ASPS Conference Proceedings 1: 799-805 (2022) Proceedings of 12th Structural Engineering Convention-An International Event (SEC 2022) Available at https://asps-journals.com/index.php/acp Effect of Vertical and Mass Irregularity on RCC Structure Subjected to Seismic Loading Nilesh Kumar 1, Jay Parmar 1, Maitri Dalal 1, Abhishek Samal 1, Jenish Patel 1, *, Y. D. Patil 2 2 1 Department of Civil Engineering, Sardar Vallabhbhai National Institute of Technology, Gujarat 395007, India Department of Civil Engineering, Associate Professor, Sardar Vallabhbhai National Institute of Technology, Gujarat 395007, India Paper ID - 060341 Abstract The greatest challenge for any structural engineer in today’s scenario is to design seismic-resistant structures. The presence of vertical geometrical irregularity in building is a matter of concern when it is subjected to devastating earthquakes. Irregular configuration either in plan or in elevation is recognize as one of major causes of failure during earthquakes. The performance of a high rise building during strong earthquake motions depends on the distribution of stiffness, strength and mass along both the vertical and horizontal directions. If there is discontinuity in stiffness, strength and mass between adjoining storeys of a building then such a building is known as irregular building which triggers structural collapse of building when subjected to seismic loading. In present study G+14 story building with mass and vertical geometrical irregularity is analysed using static method and dynamic method in ETABS v 18.0.2 as per IS-1893-2016 (part 1). Analysis is performed for zone III. Also, response spectra analysis is done for torsion check in building. For dynamic analysis linear time history data of Bhuj, Mexico, and Kobe (Medium, Low & High Intensity) is used. Comparison of behaviour of irregular building is done with G+14 regular building in form of max storey shear, story displacement, story drift. From the analysis results, it is found that the mass irregularity has maximum storey shear, story displacement, story drift compares to regular and vertical geometrical irregular building. Also, sudden change in story shear is observed at set back level. Keywords: RCC building, Equivalent Static Analysis, Response Spectrum Analysis, Linear Time History Analysis, Irregularity, Story Displacement, Inter Story Drift, Storey Shear. 1. Introduction The earthquakes are one of the most devastating natural hazards that cause great loss of life and livelihood. The earthquake is a violent shaking of the Earth during which large elastic strain energy released spreads out in the form of seismic waves that travel through the body and along the surface of the Earth. Most earthquakes in the world occur along the boundaries of the tectonic plates and are called Inter-plate Earthquakes. A number of earthquakes also occur2within the plate itself but away from the plate boundaries these are called Intra-plate Earthquakes. The losses in earthquake are due to building collapses or damages. Therefore, it is very important for structures to resist moderate and severe ground motions depending on its site location or importance of structure. During an earthquake, failure of structure starts at points of weakness. The weakness arises because of any discontinuity in mass, stiffness or geometry of structure. The structures having any such kind of discontinuity are termed as Irregular structures. One of the major reasons of failures of structures during earthquakes are vertical geometrical irregularities. Hence, changes in mass and stiffness render the dynamic characteristics of these buildings different from the regular building. In present study only vertical geometrical and mass irregularity are considered. As per IS-1893-2016 these two irregularities are classified as follow: 1) Setback or vertical geometrical irregularity: When the horizontal dimension of the lateral force resisting system in any storey is more or less than 125% of the storey below, the vertical geometrical irregularity is considered to exist. Shown in fig. 1. 