Seismic Analysis of High-Rise Building Having Lateral Load Resisting Elements with and Without Base Isolation

Abstract

In present work linear dynamic analysis of high-rise building is done and of base isolation is analyzed with lateral force resisting elements like shear wall and bracing, effect of all these structural components is calculated and analyzed on structure having 30,35 &40 Stories. Structure is Located in Earthquake Zone IV. Analysis is done with the help of ETABS 19 software. In base isolation we bifurcate the structure from foundation so the structure remains unharmed from shocks and motion at the time of seismic action. Base isolation is a widely used lateral load resisting system provided to strengthen such buildings but literature survey of Base isolation reveals that Lear Rubber Base isolation is proved to be determinantal. With the increase in urbanisation use of multi stories building is now in trending because of its high utility in commercial as well as residential. In northern part of India, maximum places are susceptible to earthquake so the basics requirement is to build the safe structure so that no damage to the life takes places. One among various methods of earthquake resistant design is Base Isolation. Various studies were carried out to check the behaviour of different parameters like time period, drift, storey displacement and overturning moment etc. with various types of base isolation. Use of LRB isolation system for high rise buildings was suggested. In the present study lead rubber bearing is used for base isolation with structure having shear wall and bracings. G+30, G+35 and G+40 storey structure are analysed. Static, linear dynamic (response spectrum) is performed to study the behaviour of building with lead rubber base isolation system. ETABSv2019 is used to perform the analysis and design. Comparison of fixed base building with LRB is shown and suitability concern is provided. It was observed from the study that Lead Rubber base isolation is more effective for earthquake prone zones.

Introduction

I. INTRODUCTION

Earthquake is a natural earth movement that causes calamity and damages to the structures. Seismic activity occurs in the earth's crust, forming waves. These waves transmit to structures through foundation. Thus due to this earthquake movements, inertia force is invoked in structure resulting in damaging the whole or part of structure.

On the other hand, earthquakes provide architects and engineers with a number of important design criteria which are unknown to the normal design process. Engineers can employ ductility to generate more displacement on a structure than the normally permissible elastic limit. The elastic limit refers to the maximum deformation of a structure before it reverts to its original shape. Cracks will develop in the structure if the building deforms more than its elastic limit. If the structure is in or near a seismic zone, the risk of an earthquake damage is quite high and unpredictable... So to save lives and to minimize the damage structural engineers are required who can help in doing so. Base isolation is the recent development for seismic resistant designs, this may not totally control the ground movement but helps in minimizing the impact of ground movement. By extending the time of vibration of the structure, base isolation helps to reduce earthquake forces. Also the structural response accelerations are less than the ground acceleration because of Base isolation. It helps in limiting the effects and after effects of earthquake and that’s why it is widely accepted in the whole world as one of the most effective approaches in past few years.

II. OBJECTIVE

In this thesis 3 cases (G+30, 35 & 40) are considered in each case 4 combination are formed i.e. shear wall+ Fixed base, shear wall+ bracing+ fixed base, and in above two case fixed base is replaced by LRB base. Dynamic analysis (Response Spectrum method) is done using CSI ETBASv19 software.

The objectives of this thesis are: -

1. To determine the effect of base isolation on time period, base shear, overturning moment, storey displacement, storey drift.
2. To determine the behaviour of Fixed base vs LRB base in Response Spectrum analysis.
3. To determine the seismic performance of LRB base as compared to Fixed structure
4. To compare the result for LRB and Fixed base in G+30, G+35 & G+40 storey.

III. MODELLING AND ANALYSIS

To study the seismic behaviour and performance of multi-storied building, three configurations are used, i.e. 30 storied, 35-storied and 40-storeid building. Each configuration is further divided into 4 cases on basis of variation of Base and bracing:

1. Frame with shear wall and Fixed Base
2. Frame with Shear wall, bracing and fixed base
3. Frame with shear wall and lrb base
4. Frame with shear wall, bracing with LRB Base

To study the behaviour, parameters selected are storey drift, storey displacement and storey shear.

IV. RESULTS AND DISCUSSION

A. Time Period

According to IS regulations, the overall height of the building and the base dimension of the building are related by a time period formula. The design of earthquake-resistant constructions is heavily influenced by that. According to the IS standards, the fundamental period of vibration is calculated using the building's overall height or the number of storeys.

