Presented by: Asst. Lecturer Kawther Hayder Abdulateef Introduction Structures are normally designed to resist the loads expected during their service life, such as dead loads, live loads, wind loads, and seismic loads. However, some structures may be subjected to unusual extraordinary events, such as explosions, impact, fires, or design and construction errors, which lead to the failure of a primary structural element such as a column or a load-bearing wall. If the structure is unable to redistribute the loads safely, the local failure may spread to the remaining elements until a large part of the structure, or the entire structure, collapses. This phenomenon is known as Progressive Collapse, and it is also called Disproportionate Collapse when the extent of the final collapse is much greater than the initial damage that triggered it. This phenomenon is considered one of the most serious problems in structural engineering, because its consequences are often severe in terms of human and economic losses, and because its causes may not be related to the usual design loads. Concept of Progressive Collapse Progressive collapse is characterized by an initial triggering event that causes a local failure, followed by the transfer of loads from the failed elements to adjacent elements, which exceed their capacity and fail in turn in a successive chain. The resistance of a structure to this phenomenon is viewed through the concept of Structural Robustness, which is the ability of the structure to withstand local damage without the final collapse being disproportionate to the initial damage. Some researchers, including Starossek, have classified progressive collapse into several mechanisms, the most prominent of which are: Pancake-type collapse: Slabs fall one on top of another after losing their support, so the falling loads on the lower floors are multiplied. Zipper-type collapse: Failure spreads horizontally or vertically along a line of adjacent elements, as a zipper opens. Domino-type collapse: The fall or overturning of one part of the structure pushes the adjacent part, which then collapses in turn. Section-type collapse: The cross-section of an element tears section after section once its capacity is exceeded. Instability-type collapse: Local loss of stability causes failure to spread through the rest of the structure. Mixed-type collapse: Combines more than one of the above mechanisms. Causes and Triggers The chain of collapse can be initiated by various events, including: Explosions: Such as gas explosions or attacks using explosives. Impact: Such as vehicles colliding with ground-floor columns. Fires: Which weaken the strength of steel and concrete and may lead to the loss of load-bearing elements. Design and construction errors: Such as inadequate punching shear resistance in slabs, weak connections, or poor material quality. Unstudied changes in use: Such as adding loads or floors, or removing load-bearing elements without a structural study. Deterioration over time: Such as reinforcement corrosion or weakening of connections. Natural disasters: Such as earthquakes, landslides, and foundation settlement. Historical Incidents That Advanced Understanding The development of interest in this phenomenon has been linked to a number of well-known incidents: 1. Ronan Point Tower (London, 1968): A gas explosion in an apartment on the eighteenth floor of a residential tower built from precast concrete panels led to the collapse of one side corner of the building along its entire height. This incident revealed the weakness of the connections between the panels, and led to a review of British building regulations and the introduction of requirements for structural tying and general robustness. 2. Alfred P. Murrah Federal Building (Oklahoma City, 1995): The explosion of a truck bomb destroyed one of the main ground-floor columns, which caused the collapse of a large part of the building, far exceeding the area directly affected by the blast. This incident had a major impact on the development of guidelines for protection against progressive collapse for U.S. federal buildings. 3. Sampoong Department Store (Seoul, 1995): This commercial building collapsed suddenly, killing hundreds of people. Investigations pointed to several factors, including inadequate punching shear resistance at the column–slab connections and increased loads resulting from modifications made to the building. The incident is used as an example of collapse linked to design and construction defects and unstudied modifications. Design Principles for Reducing Progressive Collapse Modern codes and guidelines agree that completely preventing local failure is not economically feasible in all cases, and that the goal is to contain the damage and prevent it from turning into a disproportionate collapse. Strategies are generally grouped into three approaches: 1. Indirect Design: Relies on improving the overall robustness of the structure by providing a minimum level of strength, continuity, and ductility, such as: Providing horizontal and vertical tie forces between elements. Ensuring continuity of reinforcement and detailing connections with sufficient ductility. Using structural detailing that allows plastic hinges to form instead of brittle failure. 2. Direct Design: Explicitly addresses the behavior of the structure when an element is lost, and includes: Alternate Load Path Method: A load-bearing element (such as a column) is assumed to be removed, and the structure’s ability to redistribute loads without disproportionate collapse is then verified. Specific Local Resistance Method: Critical elements are strengthened to withstand the expected extraordinary loads, such as ground-floor columns exposed to blast or impact. 