Student, CEPT UniversityDr. P V Patel
Professor, Nirma UniversityBhairav Patel
Structural Engineer, VMS Engineering
Progressive collapse is spread of an initial local structural damage causing partial or total collapse of structure. It is a kind of chain reaction where local damage of primary load carrying element causes the collapse of adjoining members which in turn, leads to additional collapse. The ultimate collapse is disproportionate to the original cause. In today’s fast growing infrastructure trend, it is necessary to accommodate the most unfavorable situations in structure design. At the same time it is equally important to make it financially viable. Progressive collapse is a situation which can be triggered from any kind of abnormal loading or event like vehicle impact, bomb blast, design or construction error etc. Failure of any load carrying structural member can lead to catastrophic failure. In present literature study, an attempt has been made to cover the basic aspects of progressive collapse like historical background, mechanism, development of guidelines etc.
Progressive collapse is the result of a localized failure of one or two structural elements that lead to a steady progression of load transfer that exceeds the capacity of other surrounding elements, thus initiating the progression that leads to a total or partial collapse of the structure
Progressive collapse as a structural engineering point of view started taking attention when partial collapse of 22 storey Ronan Point apartment building occurred in London on May 16, 1968. This collapse generated considerable concern over the adequacy of existing building codes. After the partial collapse of Ronan Point apartment building, number of other collapses around the world took place, which could be placed in to category of progressive collapse. The collapse of Skyline Plaza in Virginia, the Civic Arena roof in Hartford, the Murrah Federal Building in Oklahoma City, the Khobar Towers – Saudi Arabia, the U.S. embassies in Kenya and Tanzania, WTC Towers in New York were important collapse events in the history of progressive collapse which changed the perspective of the structural design.
In normal design practice, the abnormal events like, gas explosions, bomb attack, vehicle impacts, foundation failure, failure due to construction or design error etc are not considered. It is not economical as well to design the structures for accidental events unless they have reasonable chance of occurrence. Considering these aspects, many government authorities and local bodies have worked on developing some design guidelines to prevent progressive collapse. Among these guidelines, U.S. General Services Administration (GSA) and Department of Defense (DoD) guidelines by United Facilities Criteria (UFC) – New York, provide detailed stepwise procedure regarding methodologies to resist the progressive collapse of structure. In this procedure, one of the important vertical structural elements in the load path i.e. column, load bearing wall etc. is removed to simulate the local damage scenario and the remaining structure is checked for available alternate load path to resist the load.
This paper describes the basic aspects of progressive collapse analysis in brief along with the mechanism of progressive collapse, historical background and the development of guidelines. A little detailed information on analysis procedure, specified in GSA and UFC guidelines is also provided in ‘development of guidelines’ section.
2. Mechanism of Progressive Collapse:
Any collapse in a way could be regarded as progressive collapse, but it should be of special concern if the collapse is disproportionate to its original cause. The disproportionality refers to the situation in which failure of one member causes a major collapse of larger magnitude compared to initial event. It is similar to fall of cycles in a cycle stand when the first one is pushed. This can also be compared with “domino” effect. Another example is “house of cards” effect [fig. 1].
Based on different characteristic features, progressive collapse can be categorized in six different types as described below .
1) Pancake-type collapse: failure sequence followed in this type of collapse is; initiating event, separation of structural components, release of potential energy and the occurrence of impact forces. Depending on the size of the falling components, the potential energy of falling components can far exceed the strain energy stored in the structure. The collapse of WTC towers of New York in Sept. 2001 is example of this type of collapse where collapse is said to be initiated by weakening of the floor joists due to fire that resulted from the aircraft impact. The loss of structural member was limited to the few stories but it progressively extended throughout the height of tower. The potential energy of upper part of collapsed members converted in to kinetic energy which turned in to impact force which was far beyond the resisting capacity of the lower floors and ultimately resulted in to total collapse of the tower.
