مجله علوم و مهندسی زلزله
سال چهارم شماره 1 (پیاپی 10، بهار 1396)

The pile driving process can be easily modeled prior to installation to determine adequate and appropriate equipment selection. Pre-casted lightweight concrete (LWC) provide an attractive alternative to conventional pile materials such as steel and common concrete by improving the durability of deep foundations. In this paper, the drivability of cylindrical and tapered piles with lightweight concrete was investigated and compared with traditional pile materials using the finite difference analysis. The three-dimensional model was considered to simulate pile-soil system in drivability by FLAC3D. The vertical LWC pile was assumed to behave linear elastic, and the soil was acted in elasto-plastic material obeyed the Mohr-Coulomb failure criterion. Interface elements were also used at the soil-pile contact surfaces along the pile shaft and toe to allow the slip occur during the driving procedure. Quiet boundaries were considered to prevent waves traveling in the lateral and vertical directions for the soil. The different concrete mixtures with Leca and Scoria were assumed to compare the size of the physical properties of light aggregate, which letters S and L represent Leca and the Scoria in each concrete mix design.
The analysis of wave propagation in LWC rods without and with damping effects were performed with fixed and free end boundary conditions. The rod gravity was neglected with no soil or other supports around the rod shaft. A half-sine stress wave was applied on the rod head. To represent the dynamic responses, the force and velocity records were monitored below the rod head to prevent the mixing up the upward stress wave and downward reflection from the rod head. The obtained results were exactly in accordance with one dimensional wave propagation theory in rods. The immediate F and Z.v waves shifted down after tip reflections are the reflections from the rod free head boundary condition. In fact, the downward initial compressive wave is reflected as compression type at rod fixed-end and reflected tension type at the rod free-top boundary conditions. The F wave and Z.v wave amplitudes were attenuated with time as expected due to the damping presence in the rod.
The pile drivability with light weight concrete and cylindrical and tapered geometry was also investigated in clayey soil and the results were compared. Based on signal matching for LWC piles, as expected, residual displacements of pile S2 and L3 are 8 to 20% greater than common concrete piles, and pile S3 has approximately the same behaviour as pile CC.
The analyses results indicate that LWS piles with selecting the appropriate mixture design and ratio between elastic modulus and specific weight have a better performance compared with common used concrete and therefore, it can affect the pile optimum penetration and economic saving of pile driving procedure.

Reliable and accurate assessment of dynamic soil behavior curves is necessary for the solution of many soil dynamic problems, such as the site response analysis. The shear modulus and damping curves of silicate soils under different conditions have been investigated by many geotechnical researchers.
Carbonate sediments are located in temperate and tropical areas and cover approximately 40% of the ocean surface. This type of soil is typically observed near offshore hydrocarbon industries, such as the Persian Gulf. Carbonate sand is the accumulation of pieces of carbonate materials; it usually originates from reworked shell fragments and skeletal debris of marine organism. Foundation problems associated with carbonate soil deposits, particularly as experienced by the offshore hydrocarbon industry have led to significant research focused on understanding the behavior of these soils.
This study focuses on evaluation of dynamic properties of Bushehr calcareous sand. The shear modulus and damping ratio of the tested sand was measured at small to large shear strains using resonant column and cyclic triaxial tests. The calcareous sand specimens were tested by a fixed-free type of resonant column apparatus (SEIKEN model). By using the resonant column apparatus, shear modulus and damping ratio of the calcareous sand for the shear strain amplitude ranging from about 10-4 % to 10-2 % were measured. The cyclic triaxial tests were conducted using a fully automated GDS triaxial testing apparatus. The cyclic tests were done on samples with shear strain amplitudes ranging from about 10-2 % to 1 %. The procedure used to perform the dynamic and cyclic tests was the multi-stage strain-controlled loading under undrained condition.
