BUILDING WITH BASE ISOLATION TECHNIQUES

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1 Code: C12 BUILDING WITH BASE ISOLATION TECHNIQUES Mahmoud Sayed-Ahmed 1 Civil Engineering Department, Ryerson University, Toronto, ON, Canada ABSTRACT Base isolation (BI) system for buildings is introduced to decouple the building structure from potentially damaging induced by earthquake motion, preventing the building superstructures from absorbing the earthquake energy. The mechanism of the base isolator increases the natural period of the overall structure, and decreases its acceleration response to earthquake / seismic motion. A steel building with structural rubber bearing is introduced throughout this study. The study analysis performed to check for the adequacy of the base isolation against building lateral drift and inter-story drift as per allowance in National Building Code of Canada Two buildings were analyzed using the nonlinear time history response analysis using the dynamic MODAL analysis for fixed base (FB) building, and Isolated base (IB) building with rubber bearing. The analysis represents a case study for symmetric steel building to show the ultimate capacity of the selected structural bearing, and to make a comparison for the difference between the isolated base and the fixed base buildings. Initial results show that the presence of the structural rubber bearing reduces significantly the vertical displacement, moment and shear generated for the same mode. Keywords: building, base isolation, rubber bearing, earthquake, dynamics, time history response INTRODUCTION Base isolation (BI) is a mechanism that provides earthquake resistance to the new structure. The BI system decouple the building from the horizontal ground motion induced by earthquake, and offer a very stiff vertical components to the base level of the superstructure in connection to substructure (foundation). It shifts the fundamental lateral period, T a, dissipates the energy in damping, and reduces the amount of the lateral forces that transferred to the inter-story drift, and the floor acceleration. The Structural Engineers Association of Northern California (SEONC) published a simple regulation titled Tentative Isolation Design Requirements in 1986, which later was added as provisions in the Uniform Building Code 1997, FEMA 273 with exception of permit to pushover, and International Building Code IBC2000. The structural bearing criteria include vertical and horizontal loads, lateral motion, and lateral rotation that transferred from the superstructure into the bearing and from the bearing to 1 PhD. Candidate, Civil Engineering Department, Ryerson University, Toronto, ON, Canada. m.sayedahmed@alumni.ryerson.ca Al-Azhar University Engineering Journal, JAUES 147

2 substructure. Bearing allows for stress-free support of the structure in terms of (1) rotation in all directions, (2) deformation in all directions, (3) take horizontal forces (wind, earthquake). Reducing the effect of the horizontal forces generated from wind pressure or earthquake load is of great concern to designers. The structural bearing technique is one of those tools to reduce the lateral displacement of the building, to increase the structural safety, and to increase the human comfort during the occurrence of such event. This study tries of clarify the advantage of the base isolation technique with respect to buildings since only few researches were done into this area. Figure 1 shows the schematic diagram for the design process for building against earthquake loading as governed by the National Building Code of Canada 2010 part 4. Clause in NBCC 2010 specifies that buildings and their structural members shall be designed by one of the following methods (i) analysis based on generally established theory, (ii) evaluation of a given full-scale structure or a prototype by loading tester or (iii) studies of model analogues. Throughout this model analogue study the selected building height will be less than 60 m for regular shape building. For earthquake resistant construction using base isolation [Raufaste, 1992] it was found that more attention should be paid to four points: 1. preparation of guidelines for evaluation and approval of base isolation structures; 2. preparation of guidelines related to the performance of base isolation devices; 3. facilities to encourage exchange, collection and dissemination of technical information on the response-control structure; and 4. study of methods of evaluation of performance of response-control structures. A study run by Sener and Utku for the active-passive base-isolation systems used for the seismic response control of structures appears to be effective for small to medium strength earthquakes. Hybrid base isolation systems, which use an active system together with the passive base isolation system, may be used to control the response of structures subjected to larger ground motions created by larger magnitude earthquakes. The hybrid base isolation system using passive base isolation pads together with hydraulic type actuators is proposed. The system, placed between the foundation of the building and its superstructure, is used to minimize the forces imposed on the superstructure by the earthquake induced ground motion [Sener and Utku, 1995, 1996, 1998; Pozo et al., 2005]. In application for the base-isolation system, the Historical buildings have relatively low height, are usually massive and their natural vibration period is rather low. Hence if such buildings are located in a seismically active region, using base isolation systems will be a very effective way for improving their dynamic response. In some cases the displacements at the base isolation level are rather big and exceed the allowed limits. In such cases it is recommended to add dampers to the base isolation system [Iskhakov and Ribakov, 2007]. Analytical seismic responses of structures retrofitted using base isolation devices are investigated by Matsagar and Jangid for the retrofitting of various important structures as historical buildings, bridges, and liquid storage tanks are selected to investigate the effectiveness of the base isolation in seismic retrofitting. It is observed that the seismic response of the retrofitted structures reduces significantly in comparison with the conventional structures depicting effectiveness of the retrofitting done through the base isolation technique [Matsagar, and Jangid 2008]. Chia-Ming and Spencer presented development and experimental verification of an active base isolation system for a seismically excited building and modeling the complex nature of control-structure interaction (CSI) [Chia-Ming and Spencer, 2010]. Jung et al. investigated a smart base-isolation system using magnetorheological (MR) elastomers, which are 148