2) Zipper-type collapse: this type of collapse is initiated by rupture of one cable and propagating by overloading & rupture of adjacent cables. Example of this type of collapse is collapse of original Tacoma Narrows Bridge. After the first hangers of that suspension bridge snapped due to wind induced vibrations of the bridge girder, the entire girder peeled off and fell. Impact force does not typically occur in this type of collapse, which is the case in pancake-type collapse.
3) Domino-type collapse: mechanism behind this type of collapse is, initial overturning of one element, fall off that element in angular rigid-body motion around a bottom edge, transformation of potential energy into kinetic energy, lateral impact of the upper edge of that element on the side face of an adjacent element where the horizontal pushing force transmitted by that impact is of both static and dynamic origin because of the tilting and the motion of the impacting element, overturning of the adjacent element due to the horizontal loading from the impacting element and collapse progression in the overturning direction. This type of failure can occur in row of temporary scaffolding towers. In overhead transmission line towers also, this type of collapse is common.
4) Section-type collapse: when a member under bending moment or axial tension is cut, the internal forces transmitted by that part are redistributed in to the remaining cross section. The corresponding increase in stress at some locations can cause the rupture of further cross sectional parts, and, in the same manner, a failure progression throughout the entire cross section. This type of failure can be termed as “fast fracture” instead of progressive failure.
5) Instability-type collapse: instability of structure is characterized by small imperfection which leads to large deformations or collapse. For example, the failure of a bracing element due to some small triggering event can make a system unstable and result in collapse. Another example is failure of a plate stiffener leading to local instability and failure of the affected plate, and possibly to global collapse. Here propagating destabilization occurs when the failure of destabilized elements leads to the failure of stabilizing elements.
6) Mixed-type collapse: this type of collapse can be assigned to the structure where one or more possible failure reasons fall in to different category of progressive collapse. For example, the partial collapse of the Murrah Federal Building (Oklahoma City) seems to have involved features of both a pancake-type and domino-type scenario. The horizontal forces, induced by an initial failure, that lead to overturning of other elements. This horizontal tensile force could have been induced by falling components and transmitted to other elements through continuous reinforcing bars. Another example is collapse of cable-stayed bridges which fall in to category of zipper-type and instability-type failure. The girders and towers of cable-stayed bridges are in compression. They are braced by the stay cables. Thus, the loss of one or few cables can not only lead to unzipping, but also to instability failure.
3. Historical Background:
In this section, attempt has been made to provide brief information on some of the significant structural failures in past which have presented the world wide opportunities to evaluate the validity of engineering approaches and design procedures.
3.1 Ronan Point Apartment Building – London:
Ronan Point Apartment building, 22-storey tower block in Newham, East London was named after Harry Louis Ronan (a former Chairman of the Housing Committee of the London Borough of Newham). The tower was built by Taylor Woodrow Anglian, using a technique known as Large Panel System building or LPS. This involved casting large concrete prefabricated sections off-site, then bolting them together to construct the building. Building started in 1966, and construction was completed on 11 March 1968.
Fig. 2. Ronan Point, 1968 –
Collapse due to gas explosion 
On 16th May 1968, a gas explosion took place on 18th floor that knocked out load bearing precast concrete panels near the corner of the building which caused the floors above to collapse [fig. 2]. This impact set off a chain reaction of collapses all the way to the ground. The ultimate result was collapse of the corner bay of the building from top to bottom. It is believed that the weakness was in the joints connecting the vertical walls to the floor slabs .
3.2 Skyline Plaza – North Virginia:
Skyline Plaza was a large complex located in Bailey’s Crossroads, Virginia which included eight apartment buildings, six office buildings, a hotel, and a shopping center. The building that collapsed was to have contained 468 condominium apartments. The construction of the Skyline Plaza began in the early 1970 and was set to open in August 1973.
