Pushover analysis of base-isolated steel–concrete composite structures under near-fault excitations

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Abstract

In the present study, the seismic behavior of steel–concrete composite structures isolated by base-isolation devices under near-fault earthquake excitations is numerically investigated. The seismic analysis is performed by means of the static non-linear (pushover) analysis procedure conducted on two five-storey three-dimensional (3-D) buildings with steel columns and steel–concrete composite slabs and beams. The present 3-D building examples are assumed to be located at a near-fault area in order to take into account the effect of strong ground motion on the isolation devices. The results of this study allowed the verification of the adequacy of the attachment isolation system as well as the comparison of the behavior of the seismic-protected building with or without bracings to the unprotected buildings with or without bracings, showing the benefits of the application of the isolation devices, the limitations and the characteristics of their performance.

Introduction

For building systems, steel–concrete composite structures are known as the most economical solution to the diverse engineering design requirements of stiffness and strength. This type of construction has become a common feature in multistory steel frame buildings in several countries. In Greece, steel–concrete construction type is increasingly popular in structural applications, especially for high-raised industrial buildings. The simplest form of those steel–concrete composite structures comprise a bare-steel frame of common H-type section columns supporting I-type section beams which in turn, support the overlaid composite floor slab. On the other hand, the composite floor slab consists of cold-formed profiled steel sheets which act not only as the permanent formwork for an in situ cast concrete slab but also as the appropriate tensile reinforcement [1], [2], [3], [4]. For buildings required to resist earthquake loads, this economical structural solution is especially relevant. Moreover, the inherent ductility possessed by composite members allows a greater level of energy dissipation to be achieved, further increasing their applicability to seismic-resistant structures.

The fundamental period of a common size building is usually within the range of the predominant period of the earthquake ground motions. This leads to high dynamic amplification effect almost in ranges close to the resonance and thus, significant inertial forces can be expected to introduce on the building in the mean of a strong ground motion. This can be avoided by using a base isolation system in the foundation level of the buildings [5]. However, near-fault (NF) earthquake strong ground motions which characterized by large amplitude, long period, and pulse type of excitation tend to produce large isolator displacements. Accommodating these large displacement capabilities in the bearings requires they must be of considerable size. But these costly isolator geometries are in contradiction with the main purpose of seismic isolation to gain a more economical design solution by mitigating the forces transferred to the substructures. Moreover, in some cases, base seismic isolation in NF areas has been found to offer only limited performance benefits compared to regular buildings. Thus, one may conclude that the application of seismic base-isolation systems to buildings seems to be virtually impractical as a stand-alone seismic mitigation procedure in NF sites [6]. The combination of conventional base-isolation systems with advanced supplemental passive viscous energy absorbing devices such as viscous fluid dampers [7] and semi-active or active energy absorption devices such as magnetorheological dampers [8] was found to be an effective and alternate solution to improve the seismic behavior of base-isolated buildings under NF strong ground motions. But, these advanced isolation systems are generally expensive and at present are not commonly used for seismic protection of buildings in Greece, mainly, due to lack of experience of state departments in using such devices, requirements of a continuous source of power at the building site for semi-active and active isolation systems and maintenance associated with such devices that may be exposed to severe environmental effects at the building site. Thus, to investigate the effect of base-isolation system on the seismic performance of steel–concrete composite buildings, a conventional isolation system of the common lead–rubber bearing (LRB)-type is considered in the present study.

Inelastic time-history analysis that defines with sufficient reliability the forces and deformation demands in every element of the structural system is well known as the proper methodology of performance evaluation. However, this methodology needs the availability of knowledge and practices as far as the seismic loading, the structural modeling of all the important elements, the soil–structure interaction and the material properties are concerned. We should recognize that at this time we have not adequately developed those capabilities [9]. Taking into account these limitations, the non-linear static response or pushover analysis, in the recent NEHRP guidelines [10] has been considered as relatively simple but quite efficient non-linear methodology to evaluate the performance state of the structural system [11]. Although pushover analyses of concrete structures and steel structures have been carried out by many researchers and designers, at present, to the author's knowledge, pushover analyses for the seismic analysis of base-isolated steel–concrete composite structures are rarely reported in the technical literature. In the present study, the pushover analysis is used to estimate the expected seismic performance of steel–concrete composite buildings which are base isolated by LRB devices by evaluating their strength and deformation demands in NF earthquake strong ground motions and comparing these demands to available capacities at the desired performance levels. The estimation of the performance level of the LRB-isolated buildings is based on the assessment of important performance parameters evaluated by pushover analysis of the buildings subjected to monotonically increasing lateral forces according to UBC 1997 [12] with an invariant height-wise distribution until a pre-determined target displacement is reached. The pushover analysis is performed, in this study, by using the finite element program ETABS2000 [13] which accomplishes the pushover analysis procedure in a piece-wise linear fashion. Through the implementation of pushover analysis, plastic hinges are inserted in the finite element model as limit states are attained in successive elements according to the procedures prescribed in FEMA 273 [10] and ATC-40 [14] documents for 3-D buildings.

Herein, the seismic response of steel–concrete composite buildings isolated by LRB isolators under NF earthquake strong ground motions is investigated. The specific objectives of the present study may be summarized as: (i) to investigate seismic performance limits of LRB base-isolation system on composite buildings, (ii) to study the seismic behavior of braced as compared to unbraced composite buildings in combination with appropriate levels of base isolation for upgrading earthquake resistant design and (iii) to examine the efficiency of pushover analysis in analyzing base-isolated steel–concrete composite buildings under NF excitations.

Section snippets

Modeling and assumptions

In the present study, two five-story steel–concrete composite 3-D buildings, buildings A and B, respectively, as shown in Fig. 1, Fig. 2 have been considered. Both buildings have the same floor plan with four longitudinal bays by four transverse bays as depicted in Fig. 3. The size of the longitudinal and transverse bays is 6 m each. The height of the first floor is 4 m while that of the rest of the floors is 3 m as depicted in elevation plan of buildings A and B in Fig. 4, Fig. 5, respectively.

Pushover analysis under near-fault loading

To investigate the performance of buildings A and B under NF excitations, an earthquake strong ground motion loading is considered and assessed against UBC 1997 [12] code provisions. More specifically, it was considered: Seismic zone 4, Source A with a fault distance <=2 km (the most severe NF category in the code) and Soil type SD. Source A is code-defined as a fault with a high rate of seismic activity (slip rate ⩾5 mm per year) that is capable of producing even a M⩾7.0 earthquake event.

Recent

Results

In this section, results of the pushover analysis are presented and discussed. Comparisons are made of predicted by ETABS2000 program story drifts, base shear and plastic hinge formation to compare the seismic behavior of LRB base-isolated braced and unbraced steel–concrete composite structures (buildings A and B, respectively) with the fixed-base building ones.

Conclusions

In the present paper, the seismic behavior of various kinds of LRB base-isolated steel–concrete composite buildings under NF earthquake excitation has been investigated by using the pushover analysis. The seismic responses of the different composite buildings have been compared and some concluding remarks can be obtained as follows:

  • 1.

    The use of base-isolation systems is effective in limiting the base shear of the building excited by NF earthquakes but, unfortunately, increases the story drift of

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