2005 Louisiana Superdome Roof Failure During Hurricane Katrina August 29, 2005 - New Orleans, LA Joshua P. Progar, BAE/MAE, The Pennsylvania State University 2011
The Louisiana Superdome had portions of its single-ply membrane roof system collapse on August 29, 2005 due to wind uplift forces resulting from sustained winds of 125 mph (Cauffman 2006) from Hurricane Katrina. The 253 foot diameter domed structure had approximately half of it 9.7 acre roof membrane blown off as seen in Figure 1. With the membrane roof removed, the hurricane wind loads eventually led to two 20' x 5' sections of the roof decking failing 13 stories above street level.
This failure occurred while the facility was being utilized as a shelter for over 10,000 refugees to avoid the storm surge that came with the category 3 hurricane. Fortunately, no one in the stadium was hurt and the structural system remained intact for the remainder of the storm. Although Hurricane Katrina was a historical hurricane, the lack of an air barrier along with insufficient fasteners led to the mechanically fastened roof system being ripped apart. Furthermore, failure at weak weld points on the metal roof deck led to the collapse of two 100 square foot sections.
Summary
Hurricane Katrina
Hurricane Katrina was the costliest natural disasters and one of the top five deadliest hurricanes in the history of the United States of America. Among all recorded hurricanes in the Atlantic Ocean, it ranked as the sixth strongest (Knabb 2005). Katrina took the lives of at least 1,800 people and caused over an estimated $125 billion dollars in property damage (Graumann 2005). The hurricane formed over the Bahamas on August 23, 2005 and made landfall along the Gulf Coast of the United States on August 29, 2005 with sustained winds at landfall of 125 mph (Cauffman 2006).
Katrina was classified as a Category 5 strength storm according to the Saffir-Simpson scale, which classifies the hurricane based on wind speed and potential damage that will be inflicted by the storm. A category 5 hurricane has sustained winds of 155+mph and will result in catastrophic damage. It is important to note, that the storm was classified as a Category 5 storm while over the ocean and was downgraded to a Category 3 hurricane when it made land fall over the Gulf Coast region. With Hurricane Katrina came storm surges of 30 feet, which were responsible for the majority of the significant damage to engineered infrastructures such as the leeves, commercial and residential buildings, roads and bridges, utility distribution centers, etc (Mosqueda 2007).
History of the Superdome & its Roof
The Louisiana Superdome opened on August 3, 1975 as a multi-purpose sports and exhibition facility. It is the home of the New Orleans Saints, the city's professional football team and also is the home for the Tulane Green Wave collegiate football program. The Louisiana Superdome is known as one of the world's largest steel construction domed structures with a dome that covers 13 acres, reaches 27 stories (or 273 ft.) in height and has 125 million cubic feet of unobstructed volume (Yamin-Garone 2005). Structurally, the Superdome was designed to withstand sustained wind speeds of 150 mph and wind gusts that reach 200 mph (Yamin-Garone 2005). The structure was designed specifically for strong winds, utilizing wind tunnel tests and the stringiest wind design criteria at that time (Yamin-Garone 2005).
The Lousiana Superdome roof covers 9.7 acres and was originally designed with a fluid-applied elastomeric roofing system. This roof system consists of an elastomeric roof coating which is fluid-applied to polyisocyanurate board (iso-board) which is mechanically fastened to the ribbed steel roof deck topped with a layer of polyurethane insulation (Cauffman 2006). This roofing system is designed to have elastic properties to allow for expansion and compression during temperature cycles without damage to the roof itself. This type of roofing system is known to be cost effective with a long life expectancy of about 30 years.
The roof was replaced in 1988-1989 as part of a controversial debate which started when the facility and its district began to reconsider a new roofing system less than 15 years after the facility opened. In 1980, the Louisiana Superdome was damaged by a hailstorm that caused leakage in the roof assembly although the engineer of record stated that the 25-mil poured hypalon coating was self healing and should prevent any such leakage (Wright 2005). This sparked a controversial debate on whether or not to replace the roof system altogether. The local roofing industry convinced the Superdome representatives that their unconventional roof should be replaced by a single-ply EDPM membrane roof, which was more conventional and popular at the time. The roof was replaced in 1989 with the single-ply EDPM membrane roof which was in place until 2002 when minor membrane upgrades and renovations were completed (Yamin-Garone 2005). The single-ply EDPM roof remained throughout this renovation and was in place when Hurricane Katrina struck the Gulf Coast region in 2005.
Causes of Failure
The Louisiana Superdome partial roof system collapse was a simple wind uplift failure resulting from two different failure modes. First, the single-ply EPDM roofing membrane was ripped away from the structure and displaced to the downwind section of the dome. As a result of the membrane being displaced, wind uplift forces infiltrated the mechanically attached roofing system and ripped off two sections of roof decking at weak welds where sheets of roof deck overlap (Wright 2005). This can be attributed to two major factors in design: the lack of an air barrier in the EPDM roofing system (Lstiburek 2009) and the installation of less mechanical fasteners than required for this system during construction and renovations (Cauffman 2006).
Failure of the Single Ply Roofing Membrane
In order of chronological failure, the single-ply ethylene propylene diene monomer (EPDM) mechanically attached roofing system was ripped from the rest of the system because of a couple of major factors that have been proven to create wind uplift problems. The roofing system consisted of metal roof decking topped with sprayed polyurethane foam (SPF) and finally an elastromeric coating on top. A single-ply roofing membrane layer or facer sheet was adhered to rigid polyisocyanurate board (iso-board) which were mechanically fastened through the layer of rigid polyurethane foam into the structural metal deck (Cauffman 2006). This mechanically attached roofing system did not include an air barrier in its construction (Lstiburek 2009) and is shown in section in Figure 2. Wind uplift resistance is increased when an air barrier is included in the deck assembly because it transfers a significant amount of the wind load from the roofing membrane to the air barrier and finally to the structural metal deck (Lstiburek 2009).
