Influence at Mass of the Base Isolation System in Affecting the Higher Modes of Vibration

Article History: Received: 10 November 2020; Revised: 12 January 2021; Accepted: 27 January 2021; Published online: 05 April 2021 Abstract: Base Isolation is a technology of mitigating the effects due to earthquakes with the aspect of dissipating the seismic waves away from the superstructure, by isolating the superstructure from the ground.This concept is widely essential to be implemented in structures(buildings) irrespective of many factors. There are several materials which could be implemented as base isolator, however the need in reduction of the number of the isolators is essential dueto various factors which a developing country finds difficult to implement. In this paper, a three-storey unsymmetrical building to be considered for the study is isolated by varying the mass of the foundation beam, (Transfer beam) thereby reducing the number of isolators in the building.Furthermore,the mode shapes and frequencies of the structure without base isolation and with base isolation considering mass of the base isolation system as a key factor were analysed and compared and hence the variation in the mass of the isolation system has a promising effect in altering the higher modes of vibration. The analysis is prolonged using another methd using UBC-1997 provisions and compared. In both the methods, the influence of the mass of the isolation system has a remarkable effect in altering the higher modes of vibration.


Introduction
The consideration of higher mode effect is neglected in many of the design codes but the contribution of higer modes is very important exclusively for taller buildings. Moreover, in initial stages, higher modes causes overturning effect but it vanishes at the end [1]. The influence of higher modes have an effect in drift and acceleration of the base slab [2]. The higher frequency modes respond in static state during low frequencies. The contribution of higher mode is significant for the lateral force distribution during seismic action [3].The research was conducted to study the effect of higher mode is in affecting the vertical distribution of base shear in an isolated structure under disatarous earthqaukes. Furthermore, the non-linear behaviour isolation systems which causes variation in frequencies transmitted to the superstructure is also significantly depend on higher modes [4]. The contribution of higher modes is higly important for a base-isolated building rather than a non-isolated building. The effect of higher modes aid in shifting of the fundamental modes of the region of dominant earthquake energy and hence the damping of the structure is enhanced [5]. The dominant variation in fundamental mode is attained with a three-storey symmetrical building with STRP as the isolator and considering stiffness as the major parameter for the analysis and the authors concluded that only fundamental mode experiences anticipated change for the material considered [6].The objective of this study is to investigate the performance of a 3-storey unsymmetrical residential building located in a severe seismic prone area with the application of base isolation system. The soil at the site is of medium type. The response of this building with fixed base using seismic coefficient method has been reported elsewhere [7]. In the present investigation, the variation of higher modes are validated by varying the mass of the isolation system and compared with design principles using UBC-1997 provisions.

Proposed Methodology
The building considered for the analysis is a three-storey unsymmetrical building located in Seismic zone IV. The structure is a moment-resisting frame. The properties of the building element are presented in Table.1. The building is a residential building with three bays in x-direction and two bays in Y direction. The height of each storey is 3 m each. The investigation on the seismic isolation using Scrap tire rubber pads was analyzed and the authors eventually concluded that there is an anticipated change in first mode only [6] with the implementation of STRP bearing as the base isolator. This aspect has been significantly considered for the analysis. The investigation is enhanced by the reduction in the number of base isolators despite of the type of the isolators with the variations in mass of the isolation system. The study consists of varying the mass of the isolation system in six different cases. For instance, the building is analysed for the fixed base. The second case consists of analysis of the base isolated building with six different variations in the mass of the isolation system. The analysis is also enriched by the application of UBC-97 provisions [8] and ETABS Software[9], where the mode values are reported therein.

Analysis for the fixed Base building
From the properties in Table. (1) and Equation 1(a) are reported in [6,10] The values of the mass and stiffness is substituted [10] in eq.1a to 1b, the dynamic properties of the building is presented in Table.2  The concept of reducing the number of number of isolators is briefed in NZ Draft [11] in which the size of the transfer beam could be modified thereby reducing the number of isolators significantly. The illustrated picture is shown in Figure.2. This concept is evaluated with the mass of the transfer beam and substantially the participation of higher modes which is essential for the current research is established.

Equation 2(a)
Where TD is the design Time Period and taken as 2.5 seconds [12], mb is the mass of the isolation system. The determination of stiffness of the isolator for different cases are computed and presented in Table.1
The values of the mass and stiffness is substituted in eq.2a to 2e, the dynamic properties of the building is presented for six different cases are reported in

Method 2: As per UBC-97 Code provisions
The system with base isolation shown in Fig. 1(d) is represented by Equation (3). The solution of this equation is given by [10]: The Maximum Vertical Load (for each column) at the base of the structure are determined from ETABS Software (Static Analysis) [15] which is denoted as V1,V2,V3 and V4. The stiffness of each of the column with respect to the column loads are determined using the expression, 15] Equation (3a) where kbis the bearing stiffness, m is the mass of the isolation system and TD is the Target Design Time Period (Here it is 2.5 secs) [12].  Table.4 Composite bearing stiffness kb= Stiffness of column in each joint at the base of the structure = 2(390)+4(368)+4(300)+2(283) = 4018 kN /m. The combined mass matrix and the combined stiffness matrix for the base isolated system respectively are given by Eqs.

3(c)
The respective values of mass and stiffness are applied in the matrices and the application of MATLAB [13] is used and the mode values are determined and presented in Table.10

Higher mode participation by varying the mass of the isolation system
From the investigation, it is evident that there is a reduction in frequency with base isolation despite of the type of the isolator used. Nevertheless, the need to spot the variations in higher modes other than first mode of vibration is essential [1,2,3]. The behavior of modes in each case is presented in Table 11.

Identification of Higher mode participation using UBC-97 Provisions
In the base isolation system, the first mode is called as the Isolation modes in which isolation system actively participate in vibration and in second mode which is called as structure mode; both isolation system and structure participate in vibration [10]. From this method 2 analysis, it is evident that the first and second mode actively participates in the vibration. Nevertheless, lesser participation is attained in other higher modes and similar observations are reported [10].

Conclusion
From the study, following conclusions are drawn • The participation of all the modes is attained by considering mass of the isolation system equal to ¼ th of the mass of the structure which is the promising aspect to be considered. Moreover, the participation of first two modes which is highly attained in isolation system is reported in other cases. Furthermore, in second method, the isolation mode and structure modes actively participate in vibration.
• The need for higher mode participation is essential to reduce the drift and acceleration of the building. Hence, other factors which trigger for the participation of higher modes of vibration are to be identified.