Analysis of falling debris impact velocity in structural collapse
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Falling debris, initiates from the partial collapse of structural elements such as beams and slabs, can be triggered by various events including earthquakes, fires, and falling equipment. Current analysis methods lack the ability to predict critical aspects such as the velocity, shape, and condition of the falling debris upon impact. This makes it exceedingly difficult to conduct a falling debris analysis as allowed in the new Disproportionate Collapse Standard. Due to the limited research in this area, existing analyses use assumptions that have almost uniformly suggested that the forces exerted by falling debris are too large for structures to resist. However, real-world observations indicate that falling debris does not always lead to a full progressive collapse, prompting an important question: if the forces from falling debris are so substantial, why do some structures experience only partial collapse? Part of the answer lies with the conservative assumptions used in falling debris analysis, which assume debris falls freely over the entire height of the structure. To begin to improve our understanding, we conducted a comprehensive literature search in combination with the development of finite element (FE) models validated using previous experimental tests from the literature to assess the velocity of falling debris. The findings reveal that assuming freefall velocity for debris overestimates the impact forces due to debris often remaining partially connected to the structure. The primary objective of this research is to provide a comprehensive analysis of prior experimental studies on the sudden removal of columns in reinforced concrete (RC) frames. This study investigates failure scenarios in primary structural components, focusing on the forces generated by falling debris and the parameters influencing the actual speed of falling components compared to free-fall assumptions. Key factors such as resistance mechanisms, detailing and reinforcement, floor slabs, restraint conditions, and applied loads were analyzed to assess their influence on falling velocities. The findings demonstrate that resistance mechanisms like compressive arch action (CAA) and catenary action (CA) significantly reduce falling velocities, with values ranging between 50 percent and 75 percent of the free-fall velocity. Enhanced axial and rotational restraints lowered peak velocities by up to 45 percent, while improved reinforcement detailing at joints and the inclusion of floor slabs provided additional resistance, further mitigating falling speeds. Conversely, higher applied loads increased velocities, approaching free-fall conditions; doubling the load raised the velocity from 47 percent to 75 percent of free-fall.
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M.S.