حر خسروی

In Direct Displacement-Based Design (DDBD), the performance objective is to achieve the design displacement (target displacement) and the stiffness and strength of the structure is determined in such a way that the maximum displacement of the structure in an earthquake reaches this displacement. For this purpose, the design base shear is determined based on 3 key parameters of equivalent damping, damping modification factor and P-Delta effect. Due to the variation of relationships for each of these parameters, in this study the influence of using different relations on achieving performance objectives is investigated. In this study, 8 bridge piers with 2 different heights, 2 different span lengths and 2 seismic hazard levels were selected. Then, each of these piers was designed for 27 different design paths resulting from 3 distinct relationships for each of the 3 key parameters and a total of 216 bridge piers were designed by DDBD approach. Then, to evaluate the seismic performance of the piers, each of the 216 piers was modeled in OpenSees software and subjected to 14 earthquake records scaled on the design spectrum. After determining the maximum displacement of each pier, the proximity of this displacement to the target displacement was studied as a performance objective indicator. The results of analysis show that the use of different design relations has a significant effect on the maximum displacement of piers and their performance in earthquakes. Finally, comparing the results for 27 selected design paths, the best combination of relations for direct displacement-based design was proposed.

The basic design of any structure requires sufficient and detailed modeling of each structural element. It must be taken into consideration that the modeling of each member of the structure and performing nonlinear dynamic analysis due to the presence of multiple degrees of freedom, is time-consuming. Therefore, a wide range of structures cannot be investigated. To resolve this issue, researchers have recommended using simplified equivalent models to study a wide range of structures, provided that the equivalent ones significantly reflect the behavior of the original structure. One of these models is the fishbone model, which is used for modeling moment-resisting steel structures and it also has a suitable accuracy. Additionally, the presence of soil can significantly change the response of the structure. This is despite the fact that accurate modeling of the soil will lead to an increase in degrees of freedom. In this study, the aim is to examine the seismic performance of soil-structure systems to evaluate the accuracy of the models presented in seismic codes, and after modification, provide a simplified model for placement under the fishbone frame. In this regard, first by modeling a number of foundations on distributed Winkler springs considering nonlinear behavior for the soil, the moment-rotation capacity curve of the foundations was drawn, and then the aforementioned graphs were simplified into bilinear curves through an algorithm. The bilinear models that are presented, have greater stiffness and strength compared to the model presented by the seismic code. Two equations were suggested for determining the coefficients of the bilinear model by regression. In the next step, instead of modeling vertical distributed springs beneath the foundation, a rotational spring with modified bilinear behavior was placed under the fishbone model. The analysis of the seismic response of soil-structure systems under earthquake records shows that using the modified model instead of the model presented in the seismic code leads to responses with appropriate accuracy. In addition, by using the modified model, the time required for the time history analysis is noticeably reduced, which is important in research studies.

The capacity-based design is the main approach and the basic idea in the recent seismic design regulations. In moment frame systems, the desired mechanism is formed by the formation of plastic hinges at the two ends of the beams and the base of the first story column. To ensure preventing the formation of undesired mechanisms, the capacity design approach is used. For this purpose, the beam plastic moments are considered Displacement-Controlled (DC) actions, and the bending moments and shear forces in the columns (except the bending moment in the base of the first story column) are considered as Force-Controlled (FC) actions. Hence, these FC actions must be designed based on the capacity of DC actions and should remain linear during earthquakes. In order to prevent the formation of plastic hinges and shear failure in columns, two issues should be considered in the design: (1) design of columns on the basis of the bending moment capacity of the beam plastic hinges, and (2) taking dynamic amplification factor into account which is mainly due to the higher mode effects. Although the first issue has been considered in most of the design codes, but the dynamic amplification factor for column internal forces has only been considered in some of them. In this study, the higher mode effects were investigated in three reinforced concrete moment frames. For this purpose, three 8-, 12- and 20-story buildings, which were designed based on U.S. seismic design codes, were subjected to 11 earthquake records. The nonlinear time history analyzes were performed for both Design-Based Earthquake (DBE) and Maximum Credible Earthquake (MCE) levels.The results of the analysis show that the higher mode effect significantly influences the column moment in the columns. Hence, satisfying the strong column-weak beam criterion using the coefficient of 1.2 and ignoring the dynamic amplification factor based on U.S. design code is significantly nonconservative and leads to the formation of plastic hinges in the columns even in DBE earthquakes. However, the New Zealand design code recommendations for moment amplification factor is usually conservative. There is a similar judgement for the U.S. and New Zealand design recommendations in the case of column shear forces. It should be noted that the amplification of column shear forces is generally less than the bending moments, but the column shear failure is brittle and may lead to progressive collapse and catastrophic failure of buildings. Therefore, the design recommendations for the shear dynamic amplification factor are very important and cannot be neglected. Based on the average plus the standard deviation index, the column bending moments amplified about 50% to 60% for DBE earthquakes. This increase for shear forces in the columns reaches about 20% that should be considered in the capacity design of columns. In summary, based on the limited analysis and models used, this study recommends 1.6 and 1.2 for moment and shear amplification factors, respectively. It is notable that the dynamic amplification factor is considerably influenced by the earthquake intensity. Hence, increasing the earthquake intensity from the Design-Based Earthquake (DBE) to the Maximum Credible Earthquake (MCE) levels increase the bending moment and shear force amplification factors about 20% and less than 10%, respectively. Despite some recommendations that decrease the dynamic amplification coefficient in the lower floors and upper floors of the building; the results of this study show that the amplification is significant at these stories. Therefore, a uniform dynamic amplification coefficient is recommended along the building height.