Figure 2: Roof Section of Failed Superdome Roof Assembly. Photo Credit: Building Science Corporation.
Air barriers are used to control air leakage in a building envelope, or in this case, a roofing assembly. They control leakage both in or out of the assembly and are located between the metal decking and the rigid iso-board insulation elements in a mechanically attached roofing system. Figure 3 shows the appropriate location for an air barrier in a mechanically attached roofing assembly. Air barriers reduce fluttering, as shown in Figure 4, by resisting the membrane's tendency to want to lift off the insulation layer. This resistance occurs because of suction due to the air barrier preventing air from the interior to enter the roof assembly (Lstiburek 2009).
If an air barrier is properly installed, a significant portion of the wind uplift force is transferred to the air barrier and its attachment to the structural roof decking. Roof assemblies with air barriers that are designed to be air tight transfer more load which results in less stress on the membrane layer (Lstiburek 2009). According to laboratory testing, mechanically attached roofing systems without an air barrier have a sustained wind uplift rating of 90 psf as compared to a rating of 135 psf for systems with the air barrier (Motelli 2010).
Figure 3: Location of Air Barrier in Mechanically Attached Roofing Assembly. Photo Credit: Building Science Corporation.
Suction forces generated by Hurricane Katrina on the dome reached an estimated 80 to 100 psf due to the 125 mph sustained hurricane winds (Wright 2005). The typical mechanically attached roofing system is designed for wind pressures at the roof level of 45 psf and then applying a factor of safety equal to 2 which results in design considerations for ultimate failure capacity of about 90 psf of suction force (Prevatt 2007). With these forces exceeded during the hurricane, it was reported that smoke dampers at the top of the roof were sucked off the structure (Lstiburek 2009). This allowed for wind infiltration to intensify the building pressurization which stressed the roof assembly (Lstiburek 2009). In addition to air infiltration from these holes from the smoke dampers, typical EPDM roof assemblies are not air tight and air leakage is common from the interior. Additionally, it was reported that air borne debris punctured the roof with numerous holes which resulted in another source of air infiltration and also allowed moisture to penetrate the roof assembly (Lstiburek 2009).
Without the air barrier, wind uplift forces were intensified by the combination of wind infiltration, air leakage from the interior side of the structure, the suction force of the wind and damage done by air borne debris. Since the roof assembly lacked an air barrier and was not air tight, the membrane of the roof had to take the majority of the wind loads which led to failures of the mechanical fasteners (Lstiburek 2009). Although the roof membrane was not well documented prior to failure, many believe that the combination of forces initially lead to failure of the adhesive that bonded the rigid polyisocyanurate boards to the roof membrane that allowed fluttering of the membrane as shown in Figure 4.
Figure 4: Fluttering Membrane Due To Air Leakage. Photo Credit: Building Science Corporation.
Once the bond between these two elements was broken and air infiltration could not be stopped the result was billowing of the membrane. At this point, mechanical fasteners along the lap strips between layers of the single-ply membrane were greatly stressed in tension and began to rip away from the deck. Once the fasteners failed, the roof began to tear away from the rigid polyisocyanurate board insulation in a rapid fashion. Figure 5 shows a photograph of the membrane roof that was ripped away from the roofing assembly. The photgraph clearly shows where the adhesive bonding agent was located and the white arrows point to pieces of glass that damaged the membrane roof. This failure mode left the iso-board insulation exposed and led to the redistribution of wind uplift forces between the insulation and the structural decking. Redistributing the uplift forces, over stressed the structural decking and led to the second failure mode.
Figure 5: Damage of Superdome EPDM Roofing Membrane. Photo Credit: Robb Smith, Amtech Building Sciences, Inc.
Failure of the Metal Decking
Without an air barrier and the displaced roof membrane, the rigid iso-board insulation and structural metal decking were the lone elements in the roof assembly available to resist the uplift forces. The amplified pressurization of the interior of the building due to the smoke dampers being blown off the roof additionally stressed the structural metal decking. A third factor, the displacement of the roof membrane, continued to add to the problem by creating another form of stresses on the roof decking due to an increased load that the membrane had once resisted (Prevatt 2007). Lateral migration of moisture through the flutes of the EPDM roof assembly throughout the years prior to hurricane led to corrosion of the metal decking which greatly weakened its resistance to the large wind uplift forces (Lstiburek 2009). The fluctuating winds from the hurricane eventually led to the failure of two roof deck sections at weak weld points. The panels blew off the facility from the suction forces of the wind allowing water to pour into the building.
Although the breach in the roof did not result in any casualties or injuries, the now open sections of roof allowed moisture to freely enter into the facility for the remainder of the hurricane. This resulted in millions of dollars of damage in the interior of the dome. Figure 6 below shows damaged portions of the Superdome's roof and in particular the structural metal decking.
Figure 6: Damaged Roof Decking Sections of Louisiana Superdome. Photo Credit: Building Science Corporation.
