Progressive Collapse

Progressive collapse is the result of a localized failure of 1 or 2 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. Such damage may result in upper floors of a structure collapsing onto the lower floors.

Progressive collapse often starts with the failure of a single/group of elements or beam-column connections, which often fail in a brittle manner when subjected to abnormal loads. The response of either the elements or the structure’s system to this abnormal loading is most likely to be dynamic & nonlinear, both geometrically & in the material’s behavior (Smilowitz 2002). In the event of structural collapse, dynamic effects arise from several sources. During a collapse event following a sudden removal of an element in a structure or failure of beam-column connections, the structure will redistribute its load & stabilize in a new equilibrium position because of its geometric change. This geometric change will result in a release of potential energy & a rapid change of internal static & dynamic forces, including inertia force, which is produced by the dynamic response during the redistribution. Recent studies regarding the response of building structures have shown the importance of inertial effects for collapse analysis & verified that nonlinear behavior must be considered. The initiating events of progressive collapse are generally associated with dynamic phenomena, such as impact, explosions & sudden failure of a structural connection.

One of the 1st events that drew attention to progressive collapse as an important factor in structural design was the partial collapse of the 22-storey Ronan Point apartment building in London, on May 16th, 1968. This was caused by an accidental gas explosion on the 18th floor knocked out load-bearing precast concrete panels near the corner of the building. The loss of support at the 18th floor caused the floors above to collapse. The impact of these collapsing floors set off a chain reaction of collapse of the corner portion of the building along its entire height.

Ronan Point Building

VBIEDs have been proven to cause progressive collapse of buildings, such as the attack on the US Marine Barracks in Beirut in 1983 which was almost completely destroyed with the loss of 241 lives (Davis, 2007). Blast loads have become the focus of attention because of a number of accidental & intentional events in recent years that affected important structures all over the world which clearly indicated that this issue must be addressed in structural designs & reliability analyses. As a consequence, extensive research based on blast loads has been conducted in the past few decades (Alia and Souli, 2006).

Large amounts of explosives at short distances from the structure can cause excessive pressure forces, which cannot be accommodated in the design of an ordinary structure. It is imperative to implement measures such as perimeter control & standoff distances to reduce the possibility of a blast at close proximity to the structure.

Blast Effects on Buildings

Buildings experience the effects of explosions in several stages:

• When a VBIED is detonated the blast immediately moves out towards the structure but at the same time blast waves impact the ground beneath the vehicle thus causing ground shock waves to also move out towards the building but travelling in advance of the blast waves. The initial blast wave typically shatters windows & causes other damage to the building facade. It also exerts pressure on the roof & walls that are not directly facing the blast, sometimes damaging them as well.
• In the 2nd stage, the blast wave enters the building & exerts pressure on the structure. When directed upward, this pressure may be extremely damaging to slabs & columns because it acts counter to the design used to resist gravity loads. Air-blast pressures within a building can actually increase as the pressure waves reflect from surfaces & can cause injuries to the occupants directly by means of physical translation, ear, lung & other organ damage, or debris from building elements & contents.
• Finally, the building frame is loaded globally & responds as it would to a short-duration, high-intensity earthquake.

The blast pressures experienced by a structure are related to the amount of explosive used & the distance of the building from the explosion. The peak incident pressure, charge weight & distance are mathematically related through an expression that varies as a function of the weight of the explosive & the cube of the distance. This relationship is critical to understanding the effects of explosions on structures. The pressure experienced by a building increases with bomb size, but decreases very quickly with increasing distance between the building & the bomb. These factors form the 2 keystones of defensive design & limiting the size of a bomb through vehicle control & inspection & enforcing standoff distances from possible targets. 

The standoff distance available & the assumed size of the explosive device will determine the blast-resistant features that must be provided. Large IEDs detonated at relatively great standoff distances will produce a large but uniform pressure over the surface of the building. At shorter standoff distances, a small IED can produce locally intense effects, such as shattering load-bearing columns. While the former scenario is likely to govern design of the facade to limit the formation of hazardous debris, the assumption of a smaller, close-in device is likely to control design of the first-floor load-bearing elements to prevent localized failure leading to progressive structural collapse.