The tests were conducted in three levels of initial effective mean confining pressure equal to 40, 200, and 400 kPa. The sand specimens were constructed in relative densities lower than 50 or 80 percent, depends on the initial effective stress, in order to acquire the target relative densities (i.e. 50 or 80 percent) after consolidation.
The experimental results indicate that with an increased shear strain amplitude, shear modulus decreases and damping ratio increases. This trend, which was observed for all the tests, is typical behavior of soils under dynamic loading, as observed in the previous studies.
The effect of mean effective confining pressure and relative density on the normalized shear modulus (G/Gmax) curves of the Bushehr calcareous sand was investigated. Gmax is the small-strain shear modulus measured at shear strain amplitude about 10-4 %. The increase in mean effective confining pressure causes the normalized shear modulus to increase; however, it is more pronounced in low effective confining pressure. Changes of the normalized shear modulus curves (G/Gmax-γ) at the range of σ'm=200-400 kPa is less than that at the range of σ'm=40-200 kPa. Normalized shear modulus curves are almost independent of the changes in relative density of the sand.
The results also indicate that the increased amount of initial mean effective confining pressure leads to the smaller damping ratio for the tested sand. It is also observed that damping ratio curves (D-γ) are not affected strongly by relative density changes.
Comparison of the tests results with the ranges and models recommended by the previous researchers reveals that the normalized shear modulus and damping ratios of the studied calcareous sand are somewhat inconsistent. In fact, there might be a necessity to modify the previous recommendations for accurate prediction of the dynamic behavior curves of calcareous sands.

The recent trend in structural earthquake engineering practice is to use performance-based seismic evaluation methods for the estimation of inelastic demands in structures. Nonlinear static analysis, commonly referred to as pushover analysis, is becoming a popular simplified tool for seismic performance evaluation of existing and new structures. The pushover analysis of a structure is a static nonlinear analysis under permanent vertical load vectors and gradually increasing lateral loads until reaching the predetermined target displacement at roof level. Target displacement serves as an estimate of the global displacement of the structure expected to experience in a design earthquake. The accurate estimation of target displacement associated with specific performance objective affect the accuracy of seismic demand predictions of pushover analysis. Recently, the researchers have proposed various enhanced methods that aim to capture the true seismic-induced target displacements. Most of the reported research on development of improved Nonlinear Static Procedures (NSPs) is based on the response of analytical models subjected to Far-Fault (FF) earthquake records and less have been investigated for Near-Fault (NF) ground motions. NF motions differ from FF ones in that they often contain strong coherent dynamic long period pulses and/or permanent ground displacements. Out of the two kinds of NF ground motions, ground motions with velocity pulses caused by NF directivity effects have received a great deal of attention because of their potential to cause severe damage to structures. Capacity Spectrum Method (CSM) and Displacement Coefficient Method (DCM) are the two methods presented in FEMA-440 (2005) and ASCE/SEI 41-13 (2013) as standard methods of estimating the target displacement. DCM is considered in this paper as the basis of the presented method. In DCM, the target displacement, which corresponds to the displacement at roof level of a building, shall be calculated by applying appropriate modification factors to the elastic spectral displacement of SDOF system. In this method, C0 is modification factor to relate spectral displacement of an equivalent SDOF system to the roof displacement of the building, C1 is the modification factor to relate the expected maximum displacements of an inelastic SDOF oscillator, and C2 is the modification factor to represent the effect of pinched hysteretic shape, stiffness degradation, and strength deterioration on the maximum displacement response. It should be noted that the coefficients of this equation have been derived from FF motions in FEMA-440 (2005). Therefore, applying these coefficients to estimate the target displacement for NF ground motion may not yield accurate results. Due to this, CN, need to be used to modify the C1 coefficient when a SDOF system is subjected to NF ground motion. This modification factor was previously presented by Esfahanian and Aghakouchak (2015). This paper investigates inelastic seismic demands of the normal component of near-fault pulse-like ground motions. 20 near-fault and 20 far-fault ground motions and the responses of 10-, 15-, and 20-story multi degrees of freedom (MDOF) systems constitute the dataset. These systems are all steel moment-resisting frames, designed according to allowable stress design method. The buildings’ lateral load-resisting system is steel special moment-resisting frame. All buildings are 15 m in width.
