Effects of SoilStructure Interaction on Response of ...

Effects of SoilStructure Interaction on Response of Structures Subjected to NearFault Earthquake Records M. Ali Ghannad, Asghar Amiri, and S. Farid Ghahari Citation: AIP Conference Proceedings 1020, 642 (2008); doi: 10.1063/1.2963896 View online: http://dx.doi.org/10.1063/1.2963896 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1020?ver=pdfcov Published by the AIP Publishing

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Effects of Soil-Structure Interaction on Response of Structures Subjected to Near-Fault Earthquake Records M. Ali Ghannacf, Asghar Amirib and S. Farid Ghahari0 a

Associate Professor, Department of Civil Engineering, Sharif University of Technology, P.O.Box 11155-9313, Tehran, Iran. b M.Sc. Student, Department of Civil Engineering, Sharif University of Technology, Tehran, Iran. c Ph.D. Student, Department of Civil Engineering, Sharif University of Technology, Tehran, Iran. Abstract. Near-fault ground motions have notable characteristics such as velocity time histories containing large-amplitude and long-period pulses caused by forward directivity effects and acceleration time histories with high frequency content. These specifications of near-fault earthquake records make structural responses to be different from those expected in far-fault earthquakes. In this paper, using moving average filtering, a set of near-fault earthquake records containing forward directivity pulses are decomposed into two parts having different frequency content: a Pulse-Type Record (PTR) that possesses long period pulses, and a relatively highfrequency Background Record (BGR). Studying the structural response to near-fault records reveals that elastic response spectra for fixed-base systems, in contrast to their response to ordinary earthquakes, show two distinct local peaks related to BGR and PTR parts. Also, the effect of Soil-Structure Interaction (SSI) on response of structures subjected to this type of excitations is investigated. Generally, the SSI effect on the response of structures is studied through introducing a replacement single-degree-of-freedom system with longer period and usually higher damping. Since this period elongation for the PTR-dominated period range is greater than that of the BGR-dominated one, the spectral peaks become closer in the case of soilstructure systems in comparison to the corresponding fixed-base systems. Keywords: Near-fault earthquake records; Directivity effects; Moving average filter; Elastic spectra; Soil-structure interaction; Cone models.

INTRODUCTION Ground shaking near a fault rupture may be characterized by a short-duration impulsive motion. This pulse type motion is particular to the forward direction, where the fault rupture propagates towards the site at a velocity close to shear wave velocity. The radiation pattern of the shear dislocation on the fault causes this large pulse of motion to be oriented in the direction perpendicular to the fault, i.e., the fault-normal component of the motion is more severe than the fault-parallel component [1]. In addition, near-fault earthquake records are rich in high frequencies because the short travel distance of the seismic waves would not allow enough time for the highfrequency content to be damped as is normally observed in the far-fault records [2].

CP1020, 2008 Seismic Engineering Conference Commemorating the 1908 Messina and Reggio Calabria Earthquake,

edited by A. Santini and N. Moraci © 2008 American Institute of Physics 978-0-7354-0542-4/08/$23.00

642 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 164.67.185.248 On: Sun, 16 Mar 2014 17:41:20

Elastic and inelastic response of fixed-base structures to near-fault pulse-type records has been the subject of many studies up to now [3]. However, the role of high frequency part of near-fault records has not been studied in detail. Moreover, the effect of Soil-Structure Interaction (SSI) when the structure is subjected to near-fault records has attracted much less attention. In this paper, the response of soil-structure systems to long-period pulse type and high frequency parts of the record is investigated simultaneously. Hence, using a moving average filter, original near-fault ground motions are decomposed into two components having different frequency contents, i.e., the Pulse-Type Record (PTR) that possesses long period pulses, and the relatively high-frequency Background Record (BGR) [4]. Then the contribution of each part on elastic response of soil-structure systems is investigated. Since distance to the site can not be a suitable criterion, near-fault records collection with forward directivity effects requires a simple procedure to determine pulse-containing records. Quantitative methods used to date can be summarized by two general requirements. First, the site must fill the geometric and seismologic requirements of forward directivity, and second, visual inspection of the velocity time history reveals a pulse-like shape [5]. Recently, a quantitative algorithm to identify ground motions containing velocity pulses, such as those caused by near-fault directivity effect, is proposed by Baker [6]. In this paper, a combination of all mentioned criteria has been applied to collect near-fault earthquake records containing directivity pulses.