3 a new class of smart materials whose elastic modulus or stiffness can be adjusted depending on the magnitude of the applied magnetic field. The results further suggest that the feasibility of using MR elastomers as variable stiffness elements for enhancing the performance of conventional base-isolation systems [Jung et al. 2011]. Zhang et al. studied the influence of the action of coupling earthquake to sliding base-isolation structure for 6 story building. The results by exemplification show that the peak values of relative acceleration, relative displacement and inter-storey shear force of sliding base-isolation structure increase in different degree under the action of coupling earthquake [Zhang et al., 2011]. Regarding the slide-limited friction base isolation technology, Zhao and Ma studied the total restoring force model of isolation device. They analyzed the influential factors such as friction coefficient, elastic stiffness and yield displacement of displacement-constraint device on base isolation system [Zhao and Ma 2011]. Spyrakos et al. investigated and developed 2-DOF (degree-of-freedom) for the effect of soilstructure interaction (SSI) on the response of the base isolated multistory building founded on elastic soil layer overlaying rigid bedrock and subjected to harmonic ground motion [Spyrakos et al. 2009]. Li & Wu investigated the limitation of height-to-width ration (HWR) for base-isolated building with elastomeric rubber bearing. It was found that the isolated building with longer period may have a relatively HWR value: and the stiffness of the superstructure affects HWR limit value little [Li and Wu 2006]. The main two key conditions, which determine the HWR limit for an isolated structure, are: (1) the outermost rubber pads of the isolated layer cannot bear tensile force; (2) the compressive force that the outermost rubber pads bear cannot exceed their ultimate antipressure strength. The main objectives of this study work can be stated as follow: (1) To contribute to the efficient design of structural base isolated techniques for buildings, (2) To model and investigate a behavior of building with base isolation. BUILDING DESCRIPTION A two story building made of steel structure [SAP2000 Help, Example O], as shown in Figure 2, with 3 bays of 30 feet in each direction; the story height is 12 feet, as shown in Figure 3.1. The structural steel has the following spec; the modulus of elasticity E = ksi (A992Fy50), Poisson ratio equals to 0.3, the beam section is W24x55, the column section is W14x90. The horizontal slabs are reinforced concrete of 4000 psi and 6 in, 10 in of thickness for the roof and the floor respectively. The vertical loads for roof is 75 psf for the dead load (DL) and 20 psf for live load (LL), while for the floor is 125 psf for DL, and 100 psf for LL. Table 1. High damping bearing Properties Vertical (axial) stiffness 10,000 k/in (linear) Initial shear stiffness in each direction 10 K/in Shear yield force in each direction 5 kips Ratio of post yield shear stiffness to initial shear stiffness 0.2 Diaphragm constraints at each level are assigned to make all diaphragm rigid. This project was subjected to nonlinear time history analysis, where seismic load (Multi-Modal Pushover) is applied by SAP2000 for lacc_nor-1 file data in the X-direction and lacc_nor-2 file data in the Y- direction simultaneously. Each time history is given in units of cm/sec 2, where there are