Lessons Learned
Membrane Roofing System Failures
As a result of Hurricane Katrina, awareness of flaws in mechanically attached roofing systems were magnified within the engineering community. The Louisiana Superdome was not the only EPDM roofing system to have problems with uplift forces during Hurricane Katrina. The predominant damage to this membrane roofing system during Hurricane Katrina was membrane removal from uplift, which is consistent with the Superdome failure mode (Cauffman 2006). Generally, failures with EPDM roofing systems during Katrina included one or more of the following reasons: perimeter metal flashing performance issues, insufficient inter-laminar insulation strength or adhesive strength between the roof membrane and insulation, and non-standard installation of bituminous base sheets to metal decking using mechanical fasteners (Cauffman 2006). These three failure modes have been documented as typical problems with EPDM roofing systems in past hurricanes or other high wind level events.
Mechanical Fasteners
In addition, reports determined that the installation of these roofing systems and the quality in design and materials was not sufficient for the New Orleans geographic location. In many cases, as much as 20-40% of mechanical fasteners were not installed as recommended in the manufacturers literature. The quality of both design and installation of these roofing systems was noted due to the New Orleans Arena. This arena is located directly beside the Louisiana Superdome, and had a six year old EPDM roofing system that remained in good condition throughout the hurricane (Cauffman 2006). Many small commerical builldings, residential homes and other facilities along the Gulf Coast sustained significant roof damage from the failure modes listed above.
Mechanical Rooftop Failures
Rooftop mechanical equipment failures were common in the high-wind regions of Hurricane Katrina and frequently became detached from rooftops during the storm. The industry reacted to these failures by releasing new design provisions for wind-induced forces that are exerted on mechanical rooftop equipment. The ASCE 7 standard, which is the national code for wind loading information, adopted ASCE7-05 in 2005 which increased loads exterted on the rooftop equipment (Chowdhury 2007). ASCE 7-02, which was adopted in 2002 and was replaced by ASCE7-05, was the first edition of the ASCE 7 reference standard that attempted to address wind design loads for mechanical rooftop (Chowdhury 2007). ASCE7-05 became the first standard to address wind uplift forces and provided more in-depth recommendations for increased wind loads on the rooftop equipment. New design guidelines in ASCE7-05 now call for a factor of safety of 2 for regular structures and 3 for essential facilities (Chowdhury 2007).
It is unclear if the smoke dampers that were blown off the roof were designed under the 2002 ASCE standard or if wind uplift forces were even taken into account during design for the rooftop anchors. The new mechanical equipment connections were designed to the ASCE7-05 standard which increased the load demand at the connection points to avoid the suction forces from hurricane level winds. In 2006, the Louisiana Superdome had a $185 million roof repair completed to address the design flaws in the roofing system. The new Superdome roof is similar to the original roofing assemly but has addressed the issues that became apparent during the 2005 hurricane event. Select areas of metal roof decking was replaced due to extensive corrosion where water infiltration occurred because of air borne debris damaging the roof and the absense of an air barrier.
New Superdome Roof Design
This new roof assembly, shown in figure 7, includes an integral air barrier that consists of spray polyurethane foam (SPF) that fills the flutes of the roof deck allowing for additional resistance through increased transfer of wind loads from the air barrier to the structural metal decking. Filling the flutes with spray polyurethane foam insulation does not allow for the lateral moisture inflitration that led to corrosion of welds on the structural metal decking while also gaining the uplift resistance from the integral air barrier (Lstiburek 2009). The polyurethane foam insulation has a fully adhered spray applied membrane which does not include mechanical fasteners. Prevention of moisture infiltration and damage from air borne debris were also considered and designed for with the new roof membrane (Lstiburek 2009).
Figure 7: New Superdome Roof Section. Photo Credit: Building Science Corporation.
Additional Membrane Roofing System Failure Cases
Membrane roofing system failures were experienced throughout Hurricane Katrina's path with a variety of failure mechanisms as the cause. Trends in failures for these roofing system consisted of additional mechanisms which differed from the Superdome case. For example, an elementary school in Port Arthur, Texas lost its 15 year old roof membrane after its perimeter metal flashing was found damaged due to inadequately being attached in place (Cauffman 2006).
Two wings of a hospital roof in Pascagoula, Mississippi had its 10 year old membrane roof torn off due to a lack of interlaminar adhesion strength between the membrane and the insulation. On these two wings, the perimeter flashing was not damaged however on a third wing of the hospital the perimeter metal flashing was damaged and the roof was torn off the building (Cauffman 2006). Damage to the perimeter metal flashing on the windward side of the building surfaced as a typical failure mechanism for many small commerical buildings. The damage to flashing often occurred when the membrane roof was blown off the roof on the downwind side of these buildings.
Many EPDM membrane roofing systems throughout the region had to be repaired due to wind-borne debris damage. Small pieces of glass from damaged window glazing was often the reason for damages to large sections of EPDM roof systems. These damages sometimes led to complete roof membrane failures like the Louisiana Superdome or were limited to small leakage problems.
Conclusion
Roofing failures like the Louisiana Superdome are not always typical strength related collapses due to excessive loads such as snow loads and drifting. Failure modes for the Louisiana Superdome roof included issues with water infiltration and corrosion due to minor design detailing oversights. The Louisiana Superdome roof partially failed on August 29, 2005 in a two part failure mechanism that led to (2) 20' x 5' sections of the roof being ripped from the structure. First, the single-ply membrane roof was torn from the domed structure which resulted in failure of welds at lap joints on the roof deck due to wind uplift forces.
This example of a minor roof collapse with major economic consequences highlights the critical need for correct detailing during design and proper installation of these systems to avoid failures. The collapse happened while the facility was being used as a refuge for over 10,000 people who escaped the 30 foot of storm surge from Hurricane Katrina. Although, the roof assembly itself did have minor failures, the entire structural system was never in jeopardy. Additionally, no refugees inside the facility were injured or killed. However, millions of dollars in damages inside the facility could have been prevented.
