Abstract
The Minneapolis Metrodome, a sport stadium in Minneapolis, Minnesota (USA) faced an engineering failure in 2010, 28 years after its opening in 1982. After a major snow storm in December of 2010, large piles of snow accumulated on roof of the stadium. The roof of the Dome has an air-supported structure which mean that it has a dual layer roof made up of Teflon-coated fiberglass and fabric which is inflated by pressurizing the internal area of the stadium. This whole roof structure is externally held by cables and an edge ring revolving all around the circumference of the stadium. The snow and ice on the roof damaged the and broke one of the roof panels, which depressurized the stadium and deflated and collapsed the whole roof. There were no injuries caused because of this engineering failure and the main security measures for such a structure were followed correctly, however more maintenance check should have been done. Strength tests and deterioration checks of the roof material were not done regularly which are the reason why the roof collapsed. The collapse of the roof could have been avoided and the observations done in the official engineering evaluation report have been analyzed and explained.
Introduction
The Hubert H. Humphrey Metrodome, or more famously known as the Minneapolis Metrodome, is a sports stadium located in Minneapolis, Minnesota in the United States of America [1]. It was opened in 1982 and was the home stadium for the several American sport teams including the NFL’s (American football) “Minnesota Vikings”, the MLB’s (Baseball) “Minnesota Twins”, and the NBA’s (Basketball) Minnesota Timberwolves as well as other smaller sport teams [2]. In December of 2010 the stadium faced a strong winter storm causing almost half a meter of snow (0.43m) [4] to accumulate on its roof leading to a disastrous collapse of the roof panels. Fortunately, there were no injuries however even after renovations and reconstructions of the roof and stadium, three years later, on December 29th, 2013 the Dome was officially closed and later demolished in 2014. This incident of 2010 was the last of 4 other roof incidents that occurred in 1981, 1982, and 1983 due to accumulation of snow and in 1986 due to strong winds [3]. There was clearly a design flaw that could not withstand certain weather conditions, in this case snow, ice and strong winds, causing these continuous roof failures. The incident of 2010 was the most fatal in terms of damage done to the whole structure of the roof as the ice and snow formed holes in the panels leading to depressurization of the internal space of the stadium and massive tears in two panels around the center of the roof and midfield line. This essay will investigate how the snow and ice load caused a stress on the roof and lead to the depressurization and collapse of the roof of the stadium. The materials and structure of the roof will be analyzed and linked to the events of 2010 to explain why this engineering failure occurred.
Roof Design
The roof on top of the Minneapolis Metrodome was an inflatable roof that covered 40’500 (m2) [13] of the stadium. The total mass of the roof reached approximately 260’000 (Kg). It has a double layer design with Teflon-coated fiberglass (on its external layer) and fabric on its internal layer with specific acoustical properties for the noises and cheers in the stadium [5]. Due to Minneapolis’s cold and snowy weather it was important to find a way to melt the snow that would accumulate on top of the roof. That is why between the two layers a stream of hot air circulates around the roof to melt the snow.
Around the whole upper rim of the stadium there was an edge ring and external cables connected to the roof to allow for a stress distribution around the circumference of the dome. This ring has a slight slope to it to follow the angle of the rounded roof fabric to support more roof cable loads. In addition, this ring has a gutter to remove the melted snow or rainwater [5]. These special features of the roof allowed to lower overall costs of the stadium without compromising on quality of the materials used.
Roof Collapse
The engineers responsible to supervise, understand and calculate the stability and strength of the structure are from a company called “Geiger Engineers”. Located in northwest New York, Geiger Engineers are a structural engineering company founded in 1988 by David Geiger. They are particularly known for having worked on several domes including the stadiums for the 1988 summer Olympics in South Korea [6].
After the roof collapse of 2010, on the 2nd of April of 2011 Geiger Engineers published and official report regarding the collapse. The following report is titled: “EVALUATION REPORT OF THE DEFLATED ROOF OF THE METRODOME. For the METROPOLITAN SPORTS FACILITIES COMMISION, Minneapolis, MN.” [7] They explain that the collapse was caused by an accumulation of snow on the roof which could not be removed without endangering the workers to manually remove it. Due to the excess snow, the fiberglass panels became very unstable causing the snow to slide down panel 104 and on the edge ring. This led to a rupture in the panel, depressurization inside the dome and due to that, deflation of the internal roof panels. During the deflation, snow damaged the exhaust fans which applied an increasing stress on panels 23 and 24 in the center of the stadium, at the height of the midfield line. This stress caused the panel failures and collapse of the whole roof after 2 weeks of it being exposed to more snow and ice.
