Authors: Prof. Pushkraj. R. Admile, Akshay. R. Jagtap, Samir. D. Gaikwad, Abhay. A. Kale, Mahesh. R. Kamble
Certificate: View Certificate
This research paper focuses on the analysis of the P-delta effect on high-rise structures, which has been the subject of numerous recent studies. The P-delta effect is considered the secondary effect and is deemed more significant. It arises due to an increase in the number of stories in a structure and is particularly relevant when considering lateral loads such as earthquakes and wind loads. The objective of this paper is to incorporate axial load and displacement in the calculation of the P-delta effect in multi-storey buildings. The analysis involves RCC framed structures of different heights, specifically G+15, G+20, and G+25. The load combinations are determined according to IS 456:2000, and the software E-Tabs is utilized for the analysis. The earthquake load is applied to the structure based on IS-1839 (2002), and deflection curves are observed. Both models with and without the P-delta effect are examined in terms of bending moment, shear force, and displacement. The findings clearly demonstrate the necessity of considering the P-delta effect in multi-storey buildings. The analysis is conducted using the linear static method, and the results provide valuable insights into the structural behavior under different loading conditions.
The P-Delta effect is a second-order effect, wherein 'P' represents gravity or axial load, and 'Delta' represents displacement. The magnitude of the P-Delta effect is dependent on the magnitude of the axial load. With the increasing popularity and significance of high-rise structures, nonlinear analysis of the P-Delta effect has gained importance. The aim is to investigate the relationship between structural height and the P-Delta effect, which is a second-order effect.
As the number of storeys increases, there is an increased reduction in the resistance to collapse, indicating an increase in the P-Delta effect. If there is a change in moments, shear at the base, or displacement greater than 10%, it further amplifies the P-Delta effect. Additionally, the inclusion of shear walls leads to changes in the P-Delta effect.
It has been observed that the P-Delta effect increases with an increase in the slenderness ratio and the number of storeys. The P-Delta effect is prominent in buildings with a height exceeding 61 meters and without shear walls. The base shear and torsion values of buildings, with or without shear walls, are reduced due to the application of the P-Delta effect. These buildings are designed based on traditional building codes and are expected to experience inelastic actions during design-level earthquakes. The interaction between torsion and the P-Delta effect can be complex, particularly in the inelastic range of behavior. Inelastic actions can be influenced by both torsion and the P-Delta effect, with torsion playing a more significant role within the P-Delta effect. The type of lateral load resisting system employed in the structure also affects the degree to which torsion modifies the P-Delta effects. Hence, the characteristics of the lateral load resisting system hold greater importance than the number of storeys in high-rise buildings.
II. NEED OF THE PRESENT STUDY
In case of high-rise buildings, as the structure becomes more slender and less resistant to deformation, it is necessary to consider 2nd order and to be more specific, P-delta effects arises .As a result, codes of practice are referring engineers more and to the use of 2nd order analysis in order that P-delta and “stress stiffening effects are accounted for when appropriate in design. This is as true in concrete and timber design as it is in the design of steel work.
III. OBJECTIVES OF THE PRESENT STUDY
The primary objective of this work is to analyze the seismic capacity of gravity designed 10, 15, and 20 storied RC framed buildings using linear static and nonlinear static pushover analysis. It is also intended to check the above building for second order P-Delta effect are within acceptable limit or not and to show how to make a structure safe for second order P-delta effect. Detailed study of P-Delta effect.
IV. OBJECTIVES OF THE PRESENT STUDY
Seismic evaluation and retrofit of concrete buildings commonly referred to as ATC40 was developed by the Applied Technology Council (ATC) with funding from the California Safety Commission. Although the procedures recommended in this document are for concrete buildings, they are applicable to most building types. ATC40 recommends the following steps for the entire process of evaluation and retrofit:
VI. WORKING ON E TAB
VII. RESULT AND DISCUSSION
A. Result of 15, 20, Storey Buildings
Graphical representation of 1% increase in deflection and B.M. values due to all load cases with and without P-delta effect:-
a. Graph 6.1 represents % increase in deflection for various load cases for 15, 20 and 25 storey building.
b. From graph, it is observed that increase in deflection is more as number of storey increased.
c. Alos, P-delta effect is more observed in load cases 3, 4, 7, 8, 11, 12(the cases in which earthquake load is in y-direction). So P-delta effect is more observed in y-direction.
Graph 6.2 % increase in deflection due to P-delta
2. Graph 6.2
Graph 6.2 represents % increase in B.M. for various load cases for 15, 20 and 25 storey building.
From graph, it is observed that increase in B.M. will increase P – delta effect.
A. For 15 Storey Building
B. For 20 Storey Building
This chapter presents the major conclusions and future scope of the assessment of P-delta effect for high rise buildings. Based on the second order analysis using ETAB and verification with other authors following conclusions can be drawn. 1) As number of storey increases P-delta effect becomes more important. 2) P-delta effect is only observed in some of the beams and columns (Exterior columns and their adjacent beams) in some load cases. If these load cases are governing load cases for design of member, then only we can say that it is considerable. This condition is observed in 20 and 25 storey buildings and mostly in 25 storey building. 3) So we can say that, at least it is necessary to check the results of analysis with and without considering P-delta effect for the buildings. 4) Iterative P-delta analysis method is used. Building is by default analyzed for 10 numbers of iterations in ETAB. But the same results are observed for single iteration also. So there is no change in results by increasing the iterations. 5) Also analysis is performed by considering the seismic loading in other zones in India. Similar results are observed in the form of increase in internal forces. 6) So we should prefer form delta analysis for designing a minimum of 20 storey building considering seismic loads. And buildings up to 20 stories can be designed by conventional primary analysis or linear analysis. 7) The conclusion is valid for RCC residential buildings for seismic loading in all the zones of India and may not be applicable for commercial, educational or industrial buildings. 8) As the cross sections of members increases stiffness of a structure also increases. 9) We can bring building within permissible limit for P-delta effect by increasing the cross sections of members.
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Copyright © 2023 Prof. Pushkraj. R. Admile, Akshay. R. Jagtap, Samir. D. Gaikwad, Abhay. A. Kale, Mahesh. R. Kamble. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.