This minor failure was scrutinized because of its relevence as a well known structure and emergency shelter. Because of the failure, industry re-evaluated how mechanically attached roofing structures must be designed and constructed to resist high wind pressures. Uplift failures were observed throughout the greater Gulf Region from Texas to Mississippi and showed that the Louisiana Superdome is in no way an isolated case. Construction methods and design considerations were tweaked to avoid the failure of these roofing systems in the next hurricane event. Redesign of the new Superdome roof shows the advancement in mechanically attached roofing system design as a result of the addition of an air barrier and the replacement of mechanical fasteners with fully adhered spray membranes.
Photo Credit for all images from Building Science Corporation are reprinted for educational non-commercial purposes only.
Annotated Bibliography
Cauffman, S. (2006). "Performance of Physical Structures in Hurricane Katrina & Hurricane Rita: A Reconnaissance Report". TN-1476. National Institute of Standards and Technology (NIST), Gathersburg, MD. This is a reconnaissance report that is a large document that reviews all Hurricane Katrina related building failures and focuses on the Louisiana Superdome. In the text is important information about flashing failures that occurred on the Superdome's EPDM membrane roofing system. The article also includes key information on the failure of the fasteners, in this case, the adhesives during hurricane wind loads.
Chowdhury, A. and Erwin, J. (2007). "Rooftop Equipment Wind Load and Its Mitigation for Buildings in Hurricane Prone Regions." International Hurricane Research Center (IHRC) - Florida International University, Miami, FL. http://www.ihc.fiu.edu/lwer/docs/Year7_Section3_RoofTopEquip_RCMPY7.pdf(Dec. 4, 2011). This technical report describes rooftop equipment failures that were common during Hurricane Katrina and also gives insight into the industry's reaction to the failures. The report goes into depth about ASCE code changes resulting from the Hurricane and a historical timeline of the ASCE codes regarding wind uplift forces on rooftop equipment.
Graumann, A., Houston, T., Lawrimore, J., Levinson, D., Lott, N., McCown, S., Stephens, S. and Wuertz D. (2005)."Hurricane Katrina: A Climatological Perspective Report." Technical Report 2005-01. National Oceanic and Atmospheric Administration (NOAA), Asheville, NC. This technical report is a climatological report that documents Hurricane Katrina's development, environmental factors, statistics, etc. The report was used to discuss the statistics for the death toll and damage of Hurricane Katrina.
Knabb, R., Rhome, J. and Brown, D. (2005). "Tropical Cyclone Report: Hurricane Katrina." National Hurricane Center. National Oceanic and Atmospheric Administration (NOAA), Asheville, NC. This report gives a detailed weather related analysis of the history of Hurricane Katrina. This document was used for information on the economic signifigance and the historical signifigance of Hurricane Katrina in the United States.
Lstiburek, J. (2009). "Uplift Moments - Roof Failures". ASHRAE Journal.http://www.buildingscience.com/documents/insights/bsi-019-uplifting-moments-roof-failures/files/bsi-019_uplifting_moments.pdf (Sept. 30, 2011). This article touches on the building envelope failure (roof failure) at the Louisiana Superdome and is in depth on roof design related to problems with uplift from wind loads. The article uses a combination of figures and examples to describe key problems with the Superdome prior to the failure during Hurricane Katrina.
Mosqueda, G. and Porter, K. (2007). "Engineering and Organizational Issues Before, During and After Hurricane Katrina." Multidisciplinary Center of Earthquake Engineering Research. 4(1), 1-64. This article focuses on impacts on structural engineering before and after Hurricane Katrina. The content in this article includes information on the hurricane, wind forces generated and is in-depth about how the industry reacted to this design event.
Motelli, S., Kee Ping Ko, S. and Baskaran, B. (2010). "Effect of Fastener-Deck Strength on the Wind-Uplift Performance of Mechanically Attached Roofing Assemblies". American Society of Civil Engineers: Practice Periodical on Structural Design and Construction. 15(27), 1-13. This ASCE article deals with fastener strength of EPDM (Mechanically Attached Roofing Assemblies) and their failure modes. Since this is the leading cause of the Louisiana Superdome roof failure, it provides key information on the failure mechanism as well as diagrams for use.
Prevatt, D., Schiff, S., Stamm, J. and Kulkami, A. (2007). "Wind Uplift Behavior of Mechanically Attached Single-Ply Roofing Systems: The Need for Correction Factors in Standardized Tests." American Society of Civil Engineers: Journal of Structural Engineering. 134(3), 1-10. This ASCE article explains common failure methods of single-ply membrane roofing systems like that of the Louisiana Superdome. It is in depth with high wind loads and failure mechanisms that result on these types of roofing systems.
Wright, G., Powers, E. Michael and Armisted, T. (2005). "Corps Scrambling to Plug New Orleans Floodwall Breaches." Engineering News Record.http://enr.construction.com/news/environment/archives/050830.asp (Oct. 2,2011). This ENR article talks about the general sequencing of roof replacement on the Louisiana Superdome along with a detailed roof system description giving the different elements that were peeled away during Hurricane Katrina. Walter P. Moore is quoted in the article with analysis of what caused the failure of the roof.
Yamin-Garone, M. (2005). "Will the Louisiana Superdome Fall Victim to Katrina?" Construction Equipment Building Guide.http://www.constructionequipmentguide.com/Will-the-Louisiana-Superdome-Fall-Victim-to-Katrina/6100/ (Sept 30,2011). This short article explains the historical timeline of the Louisiana Superdome's roofing systems and replacements. It explains the size of the failures on the roof during Hurricane Katrina and comments on the structural integrity of the facility during the hurricane.