Material Analysis
The roof of the Minneapolis Metrodome is a dual layer consisting of (Teflon coated) fiberglass and acoustical fabric [5]. The main cause for the deflation and collapse of the roof is due to the excess weight of the snow on the edge ring and roof panels, therefore the materials used could not withstand the stress from that amount of snow. The following section will describe the mechanical and thermal properties of the exterior material of the roof.
The exterior layer is a Teflon-Coated fiberglass or a PTFE fiberglass. Fiberglass also known as a Glass Fiber Reinforced Polymer (GFRP) is a composite usually made of continuous fibers, in this case glass fibers, embedded in polyester or epoxy polymers. The Domes roof is an epoxy embedded fiberglass which is coated with Teflon, also known as Polytetrafluoroethylene (PTFE). PTFE is a material typically used for non-stick pans and spatulas because of its low friction, water repellant and stable material properties [10]. This material is typically used in tensile-membrane-systems[1] (like this stadium) and has been used since the 1970s and today many facilities including stadiums, museum and exhibition spaces use this roofing material [8].
It is a very durable material which has a lifespan of around 30 years and can resist temperatures between -73°C and +232°C. It has a tensile strength of around 3400 to 4000 MPa and will not become brittle over time. Even though it can sustain very high temperatures the panels will reflect most of the solar heat (73%) therefore also providing thermal insulation and only absorbs around 10% [9]. The panels can be very transparent and that is why the inside of the Minneapolis Metrodome was also lit with natural daylight. It is a non-flammable material and according to the BS476 pt6 of the fire propagation test it has a (UK) Class 0[2], meaning that: “Protects your surface from the spread of flames AND limits the amount of heat released from the surface during a fire” [8]. It is a very light and flexible material roughly weighing 1.5 Kg per m2 and very high elastic properties with a modulus of elasticity (Young’s Modulus) of around 72 GPa. These are the mechanical properties of PTFE coated fiberglass however below is a table with the mechanical and thermal properties of GFRP and PTFE according to “CESEduPack” [10]:
Mechanical Properties
Material
Fiberglass (GFRP)
Teflon (PTFE)
Young’s Modulus (GPa)
15-28
0.4-0.552
Yield Strength (MPa)
110-192
15-25
Tensile Strength (MPa)
138-241
20-30
Compressive Strength (MPa)
138-207
16.5-27.5
Elongation (%strain)
0.85-0.95
200-400
Hardness (Vickers) (HV)
10.8-21.5
5.9-6.5
Fracture Toughness (MPa.m^0.5)
7-23
1.32-1.8
Table 1 [10]
Thermal Properties
Material
Fiberglass (GFRP)
Teflon (PTFE)
Melting Point (°C)
___
315-339
Glass Temeparture (°C)
147-197
107-123
Max. Service Temperature (°C)
140-220
250-271
Min. Service Temperature (°C)
-123-(-73.2)
-263-(-253)
Conductor/
Insulator
Poor Insulator
Good Insulator
Thernal Conductivity
0.4-0.55
0.242-0.261
Specific Heat Capacity (J/Kg.°C)
1e3-1.2e3
1.01e3-1.05e3
Themal Expansion coeff. (strain/°C)
8.64-33
126-216
Table 2 [10]
The interior layer of the roof is an acoustical fabric used to alter and absorb sound for games and concerts. The Teflon coated fiberglass and this acoustical fabric formed the panels of the roof which were held by the edge ring on the exterior diameter of the stadium and inflated by keeping a constant pressurized air inside the stadium.
Air-supported Roof
The Hubert H. Humphrey is a stadium with an air supported roof. This technology was invented by the same structural engineers who worked on this stadium, Geiger Engineers, and today air supported structures are used for many sport facilities like indoor tennis courts, swimming pools and athletic fields [11]. This type of roof structures allows for much lower material costs compared to other roofing types however there are specific component which all air supported structures (including the Dome) must follow.
The roofs are made up of dual layered panels of the PTFE fiberglass and fabric and these panels are inflated by internally pressurizing the air of the stadium. High output fans are used to pressurize the whole area of the structure and these must maintain a constant internal pressure and temperature. This system will only work if the internal pressure is larger than or equal to the external pressure applied to the roof [11], [12].
Pinternal ≥ Pexternal
The Dome was built with 20, 90-Horsepower fans which pressurized the environment of the stadium. These fans will blow warmer air during the winter to heat up the panels and melt the excess snow deposited on top of the roof [14]. The stadium is not an airtight design however with Airlock type doors and the fans any air leakage can be overcome with the continuous pressurization of the structure. The airlock doors are obviously necessary as entrances/exits to the Dome [11].