If a large IED is detonated close to the structure, global damage & the size of the resulting ground crater may be increased to the point that the structure, foundation, or both will be overwhelmed & a catastrophic collapse may ensue. In other words, in large explosions the total impact on the structure results from the blast wave & ground shock resulting in greater damage to the structure.

One of the important first steps in the design of any structure is defining the loads. In security design, this must include defining the blast loading - a function of the expected charge weight & proximity. Although a VBIED with conventional explosives still appears to be the most serious bombing threat, hundreds of smaller bombing incidents occur annually.

?In South America, the Jewish community centre in Buenos Aires, Argentina, was a target of a terrorist attack on July 18th, 1994, by a VBIED which was parked approximately 5 meters away. The progressive collapse occurred in this building as a consequence of the failure of the load bearing walls; resulting in the floor slabs collapsing one on top of another. The building was totally demolished by the explosion.

Blast loading or other unforeseen events can cause progressive collapse due to damage of some key elements which can either make the structure unstable.

The response of reinforced concrete (RC) under blast loading is different from its response to typical static & dynamic loads because of the very short duration & extreme pressure loading caused by blast. The stiffness & strength of RC is likely to increase with the higher rate of loading experienced under blast conditions. This, in turn, increases the strength of RC members & translates into higher resistance. On the other hand, the high rate of loading expected during blasts may also reduce the deformation capacity & the fracture energy of RC significantly. This translates to a reduction of ductility of RC in blast loading situations, a property generally mandated by most codes & standards to preserve the integrity of a structure.

Reinforced Concrete Column

To achieve targeted integrity during blast, the redundancy of the gravity load carrying structural system takes center stage in tackling the issue of progressive collapse. The structure should be able to remain stable by redistributing the gravity loads to other members & subsequently to the foundation through an alternate load path, while keeping building damage somewhat proportional to the initial failure. The inherent mass & stiffness characteristics of RC offer distinct advantages over other building materials such as steel & timber under blast loading. RC structures are better able to resist the overall shock due to local disintegration caused by the blast.

A detonation results in a rapid release of energy in the form of light, heat, sound & a shock wave. The shock wave consists of highly compressed air that reflects off the ground surface & travels outward from the source at supersonic velocities. As the shock wave expands, the magnitude of the incident pressures decreases. When the shock wave encounters a surface it reflects, the pressure is amplified. Due to the supersonic velocity of the shock wave at impact, the waves can reflect with an amplification factor of up to 13. The magnitude of the reflection factor is a function of the proximity of the explosion & the angle of incidence of the shock wave on the surface. The resulting pressures decay rapidly with time & the shock wave becomes negative, followed by a partial vacuum, which creates suction behind the shock wave.

Blast wave action

In an external explosion, a portion of the energy is also imparted to the ground, creating a crater & generating a ground shock wave analogous to a high-intensity, short-duration earthquake.

Crater from an VBIED

Blast load generally is impulse-type high-amplitude loading that lasts for a very short period of time measured by milliseconds. The loading in many situations is localized & only those elements closest to the blast may be directly impacted. Elements farther removed from the blast site may experience little or no direct impact due to sharp attenuation of blast energy with distance. The forces experienced by members of the structure depend upon the size & proximity of the explosion. Higher explosive weights at short distances from the structure can cause excessively large forces which cannot be reasonably accommodated in design of the structure. As such, it is imperative to put in place other measures, such as perimeter fences & standoff distance to reduce the possibility of blast at close proximity to the structure.

Damage due to the air-blast shock wave may be divided into direct effects caused by the high-intensity pressures & progressive collapse. The damage caused by the pressure may cause localized failure of exterior walls, windows, roof systems, floor systems & columns. The magnitude of the pressure that affects building surfaces due to an explosion may be several times greater than the loads for which the building is designed. It is likely that the building would not have been designed for other shock wave effects such as upward pressure on the floor system. As the shock wave travels, the air blast first collides with the exterior surface of the building. The pressure wave pushes on the exterior walls, causing wall failure & window breakage. As the shock wave continues to expand, it enters the structure, pushing both upward on the ceilings & downward on the floors.

Various Progressive Collapse Simulations

When a ground column is severely damaged by an explosion, axial compressive force above this column will vanish quickly within a few milliseconds. As a result, all floors above the 1st floor will deflect identically & dynamically under uniform gravity load to seek a new equilibrium path.