The bays are 5 m on center with three bays. Story heights of all buildings are 3.2 m. The seismic masses of all level floors for each structure are assumed to be equal and consist of dead load plus 20% of live load. Dead and live loads are equal to 650 and 200 kg/m2 on the floor area that loading width of the frames is assumed to be 5 m. Design is performed based on the weak beam-strong column. In analysis and design, P-Δ (second order) effects are included. Nonlinear static and dynamic analyses were performed by the OpenSees (2013) software to simulate the performance of structural systems subjected to earthquakes. Both geometrical nonlinearity and material inelasticity were taken into account in the models. The material inelasticity was explicitly considered by employing a fiber modeling approach. Beams and columns have been modeled as finite elements with distributed inelasticity in a specified length of the member ends, using force-beam-column elements. For all of the NL-THAs, the damping matrix was defined using Rayleigh damping with a damping ratio of 5% for the first and third modes of vibration. In this paper, wavelet analysis method, presented by Baker (2007, 2008) is used for selecting pulse-like NF ground motions. NL-THA is utilized as the benchmark for comparison with nonlinear static analysis results. A new method for estimating target displacements are presented, using response spectrum analysis method and appropriate modification factors. As the proposed method considers the MDOF effects, C0 coefficient is not used in this method and only C1, C2, and CN are applied in this method. The target displacements resulting from the proposed procedure are then compared to those from the NL-THA and displacement coefficient method of ASCE 41-13, as well as to those predicted from Modal Pushover Analysis (MPA) methods. MPA is an enhanced NSP presented by Chopra, which utilizes the concept of modal combinations through several pushover analyses using invariant load patterns based on elastic mode shapes where the total response is determined with combination of each mode at the end (Chopra and Goel, 2001, 2002). It should be noted that, various methods applied to nonlinear models developed using generally accepted methods provide either overestimation or underestimation of the target roof displacement when compared to the value derived from NL-THA of recorded motions. It is shown that these procedures may lead to significantly different estimates of the target displacement, particularly for high-rise buildings responding in the nonlinear range. The results of the proposed procedure demonstrate acceptable values for target displacement, especially for near-fault earthquake records in comparison to the approximate and exact ones.

One of the new challenges in structural engineering is the mitigation of seismic hazards from structures using flexibility and energy dissipation approaches. This is in contrast with the typical seismic design methodologies in which strength and ductility resources of structural members are tapped to tackle earthquake demands. In this approach, adding to the flexibility of the structure should be in accordance with the energy dissipation potential in the system. In this case, earthquake demands for lateral strength in structures reduces, but energy dissipation devices are needed to subside the lateral deformation of such flexible structural systems.
To meet the huge demand for energy dissipation potential in these structural systems, large scale damping devices are required. Such equipment, to mention a few, comes in the form of metallic, frictional, viscoelastic, memory shaped alloys and viscous dashpots. Among the others, viscous dashpots are considered the most favorite ones to use in large structural systems due to their sizable capacity and impartiality to ambient vibration and temperature loads. Moreover, since these devices are velocity dependent energy dissipaters, they are capable of reducing both deformation and acceleration responses of the structural systems more effectively.
Viscous dashpots are typically made from a metallic cylinder, a piston, a shaft, cylinder caps and elastomeric seals (to provide confinement on the liquid inside of the cylinder). The existence of elastomeric seals in configuration assembly of these devices is considered a weak point in mechanical design of such dashpots considering maintenance issues. To address this problem, a contractible viscous dashpot was introduced earlier in which there was no need for elastomeric seals. In this work, a new version of this dashpot with variable damping constant have been tested for determination of its functionality and characteristics.
