Effect of structural characteristics distribution on strength demand and ductility reduction factor of MDOF systems considering soil-structure interaction

It is known that structural stiffness and strength distributions have an important role in the seismic response of buildings.The effect of using different code-specified lateral load patterns on the seismic performance of fixed-base buildings has been investigated by researchers during the past two decades.However,no investigation has yet been carried out for the case of soil-structure systems.In the present study,through intensive parametric analyses of 21,600 linear and nonlinear MDOF systems and considering five different shear strength and stiffness distribution patterns,including three code-specified patterns as well as uniform and concentric patterns subjected to a group of earthquakes recorded on alluvium and soft soils,the effect of structural characteristics distribution on the strength demand and ductility reduction factor of MDOF fixed-base and soil-structure systems are parametrically investigated.The results of this study show that depending on the level of inelasticity,soil flexibility and number of degrees-of-freedoms(DOFs),structural characteristics distribution can significantly affect the strength demand and ductility reduction factor of MDOF systems.It is also found that at high levels of inelasticity,the ductility reduction factor of low-rise MDOF soil-structure systems could be significantly less than that of fixed-base structures and the reduction is less pronounced as the number of stories increases.     

Optimum lateral load pattern for seismic design of elastic shear‐buildings incorporating soil–structure interaction effects

Recently, several new optimum loading patterns have been proposed by researchers for fixed‐base systems while their adequacy for soil–structure systems has not been evaluated yet. Through intensive dynamic analyses of multistory shear‐building models with soil–structure interaction subjected to a group of 21 artificial earthquakes adjusted to soft soil design spectrum, the adequacy of these optimum patterns is investigated. It is concluded that using these patterns the structures generally achieve near optimum performance in some range of periods. However, their efficiency reduces as soil flexibility increases especially when soil–structure interaction effects are significant. In the present paper, using the uniform distribution of damage over the height of structures, as the criterion, an optimization algorithm for seismic design of elastic soil–structure systems is developed. The effects of fundamental period, number of stories, earthquake excitation, soil flexibility, building aspect ratio, damping ratio and damping model on optimum distribution pattern are investigated. On the basis of 30,240 optimum load patterns derived from numerical simulations and nonlinear statistical regression analyses, a new lateral load pattern for elastic soil–structure systems is proposed. It is a function of the fundamental period of the structure, soil flexibility and structural slenderness ratio. It is shown that the seismic performance of such a structure is superior to those designed by code‐compliant or recently proposed patterns by researchers for fixed‐base structures. Using the proposed load pattern in this study, the designed structures experience up to 40% less structural weight as compared with the code‐compliant or optimum patterns developed based on fixed‐base structures. Copyright © 2012 John Wiley & Sons, Ltd.     

A novel approach to estimate the variations in stresses and fault state due to depletion of reservoirs

Geomechanical changes may occur in reservoirs due to production from reservoirs. Study of these changes has an important role in upcoming operations. Frictional equilibrium is one of the items that should be determined during the depletion as it may vary with respect to time. Pre-existing faults and fractures will slide in regions where there is no frictional equilibrium. Hence, it may be concluded that reduction in pore pressure can initiate the sliding of faults. Whereas, it is also possible that faults will not exist after a certain time as production recovers the equilibrium. Casing shearing or lost circulation may be occurred due to faulting. In reservoirs which depletion leads to frictional equilibrium, decrease of fractures and faults leads to some variations in permeability. Hence, predicting the effect of depletion on frictional equilibrium is required in dealing with casing shearing or lost circulation in drilling of new wells. In addition, permeability modeling will be more precise during the life of reservoirs. Estimation of necessary time to create or vanish faults is vital to be successful in production from wells or drilling new wells. Achieving or loosing of equilibrium mainly depends on in situ stresses and rate of production. Estimation of the in situ stresses at the initiation state of reservoir is the key to study the state of faults. The next step is to predict the effects of depletion on in situ stresses. Different models are suggested in which decrease of horizontal stresses is predicted as function of pore pressure variation. In these models, different assumptions are made such as simplifying the poroelastic theory, ignoring the passing time, and considering the geometry of reservoir. In this article, a new model is proposed based on theory of inclusions and boundary element method. This state-of-the-art model considers the geometry of reservoir. In addition, changes of in situ are obtained as a function of time to reach to a more precise model capable of applying during the reservoir life. Finally, the model is imposed on real cases. The effect of depletion on faults is studied in reservoirs of normal and strike-slip stress regimes, and conventional and proposed models are compared. For this aim, in the first step, mechanical earth models of these two reservoirs are built using logging and coring data. Stress polygon method and poroelastic horizontal strain model are used for strike-slip and normal regimes, respectively. In reservoir 1 which is in a strike-slip stress regime, a fault is distinguished in formation microimaging (FMI) log. For this reservoir, the required time to achieve to frictional equilibrium is calculated. In the reservoir 2, the potential depth of fault sliding is analyzed and required time for faulting in that depth is predicted. The predicted time for satisfaction of frictional equilibrium using the proposed model has a significant difference with the predicted time using the previous methods. In addition, the proposed model predicts that different parts of reservoir 2 are willing for faulting during depletion. The previous model determines only one point that faulting may happen. It is necessary to use this new model to consider different important factors such as geometry and time to gain more accurate predictions.