Journal of Numerical Methods in Civil Engineering
Volume:4 Issue: 2, Dec 2019

This paper investigates the applicabilty of an innovative bracing, called Articulated Quadrilateral (AQ) bracing system, which uses shape memory alloys (SMAs), for retroffiting low-rise to high-rise vulnerable SMFRs against strong ground motions. The paper investigates brace fundamental engineering characteristics, design of the system and also configuration of the brace (the proportion of SMA wire, C-shape dissipator and Post-tensioning tendons). OpenSees program is utilized for nonlinear dynamic finite element analysis and the validation of modeling using data from full-scale experimental tests performed by Speicher et al. at Georgia Institute of technology. Using 3-, 9- and 20-story steel moment resisting frames from the SAC phase II project, nonlinear pushover, incremental dynamic analysis, and fragility analysis of frames with and without AQ bracing were conducted using FEMA P695 far-field ground acceleration records. Results show that by retrofitting MRF system with AQ bracing, strength of the buildings increases up to 40%. Also, bracing of the frames yields more uniform drift distribution which reduces the likelihood of soft story formation.

The experience of past prominent earthquakes establishes the fact that the structure’s catastrophes and casualties can be dramatically decreased through the use of self-centering systems. A promising post-tensioned self-centering yielding braced system (PT-SCYBS) has been developed, comprising of two main components, including the post-tensioned wires, exhibiting the desirable self-centering properties, and steel bars, providing the energy dissipation capacity. The structural application of such systems is expeditiously expanding due to their capabilities of not only reducing the residual deformations of the structures but also improving the structure’s performance level. As such, identifying optimal design and proper placement of the proposed device in the structure is of crucial importance. In this paper, the mechanics of the proposed system, as well as a simple and efficient approach for determining the optimal design of the PT-SCYBS, have been proposed. Numerical models have been employed to examine the effect of various configurations of the device on the hysteretic behavior of the proposed PT-SCYBS. Nonlinear static and dynamic analyses are performed on the seismically deficient 3- and 9-story moment resisting frames (MRFs), enhanced with the optimal placement of the PT-SCYBS. Comparing the results of the PT-SCYBS buildings and MRFs, it can be concluded that the residual drift decreased by 96% and 77% for the 3- and 9-story buildings, respectively. As such, the optimal design of the proposed system in the building causes notably lower residual drifts as compared with the MRF buildings, resulting in enhanced seismic performance.

Input-energy is the amount of energy that is imposed by an earthquake on a structure and its correlation with structural damage has been studied and demonstrated by many researchers. Since studies concerning seismic energy in multi-degree-of-freedom systems are relatively limited compared with single-degree-of-freedom, in this paper, firstly a theoretical exact method is discussed to calculate the input-energy of the multi-degree-of-freedom elastic oscillators. It is proved that unlike the general rule in mechanics, the superposition theorem is valid for input-energy in conventional modal analysis. To estimate the input-energy, an approach based on PHSA to predict the Fourier amplitude spectrum, is proposed. The results indicate that the modal mass ratio is not the only decisive parameter in input-energy. Modal input-energy decomposition also confirms the possibility of greater input-energy of higher modes in comparison with fundamental ones or the ones with the higher mass participation ratio, especially for tall buildings located in the near-field seismic zones.

Among the methods used to design the tunnel, the Q-system is a comprehensive method that has attracted the attention of many researchers today. However, the limitations of the Q-system make it impossible to access all the required parameters as well as the time and cost of them,  which has made it impossible to classify the rock mass using the Q-system. This paper attempts to predict the value of Q by parameters that have the highest coefficient of importance in the value of Q, using the Gene Expression Programming (GEP) technique. The most effective parameters involved in the Q value have been identified using Pearson correlation analysis (PCA), and then three different input models have been used to obtain Q value so that they are more closely related to experimental values. A total number of 159 experimental data were used for training and testing of the models, respectively. The innovation of this paper is that instead of 6 parameters, only three influential ones were used for determining the value of Q. Using the three parameters RQD, Jn and Ja, which have been determined as the most effective parameters and applying Pearson correlation analysis method, the value of Q can be determined with an acceptable approximation. In the suggested relation, the coefficients of determination (R2), root mean square error (RMSE), BIAS and the scatter index (SI) obtained were 0.917, 2.31, 1.74 and 0.43, respectively that show the new equation presented by GEP, can be undoubtedly used to predict the value of Q.

This paper investigates the effects of spatially variable (non-uniform) seismic excitation incorporating incoherency effect on earthquake-induced stresses of arch dams. Coherency functions reflect the waveform variation between two different stations, which decays with an increment of distance and frequency. The response spectrum compatible non-uniform ground motions are generated utilizing the coherency functions. Besides, to study the valley shape effects and dam height on seismic responses, V-shaped and U- shaped valleys with different heights are considered. Finite element models of typical arch dams are provided, including the relevant compressible reservoirs and surrounding massed foundation rocks. Dynamic analyses are carried out for uniform and non-uniform excitations. Comparing the stress magnitudes revealed that, non-uniform ground motion inputs considering coherency functions lead to less tensile and compressive stresses than the uniform ones(identical excitation across the supports). Moreover, the stress distribution pattern depends on the utilized coherency function. Finally, the results demonstrated that the magnitude of maximum tensile stress is lower in the V-shaped valley as a general trend. Additionally, due to uniform excitation, the increase of dam height leads to an increment of tensile stresses.

In this study, the effects of the vertical component of ground motion on both the safety factor of wedge and response of dam having a foundation with joints are investigated. The Bakhtiari arch dam, with 6 wedges at each of its abutments, is chosen as a case study. The safety factor of dam abutments is obtained by the implementation of time history analysis and Londe limit equilibrium method. The thrust forces are calculated using ABAQUS, a commercial finite element software package. The safety factors of wedges are obtained using the code written within MATLAB. The results indicate that considering the vertical component of the earthquake decreases the safety factors of the wedges considerably. Moreover, the vertical component of ground motion plays a key role in the nonlinear behavior of the dam having a jointed foundation.