Additional Resources
Baskaran, A. (2002). "Dynamic Wind Uplift Performance of Thermoplastic Roofing System with New Seaming Technology". American Society of Civil Engineers: Journal of Architectural Engineering. 8(4), 1-11. This publication explains typical failure mechanisms of thermoplastic polyolefin (TPO) roofing systems. It goes into great detail about wind uplift and has useful diagrams to explain how wind uplift forces are amplified during hurricane level wind loads.
(2006). "Attachment of Rooftop Equipment in High-Wind Regions." FEMA 549. Federal Emergency Management Agency (FEMA), Washington D.C. This design guide was issued in May 2006 as a Hurricane Katrina Recovery Advisory to recommend practices for designing and constructing rooftop equipment with enhanced resistance in high-wind regions. It has concise design information on different rooftop elements along with charts that give "rule of thumb" design provisions.
August 29, 2005 - New Orleans, LA
Joshua P. Progar, BAE/MAE, The Pennsylvania State University 2011
Keywords
wind uplift forces, mechanically fastened roof system, EPDM membrane roof, fasteners, welds, air barrier, fluttering, roof failureIntroduction
Table of Contents
This failure occurred while the facility was being utilized as a shelter for over 10,000 refugees to avoid the storm surge that came with the category 3 hurricane. Fortunately, no one in the stadium was hurt and the structural system remained intact for the remainder of the storm. Although Hurricane Katrina was a historical hurricane, the lack of an air barrier along with insufficient fasteners led to the mechanically fastened roof system being ripped apart. Furthermore, failure at weak weld points on the metal roof deck led to the collapse of two 100 square foot sections.
Summary
Hurricane Katrina
Hurricane Katrina was the costliest natural disasters and one of the top five deadliest hurricanes in the history of the United States of America. Among all recorded hurricanes in the Atlantic Ocean, it ranked as the sixth strongest (Knabb 2005). Katrina took the lives of at least 1,800 people and caused over an estimated $125 billion dollars in property damage (Graumann 2005). The hurricane formed over the Bahamas on August 23, 2005 and made landfall along the Gulf Coast of the United States on August 29, 2005 with sustained winds at landfall of 125 mph (Cauffman 2006).Katrina was classified as a Category 5 strength storm according to the Saffir-Simpson scale, which classifies the hurricane based on wind speed and potential damage that will be inflicted by the storm. A category 5 hurricane has sustained winds of 155+mph and will result in catastrophic damage. It is important to note, that the storm was classified as a Category 5 storm while over the ocean and was downgraded to a Category 3 hurricane when it made land fall over the Gulf Coast region. With Hurricane Katrina came storm surges of 30 feet, which were responsible for the majority of the significant damage to engineered infrastructures such as the leeves, commercial and residential buildings, roads and bridges, utility distribution centers, etc (Mosqueda 2007).
History of the Superdome & its Roof
The Louisiana Superdome opened on August 3, 1975 as a multi-purpose sports and exhibition facility. It is the home of the New Orleans Saints, the city's professional football team and also is the home for the Tulane Green Wave collegiate football program. The Louisiana Superdome is known as one of the world's largest steel construction domed structures with a dome that covers 13 acres, reaches 27 stories (or 273 ft.) in height and has 125 million cubic feet of unobstructed volume (Yamin-Garone 2005). Structurally, the Superdome was designed to withstand sustained wind speeds of 150 mph and wind gusts that reach 200 mph (Yamin-Garone 2005). The structure was designed specifically for strong winds, utilizing wind tunnel tests and the stringiest wind design criteria at that time (Yamin-Garone 2005).
The Lousiana Superdome roof covers 9.7 acres and was originally designed with a fluid-applied elastomeric roofing system. This roof system consists of an elastomeric roof coating which is fluid-applied to polyisocyanurate board (iso-board) which is mechanically fastened to the ribbed steel roof deck topped with a layer of polyurethane insulation (Cauffman 2006). This roofing system is designed to have elastic properties to allow for expansion and compression during temperature cycles without damage to the roof itself. This type of roofing system is known to be cost effective with a long life expectancy of about 30 years.
The roof was replaced in 1988-1989 as part of a controversial debate which started when the facility and its district began to reconsider a new roofing system less than 15 years after the facility opened. In 1980, the Louisiana Superdome was damaged by a hailstorm that caused leakage in the roof assembly although the engineer of record stated that the 25-mil poured hypalon coating was self healing and should prevent any such leakage (Wright 2005). This sparked a controversial debate on whether or not to replace the roof system altogether. The local roofing industry convinced the Superdome representatives that their unconventional roof should be replaced by a single-ply EDPM membrane roof, which was more conventional and popular at the time. The roof was replaced in 1989 with the single-ply EDPM membrane roof which was in place until 2002 when minor membrane upgrades and renovations were completed (Yamin-Garone 2005). The single-ply EDPM roof remained throughout this renovation and was in place when Hurricane Katrina struck the Gulf Coast region in 2005.
Causes of Failure
The Louisiana Superdome partial roof system collapse was a simple wind uplift failure resulting from two different failure modes. First, the single-ply EPDM roofing membrane was ripped away from the structure and displaced to the downwind section of the dome. As a result of the membrane being displaced, wind uplift forces infiltrated the mechanically attached roofing system and ripped off two sections of roof decking at weak welds where sheets of roof deck overlap (Wright 2005). This can be attributed to two major factors in design: the lack of an air barrier in the EPDM roofing system (Lstiburek 2009) and the installation of less mechanical fasteners than required for this system during construction and renovations (Cauffman 2006).
