Milad Tower is a 436-m-tall telecommunications tower ranked as the fourth tallest structure of its kind in the world. The study of its seismic behaviour is of great importance as it is located in a highly seismic-prone region. Because of the existence of the world's largest revolving restaurant in the head structure, and also because of the highly sensitive communication devices such as TV and telecommunications antennas installed on the tower, nonlinear deformation under future earthquakes should be studied. In this paper, a detailed fi nite element model is developed and nonlinear dynamic analysis was carried out under the design earthquakes. The foundation, concrete shaft, head structure and 120-m antenna structure were modelled. Three components of the earthquakes are considered, and the selected earthquakes were normalized based on three design levels: design basis level, maximum design level and maximum credible level. The results of the analysis showed that all parts of the tower behave in the plastic zone except the elements of the head structure. It is also observed that in some cases, the earthquakes with lower peak accelerations and higher energy contents may have more severe effect on the tower than that with higher peak accelerations. triangular and inclined walls, respectively. Three-dimensional truss elements are used for modelling the post-tension tendons that are embedded in the exterior ring of the circular foundation. Reinforcements are accurately placed in the fi nite element model according to the structural drawings.Four-node shell elements are used from the height level of 0.0 m to the height level of 307 m, where the section of the shaft is changed to accommodate the antenna mast. Eight-node solid elements are used above the height level of 307 m for modelling the concrete shaft. The concrete shaft is modelled accurately with every structural part.All members of the head structure are modelled using a 3-D Bernoulli-Euler beam, in which the shear deformation is ignored. To consider the fl oor rigidity, all of the nodes are tied together in every storey to have equal movements (equal transition in the X and Y directions and equal rotation about the Z-axis.Four-node shell elements are used for modelling the antenna mast, and the stiffeners are modelled by 3-D Bernoulli-Euler beam elements. This part is also modelled completely. The fi nite element of the tower is shown in Figure 4. It should be mentioned that the fi nite element model consists of 13 284 elements and 17 920 nodes. MATERIAL MODELLINGThe materials used in the structure are mainly steel and concrete, and convenient models are used to defi ne the behaviour using the ABAQUS software (ABAQUS, 2006). Figure 3. The head structure of Milad Tower 882 M. YAHYAI, B. REZAYIBANA AND A. SAEDI DARYANThe isotropic trilinear steel model is used for high-strength steel, and the isotropic bilinear steel model is used for tendons and high-strength bars that do not have a plastic plateau (Table 1). ConcreteThe 28-day compressive strength of the con...
In this paper, the effect of pulse-type motions caused by forward directivity that can release huge amounts of energy in a short time period is studied on a telecommunication tower. Since telecommunication towers have longer periods, they are not as affected by seismic forces. Nevertheless, near source earthquakes characterized by high velocity and velocity pulses can change the behavior of these structures. For this reason, a telecommunication tower located near active faults was selected in this study. Considering the probable earthquake magnitude at the site and the distance of the tower from adjacent faults, nine simulated pulses and three near-fault earthquake records with forward directivity are selected and applied to a 3D fi nite element model of the tower. The results of nonlinear dynamic analysis, i.e., displacements and damage in the tower, indicate that the maximum displacement and drift ratio of the tower under the pulses are obviously affected by the ratio of the structure period to pulse period. When this ratio is decreased and close to 1.0, the maximum displacement and drift ratio are sharply increased and cause large displacements in the tower.
Due to the limitations and deficiencies in the force-based design approach, several methods are introduced and examined in order to improve this methodology. However, over the years, researchers have proposed displacement-based design methods. Among them, the direct displacement-based design (DDBD) method is one of the most thorough and accepted. The main goal of this method is to determine equivalent damping. Considering the equivalent damping and target displacement corresponding to the desired ductility, the design base shear is obtained from the displacement spectrum. Several methods are proposed to determine equivalent damping. In this study, the revised effective mass (REM) method is employed for the design of eccentrically braced frame (EBF) systems. Using this method, equivalent damping is determined for EBF's. An expression is proposed for determining the equivalent damping for EBF's in term of ductility.
Fling-step and forward directivity are two important characteristics of near-field earthquakes. Forward directivity occurs when the rupture propagates toward the site and arises in fault-normal direction for strike-slip faults. Fling-step is the consequence of permanent ground displacement imposed by near-field earthquakes and arises in strikeslip faults in the strike parallel direction. Fling-step effect produces permanent displacement at the end the culmination of displacement time history. This effect is usually omitted from the main record using the standard processing methods and therefore, cannot be determined for different structures. Many studies were performed to obtain seismic response of different structures subjected to near-field earthquake having forward directivity. On the other hand, there are few references to investigate the effect of flingstep on seismic behavior of structures. For this purpose, 14 earthquake records with flingstep are selected in a way that important factors including fling-step, PGV, PGA and the energy application type are different. The selected records are applied to eccentrically braced frame structures with 3, 6, 9 and 12 stories. A nonlinear time history analysis is used to capture displacement and story-drift ratio responses. The results show that the fling-step effect has no significant impact on any of the considered structures and its effect can be neglected in the case of eccentrically braced frame structures.
Random vibration analysis of tall structures faces multiple problems due to the large number of elements and high degrees of freedom; that is why this type of analysis is mostly used in simple structures and low degrees of freedom. In the past two decades, changes have been occurred in this type of analysis to be used in complex structures and the large number of elements. Pseudo-Excitation Method (PEM) presents a simple formulation for reducing the volume of operations. In this paper, a tall telecommunication tower is fully modeled as an example of such towers; it is analyzed by random vibration analysis with the help of the above method. Different conditions of the soil under the tower and different damping are used in modeling and analysis. The results show that structure response is strongly influenced by the soil conditions. In addition, higher modes have significant effects on the telecommunication tower response.
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