Hubert+H.+Humphrey+Metrodome+Roof+Snow+Collapse+of+2010

Hubert H. Humphrey Metrodome //Michael Kostick, BAE / MAE The Pennsylvania State University 2012 // toc
 * Minneapolis, Minnesota - December 12, 2010 **

Introduction
In the early morning hours (5:03 A.M.) on December 12, 2010, the air-supported domed roof of the Hubert H. Humphrey Metrodome in Minneapolis, Minnesota collapsed in the midst of a heavy snow storm. Opened in 1982, the Metrodome, as it is commonly referred to, has been host to numerous venues, and currently serves as the home to the University of Minnesota Golden Gophers' baseball team as well as the National Football League's Minnesota Vikings. Notable former occupants include the University of Minnesota Golden Gophers football team (1982-2008) and Major League Baseball's Minnesota Twins (1982-2009). A severe winter storm coupled with high winds dumped more than seventeen inches of wet snow in the Minneapolis region over the weekend of December 10 - 12. The resulting loading caused the fiberglass fabric roof to sag, tear, and deflate, inverting the structure in upon itself.

Events Leading Up to the Collapse
Over its twenty-nine years of service, the Hubert H. Humphrey Metrodome has been no stranger to failures involving its inflatable roof structure. In fact, the December 12, 2010 roof collapse marks the fifth time in the Metrodome's history the domed roof has failed. Three of the four previous roof collapses, in 1981, 1982, and 1983, have been attributed to snow, while the fourth failure, in 1986, was due to strong winds. (Parfitt, 2010) Figure 1 shows the Metrodome's original roof inflated in 2007. The Metrodome's fabric roof is regularly inspected by its original manufacturer and installer, Birdair Inc, with the last inspection taking place in April of 2010. The Metrodome roof structure is actually composed of two layers, a 1/32nd of an inch outer Teflon membrane and a 1/64th of an inch woven fiberglass inner liner. (Engineering News Record, 1981) According to the inspection report, Birdair rated the outer membrane as "fair to good" while the inner liner was classified as "poor" due to minor holes in the fabric and accumulation of dirt from years of service. (Birdair Inc. Inspection Report, 2010) Fabric evaluation determined that the strength of the fabric at the time of inspection was comparable to the original specified installation strength. Although Birdair classified the overall roof structure as "good," they recommended that the entire roof fabric eventually be replaced in the near future. (Duchschere, McEnroe, Brown, 2010)

Birdair suggested in a previous inspection, five years earlier, that the Metropolitan Sports Facilities Commission (MSFC) begin planning to replace the fabric membrane of the roof. They estimated the replacement cost between 12 and 15 million dollars with the planning and implementation of a new roof system taking about five years. At the time of the April 2010 inspection, after consulting with Birdair Inc., the Engineering and Facilities Staff of the MSFC decided that the roof still had serviceable life and would remain in use at least until the next scheduled inspection, four years in the future. (Agenda for the Metropolitan Sports Facilities Commission, July 15, 2010)

==Collapse== A severe winter storm, which had been developing in the upper Mid-West of the United States, moved into the Minneapolis region in the early morning hours of Saturday, December 11. Over the next day and a half, the storm system dumped more than seventeen inches of snow in the Minneapolis area, making it the fifth largest snowfall on record for the region. (National Oceanic and Atmospheric Administration, 2010)

Anticipating the approaching storm, maintenance crews at the Metrodome took preventative measures, Friday evening, December 10, 2011, by heating the internal temperature of the dome to around 80 degrees while pumping warm air into the cavity separating the inner and outer layers of the roof structure.(Depass, Zulgad, McGrath, 2010).

Early Saturday morning, the Metrodome's technical service manager, Leo Piddle, noticed the alarming rate at which the snow was falling. The snow fall coupled with high winds was causing a drifting effect on the roof of the Metrodome, and the facilities' snow removal crew was immediately called in. With the use of steam-heated water, the crew would attempt to melt the accumulating snow on the roof; however, by the time mid-day approached, the storm seemed to be winning the battle. Wind gusts were brutal all day, knocking several workers over. By about six o'clock in the evening, the conditions on the roof of the Metrodome were considered to be a threat against life safety, and all workers were subsequently removed from the dome. At this time, Steve Maki, Director of Engineering and Facilities at the Metrodome, stated that the roof was beginning to sag in the center. (Duchschere, McEnroe, Brown, 2010)

Snow continued to fall from Saturday evening through early Sunday morning. According to Weather Underground meteorologist Jeff Masters' blog, the storm system that passed through Minneapolis over the weekend of December 10 through 12, produced snow fall with a snow to water ratio of 9:1. This would create a ground snow load of approximately nine pounds per square foot. (Masters, 2010) With snow accumulating to over feet deep in some areas of the Metrodome roof, the fabric structure became overstressed.

