SEISMIC BEHAVIOUR OF RCC (G+10) SHEAR WALL BUILDING HAVING VERTICAL STIFFNESS IRREGULARITY AT DIFFERENT FLOORS

Abstract

This study explores the seismic behavior of a G+10 multi-storey shear wall building with vertical stiffness irregularities at different floors, using the response spectrum method. Various models are developed to assess key parameters, including overturning moments, storey shears, storey drifts, and maximum storey displacements across all floors. A parametric study is conducted to examine the impact of these parameters on the structural performance of the building. The study also compares the seismic response of a Reinforced Concrete (RC) frame building and a shear wall building, utilizing ETABS software for analysis. The objective is to identify the influence of vertical stiffness variations on the structural behavior and derive significant insights from the comparison, which will aid in improving the understanding of seismic performance in buildings with vertical stiffness irregularities. The findings will contribute to better design practices and enhanced earthquake resilience for high-rise structures.

Specification

Description:FIELD OF INVENTION
The field of invention pertains to the seismic behavior analysis of Reinforced Concrete (RCC) buildings, specifically focusing on a G+10 (Ground plus ten floors) shear wall building with vertical stiffness irregularity at different floors. The invention aims to study and improve the seismic performance of such buildings, which exhibit variations in stiffness along the building's height. Vertical stiffness irregularity refers to the uneven distribution of structural stiffness across floors, potentially leading to differential movements during seismic events. The invention seeks to assess how these irregularities affect the building's overall stability, response to lateral forces, and susceptibility to seismic-induced damage, providing insights into optimizing design and construction practices for enhanced earthquake resistance in multi-story RCC buildings.
BACKGROUND OF INVENTION
The background of the invention focuses on the seismic behavior of Reinforced Concrete (RCC) buildings, particularly those with vertical stiffness irregularities in their structural design. In modern construction, RCC shear wall buildings are widely used for their robustness in resisting lateral forces such as those generated by earthquakes. However, in buildings with multiple floors, variations in vertical stiffness-where different floors exhibit differing levels of rigidity-can create structural vulnerabilities during seismic events. These irregularities may arise due to factors such as varying material properties, structural configurations, or differences in floor usage and load distribution.
The vertical stiffness irregularity can lead to uneven distribution of forces throughout the structure during an earthquake, causing increased susceptibility to torsional motion, excessive drift, and potential failure at specific floors. Understanding how these irregularities affect the building's seismic performance is crucial to ensuring the safety and stability of high-rise buildings.
This invention aims to address these challenges by analyzing the seismic response of a G+10 RCC shear wall building with vertical stiffness irregularities. It seeks to offer new insights into the behavior of such buildings during seismic events, ultimately contributing to improved design guidelines and construction practices that enhance earthquake resilience and safety in high-rise buildings.
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SUMMARY
The invention focuses on the seismic behavior of a Reinforced Concrete (RCC) shear wall building with a G+10 (Ground plus ten floors) configuration, characterized by vertical stiffness irregularities across different floors. In such buildings, variations in stiffness-due to factors like differing materials, structural layouts, and load distribution on various floors-can significantly affect the building's performance during seismic events. This invention aims to analyze how these vertical stiffness irregularities influence the seismic response of the structure, especially in terms of lateral displacement, torsional effects, and overall stability during earthquakes.
The study involves detailed simulation and analysis of a RCC shear wall building with varying stiffness characteristics at different floors. The research investigates how these stiffness variations contribute to uneven distribution of seismic forces, increased floor drift, and potential damage to critical structural elements. By evaluating the impact of these irregularities, the invention seeks to identify the key factors that influence the building's vulnerability to seismic forces, providing valuable insights into its overall stability.
The invention contributes to the development of more accurate seismic design methodologies, specifically for high-rise buildings with vertical stiffness irregularities. It offers recommendations for improving building design, such as adjusting shear wall placements, optimizing floor stiffness distribution, and incorporating advanced structural reinforcement techniques. Ultimately, the invention aims to enhance the earthquake resistance of RCC shear wall buildings, ensuring greater safety and resilience in high-rise construction, particularly in earthquake-prone regions. The findings can guide revisions to building codes and inform best practices in structural engineering.
LITERATURE REVIEW
Monish and Karuna (2015) studied the seismic performance of high-rise irregular RC framed buildings, using equivalent static analysis and response spectrum analysis. Their findings showed that response spectrum analysis provided more accurate results than the equivalent static method, and structures with regular configurations performed better than those with irregular plan configurations.
Mukundan and Manivel (2015) examined the effect of vertical stiffness irregularity on multi-storey shear wall frame structures. Their analysis, which included equivalent static, response spectrum, and time history analysis on a 10-storey building, revealed that the introduction of shear walls reduced maximum storey displacements and storey drifts by over 50%.
