Xuan Dai Nguyen

Chuyển vị dư là đại lượng quan trọng phản ánh khả năng phục hồi của kết cấu dưới tác dụng của tải trọng động, do ảnh hưởng trực tiếp đến khả năng khai thác, nhu cầu cải tạo và phục hồi sau tác dụng. Các phần tử kết cấu có ứng xử phi tuyến dạng đường cong trễ song tuyến tính có khả năng tiêu tán năng lượng lớn nhưng thường tồn tại biến dạng dư đáng kể sau khi chịu tải trọng. Ngược lại, các hệ kết cấu có ứng xử phi tuyến dạng đường cong trễ hình cờ (Flag-shaped) sở hữu khả năng tự phục hồi về vị trí cân bằng, giúp giảm đáng kể chuyển vị dư. Nghiên cứu này nhằm đánh giá ứng xử động lực học và khả năng tự phục hồi của phần tử kết cấu có ứng xử phi tuyến dạng cờ so với mô hình ứng xử song tuyến tính. Hệ kết cấu một bậc tự do được sử dụng để mô phỏng ứng xử phi tuyến thông qua các phần tử lò xo dẻo phi tuyến. Hai dạng tải trọng động điển hình được lựa chọn phân tích gồm tải trọng điều hòa và tải trọng xung. Phân tích lịch sử thời gian phi tuyến được thực hiện nhằm đánh giá phản ứng chuyển vị, đặc tính đường trễ, chuyển vị dư và chỉ số tự phục hồi của kết cấu. Kết quả cho thấy sự khác biệt rõ rệt giữa hai mô hình. Kết cấu có mô hình song tuyến tính tồn tại giá trị chuyển vị dư đáng kể sau khi dỡ tải. Ngược lại, mô hình ứng xử phi tuyến dạng cờ thể hiện khả năng tự phục hồi vượt trội, cho phép hệ kết cấu quay trở lại gần vị trí cân bằng ban đầu dưới cả hai dạng tải trọng khảo sát. Các kết quả thu được cho thấy tiềm năng ứng dụng của các hệ kết cấu tự phục hồi trong việc nâng cao khả năng phục hồi và hiệu năng làm việc của công trình dưới tác động của tải trọng động.

Seismic base isolation (SBI) is widely recognized as a highly effective solution for earthquake-resistant design, extensively implemented in high-seismicity regions, and increasingly adopted in areas with moderate seismic activity. While structural analysis software provides various behavior models to simulate SBI elements, the suitability of these models for specific analytical applications remains unclear. This study conducts a numerical analysis to assess the impact of different isolator behavior models - including equivalent linear, plastic Wen, bilinear, and rubber isolators - on the seismic response of isolated buildings. Nonlinear time-history analyses are performed on a typical seismically isolated building using the 1994 Northridge earthquake record, scaled to match the target spectrum. The seismic isolators are modeled using link elements with constitutive parameters corresponding to the selected models. Key response parameters, such as isolator behavior, lateral displacements, and base shear forces, are analyzed to compare the structural performance across different isolator models. The results indicate that nonlinear models (plastic Wen, bilinear, and rubber) yield comparable and realistic seismic responses, whereas the linear model significantly overestimates displacements and forces. These findings highlight the limitations of linear assumptions in seismic analysis and underscore the necessity of employing nonlinear models for a more accurate evaluation of structural behavior during earthquakes.

Bracing systems have emerged as an effective solution for enhancing the resilience of steel frame structures under dynamic loads. Conventional steel bracings dissipate energy through bilinear hysteresis responses, ensuring structural safety but often resulting in significant residual displacements after strong earthquake impacts that necessitate costly maintenance and repairs. Shape memory alloys (SMAs), particularly superelastic alloys, have drawn interest due to their potential for both energy dissipation and self-centering capabilities. This study explores the use of SMA-based superelastic bracing systems in steel frame structures subject to seismic loading. Using nonlinear time-history analyses performed in ETABS, the study investigates the comparative response of conventional and superelastic bracing systems. The findings of this research suggest that SMA-based bracing systems may offer advantages in reducing column demands and residual displacements, indicating potential for further development in seismic design applications.

The time-history analysis is an important approach in the seismic-resistant design of structures, especially in the increasing use of performance-based seismic designs and seismic protection systems exhibiting nonlinear responses. This method requires input acceleration time-histories that match the seismic characteristics of the site and satisfy the application requirements specified in the design code. This paper focuses on the procedure of selection and scaling artificial accelerograms and natural accelerograms to meet the requirements of Vietnamese code TCVN 9386-2012. The determination of the horizontal elastic response spectra, the application requirements, and the matching criteria specified in the code are first outlined. A proposed procedure of selection and calibration of the time-history accelerations is presented in detail. A typical example is conducted to select and scale a set of accelerograms for a specific seismic region in Vietnam to illustrate the proposed method. The concordance of the calibrated accelerations is validated by matching the response spectral of considered acceleration and the target spectrum and comparing seismic responses between the modal spectral analysis and time-history analysis of two typical linear and nonlinear structures. The results show the high reliability of the proposed procedure and great efficiency in matching the acceleration.

Performance-based design is currently widely applied in the seismic-resistant design of building structures. This approach allows the presence of inelastic deformation in several structural components to dissipate the seismic energy transferred to the building. The paper aims to investigate the inelastic behavior of reinforced concrete structures of a multi- story building subjected to earthquakes. Kent and Park’s model is used to simulate the nonlinear behavior of concrete, including the effect of the confined concrete of column structures. The nonlinear model of Giuffre-Menegotto-Pinto presents the reinforcement behavior. The parameters of the materials used in the modeling are determined from a calibration of the available experimental results by the optimal parameter analysis method. The building structure is modeled by OpenSees software, analyzed with three time-history acceleration records that are selected and scaled to match the target spectrum of a specific location in Vietnam. The seismic inelastic behavior of structures is investigated by the strain-stress relationship response of typical cross-sections of considered columns. The obtained results show the high efficiency of the optimal parameter analysis to accurately identify the behavior model parameters of the reinforcement. Further, the considered models present good effectiveness for the inelastic analysis of reinforced concrete buildings.

Single-mode spectral analysis (SMSA) is a simple procedure efficiently used to evaluate the seismic demands of base-isolated bridges, particularly suitable for the design of simple bridges or preliminary design of complex bridges. However, bridge design codes, notably the Canadian code CSA-S6:19 (CSA-S6 2019), specify many limitations on the use of the method for final design. This paper evaluates the performance of SMSA and the efficiency of its limits of application as specified in the current codes through the results of a parametric study. The hysteretic properties of seismic isolation systems and the stiffness of bridge substructures are varied. Seismic demands predicted by SMSA and nonlinear time-history analyses (NLTHAs) are then compared, both inside and outside the current specified limit range in CSA-S6:19 (CSA-S6 2019). Results show that the most effective application conditions are those related to the maximum equivalent viscous damping and minimum restoring force. The upper limits of the effective period and the post-elastic period can be ignored. Further, to complement SMSA, a relation is proposed to estimate the expected residual displacement as a function of the restoring force at the design displacement. Regression relations allowing estimating the expected mean and confidence interval of the relative displacement deviation predicted by SMSA from that based on NLTHA are also proposed.