Journal of Numerical Methods in Civil Engineering
Volume:6 Issue: 2, Dec 2021

The access tunnel of the Urmia Lake for water transfer and restoration project (Kani Sib) is located in the south of West Azerbaijan Province, Iran. Part of this tunnel is located on weak and very loose soil, which in some areas, cannot be stabilized, despite the use of step drilling, and may lead to  ceiling collapse, face collapse and even deformation in support system. In these cases, it is necessary to adopt the pre-support method of the umbrella arch. Tunnel stability analysis is one of the important factors in tunnel design and support system. Indeed, the type of support system is chosen according to the required stability and permitted displacement for the tunnel. In the present article, first the permitted displacement for the tunnel is calculated by Sakurai correlation. Then, the ground reaction curve is plotted using the numerical method of finite difference, namely, FLAC3D software, and the convergence-confinement method (CCM) is used to determine the acting instant for the support system. Finally, the safety level of the proposed support system is investigated considering different safety factors. The results of this study indicate that the Sakurai displacement correlation is more reliable than the other graphs presented. The results derived from numerical modeling are verified accurately against visual observation and instrumentation results. A suitable umbrella arch pre-support system with Lattice and Shotcrete support system is recommended. The umbrella arch pre-support system encompasses forepoling pipes of 90 mm diameter with 9 m in length and 2.5 m in overlap length.

This paper introduces a procedure to risk-based optimal design of fluid viscous dampers (FVDs). To this end, the exceedance probability of specific performance level during the design lifetime as a safety criterion of the entire building is intended to be minimized. This, along with the minimization of the total damping coefficient of FVDs as the cost criterion of the dissipation system, are the considered objective functions. The damping coefficient of FVDs have been considered as design variables and the efficient configurations of damper properties over the height of the building have been determined. A multi-objective optimization framework using the non-dominated sorting genetic algorithm version II (NSGA-II) has been employed to solve the optimization problems and determine the set of Pareto optimal solutions. Linear and nonlinear FVDs with different capacities have been designed for an eight-story shear-type building with bilinear elastic-plastic stiffness behavior under 20 real earthquakes. The results show that the optimal FVDs reduce the seismic response and fragility of the building, while limiting the dampers’ cost.

In seismic design procedure, the idea of considering uniform inelastic behavior for all stories displaces materials from unnecessary locations to stories needed for damage reduction and leads to a more economic design. In this paper, a parametric study was done to assess the seismic performance of structures designed based on uniform ductility pattern. For this purpose, three artificial records whose response spectrums were matched to the codified design spectrum, were considered. Using an iterative procedure, the tuned strength patterns were obtained for a number of models with various natural periods, ductility ratios, behavior coefficients and stiffness distributions. It can be seen that the strength of stories corresponding to uniform ductility pattern decreases in all stories in comparison with distribution recommended by Standard-2800 especially for middle stories and the total strength of structures as a weight index decreases. Also, a new equation was developed by regression analysis to determine the coefficient of story strength. Assessment of the performance of the structures with tuned story strength distribution under real ground motions showed less dispersion for ductility pattern in comparison with structures which were designed according to Standard-2800. Also, it was seen that if the amplitude of the earthquake response spectrum is larger than the design spectrum, the dispersion of the ductility values over the stories increases significantly. The situation becomes critical for lower stories when the amplitude of the earthquake response spectrum is larger than the design spectrum at periods higher than the fundamental period of the structure.

Dams are important structures that are mainly constructed for water and energy supply. Dam break creates a huge flow that leads to flooding in areas downstream. Therefore, determining characteristics of this flow, including the flow depth and wave propagation velocity, is of great importance. In this research, the simultaneous effects of reservoir geometries and downstream obstacles on hydrodynamic characteristics of the flow caused by dam break have been investigated using three-dimensional numerical modeling. For this purpose, six reservoirs with different geometries, including wide, trapezoidal, L-shaped, long, hexagonal, and octagonal reservoirs, with downstream dry beds have been considered. The results of three-dimensional numerical modeling indicate that the reservoir geometric shape has a severe effect on the flow, since it plays a determining role in the inlet flow to the downstream channel. Downstream obstacles also affect the flow caused by dam break, but their effects are local and significant only over a certain length behind obstacles. This length is related to the reservoir shape and varies within the range of 19.5 to 22.5 times the pier (obstacle) diameter. Thus, the largest length in which the local effects are significant is observed in the wide reservoir, which is approximately 22.5 times the pier diameter. Meanwhile, the minimum length is related to the long reservoir, which is 19.5 times the pier diameter.

