The objective of this project is to analyze the progressive collapse behavior of 10 story reinforced concrete frame structures by removing column suddenly from different locations, as per the GSA guidelines. To achieve this, a structural model of the building has been developed in ETABS version 18.1.1 and loads are applied as per GSA guidelines. The design and detailing of the structure are done in accordance with IS codes, using the special moment-resistant frame linear static analysis method. In accordance with GSA guidelines, three-column removal scenarios have been studied on the ground floor, including the removal of a Corner column, Middle column of longer and Middle column of shorter side. The Demand/Capacity ratios (DCR) for each member have been calculated using the linear static analysis method. Members with a DCR ratio greater than 2 are expected to fail in a similar column removal case. Past studies have revealed that shear in a beam and columns are not crucial in progressive collapse. However, the linear static analysis method has demonstrated that beams are prone to fail in flexure.
Progressive collapse, a critical concern in building safety since the 1968 collapse of the Ronan Point apartment building in Canning Town, East London fig. 1.1, refers to a sequence of failure, also known as disproportionate failure. This sequence connects localized damage caused by a single structural element to the eventual collapse of an entire structure. Local failure can be understood as the loss of load-bearing capacity in one or more structural components that make up the overall structural system. Ideally, when a structural member fails, the building should have mechanisms in place to redirect the load through alternative paths. As the load redistributes within the structure, each structural element takes on new loads. However, if any load surpasses the load-carrying capacity of a component, it can trigger another local failure. Sequential failures can spread throughout the structure. When a greater number of structural components fail, it can lead to either a partial or complete collapse. The latest known example is the Twin Towers of the World Trade Center in New York, USA, fig. 1.2. The world experienced horror on September 11, 2001, as both of the Twin Towers at the World Trade Center in New York City, USA, fell down in this order: First, a Boeing 767 airplane crashed into one of the towers at high speed. This impact caused serious damage to the structure around the point of impact and set off a massive fire inside the buildings. The area around the impact, including the floors above it, lost its ability to support the weight of the floors above. As a result, the part of the tower above the impact zone collapsed. When it lost support, the heavy upper part of the tower fell, causing a chain reaction of collapses all the way down to the ground. This tragic event is an example of a complete or total collapse.
1) The collapse pattern is such that the demand capacity ratio of the beam increases when beams are closer to the removed column and decreases as beams are farther from it.
2) The beams situated both adjacent to and directly above the removed column are subjected to the greatest bending moments in comparison to beams positioned at a greater distance from the removed column.
3) The members above the removed column experience reversal of stresses in both bending and shear. To counter this, these members should be redesigned by either increasing the section size or adding more reinforcement.
4) At the lowest story, beams connected to the removed column experience failure. Similarly, beams of the same type connected to columns above the removed column at various stories also undergo failures, involving both shear and flexure failure.
5) Out of the three cases of column removal, the most severe collapse happens when the middle column of the longer side is taken out. The second most damaging scenario is when the middle column of the shorter side is removed, while the least detrimental situation occurs when the corner column is removed. Hence middle column removal case is the most critical one. In general, the RCC building under consideration exhibits a lower potential for progressive collapse when a corner column is removed compared to the removal of a middle column.
6) After the removal of a column, the axial force in the columns above it gradually diminishes and is subsequently transferred to the adjacent columns through the interconnected beams.
7) Columns located in the vicinity of the removed columns exhibit a higher PMM ratio compared to columns situated farther away from the column removal locations. The reason behind this is that when one column is lost, the neighbouring columns are burdened with carrying its load. In all three cases, the PMM ratio exceeds 2, indicating that the columns supporting adjacent bays are critical in this progressive collapse scenario.
8) A structure designed with Special Moment-Resisting Frames (SMRF) according to IS 456:2000 and detailed according to IS 13920 may not inherently resist progressive collapse. This is because SMRF design primarily resists lateral loads, while progressive collapse involves the failure of structural elements under gravity loads.
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