Scientia Iranica
Volume:28 Issue: 5, Sep-Oct 2021

Destination and departure time choices are interrelated decisions that affect urban travel demand estimation. Most previous studies ignore this interrelation and assume that these decisions are independent. Some other studies use a hierarchy structure, while the literature suggests that destination and departure time are selected simultaneously before the commencing of trips. This paper employs copula-based joint modeling to explore the interdependency between destination and departure time choices. The destination choice modeling is developed using a multinomial logit model, and a binary logit model is used for modeling departure time choice. To obtain a better-fitted model, several copula functions are used thereafter; the frank copula is selected for the final model. Results show that there are some common unobserved factors between these decisions by estimating copula dependence parameters with high statistical significance. Furthermore, there are some commonly observed factors, such as socio-demographic and travel characteristics that appear in the utility functions of both models.

Flow in river bends is associated with generation of secondary flows that leads to a rather complicated flow pattern around bridge piers located in the bend. In this study, the flow patterns around rectangular and oblong piers models with collar located at the 90o angle of a 180-degree sharp bend in an experimental plume were investigated. The 3D flow velocity data were collected using a Vectrino velocimeter. The experimental results indicated that the presence of the rectangular pier caused more intense deviation of the streamlines towards the outer bank of the bend. Furthermore, installation of the oblong pier, the maximum secondary flow power and angular velocity were decreased, respectively, by 35% and 45% in comparison with the installation of the rectangular pier. In addition, it was found that the maximum turbulence kinetic energy around the two piers was not significantly different, but by installing the rectangular pier, a region with high values of turbulence kinetic energy is formed at 90° near the inner bank. Also, installation of oblong pier reduces the values of shear stress in comparison with the use of the rectangular pier.

Lattice Boltzmann method (LBM) has emerged as a fast, precise, and efficient numerical solution to solve differential equations. There seems to be a dearth of research regarding the solution for groundwater flow in unconfined aquifer using LBM. Accordingly, in this study, an innovative numerical solution based on LBM was introduced to solve groundwater flow in unconfined aquifers, taking into account D2Q9 scheme. The solutions obtained from the proposed LBM were compared to results stemmed from three different unconfined groundwater problems with known solutions. Both steady and transient conditions for groundwater flow were considered in simulations. It was deduced that the proposed LBM could simulate the unconfined groundwater flow satisfactorily.

In this paper, we introduce a semi-analytic procedure for deriving the response spectrum of the synthetic acceleration records generated using the Double Frequency Model (DFM). DFM is a filtered white noise method for fully non-stationarity synthetic acceleration records. The proposed semi-analytic procedure is based on the theory of the first passage problem, which precludes time and computationally extensive methods e.g. Monte Carlo simulations. Assuming a slowly-varying envelope and evolutionary transfer functions, the procedure for estimating the elastic response of a structure is implemented in both time and frequency domains. Comparing the results of our model with previous models and approximations, we conclude that for a set of 10000 realizations of the DFM model, the semi-analytic model produces less than 10% error for 92% of the realizations. The accuracy of estimations is higher in the short-period compared to the long-period ranges of the response spectrum. Comparing the accuracy of approximations used to arrive at peak factors, results show that Michaelov et al's approximation executed in the frequency domain yields the best results compared to Poisson or Vanmarcke’s procedures.

In this paper, comparing the results obtained from monitoring and numerical modeling carried out to significantly help to analyze the stability of embankment dams. comparison of these analyses with the fact they are the precise instrument results, in addition to validating the numerical analysis, is the basis for performing a back analysis to obtain accurate Geotechnical parameters. Accordingly, the final parameters extracted by the back calculation method of materials in the body of earth dams to predict dam performance are the main purpose of this study. In this study, the Rudbar Lurestan dam was selected as a case study and the results of its instrumentation were processed and analyzed. Then, the monitoring results of one of the sections (C5-C5), were selected for analysis and comparison with the numerical model. At first, the model was modeled in a static state at the time of construction. FLAC2D V7 software was used for modeling and simulation was done using Mohr-Coulomb behavioral model and the initial parameters of the material. The observations show that there is a good agreement between the monitoring results of the modeling and also final parameters obtained from the back calculation show a growing trend in most parameters.

This study investigates the effects of ageing simulation on the physical properties of nano-clay modified bitumens (NCMBs). Bitumen with penetration grade 60/70 is modified with 0, 2 and 4% nano-clay, and the consistency of the samples is characterized using the penetration, softening point, and viscosity tests before and after aging. Atomic force microscopy (AFM) is used to evaluate the surface roughness and tip deflection of the modified bitumens. The results of the consistency tests revealed that the incorporation of nano-clay up to 4% NC resulted in increased hardness of the modified bitumen, and all modified bitumens were affected by ageing; the results for surface roughness and tip deflection showed a similar trend and 2NCMB exhibited the lowest roughness and highest tip deflection. However, the results of consistency tests and AFM for ageing sensitivity of the bitumens are different, indicating that the addition of nano-clay has an effect on the modified bitumen but not on the unmodified bitumen. There is a high correlation between the results of consistency tests, surface roughness and tip deflection, which implies that AFM is able to identify any changes in the fluidity of the bitumen. However, the correlation between tip deflection and penetration is very weak.