Flat slabs are RC slabs supported directly on columns without beams. Flat slabs are commonly used for construction of medium-rise office buildings & car parking structures due to their ease of construction, reduced story height & ease of routing of services. Load concentrations can be significant at edge & corner columns as well as around internal columns, making the slab-column connections susceptible to punching shear failure. Some of these collapses progressed horizontally through punching of adjoining connections due to gravity load redistribution, dynamic effects & excessive slab deformation. In many cases, failure also progressed vertically due to impact of falling slabs on lower lying ones.

Punching shear failures of flat slabs are the most common cause in RC framed buildings, as occurred at the 1997 Pipers Row Car Park in Wolverhampton in the UK & the 1995 Sampoong Superstore in South Korea. In the Pipers Row Car Park, the loss of strength due to concrete deterioration triggered punching shear failures (Wood, 1997).

Punching Shear Failure at Interior Slab-Column Connections-Piper's Row Car Park, Wolverhampton, UK, 1997

In the Sampoong Superstore the inadequate provision of reinforcement in the flat slab column region, combined with over-loading caused punching shear failures & collapse which killed 501 people (Wearne, 1999).

Sampoong Superstore in South Korea

The Alfred P. Murrah Building featured open-plan architecture combined with a glazed fa?ade, features that became vitally important when a VBIED was detonated on the curb side about 5 metres away from the building. Three columns at the 1st storey were badly damaged & caused the total collapse of the front half of the building, which accounted for 80% of the deaths. The loss of support from these 3 columns led to failure of a transfer girder. Failure of the transfer girder caused the collapse of columns supported by the girder and floor areas supported by those columns. This use of the transfer girder to support every other perimeter column has been widely attributed to the scale of the collapse, as has the lack of continuity of beam reinforcement through beam-column junctions. However, more recent forensic analysis of the building indicated that a 42m wide section of the building would still have collapsed had all the perimeter columns been continued to ground floor level & had full reinforcement continuity been provided. (In the actual event a 48m wide section of the building collapsed.) (Byfield and Paramasivam, 2012; NIST, 1995)The building comprised lightly reinforced columns which is common in non-seismic regions of the world. However, such columns are vulnerable to shear failures due to the sideways pressure from blast loading. It is believed that the column closest to the blast shattered & the 2 columns either side failed in shear. Lacking strong internal partition walls or cladding, the building had no emergency means for redistributing loads & a progressive collapse was triggered which consumed the front half of the building, killing 168 people (Corley et al, 1998). .

Alfred P. Murrah Building in Oklahoma

This highlights the ease with which VBIED’s can cause extensive column shear failures & also the importance of alternative load paths to redistribute loads away from damaged columns.

In the World Trade Centre terrorist attack, it is clear that the global collapse was initiated by local damage (i.e., immediate damage from the aircraft impacts, enhanced by high temperature effects due to fires). This fits very well with the definition of progressive collapse. The World Trade Centre towers were structurally highly redundant, comprising a rigid perimeter frame & a gravity load bearing central core, together with a truss system installed between the 107th & 110th floors which linked the perimeter frame to the central core structure (Kirk, 2005). The towers remained globally stable immediately after the impacts, despite the severing of up to 36 perimeter columns in the face of each tower. The gravity loads originally carried by the damaged perimeter columns were partially transferred to the adjacent undamaged columns via vierendeel action. In addition, perimeter columns were also believed to have become suspended from the trusses installed between the 107th & 110th floors. 

Precast concrete components are manufactured in a strictly controlled environment. It has been proven to show good behaviour under gravity & lateral loads. However, the beam to column connections remain the critical part in the precast concrete structures under the column loss scenario in a progressive collapse scenario. A connection enhancement is suggested to increase the resistance of precast concrete structures to progressive collapse.

Summary of buildings that suffered from progressive collapse.