4 time steps, at equal spacing of 0.02 sec, for total of 60 sec. There are 8 acceleration points per line. This building is analyzed under two cases; case 1 with fixed base, and case 2 with isolated base. The rubber isolator has specification listed in Table 1. ISOLATED BASE BUILDING The base isolation extends the fundamental lateral period resulting in reducing the base shear forces, enhancing the total building drift to the total height and the inter-story drift if compared with the conventional foundations [Chopra, 2001; Tedesco et al., 1998; Eggert and Kauschke, 2002]. Figure 3.a depicts the time response history for column B.1 with its three joints; Joint 13-15, the figure shows that the column from the base to the roof level moves laterally in a same rate, thus no deflection occurs at the joint 14, Figure 3.b depicts the B.1 column movement with respect to the base, and shows that to great extent the column move with base in same direction. It is worth mention that the change in the fundamental period changes the moment, and consequently changes the building deformation shape. It was observed that with the decrease of the natural period, the structure laterally deforms more. The plastic hinge showed up at the fourth and fifth period. FIXED BASE BUILDING The fixed base for the steel columns relies on the steel plate and anchored bolts connection, where the reduce of the fundamental lateral period resulted into increase of the base shear forces, increase of the total building drift to the total height and the inter-story drift if compared with the base-isolated foundations. Figure 4.a depicts the time response history for column B.1 with its three joints; Joint 13-15, the figure shows that the column from the base to the roof level moves laterally in an independent rate, thus deflection occurs at the joint 14, Figure 4.b depicts the B.1 column movement with respect to the base, and shows that Joint 13 move the base while joint 14, and 15 move independently. It is worth mention that the change in the fundamental period changes the moment values, and consequently changes the building deformation shapes. It was observed that with the decrease of the period, the structure laterally deforms more, with higher rate than that of the base isolated building. The plastic hinge location appeared from the first mode and change by the change in MODAL period. It can be located at any point along the span of member as well as the end of the member. COMPARISON OF EVALUATION RESULTS The fundamental lateral period was solved using the finite element analysis (FEA) software, SAP2000 Ver Table 2 shows that the fundamental period (T) and the corresponding frequency (ƒ=1/t) for the Modal participating mass ratio (MPMR) solved for Ritz Vector Analysis for the steel building under investigation in this study which has two scenarios; (a) fixed base, and (b) the isolated base. It was found that the natural period for the isolated base is higher than that of the fixed base by 5.699, 6.337, 6.895, 1.64, times for Modal 1 through 5 respectively. The first three modes were significantly higher, where they absorb more than 95% of the earthquake-induced load [Taranath, 2005]. Figure 5, shows the natural vibration modes for the isolated base and fixed base building against the lateral displacement. 150

5 Moment and shear forces generated from each mode are of great concern to designers, to predict the failure modes, progressive collapse of the building, or to add extra bracing to resist such lateral loading. Table 2. Modal participating mass ratio (MPMR) for fixed and isolated base building Modal Period, T [seconds] Frequency, ƒ [Hz] Mode Fixed Base Isolated Base Fixed Base Isolated Base Where ƒ 1 Hz for rigid building, ƒ < 1 Hz for flexible building Table 3 analyzes the moment (M) and shear (V) values for column B.1 and its 3 joints under five different MODAL periods (modes) for minor (V3, M2) and major (V2, M3). Selection the moment and shear values for the roof, it was found that the moment for the fixed base building is higher than that of the isolated base building by 51.38, 20455, 0.31, 2.34 and 2.23 for mode 1 through 5 respectively for the minor (M2), and 70, 106, 66, 13.7, and for mode 1 through 5 respectively for the major (M3). Hence the base isolation enhances the building capacity to resist the earthquake-induced load, and that reduction in moment could be used towards reducing the selection members sizes, reducing the total building weight and cost, after considering the new mass of the building. Drift is another point of interest to designers and must conform to code requirements. Table 4 shows the deflections in x, y, z directions for the edge column B.1 under the different 5 MODAL (periods) for the fixed base and the isolated base building. The major observation to this table is that the deflection for the base isolated building doesn t start from zero, thus reduces significantly the drift index for the building. For example in studying the drift index (DI) for MODAL mode 1, the drift index for the isolated base = ( )/288 = E- 3 in, while for the fixed base building DI = ( )/288 = 2.589E-3 in, which means that the deflection in base isolated building is less by times than that of the conventional fixed structure. It worth mention that the building efficiency is measured by four factors; the shear rigidity index (SRI), bending rigidity index (BRI), the drift index (DI) and the inter-story drift (ISD) [Taranath, 2005], where the last two criterions can be expressed as following: [1] [2] Where Δ n is the deflection at the floor; H i is the total height of the building, h i is the floor height. The joint reactions in Table 5 are obtained using modal combination applied individually to each joint. The joint reactions are represented as R i,m where is (i) is for the direction, and (m) for mode. The total reaction follows this equation 151