Maintenance on air-supported roofs is very important because of snow accumulation. This was the reason why the Dome roof collapsed in the first place and very specific maintenance procedures should be followed to avoid this problem. Firstly, the air pressure and therefore temperature should be increased inside the dome, especially during winter seasons, to heat up the panels and melt or break the snow. Secondly, snow can build up around the edge ring (circumference) of the stadium and this will cause an increased side pressure on the structure and cable holding the roof. This type of maintenance must be done manually therefore safety of humans must come first. People should not be in danger while removing the snow on top of the roof and one must always pay close attention that no one is standing below the snow buildup on the edges. And thirdly a scheduled maintenance plan should always be followed and certain weather conditions during the winter seasons and snow storms should be predicted and monitored to ensure maximum safety. In addition, according the American National Fire Protection Association (NFPA), Safety code 102 annual inspection and maintenance of such structures must be made [11].
Evaluation of Collapse
After understanding the materials and roof structure type we can understand why the roof of the Minneapolis Metrodome Collapsed according to the conclusion made after the evaluation report of Geiger Engineers.
In 2010 after the strong snow storm, half a meter of snow formed on top of the roof panels and the snow and ice damaged some parts of the panels as it slid down to panel 104 (at the edge of the roof). The pressure that the load of snow exerted on the fiberglass could not be sustained and this resulted in the panel failure and depressurization of the stadium [7]. Part V. of this essay analysed the temperature which PTFI-coated fiberglass could sustain (-73°C to +232°C) and the tensile strength of the PTFI coated fiberglass being of around 3400 MPa. Since the snow and ice would only reach around 0°C the temperature was not a factor causing the panel failure. However, with time the strength of the roof panels will decrease especially if strong forces and masses of snow act against it. Geiger Engineers completed several tensile strength tests and they found out the trapezoidal tear strength (material ability to resist continuous tear form sharp object [13]) and the strip tensile flexfold strength (flexibility, folding of material) were significantly lower than the expected results of the virgin fabric strength results during the last tests of 2003 [7]. These results were recorded after an electron microscope test of the fabric. Below is a table of the strength results as a percentage compared to the ideal material strength.
Test
% to virgin material
comments
Strip Tensile (Dry)
Warp (Direction)
Fill (Direction)
95%
100%
Good
Good
Strip Tensile (Wet)
Warp
Fill
>100%
>100%
Good
Good
Strip Tensile Flexfold
Warp
Fill
74%
79%
Not so good
Not so good
Trapezoidal Tear
Warp
Fill
75%
75%
Not so good
Not so good
Table 3 [7]
As we can see in the table the trapezoidal tear strength and flexfold were much lower than the virgin material strength. This is due to the age of the fabric and smaller deterioration (caused by snow and ice) with time on random parts of the roof panels. This decrease in strength will lead to a lower overall ultimate tensile strength, therefore with a higher stress/pressure on the panel it is more likely to fail. The Geiger Engineers report explain how these small panel deteriorations were found in several random place of the 40500 square meter roof therefore it was close to impossible to solve all the issues without replacing the entire roof. As well as the small panel deteriorations, the electron microscope test found that there was less Teflon-coating protection on the fiberglass which explains the moisture and discoloration of the fabric due to water infiltration.
Only one panel failure resulted in the complete collapse of the Domes roof. Once panel 104 broke, the depressurization of the dome caused a complete deflation of all panels. In this case:
Pinternal < Pexternal
This mean that the air supported roof structure cannot work. Once the panels completely deflated the piles of snow and ice slid down to the center of the domes roof leading to a rupture of panels 23 and 24 at the height of the midfield line. None of the other cables or edge rings were damaged during the collapse however the roof panels were unusable and these the consequences of the failure of panel 104 [7]. That is why the Engineer of recorded for the Dome concluded that: “it is my professional opinion that the only acceptable solution at this point is a complete replacement of the roof fabric to eliminate any uncertainty as to its integrity” [7].