Failure of the Single Ply Roofing Membrane
In order of chronological failure, the single-ply ethylene propylene diene monomer (EPDM) mechanically attached roofing system was ripped from the rest of the system because of a couple of major factors that have been proven to create wind uplift problems. The roofing system consisted of metal roof decking topped with sprayed polyurethane foam (SPF) and finally an elastromeric coating on top. A single-ply roofing membrane layer or facer sheet was adhered to rigid polyisocyanurate board (iso-board) which were mechanically fastened through the layer of rigid polyurethane foam into the structural metal deck (Cauffman 2006). This mechanically attached roofing system did not include an air barrier in its construction (Lstiburek 2009) and is shown in section in Figure 2. Wind uplift resistance is increased when an air barrier is included in the deck assembly because it transfers a significant amount of the wind load from the roofing membrane to the air barrier and finally to the structural metal deck (Lstiburek 2009).
Air barriers are used to control air leakage in a building envelope, or in this case, a roofing assembly. They control leakage both in or out of the assembly and are located between the metal decking and the rigid iso-board insulation elements in a mechanically attached roofing system. Figure 3 shows the appropriate location for an air barrier in a mechanically attached roofing assembly. Air barriers reduce fluttering, as shown in Figure 4, by resisting the membrane's tendency to want to lift off the insulation layer. This resistance occurs because of suction due to the air barrier preventing air from the interior to enter the roof assembly (Lstiburek 2009).
If an air barrier is properly installed, a significant portion of the wind uplift force is transferred to the air barrier and its attachment to the structural roof decking. Roof assemblies with air barriers that are designed to be air tight transfer more load which results in less stress on the membrane layer (Lstiburek 2009). According to laboratory testing, mechanically attached roofing systems without an air barrier have a sustained wind uplift rating of 90 psf as compared to a rating of 135 psf for systems with the air barrier (Motelli 2010).
Suction forces generated by Hurricane Katrina on the dome reached an estimated 80 to 100 psf due to the 125 mph sustained hurricane winds (Wright 2005). The typical mechanically attached roofing system is designed for wind pressures at the roof level of 45 psf and then applying a factor of safety equal to 2 which results in design considerations for ultimate failure capacity of about 90 psf of suction force (Prevatt 2007). With these forces exceeded during the hurricane, it was reported that smoke dampers at the top of the roof were sucked off the structure (Lstiburek 2009). This allowed for wind infiltration to intensify the building pressurization which stressed the roof assembly (Lstiburek 2009). In addition to air infiltration from these holes from the smoke dampers, typical EPDM roof assemblies are not air tight and air leakage is common from the interior. Additionally, it was reported that air borne debris punctured the roof with numerous holes which resulted in another source of air infiltration and also allowed moisture to penetrate the roof assembly (Lstiburek 2009).
Without the air barrier, wind uplift forces were intensified by the combination of wind infiltration, air leakage from the interior side of the structure, the suction force of the wind and damage done by air borne debris. Since the roof assembly lacked an air barrier and was not air tight, the membrane of the roof had to take the majority of the wind loads which led to failures of the mechanical fasteners (Lstiburek 2009). Although the roof membrane was not well documented prior to failure, many believe that the combination of forces initially lead to failure of the adhesive that bonded the rigid polyisocyanurate boards to the roof membrane that allowed fluttering of the membrane as shown in Figure 4.
Failure of the Metal Decking
Without an air barrier and the displaced roof membrane, the rigid iso-board insulation and structural metal decking were the lone elements in the roof assembly available to resist the uplift forces. The amplified pressurization of the interior of the building due to the smoke dampers being blown off the roof additionally stressed the structural metal decking. A third factor, the displacement of the roof membrane, continued to add to the problem by creating another form of stresses on the roof decking due to an increased load that the membrane had once resisted (Prevatt 2007). Lateral migration of moisture through the flutes of the EPDM roof assembly throughout the years prior to hurricane led to corrosion of the metal decking which greatly weakened its resistance to the large wind uplift forces (Lstiburek 2009). The fluctuating winds from the hurricane eventually led to the failure of two roof deck sections at weak weld points. The panels blew off the facility from the suction forces of the wind allowing water to pour into the building.
Although the breach in the roof did not result in any casualties or injuries, the now open sections of roof allowed moisture to freely enter into the facility for the remainder of the hurricane. This resulted in millions of dollars of damage in the interior of the dome. Figure 6 below shows damaged portions of the Superdome's roof and in particular the structural metal decking.
Lessons Learned
Membrane Roofing System Failures
As a result of Hurricane Katrina, awareness of flaws in mechanically attached roofing systems were magnified within the engineering community. The Louisiana Superdome was not the only EPDM roofing system to have problems with uplift forces during Hurricane Katrina. The predominant damage to this membrane roofing system during Hurricane Katrina was membrane removal from uplift, which is consistent with the Superdome failure mode (Cauffman 2006). Generally, failures with EPDM roofing systems during Katrina included one or more of the following reasons: perimeter metal flashing performance issues, insufficient inter-laminar insulation strength or adhesive strength between the roof membrane and insulation, and non-standard installation of bituminous base sheets to metal decking using mechanical fasteners (Cauffman 2006). These three failure modes have been documented as typical problems with EPDM roofing systems in past hurricanes or other high wind level events.
Mechanical Fasteners
In addition, reports determined that the installation of these roofing systems and the quality in design and materials was not sufficient for the New Orleans geographic location. In many cases, as much as 20-40% of mechanical fasteners were not installed as recommended in the manufacturers literature. The quality of both design and installation of these roofing systems was noted due to the New Orleans Arena. This arena is located directly beside the Louisiana Superdome, and had a six year old EPDM roofing system that remained in good condition throughout the hurricane (Cauffman 2006). Many small commerical builldings, residential homes and other facilities along the Gulf Coast sustained significant roof damage from the failure modes listed above.
