Civil Insight: A Technical Magazine Volume 3 | Page 45

Civil Insight (2019) 45-50 Civil Insight: A Technical Magazine Column Overdesign Factor in RC-Framed Structures 1,* 1 Student, Gunjan Karki, 2 Mahesh Raj Bhatt ME in Structural Engineering, Department of Civil Engineering, Kathmandu University, Nepal, [email protected] 2 Lecturer, Department of Civil Engineering, Kathmandu University, Nepal, [email protected] Abstract The effect of column overdesign factor cof on collapse mechanism of Reinforced Concrete (RC)-framed Buildings considers the provision of strong column-weak beam philosophy. It is nearly impossible to predict the characteristics of ground motion during an earthquake. Therefore, the determination of the complete behavior of a building during seismic excitation is a very complex task. However, it is possible to design a structure that can behave in a predefined manner in terms of damage, energy dissipation, or failure. This study focuses on the collapse mechanism of RC- framed buildings for various column overdesign factors. The collapse mechanism of a building is very important to dissipate earthquake energy and ensure its ductile failure. Two buildings, one 3-storied and the other 9-storied, were modeled using SAP2000 version 20 to represent lower-rise and middle-rise RC-framed buildings. The models were analyzed for column overdesign factor cof, from 1.2 to 2, to study the collapse mechanism. In this study, the bending moment capacities of beam and column were calculated considering under-reinforced RC section with zero axial force as recommended by IS 13920. A static non-linear analysis was carried out to study the collapse mechanism of the models. It is found that column overdesign factor 2 is not sufficient to achieve the beam sway mechanism for a 3- storied building. However, for the 9-storied case, a gradual change from column sway mechanism to beam sway mechanism is observed for the gradual increase in cof from 1.2 to 2. Keywords: Strong column-weak beam, Beam sway mechanism, Story mechanism, Column overdesign factor (cof), Pushover analysis 1) Introduction Nepal lies in a high seismic zone. The buildings in this zone are at a risk of collapse due to earthquakes. It is not feasible to design the structures to remain elastic during the earthquake due to economic consideration. Therefore, damages are allowed up to a certain limit (Mistri & Sarkar, 2016). Hence, yielding pattern of a building needs to be studied to confirm the preferable failure mechanism. In multi-storied reinforced concrete buildings, it is also desirable to dissipate earthquake-induced energy by yielding of the beams rather than columns as columns are responsible for the overall strength and stability of the structure (Agrawal & Shrikhande, 2011). The yielding of columns is avoided due to large curvature ductility demand associated with the column sideway mechanism. Failure in the beam causes local failure but failure in column of any story may cause catastrophic failure of the whole structure. Furthermore, the task of strengthening of a beam is easier than that of a column. Therefore, it is preferable to control inelasticity in columns as much as possible. Hence, to avoid the catastrophic collapse of a building and ensure safety to lives inside it, columns are *Corresponding Author Email address: [email protected] (Gunjan Karki) Submitted on October 12, 2019; Accepted on December 15, 2019 45