The effect of the angle of incidence on the structural damage and the seismic demand in far-fault earthquakes has been taken into consideration by many researchers for several decades. However, this issue has recently received more attention from researchers and design codes for near-fault earthquakes. This issue is due to the pulse-type motion caused by forward directivity phenomena in near-fault earthquakes which increases the influence of the angle of incidence on the seismic demand. Due to the importance of the issue, several researches have been conducted in the past years to improve the design criteria for near-fault regions. For example, for Nonlinear Time History Analysis (NTHA), ASCE/SEI 7-16 design code recommended that the selected near-fault ground motions should be first rotated in the fault-normal/fault-parallel directions and then applied to the main axes of the structure. This paper aims to investigate the accuracy and efficiency of the proposed method for design of structures in near-fault regions. For this purpose, using nonlinear macro modeling of an Eccentrically Braced Frame (EBF) and applying near-fault ground motions with different angle of incidence, the seismic demands of structures were evaluated. The analysis results show that not only the direction of maximum response changes with the change of near-fault ground motions and the types of seismic demand parameters, but also it varies in different stories of a building. Nevertheless, the fault-normal response usually provides an acceptable estimate of the maximum response, which is suitable for design purposes. So that in almost 70% of the cases, the ratio of the fault-normal response to the maximum response is more than 0.75.

Nowadays, extensive nonlinear dynamic analysis is widely used in different fields of research and design of structural and earthquake engineering. It has been applied in performance-based design, resilience-based design, and also other probabilistic fields and optimization. This extensive analysis imposes a high computational cost on the researcher. However, using simplified models is an appropriate approach. The Substitute Frame is a simplified model for steel and RC moment frames, which on the one hand predicts the displacement responses of the frame with very good accuracy and on the other hand, reduces the analysis time by several times. Despite numerous evaluations of the substitute frame model in nonlinear dynamic analyses, incremental dynamic analyses, and fragility analyses, the model's accuracy has not yet been investigated for predicting moment frames' force responses. In performance-based engineering, force-control actions should be designed to prevent the undesirable failure mechanism. Hence, in this study, first, the force response of the columns was predicted using the substitute frame model, and then its accuracy was evaluated comparing to the original frame response. The results of evaluations showed that the substitute frame predicts the columns bending moments, shear forces, and axial forces of the original frame with more than 90% accuracy.

Mass nonlinear dynamic analysis is unavoidable in many fields of earthquake and structural engineering, such as incremental dynamic analysis, probabilistic performance-based design, and optimization approaches. Using simplified models with fewer degrees of freedom instead of detailed original models to a great extent reduces the computational cost and prevents extremely time-consuming analysis. Among different simplified models for steel moment frames, stick models (such as shear beam models) only use the global story stiffness to estimate the original model responses, which do not consider the structural configuration. The stick models are only suitable for obtaining the general responses of the structure, such as global and interstory drift. However, simplified frame models are the more accurate simplified models that consider the details of the original frame, such as beam and column elements, nonlinear plastic hinge springs, and joint rotations. Substitute Frame is one of these models, which is a one-bay frame that predicts the structural responses for concrete and steel moment frames with very high accuracy. The purpose of this research is to develop a substitute frame model for steel moment frames with unequal bay lengths. For this purpose, the beam stiffness and nonlinear behavior of rotational springs were modified based on linear and nonlinear structural analysis approaches and the proposed model is called Modified Substitute Frame. In the following, to evaluate the accuracy of the proposed model, three types of 12-story buildings with unequal bay lengths were designed using ASCE7-16 and AISC 341-16 criteria and subjected to three different ground motion data sets, i.e., far field, near field with pulse and without pulse ground motions. The nonlinear time history analysis results showed that the Modified Substitute Frame predicts the original frame responses with very high accuracy. Moreover, the Modified Substitute Frame prediction was more precise than the Improved FishBone model which was recently presented for moment frames with unequal bay lengths.