Mechanical Rooftop Failures
Rooftop mechanical equipment failures were common in the high-wind regions of Hurricane Katrina and frequently became detached from rooftops during the storm. The industry reacted to these failures by releasing new design provisions for wind-induced forces that are exerted on mechanical rooftop equipment. The ASCE 7 standard, which is the national code for wind loading information, adopted ASCE7-05 in 2005 which increased loads exterted on the rooftop equipment (Chowdhury 2007). ASCE 7-02, which was adopted in 2002 and was replaced by ASCE7-05, was the first edition of the ASCE 7 reference standard that attempted to address wind design loads for mechanical rooftop (Chowdhury 2007). ASCE7-05 became the first standard to address wind uplift forces and provided more in-depth recommendations for increased wind loads on the rooftop equipment. New design guidelines in ASCE7-05 now call for a factor of safety of 2 for regular structures and 3 for essential facilities (Chowdhury 2007).
It is unclear if the smoke dampers that were blown off the roof were designed under the 2002 ASCE standard or if wind uplift forces were even taken into account during design for the rooftop anchors. The new mechanical equipment connections were designed to the ASCE7-05 standard which increased the load demand at the connection points to avoid the suction forces from hurricane level winds.
In 2006, the Louisiana Superdome had a $185 million roof repair completed to address the design flaws in the roofing system. The new Superdome roof is similar to the original roofing assemly but has addressed the issues that became apparent during the 2005 hurricane event. Select areas of metal roof decking was replaced due to extensive corrosion where water infiltration occurred because of air borne debris damaging the roof and the absense of an air barrier.
New Superdome Roof Design
This new roof assembly, shown in figure 7, includes an integral air barrier that consists of spray polyurethane foam (SPF) that fills the flutes of the roof deck allowing for additional resistance through increased transfer of wind loads from the air barrier to the structural metal decking. Filling the flutes with spray polyurethane foam insulation does not allow for the lateral moisture inflitration that led to corrosion of welds on the structural metal decking while also gaining the uplift resistance from the integral air barrier (Lstiburek 2009). The polyurethane foam insulation has a fully adhered spray applied membrane which does not include mechanical fasteners. Prevention of moisture infiltration and damage from air borne debris were also considered and designed for with the new roof membrane (Lstiburek 2009).
Additional Membrane Roofing System Failure Cases
Membrane roofing system failures were experienced throughout Hurricane Katrina's path with a variety of failure mechanisms as the cause. Trends in failures for these roofing system consisted of additional mechanisms which differed from the Superdome case. For example, an elementary school in Port Arthur, Texas lost its 15 year old roof membrane after its perimeter metal flashing was found damaged due to inadequately being attached in place (Cauffman 2006).Two wings of a hospital roof in Pascagoula, Mississippi had its 10 year old membrane roof torn off due to a lack of interlaminar adhesion strength between the membrane and the insulation. On these two wings, the perimeter flashing was not damaged however on a third wing of the hospital the perimeter metal flashing was damaged and the roof was torn off the building (Cauffman 2006). Damage to the perimeter metal flashing on the windward side of the building surfaced as a typical failure mechanism for many small commerical buildings. The damage to flashing often occurred when the membrane roof was blown off the roof on the downwind side of these buildings.
Many EPDM membrane roofing systems throughout the region had to be repaired due to wind-borne debris damage. Small pieces of glass from damaged window glazing was often the reason for damages to large sections of EPDM roof systems. These damages sometimes led to complete roof membrane failures like the Louisiana Superdome or were limited to small leakage problems.
Conclusion
Roofing failures like the Louisiana Superdome are not always typical strength related collapses due to excessive loads such as snow loads and drifting. Failure modes for the Louisiana Superdome roof included issues with water infiltration and corrosion due to minor design detailing oversights. The Louisiana Superdome roof partially failed on August 29, 2005 in a two part failure mechanism that led to (2) 20' x 5' sections of the roof being ripped from the structure. First, the single-ply membrane roof was torn from the domed structure which resulted in failure of welds at lap joints on the roof deck due to wind uplift forces.
This example of a minor roof collapse with major economic consequences highlights the critical need for correct detailing during design and proper installation of these systems to avoid failures. The collapse happened while the facility was being used as a refuge for over 10,000 people who escaped the 30 foot of storm surge from Hurricane Katrina. Although, the roof assembly itself did have minor failures, the entire structural system was never in jeopardy. Additionally, no refugees inside the facility were injured or killed. However, millions of dollars in damages inside the facility could have been prevented.
This minor failure was scrutinized because of its relevence as a well known structure and emergency shelter. Because of the failure, industry re-evaluated how mechanically attached roofing structures must be designed and constructed to resist high wind pressures. Uplift failures were observed throughout the greater Gulf Region from Texas to Mississippi and showed that the Louisiana Superdome is in no way an isolated case. Construction methods and design considerations were tweaked to avoid the failure of these roofing systems in the next hurricane event. Redesign of the new Superdome roof shows the advancement in mechanically attached roofing system design as a result of the addition of an air barrier and the replacement of mechanical fasteners with fully adhered spray membranes.
Photo Credit for all images from Building Science Corporation are reprinted for educational non-commercial purposes only.
Annotated Bibliography
Cauffman, S. (2006). "Performance of Physical Structures in Hurricane Katrina & Hurricane Rita: A Reconnaissance Report". TN-1476. National Institute of Standards and Technology (NIST), Gathersburg, MD.