2) Mass irregularity: When the seismic weight of any floor is more than or less than 150% of that of the floors below, mass irregularity shall be considered to exist. Shown in fig 2. Literature Survey Siva Naveen E. el al [1] studied response of nine story reinforced concrete building with plan, elevation and combination of both irregularities numerically. Total 54 configuration are made and response is compared. Out of various single irregularity analysed, stiffness irregularity has shown maximum seismic response. Among the cases having combinations of irregularities, the configuration with mass, stiffness and vertical geometric irregularity have shown maximum response. *Corresponding author. Tel: +917984535537; E-mail address: jenishypatel.19@gmail.com Proceedings of the 12th Structural Engineering Convention (SEC 2022), NCDMM, MNIT Jaipur, India| 19-22 December, 2022 © 2022 The authors. Published by Alwaha Scientific Publishing Services, ASPS. This is an open access article under the CC BY license. Published online: December 19, 2022 doi:10.38208/acp.v1.586 Kumar et al. / ASPS Conference Proceedings 1: 799-805 (2022) 3.1 Equivalent static method (ESA) All design against seismic loads must consider the dynamic nature of the load. However, for simple regular structures, analysis by equivalent linear static methods is often sufficient. This is permitted in most codes of practice for regular building (height less than 15m & zone II). It begins with an estimation of base shear, load and its distribution on each story calculated by using formulas given in the code. 3.2 Response spectrum method The representation of maximum response of idealized single degree freedom system having certain period and damping, during earthquake ground motions. The maximum response is plotted against undamped natural period and for various damping value and can be expressed in term of maximum absolute acceleration, maximum relative velocity and maximum relative displacement. Figure 1 Vertical Geometrical Irregularity 3.3 Time history method (TH) In this analysis dynamic response of the building will be calculated at each time intervals. This analysis can be carried out by taking recorded ground motion data from past earthquake database. Its solution is a direct function of the earthquake ground motion selected as an input parameter for a specific building. This analysis technique is usually limited to checking the suitability of assumptions made during the design of important structures rather than a method of assigning lateral forces themselves. Figure 2 Mass Irregularity Mahdi and Soltangharaie [2] studied seismic behavior of building with plan configurations of the structure contain reentrant corners of five, seven and ten story momentresistant space frame using the static and dynamic analysis. Found result of these two analyses are quite wide but the linear dynamic analysis has shown slightly better results than nonlinear static analysis. Jack P. Moehle et al [3] studied combined experimental and analytical study is made of the response to strong base motions of reinforced concrete structures having irregular vertical configurations. Measured responses of the structures are compared with responses computed by several conventional analysis methods and found that the inelastic static and dynamic methods were superior to the elastic methods in interpreting effects of the structural discontinuities. The main objective of present work is (i) to study behaviour of irregular RC building having vertical geometrical and mass irregularity. (ii) to analyse G+14 storey building as per IS-1893-2016 (part-1) in CSI ETABS v 18.0.2 software. Static and dynamic analysis is carried out for zone III. (iii) to compare response like storey shear, storey displacement, storey drift between regular and vertical geometrical irregular building. 