LH ATMOSPHERIC
ENGINEERING
Effects of Tornadoes on Urban Areas and Allowance
for in Structural Design
Mohammed Fahad Ashfaq
1243955
University of Birmingham
College of Engineering and Physical Sciences
School of Civil Engineering
Edgbaston
Birmingham
B15 2TT
United Kingdom
1
CONTENTS
1.0. Introduction ... 1
2.0. General Formation of Tornadoes ... 1
3.0. USA – Prime Location for Tornadoes ... 2
3.1. Tornado Effects on Urban Areas – Joplin Tornado... 2
4.0. Allowance for Tornadoes in Structural Design ... 3
5.0. Conclusion ... 4
6.0. References ... 4
Figure 1 - Tornadoes in USA, 2014 (Spc.noaa.gov, 2015) ... 2
Figure 2 - Track and magnitude of Joplin Tornado (U.S. DEPARTMENT OF COMMERCE, 2011) ... 2
1.0.
INTRODUCTION
Tornadoes are columns of air that rotate due to differences in wind velocities and temperatures, forming below cumulonimbus clouds, which make contact with ground level (Met Office News Blog, 2013). There are locations across the globe more prone to tornado formation due to factors such as humidity, temperature and geographical location (LiveScience.com, 2015).
20° to 50° from both sides of the equator locate zones, around which tornadoes are concentrated. These zones provide optimum conditions of differing air masses and high velocity air (jet stream) moving west to east; path differs during the year, both satisfying conditions for tornado production (Eagleman, Muirhead and Willems, 1975). The devastation and magnitude of tornadoes varies significantly. Wind speeds, temperature differences and geographical location all simultaneously affect the level of devastation that can occur. If a tornado is formed in an area of high-speed winds, where there are large temperature differences between air masses, whist being located in an area of high population density, the aftermath is clear to be devastation. Although computer programs can detect tornadoes from Doppler radar data (Nssl.noaa.gov, 2015). There may also be a need for protecting the built-environment and more importantly lives against these natural phenomena. It is crucial for structural designs in locations known to tornado hotspots to implement methods for resistance against such powerful wind vortices with tangential and vertical velocity components, without making designs too costly (Haan, Balaramudu and Sarkar, 2010).
2.0.
GENERAL FORMATION OF TORNADOES
2
3.0.
USA
–
PRIME LOCATION FOR TORNADOES
Formation can also relate specifically to geographical location. Figure 1 maps out the occurrence of tornadoes within the USA. The Central- Eastern regions are a target to numerous tornadoes, due to the rise to conditions suited for tornado generation. Cool, dry air moves westwards over the Rocky Mountains, simultaneously warm, moist air heads northwards from the Gulf of Mexico. The tornado activity occurs when the cool and warm air mix, giving rise to instable atmospheric conditions (thunderstorms) (Eagleman, Muirhead and Willems, 1975). The tornado then forms as in the general case.
3.1. TORNADO EFFECTS ON U RBAN AREAS – JOPLIN TORNADO
The average number of deaths yearly caused by tornadoes fluctuates massively, due to the unpredictability of magnitude and whereabouts a tornado may strike. In USA, between years 2012-2014, number of deaths occurring from tornadoes averaged only 57 yearly (Spc.noaa.gov, 2015). Whilst the year 2011 had inflicted 553 deaths to the country.
The major incident of that year in the USA that lead to such devastation occurred in Joplin, Missouri on May 22nd, which according to National Weather Service killed 158 directly (U.S. DEPARTMENT OF COMMERCE, 2011). The population of Joplin city at 2010 consensus equated to 50,150 people with population density of approximately 580 people per km2 (Quickfacts.census.gov, 2015). Even though less than 0.5% of the
population were killed, over 1,000 people were injured and displaced from their homes (U.S. DEPARTMENT OF COMMERCE, 2011). Without the ability of detecting the formation of the Joplin Tornado, it is clear that the
3 death toll would have risen. All of this damage had been caused within only 40 minutes of tornado formation (Wheatley, 2013). Alongside such an unusually high death toll for the modern era, the costs incurred through damages to properties (commercial and residential) equated to more than $2 billion (Joplin, Missouri hit by EF-5 Tornado on May 22, 2011, 2013). The tornado had a stem of 1.6km and path length of 3EF-5.6km, rated at EF-EF-5 on the Enhanced-Fujita Scale, with wind speeds exceeding 322km/h (U.S. DEPARTMENT OF COMMERCE, 2011).
Figure 2 shows the path Joplin Tornado had taken. A major reason for the unusual high death toll and
destruction was due to the width of the stem, allowing coverage of a larger land mass and the tornado’s ability to reach higher speeds closer to the centre of the urban city. As the path came closer to the inner city, the tornado levelled up further on the Enhanced-Fujita scale until it reached level 5, where it impacted city inflicting huge damages to public facilities, such as schools, hospitals, domestic and commercial property. Eye-wittness accounts confirmed steel roof trusses were rolled up like paper and extreme twisting of main suppport beams in medium rise steel structures (Crh.noaa.gov, 2015). The impact to the extent of damage didn’t end there, the loss of life ripped open the heart of the community. Whilst due to the destruction of numerous workplaces, unemployment increased as a result; Work Investment Board showed an increase in unemployment rate of 8.9% in 2011, which decreased as the recoveration process went underway to 6.9% in 2012 (Joplin, Missouri hit by EF-5 Tornado on May 22, 2011, 2013). Huge disruptions was apparent to the general living of all, as schools were forced to close, with re-construction of some schools in Joplin taking 3 years for completion (Joplinschools.org, 2015). Whilst the impact on the transport infrastructure was further felt due to the masses of debris that lay on roads and severe turning over of approximately 15,000 vehicles (Crh.noaa.gov, 2015). It is evident that the simultaneous effect of high-speed tornadoes and coincidental occurrence in urban cities is the result of such devastation.