Sayyed, Kushwah, and Rawat (2017) studied the response spectrum analysis of vertical irregular RC buildings with stiffness and setback irregularity. The results demonstrated increased storey displacements and storey drifts for irregular storeys.
Kusuma (2017) focused on seismic analysis of high-rise RC framed structures with various irregularities, such as mass irregularity, vertical geometric irregularity, and re-entrant corner irregularity. The study found that buildings with re-entrant corner irregularity experienced the highest displacements compared to other irregularities.
Syriac (2021) conducted a comparative study on the behavior of multi-storey buildings with regular and irregular plan configurations. The investigation, performed on a G+11 building with different shapes and setback irregularity, showed that storey shear forces increased significantly with the addition of setbacks.
Divya and Murali (2022) carried out a comparative analysis of horizontal and vertical irregular buildings, both with and without shear walls. The study found that the addition of shear walls increased the stiffness of the structure, reducing maximum storey displacements and storey drifts.
Harsha, Sharath, and Kavya (2023) analyzed vertical irregular structures with and without lateral force-resisting systems. They considered various irregularities, including vertical stiffness, mass, and geometric irregularity, and concluded that storey shear and maximum storey displacements were significantly reduced for braced frame structures.
Pandey and Kodwani (2023) presented an analysis of unsymmetrical building frames under seismic loads using ETABS, considering different types of irregularities in T-shaped and L-shaped buildings. Their results indicated that L-shaped structures experienced greater displacements compared to T-shaped structures.
DETAILED DESCRIPTION OF INVENTION
Seismic forces act on a structure at the point where mass is concentrated, often referred to as the center of mass. The structure responds to these forces at the center of stiffness. When there is a mismatch between the center of mass and the center of stiffness, irregularities occur. Regular structures with continuous members generally perform better under seismic forces compared to irregular structures with discontinuous members. Therefore, it is crucial to maintain regularity and continuity in a building's design to enhance its performance during ground motions.
According to IS 1893:2016 (Part-I), two primary types of irregularities are recognized: plan/horizontal irregularity and vertical irregularity. The vertical irregularities identified in the code include:
1. Stiffness irregularity due to a soft storey: This occurs when the lateral stiffness of a storey is significantly lower than that of the storey above it. Specifically, if the lateral stiffness of a storey is less than 70% of the average stiffness of the storey above, or less than 80% of the average stiffness of the three storeys above. For an extremely soft storey, the stiffness must be less than 60% of the storey above, or less than 70% of the average stiffness of the three storeys above.
2. Mass irregularity: This happens when the seismic weight of a floor exceeds 150% of the weight of the floors below it, creating a disproportionate mass distribution.
3. Vertical geometric irregularity: This occurs when the horizontal dimension of the lateral force-resisting system in a storey is more than 125% of the horizontal dimension of the storey below it, leading to irregular distribution of forces.
4. Strength irregularity: This arises when a storey has less lateral strength than the storey above it, weakening the overall structural integrity.
5. In-plane discontinuity in vertical elements: When vertical elements that resist lateral forces have offsets greater than 20% of the original width of the element, it results in a discontinuity in force transfer, compromising structural integrity.
6. Floating/stub columns: These columns are not fully connected to the rest of the structure, leading to concentrated damage in the structure during seismic events.
In summary, vertical irregularities in a building-such as changes in stiffness, mass, geometry, strength, and continuity of elements-can significantly impact the seismic performance. Ensuring proper regularity and continuity is essential for the building's ability to resist lateral forces during earthquakes.
The objective of this research is to examine the impact of introducing shear walls in structures with stiffness irregularity. According to IS 1893:2016 (Part-1), it is recommended to maintain regular structural configurations wherever possible. However, due to various architectural, aesthetic, and functional considerations, many modern residential and commercial buildings are irregular. As a result, some degree of irregularity is often present in these structures. This study aims to compare two structural systems-moment frame and shear wall systems-focusing on a parametric analysis of key seismic parameters, including overturning moments, storey shear forces, maximum storey displacements, and storey drifts.
Problem under Investigation
The study investigates the seismic performance of a 12m x 12m building with a column spacing of 4m in both the X and Y directions. Stiffness irregularity is introduced at a specific floor level in each model, with one regular model (no stiffness irregularity) also considered for comparison. The models analyzed are as follows:
• Regular Model (R1): No stiffness irregularity
• Model with vertical stiffness irregularity at the ground storey (S1)
• Model with vertical stiffness irregularity at the fourth storey (S2)
• Model with vertical stiffness irregularity at the seventh storey (S3)
Each of these models is analyzed using two structural systems: one with a moment frame structure and the other with a shear wall structure. The following table outlines the geometrical and material data for the analysis:
Table 1: Basic Data for Moment Frame and Shear Wall Frame Systems


In the ETABS modeling, four building models with two different structural systems (moment frame and shear wall frame) are analyzed. Stiffness irregularity is introduced by increasing the storey height from 3m to 4.5m, leading to a stiffness value of 0.3 for the irregular storey. This change indicates the presence of stiffness irregularity as per IS 1893 (Part-1): 2016.