• St Mark’s in Campanile Venice in Italy on July 1902. Caused by fire.
• Ronan Point apartment in London, UK on May 1968. Caused by gas explosion.
• Skyline Towers Building in Virginia, USA on March 1973. Caused by early framework removal.
• Civic Arena Roof in Kansas, USA on June 1979. Caused by heavy snow load.
• Pavie Civic Tower in Pavia, Italy on March 1987. Caused by excavation.
• Ancient bell tower in Goch, Germany in 1992. Caused by dynamic effects.
• Alfred P. Murrah federal building in Oklahoma, USA on April 1995. Caused by terrorist attack.
• Khobar Towers in Saudi Arabia on June 1996. Caused by terrorist attack.
• World Trade Center in New York, USA on Sept 2001. Caused by terrorist attack.
• Windsor Tower in Madrid, Spain on Feb 2005. Caused by fire.

Reducing the risk of progressive collapse

The goal for a blast resistant design includes preventing progressive collapse of a component or structural system, minimizing global damage, or localizing damage to absorb the blast energy. One approach would be to harden the first 2 or 3 stories of a building to make it blast-resistant. Another alternative is to embed steel cables in the floors at the building perimeter. The cables would redistribute gravity loads to other columns if an exterior column were to be severely damaged. Yet another method would be having redundant load paths in the vertical load carrying system to help ensure alternate load paths are available in the event of local failure of structural elements. The loss of major structural elements result in load redistributions & member deflections which require the vertical & horizontal transfer of loads throughout the structure through load paths.

Structural members & their connections need to maintain their strength through load redistributions associated with the loss of key structural elements. Structural elements such as perimeter beams or slabs must be designed to withstand shear load in excess of that associated with the ultimate bending moment in the event of loss of an element.

Columns, girders, roof beams, lateral load resisting system & floor beams & slabs must be designed to resist reversals in load direction. Large column spacing decreases the likelihood that the structure will be able to redistribute load in the event of column failure.

References:

Primer for Design of Commercial Buildings to Mitigate Terrorist Attacks, Federal Emergency Management Agency, FEMA 427, December 2003.

U.S. Department of the Army Technical Manual, TM5-1300, Design of Structures to Resist the Effects of Accidental Explosions, Washington, DC, 1990.

U.S. Department of the Army Technical Manual, TM5-885-1, Fundamentals of Protective Design for Conventional Weapons, Washington, DC, 1967.

Bazant, Z. P. and Zhou,Y. ,“Why Did the World Trade Center Collapse? – Simple Analysis”, Journal of Mechanics, ASCE in press, posted since Sep. 14 at https://www.civil.northwestern.edu. 

Byfield MP and Paramasivam S (2012) The Murrah Building: A reassessment of the transfer girder. Journal of Performance of Constructed Facilities, No. 4, DOI: 10.1061/(ASCE)CF.1943- 5509.0000227.

Corley WG et al. (1998) The Oklahoma City bombing: summary and recommendations for multihazard mitigation. Journal of Performance of Constructed Facilities, 12: p. 100.G. C. Mays and P. D. Smith, Blast Effects on Buildings, Editors, Thomas Telford, 1995.

Davis (2007) Buda's wagon: a brief history of the car bomb. Verso Books

Kirk JA (2005) The world trade centre collapse: Analysis and recommendations, in Civil and Environmental Engineering. Thesis, Master of Engineering, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology: Massachusetts.

Rusnardi R, Kiyono J, Ono Y. Estimation of earthquake ground motion in Padang, Indonesia. International Journal of GEOMATE. 2011;1(1 SERL 1):71-8. , R. (2002), “Analytical tools for progressive collapse analysis”, Paper Presented at the National Workshop on Prevention of Progressive Collapse, July 10-12, 2002, Rosemont, Illinois. Available from National Institute of Building Sciences (NIBS).

Wearne P (1999) Collapse: Why Buildings Fall Down. Channel 4 Books

Wood J (2003) Pipers Row Car Park, Wolverhampton – Quantitative Study of the Causes of the Partial Collapse on 20th March 1997. SS&D Contract Report to British Health and Safety Executive (HSE)

------------------------------------

Endro Sunarso is an expert in Security Management, Physical Security & Counter Terrorism. He is regularly consulted on matters pertaining to transportation security, off-shore security, critical infrastructure protection, security & threat assessments, & blast mitigation. He is also a Certified Identity & Access Manager (CIAM).

Endro has spent about 2 decades in Corporate Security (executive protection, crisis management, business continuity, due diligence, counter corporate espionage, etc). He also has more than a decade of experience in Security & Blast Consultancy work, initially in the Gulf Region & later in SE Asia.