Tendency of engineers toward continuous welded rails and its beneficial effects have raised the importance of lateral stability in railways. Small radius curves of tracks and temperature variations cause lateral force in tracks. There are different procedures to increase the lateral resistance of railway tracks. These methods are implemented using different materials by changing the size, geometry and dimensions of track components, especially sleepers. Although there are studies conducted on winged sleepers, utilizing mid-winged sleepers with modified geometry and dimension is considered in this research. Winged sleepers considerably increase the lateral resistance of tracks. However, some operational problems exist in its maintenance as the collision between wings of sleepers and tamping machine tines occurs. In this study, a number of experimental tests and numerical modeling were conducted on the lateral resistance of conventional and mid-winged sleepers. The lateral resistant force of tracks was measured by track panel loading test and single sleeper push test. The results revealed that by changing the conventional track to the mid-winged track, the lateral resistance increased considerably. The mid-winged panels tests, single mid-winged sleeper tests and numerical modeling indicate 58% to 64% increase in lateral resistance of mid-wing track compared with the conventional tracks.

This study investigated the effects of applying a uniform magnetic field (UMF) of flux density 500 mili Tesla (mT) to fresh and hardened concrete specimens with steel fibers at volume ratios of 1 and 1.5% on mechanical and microstructural properties. To attain these objectives, compressive and splitting tensile strengths tests were carried out on the specimens with steel fiber-reinforced concrete (SFRC) at 28 days. Furthermore, the microstructure of SFRC, subjected to the UMF, was assessed via scanning electron microscopy (SEM) images. An electromagnetic instrument, capable of generating a density of 500 mT, was used to produce UMF. Finally, a model equation was proposed to predict the splitting tensile strength of SFRC subjected to the UMF as a function of its compressive strength. The application of UMF to SFRC specimens incorporating 1.5% steel fibers revealed an increase in both compressive and splitting tensile strengths up to about 18.2% and 9.5%, respectively. The SEM analysis indicated that the UMF enhanced the cement hydration process, which is responsible for the higher mechanical strength development of SFRC compared to the control specimen.

This paper studies the mechanical behavior of two types of KLP sleeper, namely low-density polyethylene sleeper (LDPE-16) and high-density polyethylene sleeper (HDPE-25) with 16 mm and 25 mm steel bars diameter, respectively, in static, dynamic and longtime static three points bending moment tests. Therefore, HDPE-25 and LDPE-16 with six strain gauges mounted on their steel bars, were manufactured to assess their mechanical responses. Moreover, a finite element method (FEM) model is developed to perform a sensitivity analysis based on different diameters of steel bars for HDPE with 16 mm (HDPE-16) and LDPE with 25 mm (LDPE-25). The results show that steel bars of LDPE-16 yielded under 4 hours of 30 kN load, while, HDPE-25 shows significant resistance. Numerical results show that HDPE-25 is overdesigned and can be replaced by LDPE-25 which has lower weight and price. The natural frequencies of HDPE-25 are almost 16%, 19%, 16% and 33% higher than the three first bending frequencies and first torsion frequency of LDPE-25, respectively, that proves the better performance of LDPE-25 in case of preventing resonance. Moreover, the bending modulus HDPE-25 is almost 42%, 45% and 65% is higher than HDPE-16, LDPE-25 and LDPE-16, respectively.

This study investigated the flexural strength of geopolymer concrete (GPC) beams produced with ground granulated blast furnace slag (GGBFS) and corn cob ash (CCA). The design of experiment (DOE), Box-Behnken design (BBD) of the response surface methodology (RSM), was used to optimize the strength. GGBFS was replaced at 0, 20, and 40 wt. % of CCA. The mixes were activated with 14 molar concentration (14 M) of both sodium silicate (Na2SiO3) and sodium hydroxide (NaOH) solutions. The mix design properties such as alkaline liquid to binder ratio, binder to aggregate ratio, binder ratio, and curing time were statistically applied as continuous (independent) variables to optimize the response factor (flexural strength). Compared with the control sample (Portland cement concrete), GPC exhibited higher compressive and flexural strengths at up to 40 wt. % of CCA replacement. The models predicted the response of flexural strength with less than 5% variability. Besides, the correlation between the experimental and the optimized flexural strength yielded a high precision with 99.6% “R2”. Therefore, this study's response models would be advantageous in the optimization of mix design proportions for obtaining target flexural strength of GPC beams produced with GGBFS and CCA.

In the performance-based optimal seismic design, one attempts to obtain structural design variables to meet the minimum objective function satisfying the strength-based and performance-based constraints. A limited number of studies have been conducted on the performance-based optimal seismic design of reinforced concrete frames. On the other hand, due to the importance of environmental impacts, further study is necessary for the design of RC buildings with the aim of reducing CO2 emissions. In this study, a computational procedure is developed for performance-based optimal seismic design of RC frames with prismatic and non-prismatic beams. The objective functions consist of minimizing the cost and CO2 emissions. Nonlinear pushover analysis is performed for analysis of the structures. The described procedure is applied to 4-story reinforced concrete frames and the relationship between optimal cost and optimal CO2 emissions is studied for frames with prismatic beams and frames with non-prismatic beams.