Contractible viscous dashpots are made from two flexible chambers that axially contract or expand to accommodate liquid movement between the two. In this mechanism all parts of the device are made of steel and there is no relative movement between cylinder caps and the main shaft. Therefore, there is no need for elastomeric seals to confine the liquid inside of cylinders at cylinder caps.
The Dashpot was designed for load capacity of and the maximum stroke of . In the test procedure, however, due to some limitations in the test setup, the attainable load was around 310 KN. The test results show stable hysteretic loops under sinusoidal excitations with the amplitude of in the frequency range of 0.1-0.25 Hz. The hysteresis loops resemble a viscous device with viscoelastic behavior that can be roughly represented by Kelvin model. As expected, damping constant of the dashpot reduces by an increase in excitation frequency. The capability of change in damping characteristics of the dashpot was embedded in the device. This ability was shown in the experiments where damping constant of the device became almost tripled during the test process by adjusting the embedded mechanism in the device.
The contractible dashpot used in this study has shown an initial frictional behavior due to imperfection in its manufacturing process. Increase in the internal liquid pressure in the device expands this frictional behavior to about 10% of total capacity of the dashpot. The initial frictional force in the device can be easily improved to help the functionality of the dashpot.
This dashpot has shown acceptable performances in all the experimental investigations carried out in the course of this study. Considering its simplicity and practicality (low maintenance costs), there would be a good chance for such devices to be used in large structures in near future.

Mazandaran is located in the north of Iran. Tourism industry promotes high rise building structures in this province. According to the Iranian code of practice for seismic resistant design of buildings, we cannot use the common procedures to estimate the base design acceleration in high rise buildings, which have some special conditions.
In order to determine the base design acceleration, we need the site special spectrum. Seismic hazard analysis is considered one of the most important methods to determine the base design acceleration. Attenuation relationship is one of the main factors in this method as the incorrect spectrum design will obtain in using an inappropriate attenuation relationship. This study aims to represent the constant coefficients of existing attenuation relationships in Iran plateau, which have been studied by other researchers previously, for Mazandaran province. Seismic data of Mazandaran have been used for this purpose. The total number of accelerogram records used with the moment magnitude more than 4 between 1975 and 2016 was 105 records in which 71 records had the shear wave velocities in the top 30 meters more than 375 m/s, which can be considered as rock. Moreover, 34 records with the shear wave velocity of less than 375m/s had been considered as soil. Baseline correction and frequency filtering have been done on records by using Seismosignal software to make them more precise. In this paper, four different attenuation models were selected to analyze by using SPSS software. Between the existing models the first ground motion model which has been studied, was presented by Nowroozi in 2005. The 178 reported data by Bard et al, was used in Nowroozi paper. The second one was from Ghodrati et al presented in 2007 for Iran plateau. They considered Iran into two main seismic zones of Zagros and Alborz and Central Iran according to tectonic conditions. All of the earthquake records in their databank had distances between 5 to 150 kilometers from site to source. They chose the shear wave velocity of 375 M/S for the boundary of soil and rock. The third attenuation relationship was Saffari et al work published in 2012. They used 351 records to present an attenuation ground motion model for Iran. They developed attenuation relationships for peak ground acceleration, peak ground velocity and acceleration response spectra with 5% damping. The last but not least attenuation relationship studied in this paper was the ground motion prediction equation by Soghrat and Ziyaeifar published in 2016. They used 325 three-component records of 55 earthquakes with magnitude ranging from 4.1 to 7.3 for estimation on the regression coefficients.
In the presented attenuation relationships in this paper, the effects of earthquake magnitude, site to source distance and ground type on the peak ground acceleration have been investigated. According to this study, using the records of Mazandaran province leads to homogeneous results indicating the accuracy of extracting constant coefficient of this study. Standard deviation is such an important factor in attenuation relationships, which highly recommended to study. Furthermore, it has been found that mathematical models do not alter the final results significantly; probably, used data affects the attenuation relationship more. Accordingly, using the seismic data in each region is recommended in order to achieve the precise attenuation relationships.