Fig. 3. Skyline Plaza, 1973 –
Premature formwork removal 
In the midst of construction on March 2, 1973, progressive collapse occurred during construction of 24th floor. The collapse involved the full height of the tower, and the falling debris also caused horizontal progressive collapse of an entire parking garage under construction adjacent to the tower [fig. 3]. The incident occurred at around 2:30 in the afternoon and resulted in the death of 14 construction workers and the injury of 34 others. The most likely cause of the collapse was a punching shear failure of the 23rd floor slab. The two factors that contributed to this were premature removal of shores below the 23rd floor slab, and the low strength of the 23rd floor concrete in the area supporting the weight of the 24th floor slab .
Civic Arena Roof – Hartford:
The Civic Arena roof in Hartford, a steel space frame collapsed after heavy snow fall in January, 1978 [fig. 4]. The premature buckling of one of the members, resulted due to design error, caused progressive collapse. During construction in 1972 and 1973, the inspection agency notified the engineers of excessive deflections. The measured deflection of the roof was twice that predicted by computer analysis. On the day of the collapse, the sum of dead and live loads was less than the design load .
3.4 Hyatt Regency Hotel, Kansas City, U.S.:
Fig. 4. Civic Arena Roof, 1978 – Collapsed due to design error 
Construction of the 40-storey Hyatt Regency Crown Center of Kansas city, U.S. began in 1978, and the hotel opened on July 1, 1980 after construction delays including an incident on October 14, 1979, when 2,700 square feet (250 m2) of the atrium roof collapsed because one of the roof connections on the north end of the atrium failed.
On July 17, 1981, the Hyatt Regency Hotel in Kansas City, Missouri, held a videotaped tea-dance party in their atrium lobby. Construction of the walk way was like; the fourth floor bridge was suspended directly over the second floor bridge, with the third floor walkway set off to the side several meters away from the other two. Construction difficulties led to a minor but flawed design change that doubled the load on the connection between the fourth floor walkway support beams and the tie rods carrying the weight of both walkways. This new design could barely handle the dead load weight of the structure itself, much less the weight of the spectators standing on it. The connection failed and both walkways crashed one on top of the other and then into the lobby below as shown in fig. 5, killing 114 people and injuring more than 200 others .
3.5 U.S. Marine Barracks – Lebanon:
The car bomb was detonated by a suicide bomber driving a delivery van packed with about 2,000 pounds (910 kg) of explosives at approximately on April 18, 1983 that killed over 60 people, mostly embassy staff members and United States Marines and sailors. The blast collapsed the entire central facade of the horseshoe-shaped building, leaving the wreckage of balconies and offices in heaped tiers of rubble, and spewing masonry, metal and glass fragments in a wide swath [fig. 6]. The explosion was heard throughout West Beirut and broke windows as far as a mile away .
L’ambiance plaza, a 16-story apartment tower in Bridgeport, Conn., totally collapsed on April 1987, during construction, killing 28 construction workers. L’Ambiance Plaza was to be a 16 story structure with 3 parking levels and 13 apartment levels. It was composed of two offset rectangular towers, separated by a construction joint at a central elevator lobby.
The flat plate floors were designed to be constructed by the “lift slab system.” The design used unbonded plastic-sheathed post-tensioning tendons in each direction. The major design or construction deficiencies which led to total collapse were ; improper drape of post-tensioning tendons adjacent to elevator openings, overstressed concrete slab sections adjacent to two temporary floor slots for cast-in-place shear walls, overstressed and excessively flexible steel lifting angles during slab lifting, and unreliable and inadequate temporary slab-column connections to assure frame stability [fig. 7].
3.7 Murrah Federal Building, Oklahoma City:
Fig. 7. L’Ambience Plaza, Bridgeport, 1987
– Construction and Design Error 
Alfred P. Murrah Federal Building in Oklahoma City was the target of terrorist attack in 1995. The truck bomb explosion caused extensive damage to the exterior columns. Suspended transfer girder resting on the exterior columns failed due to loss of support which triggered the progressive collapse of upper stories [fig. 8]. It was the most destructive act of terrorism on American soil until the September 11, 2001 attacks. The Oklahoma blast claimed 168 lives, including 19 children under the age of 6 and injured more than 680 people. The blast destroyed or damaged 324 buildings within a sixteen-block radius destroyed or burned 86 cars and shattered glass in 258 nearby buildings .