Results and Discussion
Overturning Moments
Figures 3(B) and 3(C) illustrate the stiffness irregularity at the fourth and seventh storeys, respectively. Figures 4(A) and 4(B) depict the variation in overturning moments along the storey height for the moment frame system and the shear wall frame system across models R1, S1, S2, and S3.



Table 2 to Table 5 present the overturning moments at different storey levels for models R1, S1, S2, and S3. The overturning moments in the shear wall frame system are generally lower than those in the moment frame system. Specifically, for models R1, S1, and S2, the maximum overturning moments at the base level in the shear wall frame system are approximately 17% lower compared to the moment frame system. However, for model S3, the reduction is around 15%. The overturning moments at the base level for models S1 and S2 are higher in comparison to R1, while in S3, they are lower for both structural systems.
The variation in overturning moments at the base level for model S3 shows an increase of approximately 2.3% in the moment frame system and 0.3% in the shear wall frame system when compared to R1. In contrast, model S1 exhibits a higher increase-around 4.5% for the moment frame system and 5% for the shear wall frame system, when compared to R1. For model S2, the variation is about 2% for both structural systems.
From Figures 4(A) and 4(B), a significant increase in overturning moments is observed in irregular storeys for both structural systems. The lowest overturning moments are observed in model S3, while the highest occur in model S1. This indicates that an increase in storey height leads to greater vertical stiffness irregularity, which, in turn, increases the overturning moments.
Storey Shears

Figures 5(A) and 5(B) show the variation of storey shears along the storey height for the moment frame system and shear wall frame system for models R1, S1, S2, and S3.


The storey shears for the shear wall frame system are generally lower than those for the moment frame system. For models R1, S1, and S2, the maximum storey shears at the base (B) level are approximately 20% less in the shear wall frame system compared to the moment frame system, with the variation for model S3 being around 18%. Additionally, the storey shears for S2 and S3 are found to be lower than R1, while for S1, the shear wall frame system shows similar values to R1 for both systems.
For model S2, the variation in storey shear forces is approximately -2.6% for the moment frame system and -3.2% for the shear wall frame system compared to R1. In model S3, the variations are about -4.5% for the moment frame system and -3.2% for the shear wall frame system compared to R1. The lowest storey shear force is observed in model S3, while the highest is observed in S1, indicating that an increase in storey height and vertical stiffness irregularity does not necessarily lead to an increase in storey shears.

Storey Drifts

Figures 6(A) and 6(B) illustrate the variation of storey drifts along the storey height for the moment frame system and shear wall frame system for models R1, S1, S2, and S3.


The storey drifts for the shear wall frame system are generally lower than those for the moment frame system. However, in the case of storey drifts, the shear wall frame system provides more stiffness, reducing the maximum displacement compared to the moment frame system, making it a more effective solution for limiting drift under seismic loading.
The following tables (Table 10 to Table 13) present the variation in storey drifts along the storey height for both the moment frame system and shear wall frame system across models R1, S1, S2, and S3. In both structural systems, it was observed that storey drifts increased significantly at the irregular storeys. Specifically, for model R1, the maximum storey drift occurs at storeys 2, 3, and 4 in the moment frame system, whereas for the shear wall frame system, the maximum storey drift is observed at storeys 3, 4, and 5. The maximum storey drift for the shear wall frame system is found to be 7% higher than that of the moment frame system in model R1.
For model S1, the maximum storey drift is observed at storey 2 for both structural systems. However, the shear wall frame system has a 6.5% higher maximum storey drift than the moment frame system. The comparison between S1 and R1 shows a 7% increase in the maximum storey drift for the moment frame system and a 6.5% increase for the shear wall frame system.
In model S2, the maximum storey drift for both systems is found to be similar. However, when compared to R1, the moment frame system shows a 28.5% increase in maximum storey drift, while the shear wall frame system exhibits a 16.5% increase.
For model S3, the maximum storey drift occurs at storey 7. Interestingly, for this model, the shear wall frame system shows a 50% reduction in maximum storey drift compared to the moment frame system. Additionally, the maximum storey drifts for the shear wall frame system in models S2 and S3 are quite similar. In comparison with R1, the moment frame system shows a 55% increase in maximum storey drift for model S3, while the shear wall frame system shows a 22% decrease. This indicates a steep increase in storey drifts for the moment frame system, whereas the shear wall frame system exhibits a relatively linear increase.

Maximum Storey Displacement

Figures 7(A) and 7(B) show the variation of maximum storey displacements along the storey height for the moment frame and shear wall frame systems across models R1, S1, S2, and S3.


