapsed and 142 lives were ended abruptly. Single family homes simply are not going to be designed to completely withstand the worst tornadoes, but it is possible to save the lives of the occupants. In tornado regions buildings should be building with safe-zones made entirely of concrete. These safe zones or rooms would be able to withstand the wind, suction, and even debris acting as projectiles. The rest of the home should have hurricane ties and continuous load paths. Pre-engineered metal buildings should have backup lateral resistance for when the cladding and roofs are blown off the structure. Heavy construction, concrete and steel, performed well against the tornado and were left pretty much intact, however the exterior walls were destroyed and in the process took out some of the floors. In conclusion, it is extremely hard to “tornado-proof” a building, but certain precautions can be made in order to save lives. No buildings are going to be perfect after a storm of this magnitude but if more buildings can stay standing, like St. John’s Regional Medical Center, with just a few more modifications, lives would be saved. We need to learn from the tornadoes in Joplin, MO.

Additional Resources
Gregerson, John. (July 4, 2011) “Kansas Tornado Town Rebuilding with an Eye Toward Sustainability.” Engineering News-Record.
 * An article that follows how another town chaged building practices even though the code never changed.

Joplin Tornado: Rapid Deployment Damage Assessment Team. Hosted by CAPS, University of Alabama. <http://esridev.caps.ua.edu/JoplinTornado/> (November 14,2011)
 * This site is an interactive map of the city of Joplin, MO with images and case studies of buildings and damage throughout the area.

Parfitt, Kevin. (May 24, 2011) “Joplin Disaster: Tornado Causes Almost Total Destruction.” Building Failures Forum. < http://buildingfailures.com/2011/05/24/joplin-disaster-tornado-causes-almost-total-destruction/> (October 1, 2011)
 * This article gives background information on the tornado. This article also gives the three second gust speeds for tornado classifications.

Staff. (May 25, 2011) “Joplin EF-5 twister deadliest single tornado in 60 years; 123 dead, 750 injured; 8,000 structures damaged.” News-Leader. <http://www.news-leader.com/article/20110525/NEWS01 /105250418/Joplin-EF-5-twister-deadliest-single-tornado-60-years-123-dead-750-injured-8-000-structures-damaged> (October 2, 2011)
 * This article is a quick overview of the tornado.