Accelerated construction methods are extensively used worldwide to reduce the negative impacts of bridge construction on urban traffic. These methods usually require prefabricating parts of the bridge off-site, which reduces on-site construction time and improves the quality and safety of construction. While the use of precast elements for bridge decks is relatively common, using precast elements for bridge piers is a recent development, especially in high-seismicity regions. Prefabrication of bridge piers can further expedite the construction of bridges. Moreover, the use of precast elements can be combined with a self-centering capability, through which the earthquake-induced damage and cost of post-earthquake repairs are greatly reduced. Despite a number of previous numerical and experimental studies on the behavior of precast, self-centering bridge piers, limited information is available on the selection of design parameters for such piers, and important decisions such as the prestressing force needed to achieve suitable seismic behavior remains to a large extent uncertain. This study aims to investigate the seismic behavior of concrete bridges consisting of precast self-centering piers, in which unbonded, post-tensioned tendons are used for self-centering and reinforcing steel is used to dissipate earthquake energy. A two-dimensional numerical model was developed in OpenSees to simulate the behavior of concrete bridges consisting of precast self-centering piers. The model consisted of fiber elements to model concrete and mild steel, as well as truss elements to model unbonded post-tensioning steel. The model also involved the use of zero-length sections to model the bond-slip behavior of mild steel bars. The modeling approach was validated based on experimental results available in the literature on cyclic loading of four bridge piers. To evaluate the effects of various design parameters on the behavior of precast segmental bridge piers, 9 segmental piers with different percentages of prestressing force and reinforcing steel were designed according to 2017 AASHTO LRFD Bridge Design Specifications. All piers were designed to possess similar nominal flexural capacities. The piers were then subjected to monotonic, cyclic, and dynamic time history analyses. The results showed the positive effects of prestressing in delaying cracking and reducing the residual drifts of precast bridge piers. Increasing the prestressing force ratio up to 10 percent of compressive strength of the pier cross section was observed to improve the overall seismic behavior of the structure, above which a further increase in the prestressing level may result in a diminished performance. The optimal value for the prestressing force ratio, which resulted in the most desirable behavior for cyclic and dynamic loadings was therefore found between 0.1 and 0.15. In piers with a prestressing ratio above 0.15, a decrease was observed in the area of hysteresis loops, which was accompanied by negative stiffness of the base shear versus drift curve. Moreover, the residual drift of the pier increased when prestressing ratios greater than 0.15 were used. The maximum drift of the structure was found to be insensitive to the prestressing force ratio. The results of this study are of great value for optimal design of precast, self-centering bridge piers in high-seismicity regions.

Mass dynamic analysis of structures requires high computational cost which leads to time consuming process. Therefore, it is necessary to use simplified models that significantly reduce the time of the analysis with appro-priate and acceptable accuracy compared to the detailed model. In this research, a simplified model of multi degrees of freedom for the dual system of moment resisting frame and shear wall with regular plan is proposed. MVLEM model is used for shear wall and Substitute Frame is used for RC moment resisting frame. The MVLEM model consists of axial elements that simulate the flexural behavior of the shear wall and the substi-tute frame uses one bay frame instead of multi-bay moment resisting frames. In the end, the accuracy and speed of modeling and analysis of the simplified model compared to the original model as a detailed frame. The results show that the simplified model provides seismic responses with less than 10% error and analysis time less than 30 times compared to the detailed model.

