Seismic earthquakes are a real danger for the construction evolution of high rise buildings. The rate of earthquakes around the world is noteworthy in a wide range of construction areas. In this study, we present the dynamic behavior of a high-rise RC building with dynamic isolators (lead-rubber-bearing), in comparison with a traditional shear wall system of the same building. Seismic isolation has been introduced in building construction to increase the structural stability and to protect the non-structural components against the damaging effects of an earthquake. In order to clarify the influence of incorporating lead rubber bearing isolators in the seismic response and in reducing seismic damages; a comparative study is performed between a fixed base system (shear wall system) and an isolated base system (Lead Rubber Bearing) on an irregular high rise reinforced concrete (RC) building located in Beirut consisting of 48 storeys almost asymmetric orthogonally. For this purpose, a non-linear analysis of a real earthquake acceleration record (EI Centro seismic signal) is conducted, so that the mode shapes, the damping ratio and the natural frequencies of the two models are obtained using ETABS software. The results prove a substantial elongation of the building period, as well as a reduction in the building displacement, the roof acceleration, the inter-storey drift ratio and the base shear force of isolated building relative to fixed-base building. This study proves that this technology is applicable to high rise buildings with acceptable results.

Buildings and more precisely high buildings are very sensitive to earthquake ground motions. Since the 1960–1970’s, the study of structural behavior subjected to seismic motion has been developed and continues to evolve, after strong earthquakes that struck different urban regions around the world caused buildings destruction and loss of lives. These buildings should be protected against seismic motions in order to make their structures safer. Many researchers use the principle of energy dissipation by adding damping devices and other systems to the structure as protective systems [

Furthermore, the traditional design approach increases the stiffness of the building, i.e., increases the stiffness of the structural components. However, the non-structural components may be subjected to significant damages during a major earthquake. To reduce the inter-storey drifts, the storey accelerations and the storey displacements, the concept of base isolation is increasingly being adopted [

The application of isolators is well known and familiar in the literature, but there is no proper research for a tall irregular building using real floor plans. This study presents the seismic analysis for a high-rise building located in Beirut (Lebanon), which is irregular in shape using software package ETABS. It is the first implementation of this isolation technology (Lead Rubber Bearing) in this country. Hence, a non-linear time history analysis will be performed to understand the effect of seismic loading on the structural response throughout the loading period. In order to demonstrate the effectiveness of using LRB as an isolator system instead of a conventional shear wall system, a comparative analysis of the response results (such as the fundamental time period, storeys acceleration, storeys displacement, base shear force and inter-storey drift ratio) is accomplished for two structural models. In the following, Section 2 describes the type of isolator chosen for this work and its advantages. Section 3 describes the developed structural models with their properties. Section 4 presents the design of the isolators and their properties calculation. In Section 5, the results of the modeling for the two structural systems and the effect of isolator system on the dynamic behavior of the building are presented. Finally, Section 6 concludes the main findings of this work and opens a brief discussion of its possible extensions.

The development of isolation techniques has been progressed to provide more flexibility and damping to the structure during the seismic attack. Nowadays, lead rubber bearing (LRB) is one of the most used isolator types among all other categories. The first use of LRB was in New Zealand (1970’s) giving a new concept to the design of base isolated structures [

Dynamic Isolation System [_{e}) and the post-yield stiffness (_{d}) for the material. The force-displacement relationship of a typical LRB is non-linear [

In the present study, a 168.2 m height (44-storey, 3 basements, and a roof) high rise RC building has been modeled using software package ETABS. In order to proceed with the study two structural models has been designed. The first model is based on a shear wall system (1 m thickness) with fixed support at the base, as shown in

^{2}. The ground motion processes during 39.1 s with 0.436 g as the peak ground acceleration for the two structural models. The soil acceleration

Floors | Area (m^{2}) |
Eccentricity _{x} (m) |
Eccentricity _{y} (m) |
_{x} % to floor dimension |
_{y} % to floor dimension |
---|---|---|---|---|---|

Basements | 1710 | 0.8667 | 1.0003 | 2.3% | 2.0% |

Ground floor | 1717 | 0.8667 | 1.0003 | 2.3% | 2.0% |

F2-F12 | 1630 | 1.2772 | 1.3614 | 3.4% | 2.7% |

F14-F25 | 1160 | 1.7489 | 1.8535 | 4.6% | 3.7% |

F26-F32 | 920 | 1.4035 | –0.8471 | 3.7% | 2.1% |

F33-F44 | 680 | 1.7565 | 2.2618 | 4.6% | 7.5% |

A static design of the high-rise building has been performed on ETABS, in order to calculate the vertical load acting on each isolator. This was the first step of LRB design. After that, the LRB isolators was divided into 4 types regarding the range of load acting on them, as shown in