This is a reconnaissance report that is a large document that reviews all Hurricane Katrina related building failures and focuses on the Louisiana Superdome. In the text is important information about flashing failures that occurred on the Superdome's EPDM membrane roofing system. The article also includes key information on the failure of the fasteners, in this case, the adhesives during hurricane wind loads.
Chowdhury, A. and Erwin, J. (2007). "Rooftop Equipment Wind Load and Its Mitigation for Buildings in Hurricane Prone Regions." International Hurricane Research Center (IHRC) - Florida International University, Miami, FL.
http://www.ihc.fiu.edu/lwer/docs/Year7_Section3_RoofTopEquip_RCMPY7.pdf (Dec. 4, 2011).
This technical report describes rooftop equipment failures that were common during Hurricane Katrina and also gives insight into the industry's reaction to the failures. The report goes into depth about ASCE code changes resulting from the Hurricane and a historical timeline of the ASCE codes regarding wind uplift forces on rooftop equipment.
Graumann, A., Houston, T., Lawrimore, J., Levinson, D., Lott, N., McCown, S., Stephens, S. and Wuertz D. (2005). "Hurricane Katrina: A Climatological Perspective Report." Technical Report 2005-01. National Oceanic and Atmospheric Administration (NOAA), Asheville, NC.
This technical report is a climatological report that documents Hurricane Katrina's development, environmental factors, statistics, etc. The report was used to discuss the statistics for the death toll and damage of Hurricane Katrina.
Knabb, R., Rhome, J. and Brown, D. (2005). "Tropical Cyclone Report: Hurricane Katrina." National Hurricane Center. National Oceanic and Atmospheric Administration (NOAA), Asheville, NC.
This report gives a detailed weather related analysis of the history of Hurricane Katrina. This document was used for information on the economic signifigance and the historical signifigance of Hurricane Katrina in the United States.
Lstiburek, J. (2009). "Uplift Moments - Roof Failures". ASHRAE Journal. http://www.buildingscience.com/documents/insights/bsi-019-uplifting-moments-roof-failures/files/bsi-019_uplifting_moments.pdf (Sept. 30, 2011).
This article touches on the building envelope failure (roof failure) at the Louisiana Superdome and is in depth on roof design related to problems with uplift from wind loads. The article uses a combination of figures and examples to describe key problems with the Superdome prior to the failure during Hurricane Katrina.
Mosqueda, G. and Porter, K. (2007). "Engineering and Organizational Issues Before, During and After Hurricane Katrina." Multidisciplinary Center of Earthquake Engineering Research. 4(1), 1-64.
This article focuses on impacts on structural engineering before and after Hurricane Katrina. The content in this article includes information on the hurricane, wind forces generated and is in-depth about how the industry reacted to this design event.
Motelli, S., Kee Ping Ko, S. and Baskaran, B. (2010). "Effect of Fastener-Deck Strength on the Wind-Uplift Performance of Mechanically Attached Roofing Assemblies". American Society of Civil Engineers: Practice Periodical on Structural Design and Construction. 15(27), 1-13.
This ASCE article deals with fastener strength of EPDM (Mechanically Attached Roofing Assemblies) and their failure modes. Since this is the leading cause of the Louisiana Superdome roof failure, it provides key information on the failure mechanism as well as diagrams for use.
Prevatt, D., Schiff, S., Stamm, J. and Kulkami, A. (2007). "Wind Uplift Behavior of Mechanically Attached Single-Ply Roofing Systems: The Need for Correction Factors in Standardized Tests." American Society of Civil Engineers: Journal of Structural Engineering. 134(3), 1-10.
This ASCE article explains common failure methods of single-ply membrane roofing systems like that of the Louisiana Superdome. It is in depth with high wind loads and failure mechanisms that result on these types of roofing systems.
Wright, G., Powers, E. Michael and Armisted, T. (2005). "Corps Scrambling to Plug New Orleans Floodwall Breaches." Engineering News Record. http://enr.construction.com/news/environment/archives/050830.asp (Oct. 2,2011).
This ENR article talks about the general sequencing of roof replacement on the Louisiana Superdome along with a detailed roof system description giving the different elements that were peeled away during Hurricane Katrina. Walter P. Moore is quoted in the article with analysis of what caused the failure of the roof.
Yamin-Garone, M. (2005). "Will the Louisiana Superdome Fall Victim to Katrina?" Construction Equipment Building Guide. http://www.constructionequipmentguide.com/Will-the-Louisiana-Superdome-Fall-Victim-to-Katrina/6100/ (Sept 30,2011).
This short article explains the historical timeline of the Louisiana Superdome's roofing systems and replacements. It explains the size of the failures on the roof during Hurricane Katrina and comments on the structural integrity of the facility during the hurricane.
Additional Resources
Baskaran, A. (2002). "Dynamic Wind Uplift Performance of Thermoplastic Roofing System with New Seaming Technology". American Society of Civil Engineers: Journal of Architectural Engineering. 8(4), 1-11.This publication explains typical failure mechanisms of thermoplastic polyolefin (TPO) roofing systems. It goes into great detail about wind uplift and has useful diagrams to explain how wind uplift forces are amplified during hurricane level wind loads.
(2006). "Attachment of Rooftop Equipment in High-Wind Regions." FEMA 549. Federal Emergency Management Agency (FEMA), Washington D.C.This design guide was issued in May 2006 as a Hurricane Katrina Recovery Advisory to recommend practices for designing and constructing rooftop equipment with enhanced resistance in high-wind regions. It has concise design information on different rooftop elements along with charts that give "rule of thumb" design provisions.