Safety measures and Responsibilities
Part VI. of this essay explains about the maintenance that should be done on the roof. Even though this resulted in a collapse of the roof all safety measures were followed successfully and most procedures to avoid this disaster were done. Firstly, the air pressure and temperature of the dome was increased to 26.7°C while releasing warm air through the panel layers to melt the snow. Secondly the reason why the amount of snow from the roof could not be removed is because the life of the workers would be out at risk. There was too much snow and the possibility for the panels to collapse was very high and that is why, thankfully, no one got injured because of this collapse. And finally, the weather conditions were expected to be like this as they are very common for a city like Minneapolis and the high wind speeds as well as snow is normal to occur during winter seasons. On the other hand, more, maintenance should have been done to keep this stadium in the conditions for a such a disaster to not occur. The roof collapse occurred in 2010 and the last strength test were 7 years earlier in 2003. This is a too long time period between tests and a proper maintenance on the complete roof was not done since its construction. It would have costed a lot of money to replace the roof multiple times, but this could have happened, for example during a Baseball game, and could lead to injuries and deaths. The people’s safety should always be the main priority. The evaluation report from the engineer of record from Geiger Engineers was very honest and showed professionality on the workplace and stated & justified that mistakes his company made. However, as said in part V. of this essay, Teflon-coated fiberglass will last for around thirty years and from 1982 to 2010 the 5 roof collapses that occurred during its lifetime were at the beginning during construction and at the end after this almost exact 30-year period [7] after the definitive demolition in 2013 [1].
ROOF BREAKES WITH ONE PANEL ONLY????
References
[1] “Metrodome (Demolished) – Minneapolis, Minnesota – Bob Busser.” Bob Busser Clemson University Littlejohn Coliseum Pt1 Clemson South Carolina Gallery RSS, ballparks.smugmug.com/Metrodome-demolished-Minneapolis-Minnesota/.
[2] “Hubert H. Humphrey Metrodome.” Wikipedia, Wikimedia Foundation, 10 Dec. 2018, en.wikipedia.org/wiki/Hubert_H._Humphrey_Metrodome#Roof.
[3] Osornio, Julian. “2010: Metrodome.” Learning from Building Failures, 22 Feb. 2015, buildingfailures.wordpress.com/2010/02/05/metrodome/.
[4] Wiegler, Laurie. “Tearing into the Metrodome: Are Other Air-Pressurized Stadiums Unsafe and Outmoded?” Scientific American, 20 Jan. 2011, www.scientificamerican.com/article/tearing-metrodome-pressurized-stadiums-unsafe/.
[5]Xu Bing: Square Word Calligraphy Classroom – The Miriam and Ira D. Wallach Art Gallery, Columbia University. Libraries. Digital Program Division, www.columbia.edu/cu/gsapp/BT/DOMES/METRODM/m-fabric.html.
[6]“About.” About | Geiger Engineers, www.geigerengineers.com/about.
[7]Evaluation Report of the Deflated Roof of the Metrodome. Geiger Engineers, 2 Apr. 2011, www.leg.state.mn.us/docs/2017/other/170773.pdf.
[8] “PTFE Fiberglass.” Structurflex, www.structurflex.com/materials/ptfe-fiberglass/.
[9] “PTFE Fiberglass.” PTFE, or Polytetrafluoroethylene, Is a Teflon®-Coated Woven Fiberglass Membrane That Is Extremely Durable and Weather Resistant., BIRDAIR, www.birdair.com/tensile-architecture/membrane/ptfe-fiberglass.
[10] CESEduPack, RAS Newcastle University, https://ras-gateway.ncl.ac.uk/Citrix/RASWeb/
[11] “Air Supported Structures.” Key Guidelines for Securing Public Web Servers | The Hanover Insurance Group | Risk Solutions, www.hanover.com/risksolutions/air-supported-structures.html.
[12] “Air-Supported Structure.” Wikipedia, Wikimedia Foundation, 9 Nov. 2018, en.wikipedia.org/wiki/Air-supported_structure.
[13] O’Fallon, Jolyssa. “Home.” TestResources | Test Machines, Grips and Fixtures, www.testresources.net/applications/test-types/tear-test/trapezoid-tearing-strength-of-fabrics/.
[14] Xu Bing: Square Word Calligraphy Classroom – The Miriam and Ira D. Wallach Art Gallery, Columbia University. Libraries. Digital Program Division, www.columbia.edu/cu/gsapp/BT/DOMES/METRODM/intro.html.
[15] Image 1, Guy, Minnesota Ticket. “Minneapolis Metrodome.” Story of the Decade: Metrodome Roof Collapse, metrodome.blogspot.com/2010/12/story-of-decade-metrodome-roof-collapse.html. Image
[16] Image 2, http://www.meteorologynews.com/wp-content/uploads/2010/12/MetrodomeCollapse3.JPG
[17] Image 3, https://www.albertleatribune.com/wp-content/uploads/2013/12/1227.dome_.1.jpg
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