RECORD DECOMPOSITION By applying a middle-assigned moving average filter on collected near-fault database, all original records are decomposed into PTRs and BGRs using the method proposed in [7]. As is mentioned there, the PTR is considered as the low-frequency smoothed part of each record which may have the same duration as the original record. The number of points for moving average filtering, which is related to the cut-off frequency of the filter, is directly dependent on pulse period and inversely on the length of the time intervals of input record, i.e.: m = a^ dt

(1) '

K

where m, TP and dt are the number of points for moving average filter, period of the dominant pulse, and the length of the time intervals of input record, respectively, and a is a coefficient which is determined empirically and set to 0.25. 7> is calculated using Short-Time Fourier Transform (STFT) [7]. BGR would be considered as the difference between original record and PTR. Figure 1 presents the results of this decomposition for two records. This figure reveals the appropriate performance of moving average filtering for records with only one main pulse (left) or multiple pulses (right).


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(c) BGR acceleration FGURE 1. Samples of PTR and BGR extracted from original records using moving average filter.

STRUCTURAL RESPONSE Fixed-Base Structures Easttc response spectra computed for all records containing their PTR and BGR components confirm the idea that their elastic response spectra, in contrast to ordinary ear er hquakes, have two distinct local maximum regions. In one region, usually short related response of structures is dominated by BGR, and in the second part, typically related tto long periods, by PTR. Figure 2 represents pseudo acceleration response spectra of four near-fault earthquake records and their BGR-PTR components in which two mentioned regions are apparent Therefore, considering directivity pulses alone as a representative of near-fault earthquake records for computing response of s tructures may be unsafe in some situations, especially for short buildings. Following the discussion above, we can say that there is usually a frequency gap in elastic response spectrum of structures against near-fault earthquake records. In other words, these spectra are a combination of two conventional design response spectrums . with two constant-acceleration regions corresponding to BGR and PTR components. Note that tn some cases, these regions may overlap, or even cover each other.


TCU031, Chl-Chl 1999

(d)CHY101,Chi-Chil999 (c)WGK,Chi-Chi!999 FIGURE 2. Examples of pseudo acceleration response spectra for near-fault earthquake records.

Soil-Structure Systems In this section, the effects of SSI on response of linear structures subjected to nearfault earthquake records are investigated. For this purpose, a simplified discrete model shown in Fig. 3 is used to represent the real soil-structure system. This model is based on the following assumptions: 1. The structure is replaced by an equivalent elastic SDOF system. 2. The foundation is replaced by a circular rigid disk of mass m/, and mass moment of inertia If. 3. The soil beneath the foundation is considered as a homogeneous half-space and replaced by a simplified 3DOF system based on the concept of Cone Models [8]. m,I

FIGURE 3. Soil-structure model [9].


The non-dimensional parameters for studying SSI effects are summarized in Table 1. TABLE 1. SSI non-dimensional parameters. Definition non-dimensional frequency natural frequency of fixed-base structure height of the structure shear wave velocity of the soil aspect ratio of the building structure-to-soil mass ratio index mass ratio of the foundation to structure rrif/m V Poisson's ratio of the soil material damping ratios of the soil and the structure io, is More details of parameters are presented in reference [9]

Parameter a0= cosh/Vs cos h Vs h/r m

Assigned Values 0,1,2

1,3 0.5 0.1 0.25 0.05

The first two items, i.e. ao and h/r, are the key parameters that define the principal SSI effects. The other parameters, however, are those with less importance and are set to some typical values for ordinary buildings [10]. As mentioned in literature, the effects of SSI on elastic response of a structure can be summarized in two general aspects: first, it increases the period of the system because of the soil softness which is ignored in fixed-base models; second, it usually increases the damping ratio of the system via introducing new sources of damping in the soil, including hysteretic and radiation damping [11]. Elastic response spectra for soil-structure systems having three different values of non-dimensional frequency (ao=0, 1, 2) and two values of aspect ratio (/?/r=l, 3) subjected to all records containing PTR and BGR components are calculated. Note that a system with ao=0 represents a fixed-base structure such as those presented in the previous section. As an illustration, Fig. 4 shows pseudo acceleration response spectra of different soil-structure systems subjected to the four near-fault accelerograms recorded during the Chi-Chi, Taiwan earthquake. As it can be seen, increasing in structure-to-soil stiffness, i.e. ao, results in movement of response spectrum to lower periods and reduction in spectral ordinates. Because this period shifting for PTR-dominated region is greater than BGR-dominated region, frequency gap between these two spectral peaks decreases in comparison to the fixed-base systems. Moreover, the greater is the value of h/r, the lower is the damping effect of SSI. Therefore, as the value of ao and h/r get larger, the structures which are located in gap region and have low fixed-base response may be located in peak spectral regions.


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(b) TCU036, Chi-Chi earthquake 1999 FIGURE 4. Pseudo acceleration response spectra of SSI systems subjected to near-faull records.


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