6 [3] For example the joint reaction, for the isolated base building in X-direction equals to SQRT ( ) = 1.33 kips, while for the fixed base building it is equal to SQRT (3.134E E ) = kips. Apparently, the joint reaction in fixed base building for column B.1 in X-direction is higher by times than that of the base isolated building. While the base reactions for response spectrum are computed for each mode and then the modes are combined using complete quadratic combination (CQC) or square root of sum of squares (SRSS) modal combination rule: [4] [5] Where for the base reaction, all joint reactions from all columns must be computed [CSI, 2012]. Alternative simplified analysis simulating the dynamic response of multi-story building can be done by converting the multi-degree of freedom (MDOF) system to Single-degree of freedom (SDOF) system once the equivalent mass and stiffness is obtained [Taranath, 2005]. CONCLUSIONS Based on the theoretical and modeling findings, the following conclusions can be drawn: The main observation from the modeling study on the accuracy of seismic effect and lateral load patterns utilized in the Multi-Modal Pushover analysis (MPA) in predicting earthquake effect showed that the accuracy of the pushover results depends strongly on the earthquake load path, properties of the structure and the characteristics of the ground motion. The lateral deflection for MDOF for multi-story building can be represented as SDOF once the equivalent mass and stiffness is obtained. The plastic hinge location varies by the type of loading, and the change in MODAL period. It can be located at any point along the span of member as well as the end of the member. Drift index and inter-story drift should be predicted using the multi-modal (SRSS) and the elastic first mode with long period for the lateral load pattern which corresponds to the average in most cases. Base-isolated structure exhibit less lateral deflection, as the lateral displacement at the base never equals to zero, and less moment values than the fixed base structure. The base isolation decouples the building from the earthquake-induced load, and maintain longer fundamental lateral period than that of the fixed base. ACKNOWLEDGMENTS 152

7 The author would like to thank Prof. Dr. K.M. Anwar Hossain, P.Eng. for his helpful directions during the course of this research. The author also appreciate the support from Ryerson University, ON, Canada; library for support and making the available database for literature review and civil engineering department for offering the SAP2000 (Ver. 14) to run the modal analysis. REFERENCES Chia-Ming, C. and Spencer Jr., B. F. (2010). "An Experimental Study of Active Base Isolation Control for Seismic Protection," in Sensors and Smart Structures Technologies for Civil, Mechanical, and Aerospace Systems, 8-11 March, USA, p V (12 pp.). Chopra, A.R. (2001). Dynamics of structures. Prentice-Hall, New Jersy, USA. CSI. (accessed March 2012). Base reactions for response spectrum, website: Eggert, H., Kauschke, W. (2002). Structural Bearings, Ernst & Sohn, Germany. FEMA. (1997). NEHRP Guidelines for the seismic rehabilitation of buildings, FEMA 273. Federal Emergency Management Agency, California, USA Jung, H.-J.: Seung-Hyun, E.: Dong-Doo, J.: Jeong-Hoi, K. (2011). "Seismic performance analysis of a smart base-isolation system considering dynamics of MR elastomers." 55 City Road, London, EC1Y 1SP, United Kingdom, pp International Code Council. (2000). International Building Code. ICC Inc., Country Club Hills, IL, USA. International Conference of Building Officials. (1997). Uniform Building Code. ICBO, Whittier, California, USA. Iskhakov, I. and Ribakov, Y. (2007). "Modern trends in base isolation applications for seismic protection of historic buildings." in 10th International Conference on Studies, Repairs and Maintenance of Heritage Architecture, STREMAH 2007, June 4, June 6, 2007, Prague, Czech republic, pp Li, H.-N, and Wu, X.-X. (2006). Limitation of height-to-width ration for base-isolated buildings under earthquake. Structural Design of Tall Special Building, vol. 15, pp Matsagar, V. A. and Jangid, R. S. (2008). "Base isolation for seismic retrofitting of structures." Practice Periodical on Structural Design and Construction, Vol. 13, pp National Research Council of Canada. (2010). National Building Code of Canada. NRCC, Ottawa, Canada. Pozo, F., Ikhouane, F., and Pujol, G. (2006). Adaptive backstepping control of hysteretic basedisolated structures. Journal of Vibration and Control, Vol. 12, No. 4, pp Raufast, N.J. (1992). Earthquake resistant construction using base isolation. U.S. Department of Commerce, Technology Administration, National Institute of Standards and Technology, USA. Sener, M. and Utku, S. (1995). "Active-passive base isolation system for seismic response controlled structures." in Proceedings of the 36th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference and AIAA/ASME Adaptive Structures Forum. Part 1 (of 5), April 10-13, New Orleans, LA, USA, pp Sener, M. and Utku, S. (1996). "Control of torsional modes in buildings under seismic excitation by adaptive base isolation." Smart Structures and Materials 1996: Passive Damping and Isolation, Febrary 26-27, San Diego, CA, USA, pp Sener, M. and Utku, S. (1998). "Adaptive base isolation system for the control of seismic energy flow into buildings." Journal of Intelligent Material Systems and Structures, vol. 9, pp