3. Problem formulation Table 1. Building Model Nomenclature Type of Irregularity Model Name Regular REG Vertical Irregularity - I VI1 Vertical Irregularity - II VI2 Mass MI Table 2. Geometrical and Material Data Number of Storeys 15 (G+14) Ground story height 3m Typical story height 3m Bay width 4m No of bay 7 Column size 700 mm X 900 mm up to 5th storey 700 mm X 800 mm up to 10th storey 700 mm X 700 mm up to 15th storey Beam size 550 mm X 450mm Slab thickness 150 mm Outer wall thickness 230 mm Inner wall thickness 150 mm Parapet wall height 1m Concrete grade M 30 Steel grade HYSD 500 Brick masonry 20 kN/m3 density 2. Seismic analysis method Seismic analysis is an important tool in earthquake engineering which is used to understand the responses of building due to seismic excitations in a simpler manner. In the past, the buildings were designed just for gravity loads and seismic analysis is a recent development. It is a part of structural analysis and structural design where earthquake is prevalent. Different types of earthquake analysis methods are discussed below. 800 Kumar et al. / ASPS Conference Proceedings 1: 799-805 (2022) Table 3. Loading Data Deal load (DL) 2 kN/m2 & 3 kN/m2 on roof DL of external wall 11.27 kN/m DL of inner wall 7.35 kN/m DL of parapet 4.6 kN/m Live load 4 kN/m2 Earthquake Zone III Damping Ratio 5% Importance Factor 1.2 Type of Soil Medium Type of Structure OMRF Response Reduction Factor 3 Natural Time Period 0.075h0.75 = 1.303 sec (a) Table 4. Specific Data for Mass Irregularity Building Model Modification for Mass Irregularity on 7th Storey Live Load 6 kN/m2 Slab Thickness 200 mm Dimension of Water Tank provided at the top of building Column Size 700 mm x 900 mm Beam Size 450 mm x 1600 mm Table 5. Time History Linear Model Analysis Data Earthquake PGA Time (Sec) Frequency (Hz) Mexico 0.1g 180 0.0055 Bhuj 0.25g 133 0.0075 Kobe 0.8g 36 0.0277 (b) Results and Discussions 5.1 Storey Shear (c) (a) (d) Figure 3 (a) REG (b) VI1 (c) VI2 (d) MI (b) Figure 4 ESA Results for (a) EQX and (b) EQY Load Case 801 Kumar et al. / ASPS Conference Proceedings 1: 799-805 (2022) Maximum Storey Shear Force of Models VI1, VI2 and MI are 62.9%, 60.66% and 134.4% of Maximum value of Storey Shear Force of REG for Equivalent Static Analysis (ESA). In both types of analysis (ESA and THA), asymmetric building will have more Storey Shear than that of symmetric building. 5.2 Storey Displacement (a) (a) (b) (b) (c) Figure 5 Time History Results (a)Mexico, (b)Bhuj and (c)Kobe (c) (d) Figure 7 Storey Disp. Results for (a)ESA, (b)TH Mexico, (c)TH Bhuj and (d)TH Kobe – EQX Load Case Figure 6 Storey Shear Comparisons with Analysis Methods 802 Kumar et al. / ASPS Conference Proceedings 1: 799-805 (2022) Table 6. Storey Displacement Results for different Time History Analysis Earthquake Mexico Bhuj (a) Kobe Load Case EQX EQY EQX EQY EQX EQY Maximum Storey Displacement with respect to REG Model (%) VI1 VI2 MI 130.15 88.96 113.69 106.22 101.28 108.48 127.39 107.66 141 119.5 107.73 139.62 143.21 119.06 129.2 87.75 101.98 120.1 In case of ESA, Maximum Storey Displacement for Load Case EQX of Models VI1, VI2 and MI are 96.6%, 91.9% and 132.9% of Maximum value of Story Displacement of REG. This signifies that for same seismic weight buildings, building which is having vertical geometrical Irregularity will have more displacement than regular building. Asymmetric building will have rotational characteristics along with translational displacement in direction of asymmetricity when seismic force is acting in the direction of asymmetricity. In Time History analysis, Maximum Storey Displacement depends upon PGA (peak ground acceleration) and time duration of earthquake along with distribution of load and storey stiffness. (b) 5.3 Storey Drift (c) (a) (d) Figure 8 Storey Disp. Results for (a)ESA, (b)TH Mexico, (c)TH Bhuj and (d)TH Kobe – EQY Load Case (b) Figure 9 Storey Displacement Comparisons with Analysis Methods 803 Kumar et al. / ASPS Conference Proceedings 1: 799-805 (2022) (c) (d) Figure 11 Storey Drift vs Storey Graphs for EQY Load Case (d) Figure 10 Storey Drift vs Storey Graphs for EQX Load Case Figure 