Time period for G+30 storey building in case of model having shear wall+ fixed base is 5.12 and in case of modal with shear wall+ bracing + fixed base is 4.34 while time period is observed to decrease in modal with shear wall + isolation to 5.9392 & in modal with shear wall+ bracing+ isolation is 5.7856

Time period for G+30 storey building in case of model having shear wall+ fixed base is 5.12 and in case of modal with shear wall+ bracing + fixed base is 4.34 while time period is observed to decrease in modal with shear wall + isolation to 5.9392 & in modal with shear wall+ bracing+ isolation is 5.7856.

Time period for G+35 storey building in case of model having shear wall+ fixed base is 5.00305 and in case of modal with shear wall+ bracing +fixed base is 4.495306 while time period is observed to decrease in modal with shear wall + isolation to 5.798944 & in modal with shear wall+ bracing+ isolation is 5.304461

Time period for G+40 storey building in case of model having shear wall+ fixed base is 5.876 and in case of modal with shear wall+ bracing +fixed base is 4.968 while time period is observed to decrease in modal with shear wall + isolation to 5.00305 & in modal with shear wall+ bracing+ isolation is 6.7574.

The model time period of LRB base as compared to fixed base is increased, this is due in base isolation, super structure is separated from sub structure so the effect of earthquake is minimized hence time period of the structure is increases and increase in time period is stated above.

B. Storey Displacement

The lateral displacement of the story in relation to the base is called story displacement. Response spectrum analysis of for uniform and optimized section are performed. Storey drift is the difference of displacements between two consecutive storeys divided by the height of that story and Story displacement is the absolute value of displacement of the storey under action of the lateral forces The displacement result of this analysis is shown in graph

Max Storey displacement for G+30 storey building in case of model having shear wall+ fixed base is 153.37 mm and in case of modal with shear wall+ bracing + fixed base is 89.34 mm while Storey displacement is observed to increase in modal with shear wall + isolation to 173.05 mm & in modal with shear wall+ bracing+ isolation is 113.6 mm. Storey displacement for G+35 storey building in case of model having shear wall+ fixed base is 191.305 mm and in case of modal with shear wall+ bracing +fixed base is 110.97 mm while Max storey displacement is observed to increase in modal with shear wall + isolation to 219.235 mm & in modal with shear wall+ bracing+ isolation is 125.51 mm.

Max storey displacement for G+40 storey building in case of model having shear wall+ fixed base is 260.92 mm and in case of modal with shear wall+ bracing +fixed base is 158.954 mm while Max storey displacement is observed to increase in modal with shear wall + isolation to 304.75 mm & in modal with shear wall+ bracing+ isolation is 183.75 mm.

The storey displacement of LRB base as compared to fixed base is increased, and increase in displacement is observed because base isolation make structure more ductile, due to this ductility in the structure tends to displace.

C. Storey Drift

The storey shear and storey drift graphs are useful when analysing the effect of lateral loading on a multi-story building due to seismic or wind loads. The storey drift ratio is the storey drift divided by the storey height. Storey drift is the lateral displacement of a floor relative to the floor below. The storey drift ratio is an useful quantity that can be directly compared to the code requirements because seismic loading rules often impose limits on storey drift as a percentage of the storey height.

Storey drift for G+30 storey building in case of model having shear wall+ fixed base is 0.00217 and in case of modal with shear wall+ bracing + fixed base is 0.00129 while storey drift is observed to increase in modal with shear wall + isolation to 0.00228 & in modal with shear wall+ bracing+ isolation is 0.00126.

Storey drift for G+35 storey building in case of model having shear wall+ fixed base is 0.00234 and in case of modal with shear wall+ bracing +fixed base is 0.00165 while storey drift is observed to increase in modal with shear wall + isolation to 0.00251 & in modal with shear wall+ bracing+ isolation is 0.00147.

Storey drift for G+40 storey building in case of model having shear wall+ fixed base is 0.00271 and in case of modal with shear wall+ bracing +fixed base is 0.00153 while storey drift is observed to increase in modal with shear wall + isolation to 0.00273 & in modal with shear wall+ bracing+ isolation is 0.0016.