8 Spyrakos, C.C.: Koutromanos, I.A.: Maniatakis, Ch.A. (2008). Seismic response of base-isolated buildings including soil-structure interaction. Soil dynamics and earthquake engineering, Vol. 29, No. 4, pp Structural Engineers Association of Northern California. (1986). Tentative Isolation Design Requirements, Blue book. SEONC, California, USA. Taranath, B.S. (2005). Wind and Earthquake Resistant Buildings: structural analysis and design. Marcel Dekker, New York, USA. Tedesco, J.W., McDougal, W.G., and Ross C.A. (1998). Structural dynamics: Theory and applications. Prentice Hall, USA. Zhang Y.: Yuanging, W: Yongjiu, S. (2011). "Parameters optimization of sliding base-isolation structure under the action of coupling earthquake." 1st International Conference on Civil Engineering, Architecture and Building Materials, CEABM, June 18-20, Haikou, China, pp Zhao, G.-F. and Ma, Y.-H. (2011). "Parameters study of rural buildings structures supported on slide-limited friction base isolation system." Journal of Vibration and Shock, vol. 30, pp

9 Major (V2, M3) Fixed-Base Minor (V3, M2) Major (V2, M3) Isolated-Base Minor (V3, M2) Table 3. Modal moment and shear values for edge column B.1 H Modal 1 Modal 2 Modal 3 Modal 4 Modal 5 Moment Shear Moment Shear Moment Shear Moment Shear Moment Shear E E E E E-3 2.3E E E E E E H is the building height in [in], M is the moment in [kip-in], V is the shear force in [kip 155

10 Table 4. Joint displacement in column B.1 Modal Joint Fixed Base Isolated Base Mode [Height] U1 U2 U3 U1 U2 U3 15 [288] -9.2E E [144] -5.4E E [0.00] E E-5 15 [288] E E [144] E E-5 13 [0.00] E E-5 15 [288] E E [144] E E-5 13 [0.00] E-5 15 [288] 1.19E E [144] -1.03E E [0.00] E [288] [144] [0.00] Where U1, U2, U3 are displacement in x, y, z directions respectively in [in]; Height in [in] Table 5. Joint reactions for column B.1 at the base (Joint 13) Structure Type Type Joint reaction [kip] Modal Modal Isolated Base Modal Modal Modal Gravity Modal E Modal Fixed Base Modal E Modal E Modal Gravity Directions 1, 2, 3 represent X, Y, Z axis respectively; Gravity load equals to dead and live load Al-Azhar University Engineering Journal, JAUES 156

11 Figure 1. Design procedure for Base Isolation buildings according to NBCC 2005 Joint 15 Joint

12 Height, in Height, in Figure 2. 3D Finite element model a. displacement of column (joint 15, 13) b. displacement of column w.r.t. base Figure 3. Isolated base building response histories a. displacement of column (joint 15, 13) b. displacement of column w.r.t. base Figure 4. Fixed base building response histories Mode Mode 2 Mode Mode 3 Mode 2 Mode 4 Mode Mode 5 Mode Mode Displacment, in Displacement, in a. Isolated base building b. Fixed base building Figure 5. Natural vibration modes 158

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