There is no Consensus among researchers about the influence of ground motion duration on the structural vulnerability. Most of disagreement is mainly due to considering different damage indices in different investigations. In this study, maximum displacement of structure due to ground motion duration is investigated. For this purpose, SDOF systems with P-Delta effect are subjected to spectrally matched record pairs. In the present study, a vast number of (more than 150000) nonlinear dynamic analysis is performed for SDOF models with 41 structural period from 1 to 5 second, 2 ductility reduction factor and 7 quantity of gravity load subjected to 146 spectrally matched ground motion pairs. The analysis results indicate that maximum displacement of structure under long duration and short duration records is almost equal, when no collapse has occurred. However, for large ductility demands, the number of collapse in ling duration ground motions is almost twice their short duration pairs.

In recent years, dynamic analysis has been considered as an appropriate tool in the earthquake engineering field. On the other hand, the number of analyzes required in the research and design field has grown. So, in some fields of research with a probabilistic approach, resilience or optimization, requires extensive nonlinear dynamic analyses. Performing such a mass analysis has a very high computational cost and requires a lot of analysis time. One of the practical solutions to this problem is the use of simplified models that significantly reduce the total analysis time with appropriate and acceptable accuracy compared to the detailed model. In this research, in order to determine the collapse margin ratio of structures, three 8-,12-, and 20- story buildings have been considered. By performing the incremental dynamic analysis (IDA) on the detailed model and the substitute frame model, their collapse intensities and collapse margin ratio have been calculated. The results show that the substitute frame model determines the collapse margin ratio with an error rate of 3%, and also reduces the analysis time by an average of 2/5%.

The unique features and destructive effects of near-fault records are of high interest to many researchers; the issue of the location of structures against these records has received less attention due to the lack of data. Therefore, in this study, the focus is on the effect of the location of the structure vs. causative fault on the amount of damage. To this end, we need enough near-fault records, which can be simulated using the synthetic generation technique due to the lack of real data. Using the fault parameters obtained for the 1999-Kocaeli earthquake, 273 earthquake records were generated by different location coordinates using the theoretical-based Green's function. To evaluate the seismic performance of the structures, OpenSEES software was used to carry out 9828 dynamic time-history analyses. The studied structures are SDOF with constant ductility. The records were applied according to the position of the structure against the causative fault and the relevant spectra were drawn as colored contours. The results showed that the location of the maximum responses in the inelastic state is almost the same as in the elastic state, so the critical location can be determined by a simpler elastic analysis; Stations that showed a maximum value at low periods have a larger amplitude, and stations that showed a maximum value at high periods have a higher pulse period. Both the distance and the angle of the SDOF location are influential in determining the location of the more severe failure.

In structural analysis, P-Delta effect refers to the changes in internal forces due to P-Delta moment which can be found by multiplying gravity load (P) by the lateral displacement of structure for earthquake lateral load (Delta). The influence of P-Delta in elastic response of structures is ignorable, but P-Delta effect should be given more attention when the structure responds in inelastic range. On the other hand, the influence of ground motion duration is magnified when P-Delta effect is considered in the structural analysis. In this paper, to consider the simultaneous effect of ground motion duration and P-Delta effect, an algorithm is implemented in MATLAB which employs OPENSEES for linear and nonlinear dynamic analysis of elasto-plastic systems subjected to long- and short-duration records. The analysis results indicate that the frequency of structural collapse for long-duration ground motions is about twice of short-duration ground motions. The frequency of collapse also increases when ductility and stability index increase, but decreases when the period of structure increases. Furthermore, the increase of ground motion duration has no influence on the residual displacement of structures, but nonlinear behavior of structures and gravity load effect increase the residual displacements.

The prediction of nonlinear behavior of shear walls under lateral loads requires simple and precise analytical models representing the nonlinear behavior of the shear walls as well as the experimental results. In recent decades, various analytical models have been proposed by researchers to predict the nonlinear behavior of reinforced concrete shear walls in order to consider the most important behavioral properties of the wall. The model used for analysis and design of the wall should be simple and accurate enough to predict the hysterical behavior. Using the method of fiber section elements can be considered as one of the most accurate methods in wall modeling. But due to the complexity of the modeling and high duration of the analysis, the use of other methods is suggested. Considering the recent progress in modeling nonlinear behavior of materials to provide more accurate behavior against cyclic and seismic loads, these material models can be directly used in modeling of structural systems. In this study, in addition to using the fiber section model, a method based on axial elements is also used. It is implemented by the concept of equivalent spring’s behavior and using the advanced material models in the platform of OpenSees. The results show that the use of this simplified model compared to the fiber method, in addition to reducing the computational cost in both aspects of the duration of modeling and analysis, also satisfies the desired accuracy.