At approximately 5:03 A.M. on Sunday December 12, 2010, a sliding mass of snow and ice broke free and slid down the roof, slicing a gaping hole in fabric panel number 104, seen in Figure 3. Although the internal pressurization system of the Metrodome was designed to compensate for minor tears, a hole of this magnitude resulted in the depressurization of the space and ultimately the collapse of the dome. As the structure inverted, the accumulated snow and ice rushed towards the center. Upon impact with roof equipment, the sliding mass caused tears in fabric panel numbers 43 and 44, directly over midfield. (Maki, 2011) Figure 4 shows the accumulation of snow at midfield due to the failure of fabric panel numbers 43 and 44. Maki stated that it was his intention to begin clearing snow again that morning at 8 o'clock A.M. (Riddle, Ernster, 2011) The collapsed fabric roof came to rest atop the inverted support cables as can be seen in Figure 2.
 * = [[image:failures/Metrodome_Fabric_Panel_104_MSFC.jpg width="400" height="300" caption="Figure 3: Tear in Metrodome roof at fabric panel 104. (Photo courtesy of Metropolitan Sports Facilities Commission)"]] ||= [[image:failures/Metrodome_Fabric_Panels_43_&_44_midfield_MSFC.jpg width="400" height="300" caption="Figure 4: Interior view of Metrodome post collapse. Snow is due to rupture of fabric panels 43 & 44. (Photo courtesy of Metropolitan Sports Facilities Commission)"]] ||

Investigation & Cause of Failure
Immediately following the dome's deflation on December 12th, a series of firms were contacted to assess the damage that had been done to the Metrodome roof structure. Included in the list of firms contacted were Birdair Inc, the manufacturer and installer of the roof structure, Geiger Engineers, the designers of the fabric roof and engineer of record, and Walter P. Moore and Associates, a third party known for their for their expertise in commercial roof structures. According to the Metropolitan Sports Facilities Commission, it was unanimously accepted that the deflation of the dome was caused by sliding snow and ice impacting the ring beam along the perimeter of the roof structure, rupturing the roof fabric at panel number 104. The tear in this panel caused the loss of internal building pressure resulting in the deflation of the air supported fabric. (Agenda for the MSFC Special Meeting, February 10, 2011) Knowing this, investigative actions were focused on the salvageability of the damaged roof structure. The following document illustrates the condition of the Metrodome after the deflation on December 23, 2010. Panels marked with an "X" indicate locations where the roof fabric had ruptured.



Birdair Inc., who visually inspected the state of the deflated roof, reported numerous minor tears and abrasions in the fabric, as seen in Figures 5 and 6, attributed to the effects of sliding snow and ice. Creases and discoloration in the fabric occurred in locations where ice and snow had accumulated. Discoloring of the material indicates that the inner fabric of the membrane had been exposed and allowed moisture to infiltrate. Through the use of an electron microscope, Geiger Engineers were able to further observe and assess the condition of the fabric. Their examination revealed the loss of the Polytetrafluoroethylene (PTFE) protective coating on the fabric membrane and the formation of cavities at the intersection of glass yarns within the inner fabric. If these depressions reached the depth of the inner woven glass yarns they would have been exposed to the elements, and the infiltration of moisture would ultimately weaken them. Overall, Geiger Engineers could not approve a partial repair of the fabric membrane, and in turn called for its complete replacement. (Geiger Engineers Report)

Findings from Walter P. Moore and Associates (WPM) were conclusive with those of Geiger Engineers. Birdair Inc. tested nine fabric samples taken from the deflated roof or the Metrodome. Upon reviewing the results, WPM was mainly concerned with the biaxial testing results of fabric panel number 72. According to documentation, the material exhibited unusual behavior, failing at minimal loading. The failure had appeared to stem from a region of darkened yarns, associated with infiltration of moisture due to damage of the PTFE coating. Further investigation of panel number 72 indicated that there was indeed damage done to the protective coating of the membrane. A series of holes were found in the coating, similar to what was reported by Geiger Engineers. WPM described the holes as dirt filled, forming over the intersection of glass yarns. Although it was unclear whether these holes allowed the yarns to be exposed to the elements, conservative action was taken by WPM proposing that at a minimum, 30 diamond panels, 22 rectangular panels, and 10 triangular panels would have to be replaced in addition to the panels that failed during the initial deflation. Upon further risk assessment, WPM recommended that the entire roof membrane be replaced. (Walter P. Moore and Associates Report)
 * = [[image:failures/Metrodome_Exposed_Yarn_WPM.png width="419" height="312" caption="Figure 5: Metrodome outer membrane showing typical exposed yarns. (Permission - Walter P. Moore & Associates; Photo courtesy of Metropolitan Sports Facilities Commission)"]] ||= [[image:failures/Metrodome_Typical_Yarn_Tear_WPM.png width="438" height="312" caption="Figure 6: Metrodome outer membrane showing typical tear. (Permission - Walter P. Moore & Associates; Photo courtesy Metropolitan Sports Facilities Commission)"]] ||

According to Geiger Engineers, the ring beam and columns, and cables were inspected and deemed useable for future use. Bent and broken rods were suggested to be replaced. (Geiger Engineers Report) Walter P. Moore and Associates agreed with Geiger Engineer's findings and recommendations.