3.8 Khobar Towers, Saudi Arabia:
Fig. 8. Alfred P. Murrah Building Collapse – Terrorist Attack, 1995 
The Khobar Towers bombing was a terrorist attack on part of a housing complex in the city of Khobar, Saudi Arabia, located near the national oil company (Saudi Aramco) headquarters of Dhahran. In 1996, Khobar Towers was being used to house foreign military personnel.
The attackers were reported to have smuggled explosives into Saudi Arabia from Lebanon. In Saudi Arabia, they purchased a large gas tanker truck and converted it into a bomb. They prepared for the attack by hiding large amounts of explosive materials and timing devices in paint cans and 50-kilogram bags, underground in Qatif near Khobar. The bomb was a mixture of gasoline and explosive powder placed in the tank of a tanker truck. On June 25, 1996, a terrorist truck bomb exploded outside the northern perimeter of Khobar Towers resulted in to 19 fatalities and approximately 500 US wounded resulted from the attack [fig. 9]. The force of the explosion was enormous. It heavily damaged or destroyed six high rise apartment buildings in the complex. Windows were shattered in virtually every other building in the compound and in surrounding buildings up to a mile away. An enormous crater, 85 feet (26 m) wide and 35 feet (11 m) deep, was left where the truck had been and within a few hours was filling up partially with salt-water from the Persian Gulf, which is less than one mile (1.6 km) away. The blast was felt 20 miles (32 km) away in the Persian Gulf state of Bahrain .
4. Development of Guidelines:
After the collapse of the Ronan Point Apartment Tower in 1968, many codes and standards around the world, attempted to address the issue of progressive collapse by providing provisions in their relevant guidelines. They were based on either explicit design requirements or on general structural integrity requirements. Most of the European, UK guidelines and building codes followed the explicit design requirement and U.S. codes followed the general structural integrity requirement. Several US government agencies developed the specific design guides which closely resembled the requirements in the UK and European Codes .
A requirement to consider progressive collapse due to “Local failure, caused by severe overloads” was first introduced in the United States in the General Design Requirements section of ANSI Standard A58.1, 1972, the first edition that appeared after the Ronan Point Collapse . Later editions of 1982, 1995 and 2005 contained more comprehensive performance statement and check of strength & stability of structural systems with recommended set of load combinations. The National Institute of Standards and Technology (NIST 2005) presented a report on collapse of WTC, which provided recommendations for “improving the safety of buildings, occupants and emergency responders.” In 2006, NIST presented a document to provide owners and practicing engineers with best practices to reduce likelihood of progressive collapse of buildings in the event of abnormal loading .
The U.S. General Services Administration (GSA) published the general guidelines to assess the potential for progressive collapse in RCC and Steel buildings in 2000 and 2003. These guidelines provide “threat independent” methodology to minimize the progressive collapse potential i.e. cause of element failure is not considered. Guidelines provide simplified approach i.e. Linear Procedure for low to medium rise buildings i.e. building up to 10 storey and sophisticated approach i.e. Nonlinear Procedure for buildings above 10 storey or building with asymmetrical configuration.
The simplest analytical procedure to evaluate the progressive collapse potential is Linear Static Method where the performance of structure is evaluated by demand to capacity ratio (DCR), which should not exceed 2 for regular structures and 1.5 for irregular structures or else they are considered as severely damaged or failed . GSA has defined the DCR as the ratio of Demand to Capacity where Demand is the acting force in terms of moment, axial force, shear and possible combined force while capacity is ultimate, un-factored capacity of the component and/or connection/joint. In this procedure, one of the vertical load bearing structural elements is removed as per the locations specified in the guidelines and the remaining structure is checked for the alternate load path for gravity load combination. For static analysis procedure, load combination defined in the guidelines is 2(DL + 0.25LL); where factor 2 is provided to function as dynamic magnification factor to simulate dynamic response. For dynamic analysis procedure, the load combination defined is (DL + 0.25LL).