12 Storey Drift Comparisons with Analysis Methods In case of ESA, Maximum value of Storey Drift in all Building Model is 0.0084 which is less than 0.012. So, Storey Drift of all Building Models are well within the limit specified by IS: 1893 (Part 1) – 2016. While in case of Time History Analysis for Mexico Earthquake, Storey Drift values are beyond 0.012 due to large time duration of earthquake. For Bhuj and Kobe earthquake all buildings are safe for storey drift criteria. From figure 10 and 11, it is observed that asymmetric building will have more Storey Drift than symmetric building. (a) (b) Table 7. Storey Drift Results for different Type of Analysis Maximum Storey Drift with respect to REG Analysis Model (%) Type VI1 VI2 MI ESA 109 86.8 135 TH 126.22 86.65 117 Mexico TH 130 104.5 114.65 Bhuj TH 243.08 147.12 129.85 Kobe 5.4 Torsional Check (c) 804 Kumar et al. / ASPS Conference Proceedings 1: 799-805 (2022) Table 8. Torsional Check Results using Response Spectrum Method Zone - 3 Modal Participating Mass Ratio [Sum-Rz] Mode REG VI1 VI2 MI 1 0 0.208 0 0.0001 2 0 0.208 0 0.0001 3 0.7854 0.6303 0.6376 0.7883 Type M.P.M.R. Check c. Ok Mode 1 2 Sum - Ux 0 0.7775 Sum - Uy 0.7896 0.7896 Sum - Ux 0 0.6827 VI1 Sum - Uy 0.559 0.559 Sum - Ux 0 0.6877 VI2 Sum - Uy 0.7099 0.7099 Sum - Ux 0 0.7809 MI Sum - Uy 0.793 0.793 It is observed that all models do not exhibit Irregularity. REG b. 3 0.7775 0.7896 0.6827 0.7122 0.6877 0.7099 0.7809 0.7931 Torsional d. Disclosures Free Access to this article is sponsored SARL ALPHA CRISTO INDUSTRIAL. 4. Conclusion The seismic response analysis has been evaluated for models of vertical geometrical and mass irregular buildings with the help of ETABS v 18.0.2 software. The seismic performance of regular, vertical geometrical irregular and mass irregular buildings is studied by plotting graph of storey displacement, storey drift, storey shear by equivalent static analysis and 3-time history analysis. The concluding remarks are: a. distribution of load, storey stiffness, PGA and time duration of earthquake. Buildings models with vertical geometrical irregularities (VI1 and VI2) have 25.03% to 31.43% more storey displacement and 21.08% to 52.25% more storey drift than that of regular building model (REG). In THA, as time duration decreases from Mexico earthquake to Kobe earthquake, decrease of 88.57% to 95.47% is observed for storey displacement and storey drift results of models with respect to results of THA of Mexico earthquake. This shows that as time duration decreases, values of storey displacement and storey drift reduces. There are many other factors which are affecting the results of THA like PGA, frequency etc. To find out all these parameters that are affecting results of THA, further study is needed. In both types of analysis (ESA and THA), symmetric and regular building performed better than that of asymmetric building and buildings with irregularities. by References As observed from data of Storey Shear, for same seismic weight, VI1, VI2 and MI model will exhibit 85.6%, 82.6% and 99.25% of Storey Shear of REG model. This shows that in ESA, Storey Shear depends upon distribution of load and storey stiffness. While in case of Time History Analysis, Storey Shear depends upon dynamic response of building which includes combination of 805 1. Siva Naveen E et al. (2018); “Analysis of Irregular Structures under Earthquake Loads”, 2nd International Conference on Structural Integrity and Exhibition, Vol. 14, 2018. 2. T. Mahdi, V. Soltangharaie (2012); “Static and Dynamic Analyses of Asymmetric Reinforced Concrete Frames”, 15th World Conference on Earthquake Engineering, Lisboa, 2012 3. Jack P. Moehle et al. (2004); “Seismic Analysis Methods for Irregular Buildings”, ASCE Journal of Structural Engineering, Vol. 112, No. 1, January 1986, pp. 35-52. 4. IS-875 (Part 1,2) 1987: Code of Practice for Design Loads other than earthquake Load. BIS, New Delhi. 5. IS 1893 (Part 1): 2016, Criteria for Earthquake Resistant Design of Structures, BIS, New Delhi.