LRB isolators can be designed to carry different values of displacement by changing their diameter and their design parameters. The main parameters calculated for the four isolator types are: the effective stiffness _{eff} (the isolator force divided by the displacement), the effective damping (25%), the isolator diameter (_{i}), the lead diameter (_{L}), the isolator Height (H), and finally the number of layers (n). The elastic stiffness _{e} and the yielding stiffness _{d} (_{eff}. The design limit of rubber shear strain is 250%, and the rubber shear modulus is _{max} is the control parameter of each LRB isolator. After the time history analysis, the displacement calculated in each isolator should be less than the maximum displacement property provided in the manufacturer data, in order to have a safe design and avoid isolator failure. The design properties of the 4 isolator types are shown in

Type | _{eff} (KN/m) |
_{i} (mm) |
_{L} (mm) |
_{max} (mm) |
_{max} (KN) |
||
---|---|---|---|---|---|---|---|

LRB1 | 1120.92 | 1160 | 330 | 649 | 45 | 760 | 13800 |

LRB2 | 1665.14 | 1260 | 355 | 742 | 39 | 810 | 20500 |

LRB3 | 2241.84 | 1360 | 380 | 635 | 33 | 860 | 27600 |

LRB4 | 2712.95 | 1450 | 405 | 582 | 30 | 910 | 33400 |

The results of the non-linear time history analysis have been performed for the two structural models, for fixed base model (shear wall system) and for isolated model, in terms of the time history functions of storey responses (accelerations, displacements, base shear forces) and also storey responses in function of each level. These functions give better insight into the response behavior of the structure at each time step of the analysis. The following sub-sections detail these results.

The structural period is one of the most important factors in building seismic analysis. Structural period for both models was compared and presented in

System | Structural period (s) |
---|---|

Fixed Base | 7.8 |

Isolated Base | 16.4 |

The increase of the fundamental time period length of a building structure causes a reduction of the induced storey acceleration, in other words a reduction of the earthquake induced inertia forces of the building. ^{2}, whereas the maximum acceleration of the isolated base system is 3.71 m/s^{2}, comparatively less than the first one. Then, the acceleration reduction at the roof storey is 55.24%. As seen in

In other words, one of the main objectives of the seismic isolation system is to shift the fundamental frequency of a structure away from the dominant frequencies of an earthquake ground motion.

The displacements of the diaphragm centre of mass at each storey were reduced due to the use of the rubber bearings, which in turn reduced the impact of earthquakes on the structure. As seen in _{building}/200 (16820 cm/200 = 84.1 cm) according to UBC-97. On the other hand, the maximum displacement value of 85.56 cm obtained for the fixed base building is more than the UBC maximum allowable displacement. This means that this building even with a high thickness of a shear wall system (1 m thickness) is not safe against an earthquake attack. However, the reduction in roof storey displacement is 46.11%. This gives evidence that the base isolation buildings are more flexible than fixed base buildings. The displacement reduction at roof storey joint is less than the reduction obtained in the case of low rise or medium rise buildings [

The spectral accelerations at roof storey for both models are shown in ^{2} in case of the fixed base model and 1.68 m/s^{2} in case of the isolated base model. The value of spectral acceleration is greater in the fixed base model than in the isolated base model for the same building and seismic load case. It can be observed that the effect of high seismic excitations on low period values has been reduced due to the presence of the isolation system. On the other hand, the energy dissipation by the isolators and the forces developed within the isolators themselves are the two factors affecting the spectral displacement [

A The base shear force is also reduced due to the inducing of Rubber isolators during the vibration of an earthquake.

The maximum drift evaluation in the fixed base case is 0.058 > 0.02. This means that this building is not safe in terms of drift damages during an earthquake. For the isolated model the maximum drift is 0.00317, that should be less than 0.02/

In this study, a non-linear structural response analysis of high rise building in an active seismic region was performed by the incorporation of the innovative LRB isolators. The seismic analysis compared two structural models of the same high rise building. After providing LRB isolator system, the fundamental period of the isolated structure is increased by approximately 3 times compared with the fundamental period of the fixed base structure. We can also conclude by the comparison of the two models that:

The reduction in the energy dissipation, i.e., acceleration, at the roof storey is 55.24%.

There is increase in the storey acceleration at the first floor.

The lateral roof storey displacement is reduced by 46.11%, which gives evidences of the flexibility induced in the building.

The seismic excitations represented by the spectral acceleration history at roof level disappear in case of the isolated base model.

The spectral displacement at roof level decreases 84.6% with the use of the isolation system.

The base shear force is reduced from 8368 Ton in the fixed base, to 1169 Ton in the isolated base structure.

There is a decrease in the inter-storey drift ratio.

The lead rubber bearing can be used as an isolator system for high-rise buildings with efficient results.

Under seismic loading the response performance of a conventional fixed base structure is greatly improved by the incorporation of the LRB isolation system. The efficiency of this system depends on the characteristics of the seismic earthquake, its designed properties, and on the structural criteria. Therefore, it is important to perform a detailed analysis to make an efficient design of the rubber bearings according to the vibration data. The main goal of this isolation system is the gained flexibility in the structure and the reduced damages in the vibration movements of the structure. This study also proved also the feasibility of this system in high rise buildings. It should be mentioned that in this study, only one earthquake record has been used to find the optimum seismic response of the building. However, more generalized solutions can be achieved using additional records.