4.0.
ALLOWANCE FOR TORNAD OES IN STRUCTURAL DESIGN
It is of utmost importance for the designer to consider designing a structure to withstand load effects from tornadoes in tornado prone areas in a cost-effective manner. Whilst also noting probability of occurrence of tornadoes within the region and having an appreciation of the duration of these extreme events, as they only last for a few hours at most (Wheatley, 2013).
4 vortices. These results were obtained from wind tunnel testing, hence up-sizing the outcomes for such results doesn’t necessarily reflect actual results from tornadoes in reality, due to effects e.g. debris impact on structures that can accumulate with time.
For application in design, from wind tunnel modelling, sensors initiated to detect the surface pressures acting on the walls and roof of the building. These pressures integrated across the area, in which overall force and moment coefficients are calculated. These coefficients are then normalised with respect to the maximum horizontal velocity for modelling the effect of the tornado-like vortices on the structure (Haan, Balaramudu and Sarkar, 2010).
In comparison to normal atmospheric boundary layer conditions used in codes of practice, Haan, Balaramudu and Sarkar, 2010 estimate lateral forces acting on the structure can equate to an increase of 50% the original load induced from straight-lined wind over open terrain from codes. Whilst vertical uplift of 2-3 times greater than suggested in the codes for tornadoes. Case, Sarkar and Sritharan, 2014 showed weaknesses in the structure were heavily induced by the lack of strength from roof connections under these extreme conditions. They concluded that due to the higher probability of lower speed tornadoes occurring, building a structure designed for resistance against wind speeds of up to 73m/s will ensure the safety and cost-effectiveness of the design.
5.0.
CONCLUSION
It is obvious the effects of tornadoes are of damage and destruction to the built-up environment. Whilst the formation of tornadoes only occur in particular regions of the world. Where mixing of hot and cold air with large temperature differences give rise to thunderstorms that indirectly form tornadoes. Their probability of occurrence and geographical location is for the designer to consider with highest regards for stability and safety of their structural design. Lateral loads were 1.5 times greater than those from ASCE-705 provisions, whilst uplift forces were 2-3 times greater than values from provisions, showing the power such phenomena have. It was seen that roof connections are crucial for structural stability of structures in presence of tornadoes for they are the weakest part of the structure, causing structural collapse. Justification between cost, probability of occurrence and safety is key for designing against tornadoes.
6.0.
REFERENCES
Case, J., Sarkar, P. and Sritharan, S. (2014). Effect of low-rise building geometry on tornado-induced loads. Journal of Wind Engineering and Industrial Aerodynamics, 133, pp.124-134.
Crh.noaa.gov, (2015). National Weather Service Springfield, MO - Event Review - May 22, 2011. [Online] Available at: http://www.crh.noaa.gov/sgf/?n=event_2011may22_survey [Accessed 22 Feb. 2015]. Eagleman, J., Muirhead, V. and Willems, N. (1975). Thunderstorms, tornadoes, and building damage. Lexington, Mass.: Lexington Books.
Haan, F., Balaramudu, V. and Sarkar, P. (2010). Tornado-Induced Wind Loads on a Low-Rise Building. Journal of Structural Engineering, 136(1), pp.106-116.
Joplin, Missouri hit by EF-5 Tornado on May 22, 2011. (2013). 1st ed. [eBook] Joplin: Lynn Iliff Onstot Public Information Office. Available at: http://www.joplinmo.org/ [Accessed 22 Feb. 2015].
5 LiveScience.com, (2015). Tornado Alley: Where Twisters Form. [Online] Available at:
http://www.livescience.com/25675-tornado-alley.html [Accessed 20 Feb. 2015].
LiveScience.com, (2015). Tornado Facts: Causes, Formation & Staying Safe. [Online] Available at: http://www.livescience.com/21498-tornado-facts.html [Accessed 20 Feb. 2015].
Met Office News Blog, (2013). What are tornadoes? [Online] Available at:
https://metofficenews.wordpress.com/2013/05/21/what-are-tornadoes/ [Accessed 20 Feb. 2015]. Nssl.noaa.gov, (2015). NSSL: Severe Weather 101: Tornado Detection. [Online] Available at: https://www.nssl.noaa.gov/education/svrwx101/tornadoes/detection/ [Accessed 20 Feb. 2015]. Quickfacts.census.gov, (2015). Joplin (city) QuickFacts from the US Census Bureau. [Online] Available at: http://quickfacts.census.gov/qfd/states/29/2937592.html [Accessed 21 Feb. 2015].
Spc.noaa.gov, (2015). [Online] Available at: http://www.spc.noaa.gov/climo/torn/STATIJ11.txt [Accessed 21 Feb. 2015].
Spc.noaa.gov, (2015). Annual Fatal Tornado Summaries. [Online] Available at: http://www.spc.noaa.gov/climo/torn/fatalmap.php [Accessed 21 Feb. 2015].
Spc.noaa.gov, (2015). Storm Prediction Center Annual Report Summary - 2014. [Online] Available at: http://www.spc.noaa.gov/climo/online/monthly/2014_annual_summary.html# [Accessed 20 Feb. 2015]. U.S. DEPARTMENT OF COMMERCE, (2011). NWS Central Region Service Assessment Joplin, Missouri, Tornado – May 22, 2011. Kansas City, MO.