In both structural systems, the maximum storey displacements are greatest at the roof level. It is evident that the moment frame system exhibits greater displacements than the shear wall frame system across all models (R1, S1, S2, and S3). The displacement increases significantly at irregular storeys for the moment frame systems.
For model R1, the storey displacements are the least, while for model S2, they are the highest for both structural systems. For model S1, the maximum storey displacement increases by 14% for the moment frame system and 8% for the shear wall frame system compared to R1. The maximum displacement for the moment frame system is 43% greater than that of the shear wall frame system in model S1. For S1, a sharp increase in displacements occurs at storey 1 for the moment frame system, reaching 2.5 times the displacement of R1 at the same storey.
In model S2, the variation in maximum storey displacement is 15% for the moment frame system and 12% for the shear wall frame system compared to R1. The maximum displacement for the moment frame system in S2 is 41% greater than the shear wall frame system. A sudden increase in displacement of 75% is observed at storey 4 compared to the storey below, in contrast to a 35% increase in R1 for the moment frame system.
In model S3, the variation in maximum storey displacement is found to be 7% for both structural systems compared to R1. The moment frame system shows a 40% higher displacement than the shear wall frame system in model S3. For S3, there is a sudden increase in displacement of 35% at storey 4 for the moment frame system, compared to a 12% increase in R1. This sharp increase in displacement is observed only in the moment frame systems, while the shear wall frame system shows a more linear increase in displacement.
From the results, it can be concluded that the moment frame structural system experiences higher overturning moments and storey shears due to its greater stiffness. In contrast, the shear wall frame system, with its lower stiffness, exhibits increased ductility. Ductile structures, when designed appropriately, generally perform better under lateral loads.
In terms of storey drifts and displacements, S3 in the moment frame system undergoes the highest storey drifts, while S2 experiences the greatest storey displacements. For the shear wall frame system, S2 experiences the highest storey drifts and displacements. Despite the higher storey drifts observed in S3, they remain within the permissible limits defined by IS 1893 (Part-I): 2016, Clause 7.111.1. Additionally, S3 in the moment frame system is subjected to the lowest overturning moments and storey shear forces.
Thus, S2 is identified as the most critical model in both structural systems, while S3 is the least critical. The inclusion of shear walls in the building frame results in smoother variations in storey drifts and displacements at irregular storeys, as opposed to the steep variations observed in the moment frame system.
From an economic perspective, the shear wall frame system is more cost-effective. The concrete volume required for the moment frame system is 0.20m³ per meter, while the shear wall frame system requires only 0.16m³ per meter-approximately 20% less. Additionally, in residential buildings, architectural needs often influence structural decisions. Moment frame systems typically necessitate larger column dimensions due to stricter design codes, such as the beam-column capacity ratio outlined in IS 13920:2016 (Cl. 7.2). This often results in offsets between columns and walls. On the other hand, the shear wall frame system offers more flexibility in adjusting wall thickness, enabling it to meet both structural and architectural requirements more efficiently.
DETAILED DESCRIPTION OF DIAGRAM
Figures 1(A) and 1(B): Regular Model (R1) with moment frame and shear wall systems, respectively.
Figures 2(A), 2(B), and 2(C): Models with stiffness irregularity at the ground (S1), fourth (S2), and seventh (S3) storeys for the moment frame system.
Figures 3(A), 3(B), and 3(C): Models with stiffness irregularity at the ground (S1), fourth (S2), and seventh (S3) storeys for the shear wall frame system.
Figure 4: (A) - Moment Frame Model (B) - Shear Wall Frame Model
Figure: 5(A) and 5(B) show the variation of storey shears
Figure: 6(A) and 6(B) illustrate the variation of storey drifts
Figures 7(A) and 7(B) show the variation of maximum storey displacements , Claims:1. Seismic Behaviour of RCC (G+10) Shear Wall Building having Vertical Stiffness Irregularity at Different Floors claims that the study investigates the seismic behavior of a G+10 RCC shear wall building with vertical stiffness irregularity.
2. Vertical stiffness irregularity is considered by varying the stiffness of shear walls at different floors.
3. The analysis focuses on the impact of vertical stiffness irregularity on the building's response during seismic events.
4. The study uses response spectrum analysis to evaluate the seismic performance of the structure.
5. The irregularity is introduced by altering the dimensions or material properties of shear walls at selected floors.
6. The results show increased lateral displacement and inter-story drifts at floors with reduced stiffness.
7. The base shear force distribution changes with the variation in vertical stiffness, affecting the overall structural stability.
8. Floor stiffness irregularity leads to uneven distribution of seismic forces, particularly in higher floors.
9. The study concludes that vertical stiffness irregularity can significantly reduce the seismic resistance of tall buildings.
10. Mitigation strategies, including uniform distribution of stiffness, are recommended to enhance seismic performance.

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