D. Base Shear

The maximum expected lateral stress on the base of the structure caused to seismic activity is called base shear. It is calculated using the seismic zone, soil material, and building code lateral force equation, it is observed that as shear at the bottom of the storey is maximum and critical so the base shear of all the model with linear dynamic analysis is carried out and result obtained are plotted in below graph,

The Base shear of lrb base as compared to fixed base is reduces, this reduction in base shear is due to base isolation effect of earthquake forces has been reduced significantly on to the structure also it provides damping effect to the base , and reduction in base shear is as follows:

Base shear for G+30 storey building in case of model having shear wall+ fixed base is 3621 and in case of modal with shear wall+ bracing + fixed base is 2670 while base shear is observed to decrease in modal with shear wall + isolation to 2111.76 & in modal with shear wall+ bracing+ isolation is 1694.38

Base shear for G+35 storey building in case of model having shear wall+ fixed base is 4099 and in case of modal with shear wall+ bracing +fixed base is 3438.25 while base shear is observed to decrease in modal with shear wall + isolation to 2516.786 & in modal with shear wall+ bracing+ isolation is 2253.42.

Base shear for G+40 storey building in case of model having shear wall+ fixed base is 4369.81 and in case of modal with shear wall+ bracing +fixed base is 4041.907 while base shear is observed to decrease in modal with shear wall + isolation to 2875.33 & in modal with shear wall+ bracing+ isolation is 2837.41

E. Overturning Moment

By multiplying the story shear by the distance to the centre of mass above the height in concern, the overturning moments can be calculated. As storey shear of the building increases from top to base floor overturning moment also increases from top to base . Response spectrum analysis of all the structure for all the model are performed. The storey overturning moment result of this analysis is shown in graph

Overturning moment for G+30 storey building in case of model having shear wall+ fixed base is 141250.7 KN-m and in case of modal with shear wall+ bracing + fixed base is 169218.3 KN-m while overturning moment is observed to decrease in modal with shear wall + isolation to 99412.22 KN-m & in modal with shear wall+ bracing+ isolation is 124273.9 KN-m

Overturning moment for G+35 storey building in case of model having shear wall+ fixed base is 225747.2 KN-m and in case of modal with shear wall+ bracing +fixed base is 268639.2112 KN-m while overturning moment is observed to decrease in modal with shear wall + isolation to 164343.988 KN-m & in modal with shear wall+ bracing+ isolation is 205132.9 KN-m

Overturning moment for G+40 storey building in case of model having shear wall+ fixed base is 305247.6837 KN-m and in case of modal with shear wall+ bracing +fixed base is 305247.6837 KN-m while overturning moment is observed to decrease in modal with shear wall + isolation to 223441.3 KN-m & in modal with shear wall+ bracing+ isolation is 217014.3 KN-m.

It is observed that overturning moment is reduced in LRB case as compared to fixed base, as the base shear values are reduced significantly and moment generated by the earthquake forces is observed to be reduced significantly which results in reduction of overturning moment

V. FUTURE SCOPE

The following conclusions are drawn from the results within the scope of this project:

1. The maximum Storey displacement of LRB base for response spectrum analysis of 30,35 and 40 storey building in X-direction, are 10.68-16.8% times more as compared to fixed base, which suggest that building has gain some flexibility which will results in absorbing more earthquake energy.
2. The above points conclude that use of LRB isolation system in low storey structure is more suitable than high rise structure.
3. Average percentage reduction in base shear of LRB building w.r.t. fixed base building is 29.80% to 41.68% in 30,35 and 40 storey building with respective lateral supporting elements. which will result in reducing steel reinforcement of the building
4. Time period of building can be adequately increased by using LRB as compared to fixed base which will result in reducing natural frequency of the building
5. Intermittent storey drift can be reduced and it will help in enhancing human comfort criteria of the building

Conclusion

1) The results of analyzed LRB base and fixed base for G+30, G+35&G+40 storey are presented in this chapter. Comparing the results of FIXED and LRB base models, the results show, the LRB base structure option is better than the fixed base. LRB base structure for high-rise buildings is the best of all options, the displacement values of the floors are within the allowable limits according to the code\'s limits. LRB base gives more ductility to the structure than Fixed base most suitable under the action of lateral force. Also, the performance of LRB base is good compared to Fixed base. Effect of earthquake on the structure is reduces which helps to reduce the costof the foundation, due to less overturning moment. Base isolated structures are the best solution for tall structures in earthquake prone zone. 2) Considering the earthquakes, due to inherent flexibility properties of lead and rubber, LRB will perform better than conventional fixed base structures. 3) After analysis of model and results are discussed in previous chapter. Some concluded points are listed below.

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Copyright © 2022 Abhilash Naik , Dr. Savita Maru. 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.