Prevention
Polytetrafluoroethylene coated roof structures are rated to have a useful life span of approximately 20-30 years. Many times, with the aid of good maintenance, the life of these structures are often pushed past their expectations. Walter P. Moore and Associates suggests that in addition to approaching the max useful life of the roof membrane of the Metrodome, the previous three collapses have done much to weaken the integrity of the woven fiberglass liner and PTFE coated outer membrane. In addition, air-supported roof structures are extremely vulnerable to progressive collapse. (Walter P. Moore and Associates Report) For example the local failure of fabric panel number 104, tearing under the sliding mass of snow and ice, directly resulted in the global failure of the entire dome, deflation and inversion.

The maintenance crew in charge at the Metrodome took preventative measures preparing for this winter storm just as any other beforehand. However, the aging roof fabric in combination with a fierce winter storm proved to be too much for the roof structure of the Metrodome. At twenty-eight years old, the roof fabric was towards the upper end of the spectrum of its life expectancy. It was clearly time to replace the deteriorating roof fabric.

Similar Cases
The BC Place Stadium in Vancouver is a similar style venue to the Metrodome; it too having an air-supported domed roof. Constructed around the same time, the BC Place fell victim to a partial roof collapse due to accumulation of snow and sleet on the roof of the structure. According to the service plan intended for the BC Place Stadium, the roof was due for replacement around its time of collapse in January of 2007. However, officials claimed that the structure was capable of lasting an additional fifteen to twenty years so long as proper upkeep was maintained. (Dallessandro, Levash, Patrick, 2010)

According to investigations carried out following the deflation of the dome, it was determined that the major cause for failure of the air-supported structure was human error. On, the morning of the collapse, workers noticed the fabric roof of the stadium hanging lower than normal. Assuming this was caused by under-inflation of the space, workers started fans to increase pressure inside the dome. Due to a damaged roof panel, the increased pressure in turn, actually caused an over pressurization of the space. This accompanied with high winds and snow from the storm outside caused the damaged roof panel to tear open. The roof was then deflated by orders of the facilities' staff. (CBC News, 2007)

As mentioned earlier, the 2010 collapse of the Metrodome roof was not the only time the roof deflated in its history. In fact, the three previous collapses, in 1981, 1982, and 1983, were all caused by rapid heavy snowfall. The 1981 deflation resulted from the sheer weight of the snow on the roof where as the 1982 and 1983 deflations resulted from tears in the roof fabric due to sliding snow and ice. These later two collapses were similar in nature to the 2010 collapse, but were not as severe. (Coleman)

The following video, courtesy of Fox Sports, captures the entire December 12, 2010 collapse from inside the Metrodome. Initial deflation depicted in the video is attributed to the loss of internal pressure within the dome due to the tear in fabric panel number 104, whereas the rupture and infiltration of snow onto the field is cause by the failure of fabric panels number 43 and 44.

media type="youtube" key="X_uscBJn0p0?version=3" height="360" width="640" align="center"

Conclusion
Air-supported roof structures are becoming a thing of the past. As sports facilities evolve, they are expected to provide more and more amenities such as restaurants, bars, and digital technologies. Air-supported structures just are not able to accommodate such luxuries. In addition, these air-supported domed structures have rather large heating and cooling costs due to poor insulation values, and require constant monitoring. They do offer a vast unobstructed space for spectators, and the ability to hold events through all seasons of the year. However, tension-supported fabrics are becoming more prevalent over air-supported fabric structures, mainly because of their higher level of reliability. (Riddle, Mason, and Ernster)

The collapse of the Metrodome's roof structure was unexpected to say the least. The poor condition of the roof coupled with blizzard like conditions really created a perfect storm which ultimately led to the structure's failure. In general, the roof had ultimately seen its useful life period and simply needed to be replaced. It was extremely fortunate that the collapse occurred when it did, injuring nobody.

Careful assessment had been placed into the replacement of the roof membrane at the Metrodome. Heeding the suggestions of Walter P. Moore Associates and Geiger Engineers, the Metropolitan Sports Facilities Commission devised a plan to replace the entire roof fabric and repair any damages to the underlying cable supports. Birdair Inc. was contracted to complete the job primarily for their expertise with air-supported roof structures and their guarantee to have the structure complete before the Minnesota Viking's first home game, August 27, 2011. (Duchschere) The roof structure repair and fabric replacement were completed July 12, 2011.