Maryam Mokhtari, Kazem Barkhordari, Saeid Abbasi Karafshani,
Volume 13, Issue 5 (12-2019)
Abstract
In recent years, with the growing use of the nailing method for stabilizing excavation walls, there has been a need for a comprehensive investigation of the behavior of this method. In the previous studies, the behavior of nailed walls has been investigated in static and dynamic states and under different conditions. However, due to the different feature of near-field ground motions, it is necessary to study the effect of these motions on the behavior of the nailed walls. Near-fault ground motion is significantly affected by the earthquake record direction and the rupture mechanism. So, in this study, to compare the effects of near-field and far-field ground motions, a two-dimensional (2D) soil- nailed wall was considered. PLAXIS 2D was used for the modeling of the soil-nailed wall system. An excavation with a dimension of 10 meters in height was taken into the account. In this study, 10 records (Five fault-normal near-field ground motion records and five far-field ground motion records), were recorded on the rock and applied to the model. These ground motion records were derived from the near-fault ground motion record set used by Baker. These records were scaled to the Peak Ground Acceleration (PGA) of 0.35g and then applied to the bottom of the finite element models. Mohr-Coulomb model was then used to describe the soil behavior, and Elasto-plastic model was employed for the nails. A damping ratio of 0.05 was considered at the fundamental periods of the soil layer. The results showed that the generated values of bending moment, shear force and axial force in nails under the effect of the near-fault ground motions were more than those in the far-ault ground motions. These values were almost equal to 23% for the maximum bending moment, 30% for the shear force, and 22% for the axial force. The created displacement under the effect of near-fault ground motions was more than that in the far-fault since a higher energy was applied to the model in the near-field ground motions during a short time (pulse-like ground motions). In contrast, in the far-fault ground motions, due to the more uniform distribution of energy during the record, such pulse-like displacements were not observed in the system response. Increasing in nail length and soil densification, decreases the displacement of the soil-nailed wall but does not change the general behavior of the soil under the effect of near-field ground motions. Based on the obtained results, for a constant PGA, there were positive correlations between the values of the maximum displacement on the top of the wall and the PGV values of near-fault ground motion records. However, the mentioned correlations were not observed in the case of far-fault ground motions.
Dr. Seyed Hossein Hosseini Lavassani, Mr. Hamed Alizadeh, Pro. Ali Massumi,
Volume 16, Issue 1 (5-2022)
Abstract
Many researches have been currently conducted on the effects of fault distance on structures revealing that their seismic response can differ according to their distance from the fault. Suspension bridges due to their long period and high flexibility can be more sensitive to this phenomenon, especially in vertical vibration. Since the engineers tend to use longer spans, the length factor should be studied more accurately. In this paper, the effects of length factor on the seismic response of the suspension bridge under near and far-fault ground motions were addressed. The Vincent Thomas and Golden Gate suspension bridges as short and long ones, respectively, are selected as the case studies. The seismic responses of two bridges under five main worldwide ground motions contained both near and far-fault ones, with the same peak ground’s acceleration, are evaluated. The results indicated that the response of both bridges to the near and far-fault ground motions are perfectly different. Short span suspension bridges are vulnerable to near-fault ground motions, whereas long span ones are completely susceptible to both near and far-fault ground motions, and by increasing the length of span, the sensitivity of bridge was increased against far-fault low frequency excitations. Also, maximum displacement responses of spans in both bridges did not increase by maximizing peak ground’s acceleration.
