جستجوی مقالات مرتبط با کلیدواژه « uplift » در نشریات گروه « عمران »

In recent earthquakes, many buried structures, including tunnels and lifelines, have been seriously damaged. It is noteworthy that the phenomenon of soil liquefaction has a significant role in the occurrence of these damages. The enormous damage because of soil liquefaction, has motivated the study of uplifting of lifelines. Therefore, in this research, it has been attempted to study uplift of buried pipe in liquefiable soil by physical modeling. The soil used in this study was Gum tape sand and the shaking table was used to simulate the seismic load. Also, due to the importance of the deformation mechanism in this process, particle image velocimetry technique has been used. The buried pipe (at three depths of 1.5, 2.5 and 5 times the diameter of the pipe) has been subjected to seismic load and then deformation mechanism of it has been discussed. The results show that by decreasing the buried depth of the pipe, due to the relatively high pore water pressure at the lower soil depth, the overpressure created after the dynamic loading tends to dissipate and flow to low pressure points (surface area) and because of upward flow in the surface area, the uplift will continue to some extent. Also, the displacement vectors around the pipe are circular rings that try to uplift the pipe.

The science of seismic control of the structures always seeks to reduce and control the destructive effects of the forces which are produced during an earthquake event. However, the yield of the soil under intensified seismic loads can cause some irreversible effects on the structural elements and the structure may withstand the increased moments and the forces for which it is not formerly designed. This unfavorable phenomenon significantly affect the seismic response and performance of the structures, and will ultimately leads to disappearance of all functional goals that are sought to create in the structural design stage by the designer of the structure. In this research, by a relatively accurate three dimensional finite element modeling, from a high-rise concrete structure equipped with the active mass damper, and by examining the lesser-known aspects of the problem such as uplift of the foundation and the effects of nonlinear interaction of soil and structure, an attempt has been made to conduct a relatively comprehensive study on the questions of seismic control of structures equipped with active mass dampers due to the nonlinear effects of the underneath soil behavior. For this purpose, time history dynamic analysis was performed on the structural model under the effect of 22 horizontal records of distant basin earthquakes in x and y directions followed by the Appendix A of FEMA P695. The Ibarra model (a reviewed and modified model based on the Clough model) is used for modeling of the hysteresis behavior of concrete materials. The underneath soil is modelled by three springs approach presented in ATC 40 and FEMA 440 with equivalent stiffness based on soil modulus of elasticity and Poisson’s ratio of three categories (the main C category and upper and lower C and E categories ). To achieve the goal of optimization and evaluation of the active mass damper parameters (consist of tuning ratio, mass ratio and damping ratio), the mass damper spectra method with investigation of changes in structural responses has been used. The Fuzzy theory has been used to calculate the control force of an active mass damper. The results indicate that with respect to entrance of the structure to non-linear zone and its interaction with non-linear behavior of the soil, the efficiency of the active mass damper in uplift control of the foundation decreases, but a good efficiency is observed in the lateral displacement and inter story drift control. By evaluation of the three dimensional analysis results of a nonlinear soil structure system equipped with the active mass damper, the researchers observed that for the set of the recorded earthquake and based on the specifications considered for fuzzy algorithm, the active mass damper has a satisfactory effect in the control of displacements and drifts of the structure, in the amount of almost 15 percent. It was also identified that the active mass damper has a negligible effect on the foundation uplift in the structures which constructed on the hard soil. But when the soil becomes softer, a 3 percent mean decrease is observed in the uplift displacements in foundation.