3. Event Control: Such as controlling access to the building, keeping vehicles away from facades, and reducing explosion and fire risks through planning and security measures. Analysis Methods A number of analysis methods are used to assess the resistance of a structure to the loss of a load-bearing element, including: Linear static analysis: The simplest method, and it usually relies on amplification factors to represent the dynamic effect of element loss. Nonlinear static analysis: Takes into account the plastic behavior of materials and large deformations, and gives an estimate closer to reality. Nonlinear dynamic analysis: Considered the most accurate because it simulates the time-dependent effect of element loss and load transfer, but it is the most complex and computationally costly. The importance of considering the dynamic effect resulting from the sudden removal of the element stands out in these analyses, because the loads transferred to adjacent elements are greater than those given by simple static analyses. Factors Affecting the Resistance of the Structure The ability of a structure to resist progressive collapse is affected by several factors, including: Ductility and continuity: Structures with ductile detailing and continuous connections tolerate large deformations and redistribute loads better. Degree of structural redundancy: The presence of multiple alternative load paths reduces the effect of losing any single element. Structural system: The behavior differs among framed systems, wall systems, precast panels, and systems with flat slabs. Connection behavior: Connections are often the weakest link in precast and steel systems. Punching shear in flat slabs: A local failure at one column can spread to adjacent columns because of the increased loads on them. Quality of construction and materials: Construction defects can turn a local failure into a widespread collapse. Codes and Guidelines Several international codes and guidelines have addressed structural robustness and progressive collapse, including: The American loads standard ASCE 7, which includes general requirements for structural integrity. The U.S. guidelines issued by the General Services Administration (GSA), and the Department of Defense criteria UFC 4-023-03, which address the design of buildings to resist progressive collapse. The European code EN 1991-1-7 on accidental actions, which adopts a classification of buildings according to consequence classes. The structural integrity requirements in the ACI 318 code for reinforced concrete. Most of these codes adopt the principle of graduated requirements according to the importance of the building and the consequences of its collapse, so that requirements become stricter for tall, highly occupied, or vitally important buildings. Progressive Collapse in Existing Structures The problem is not limited to new structures; existing structures, especially aging ones, may lack sufficient structural robustness because they were designed before the development of modern codes. Assessing them requires examining connections, reinforcement continuity, and alternate load paths, in addition to reviewing the modifications made to the structure. Available improvement methods include strengthening critical columns and connections, adding tie elements or backup support systems, reducing excessive loads, and implementing periodic inspection and maintenance programs. Challenges and Research Trends This issue still faces challenges, including balancing economic cost against the required level of safety, the difficulty of predicting extraordinary events, and the variation in requirements among different codes. Recent research is moving toward studying connection behavior under large deformations, developing advanced numerical models to simulate collapse, assessing robustness using probabilistic approaches, improving the performance of precast structural systems and flat slabs, and studying the effect of fire accompanying the loss of load-bearing elements. Conclusion Progressive collapse represents one of the most dangerous modes of structural failure, as it turns limited local damage into widespread collapse. Historical incidents have shown that its causes may be extraordinary or may result from defects in design and construction, and that preventing it requires an integrated system that includes structural tying, ductility, structural redundancy, alternate load path analysis, construction quality, and periodic maintenance. With the increasing height and complexity of buildings, the importance of incorporating the concept of structural robustness into routine design practice grows, as a fundamental part of structural safety and the protection of lives. References Starossek, U. (2009). Progressive Collapse of Structures. London: Thomas Telford. Starossek, U., & Haberland, M. (2010). Disproportionate collapse: terminology and procedures. Journal of Performance of Constructed Facilities, 24(6), 519–528. Adam, J. M., Parisi, F., Sagaseta, J., & Lu, X. (2018). Research and practice on progressive collapse and robustness of building structures in the 21st century. Engineering Structures, 173, 122–149. Ellingwood, B. R., Smilowitz, R., Dusenberry, D. O., Duthinh, D., & Carino, N. J. (2007). Best Practices for Reducing the Potential for Progressive Collapse in Buildings. NISTIR 7396. Gaithersburg, MD: National Institute of Standards and Technology. U.S. General Services Administration (GSA). (2013). Alternate Path Analysis and Design Guidelines for Progressive Collapse Resistance. Washington, DC. U.S. Department of Defense. (2016). Design of Buildings to Resist Progressive Collapse. UFC 4-023-03. Unified Facilities Criteria. American Society of Civil Engineers (ASCE). (2016). Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE/SEI 7-16). Reston, VA. European Committee for Standardization (CEN). (2006). Eurocode 1: Actions on Structures – Part 1-7: General Actions – Accidental Actions (EN 1991-1-7). Brussels. Izzuddin, B. A., Vlassis, A. G., Elghazouli, A. Y., & Nethercot, D. A. (2008). Progressive collapse of multi-storey buildings due to sudden column loss, Part I: Simplified assessment framework. Engineering Structures, 30(5), 1308–1318. Griffiths, H., Pugsley, A., & Saunders, O. (1968). Report of the Inquiry into the Collapse of Flats at Ronan Point, Canning Town. London: HMSO. Pearson, C., & Delatte, N. (2005). Ronan Point apartment tower collapse and its effect on building codes. Journal of Performance of Constructed Facilities, 19(2), 172–177. Corley, W. G., Mlakar, P. F., Sozen, M. A., & Thornton, C. H. (1998). The Oklahoma City bombing: summary and recommendations for multihazard mitigation. Journal of Performance of Constructed Facilities, 12(3), 100–112. Gardner, N. J., Huh, J., & Chung, L. (2002). Lessons from the Sampoong department store collapse. Cement and Concrete Composites, 29(6), 523–529.