There are various analysis approaches prevailing through which, the potential for progressive collapse can be evaluated. They are mainly,
1. Direct Design Approach:
Direct design approach includes (a) Alternate Path Method, which requires that the structure be capable of bridging over a missing structural element (b) Specific Local Resistance (SLR) Method, which requires that the building, or parts of the building, provide sufficient strength to resist a specific load.
2. Indirect Design Approach:
This approach is based on provision of minimum level of strength, continuity and ductility. It emphasizes on good plan layout, integrated tie system, ductile detailing and extra reinforcement for accidental loading and load reversal. Thus Indirect Method is likely to be primary method used to enhance the robustness of the building
Department of Defense (DoD) of U.S. has also published the similar guidelines under Unified Facilities Criteria (UFC) in 2002 to reduce the potential of progressive collapse for the buildings that experiences significant structural damage under abnormal loading condition . This guideline provides three analysis approaches for progressive collapse analysis requirement;
1. Tie Force Method: In this approach, the building is mechanically tied together to enhance continuity and ductility which also help to develop the alternate load paths.
2. Alternate Load Path Method: This method is used to determine the bridge over capacity of the structure when one of the load carrying elements is removed.
3. Enhanced Local Resistance Method: In this method, enhance local resistance is provided through flexural and shear resistance of perimeter building columns and load bearing walls.
Load combination defined by the guidelines is
Static analysis: 2[(0.9 or 1.2)DL + (0.5LL or 0.2SL)] + 0.2WL
Dynamic analysis: [(0.9 or 1.2)DL + (0.5LL or 0.2SL)] + 0.2WL
Upward loads on floor slabs: 1.0 DL + 0.5LL ; Where DL = Dead Load, LL = Live Load and WL = Wind Load and SL = Snow Load
This way, in this analysis procedure, additional effect of lateral load and upward component of gravity load is considered to assess the progressive collapse potential. Also these guidelines specify column removal conditions on each floor one at a time, which was just on ground floor in GSA guidelines.
The Indian Institute of Technology, Kanpur (IITK) and Gujarat State Disaster Management Authority (GSDMA) has published a document called “Measures to Mitigate the Terrorist Attacks on Buildings” in 2007, which covers mostly the qualitative aspects .
It is not always feasible to design structures for absolute safety, nor is it economical to design for abnormal events unless they have a reasonable chance of occurrence. Alternatively, proper structural design can greatly reduce the possibility of progressive collapse by paying due attention to structural details and material properties.
The mechanism and history of building collapses presented in the study, provide the information on probable cause of failure and behavior of building at the time of collapse, which helps to design the building in better way.
 US General Service Administration (GSA 2003), “Progressive Collapse Analysis and Design Guidelines for New Federal Office Buildings and Major Modernization Projects, 2003, USA”
 IITK – GSDMA, “Guidelines on measures to mitigate effects of terrorist attacks on buildings,” Indian Institute of Technology, Kanpur & Gujarat State Disaster Management Authority, July 2007.
 H.S. Lew, “Best practices guidelines for mitigation of building progressive collapse,” May 2003.
 Bruce R. Ellingwood, “Mitigating risk from abnormal loads and progressive collapse,” Journal of performance of constructed facilities / ASCE, pp. 315-323, Nov. 2006.
 National Institute of Standards and Technology (NIST), “Best practices for reducing the potential for progressive collapse in buildings”, U.S. Department of Commerce, Technology Administration, NIST, U.S.A., August 2006.
 United Facilities Criteria (UFC 4-023-03), “Design of buildings to resist the progressive collapse”, Department of Defense, New York