Gravity dams are vital structures whose proper design and evaluation for stability are quite important. Effective issues on the stability of gravity dams are the uplift force and its distribution below the dam base. The uplift load pattern and distribution according to common codes are influenced by some factors such as head and tail water, assuming a segmented linear load distribution below the dam. In this research, to investigate the sensitivity of the load pattern to dam height, a number of gravity dams of Pine Flat type with different heights and their foundations are modeled. Coupled p-u finite element analysis is performed accounting for the seepage and stress field simultaneously. Dam body is considered to be completely impervious. The foundation rock is assumed as homogeneous and uniform, in terms of elasticity and permeability. The stresses generated in the dam interface for each case of the coupled hydro-mechanical analysis is compared against that of the conventional load pattern according to the USACE regulation for the same dam model. It was found that the error magnitude due to the conventional pattern has a direct relationship with the dam height. As the dam height increases, the amount of error of calculated stress increases. In particular, the error at the critical zones of the foundation such as at the dam heel, may raise even up to 40%. In the group of dams studied, the error increases even up to 12 times in respect to the expected error in the shorter dams. The deficiency could in some cases completely affect the safety of the dam. This research indicates the necessity of using more accurate methods of estimating uplift load under high gravity dams. Gravity dams are vital structures whose proper design and evaluation for stability are quite important. Effective issues on the stability of gravity dams are the uplift force and its distribution below the dam base. The uplift load pattern and distribution according to common codes are influenced by some factors such as head and tail water, assuming a segmented linear load distribution below the dam. In this research, to investigate the sensitivity of the load pattern to dam height, a number of gravity dams of Pine Flat type with different heights and their foundations are modeled. Coupled p-u finite element analysis is performed accounting for the seepage and stress field simultaneously. Dam body is considered to be completely impervious. The foundation rock is assumed as homogeneous and uniform, in terms of elasticity and permeability. The stresses generated in the dam interface for each case of the coupled hydro-mechanical analysis is compared against that of the conventional load pattern according to the USACE regulation for the same dam model. It was found that the error magnitude due to the conventional pattern has a direct relationship with the dam height. As the dam height increases, the amount of error of calculated stress increases. In particular, the error at the critical zones of the foundation such as at the dam heel, may raise even up to 40%. In the group of dams studied, the error increases even up to 12 times in respect to the expected error in the shorter dams. The deficiency could in some cases completely affect the safety of the dam. This research indicates the necessity of using more accurate methods of estimating uplift load under high gravity dams.

In design of structures for applied loads, the common assumption is that the structure is tightly attached to its base, i.e. no vertical rigid-body motion can happen at the same place. During a major earthquake, the couple of axial forces associated with the overturning moment can overcome that of the gravity loads in lateral load bearing columns and result in an uplifted column. With a building losing some of its contact points to the ground, a reduced lateral stiffness can be expected. This in turn can result in a reduced seismic base shear. At the same time, the structural members around the uplifted part can undergo large local deformations and perhaps a more extensive seismic damage. Illuminating the above predictions is the incentive of this research. In this paper, in the numerical part, a number of steel frames having various numbers of stories and bays are studied in two cases of without uplift (fxed base) and with uplift. The methods of analysis are the non-linear static and dynamic analysis procedures.According to the fndings, the uplift phenomenon generally has an important role in changing the behavior of structure and reduction of its response. At the same time, it can result in locally increased damages that sometimes can add up to total failures. The results showed that uplift increases the structural period and the absorbed energy, and decreases the displacements of most parts of the structures and their internal forces.

Earthquakes could have catastrophic impacts on tunnels and buried structures. Liquefaction phenomenon is one of the most destructive effects of earthquakes. Liquefaction results in decreasing effective stress and this consequently leads to decreases in shear strength of the soil. As a result, large deformation in both soil and buried structure occurs. Moreover, liquefaction causes soil boiling, lateral spreading, structure settlement and the uplift of buried structure. In many earthquake cases, uplift of the buried structure due to liquefaction was reported as one of the major damaging factors. This research sought to model earthquake induced uplift of buried structures including tunnels in the soil susceptible to liquefaction under seismic loading. Two dimensional finite difference modeling is employed to simulate centrifuge experiments. The UBCSAND model was adopted as constitutive model in numerical simulations. The modeling results are compared with the measurements of centrifuge experiment and the capabilities of the model to predict the liquefaction of soil around tunnels are assessed.