Compared to traditional methods based on mean response evaluation of seismic parameters with significant confidence margin, the growing use of the new generation of performance-based design methods, which are based on loss and financial assessment, necessitates an increase in accuracy and reliability in probabilistic evaluation of structural response for all values of seismic parameters. Even with the same limited number of common nonlinear analyses, utilizing the Bayesian approach, which allows the use of diverse and even inaccurate data to form beliefs, is a powerful method to predict and enhance seismic response results. In this paper, the practicability of using linear analysis data in a Bayesian inference model to predict nonlinear responses is evaluated. A 20-story reinforced concrete special moment resisting frame is being considered, and a Bayesian model for prediction of the maximum story drift and the peak floor acceleration has been investigated. The Bayesian model was developed on linear results and finally updated with a limited number of nonlinear results. The predictability power of predictors, Bayesian model comparison among different likelihood functions, and common diagnostics tools in numerical solution of the Bayesian model developed on linear results, have all been examined. The results demonstrate a significant improvement in the outcomes, while proving the practicability of developing a stable and reliable model based on linear analysis data.

Today, one of the most important engineering requirements is to ensure optimal design with best possible seismic performance of structures. To this end, the present paper aims to apply the optimization process for the design of the through-bolt steel beam connection to the concrete-filled steel tube (CFST) column reinforced with rib plates. This study employs a multi-level cross-entropy optimizer (MCEO) algorithm along with response surface method (RSM) and finite element method (FEM) to establish the objective functions and constraints. The variables considered are the rib plate geometry and the steel and concrete strength parameters. In order to overcome problems, optimization is performed to increase the load-bearing capacity of the connection and to satisfy the constraints. Adopting this smart solution eliminates the need to connect finite elements for loop optimization and provides an explicit function for system performance. The results show that a very accurate analytical model can be developed to describe system performance using this process. This solution can optimize the performance of several systems that require a large amount of analysis and solve a wide range of structural optimization problems.

The present study is focused on fragility analysis of steel moment resisting frames (MRFs) incorporating nonlinear soil-structure interaction (SSI) effects. To this end, incremental dynamic analyses are performed using a suit of real ground motion records. A MRF structure is considered which is supported by single footings. To evaluate the SSI effects, four cases are compared including (i) fixed base, (ii) linear SSI and uncoupled footings (i.e. without tie beams), (iii) nonlinear SSI and uncoupled footings and (iv) nonlinear SSI and coupled footings (i.e. with tie beams). The SSI effect is represented by modified Beam-on-nonlinear Winkler foundation (BNWF) model. An appropriate structural damage index based on summation of cumulative plastic hinges’ rotations is employed. The seismic fragility curves of the structures are derived and compared for the above-mentioned cases. The results show that nonlinear SSI has significant effects on seismic fragility curves. Evidently, these effects are mitigating especially in case of footings with tie beams. To assess the effect of ground motion type, fragility curves are also derived for each type of ground motion comparatively. It is observed that near field pulselike records are more destructive than far field or near field no-pulse records in terms of fragility curves. Overall, based on findings of this study, the obtained modified fragility curves are supposed to be helpful for the earthquake engineers to conduct more realistic loss estimations considering SSI effects. These modification factors need to be generalized with respect to a variety of structural systems, site types and foundation configuration.

It is well accepted that an urban region's seismic resilience is directly related to the seismic resilience of the local water systems. Pipelines having low earthquake resistance generally include old pipes and those susceptible to corrosion. The seismic vulnerability of the water transmission pipelines can be evaluated along with the geologic hazards such as landslides, liquefaction, fault movement, etc. In this study, GIS-based analyses are implemented for one of Tehran's main water transmission pipelines, which transfer Mamloo Dam water to Tehran's southern regions, by considering the four most probable earthquake scenarios to evaluate post-earthquake serviceability of the studied pipeline. Transient Ground Deformation (TGD) due to seismic wave propagation, and also Permanent Ground Deformation (PGD), which may result in liquefaction (lateral spreading, and ground settlement) and landslide, are regarded as destructive earthquake effects on the water transmission pipelines. A restoration curve is also developed for the worst scenario to investigate the adequate post-earthquake water supply throughout the service area and ensure rapid system recovery. Results show that the water serviceability index regarding the worst earthquake scenario (Rey fault activated) is 28%, which means that more than 72% of the study area's population will experience severe disruption of water availability in a potential earthquake.