Scientia Iranica
Volume:28 Issue: 1, Jan-Feb 2021

Placement of bridge piers along rivers creates a complex 3D flow pattern which disrupts normal river flow and the resulted turbulence erodes alluvial sediments around the pier. In this research, SSIIM model has been used to simulate flow and scour patterns when no pier, one pier, and triple piers are placed at a 180 degrees bend and the results are compared with those of experiments. The piers are vertical. The simulated channel was 1 m wide with a U-turn having the relative curvature radius of 2 carrying a volumetric flow of 70 l/s the flow depth at the beginning of the bend is 18 cm. Results showed that SSIIM is well capable of simulating bed form changes and flow patterns such that at the bend with triple piers error in maximum scour and sedimentation was only 4%, in maximum transverse velocity 12%, in maximum longitudinal velocity 13%, and in maximum vertical velocity 19%. In general, SSIIM model satisfactorily simulates the location and value of local scour arising from single and series piers in numerical simulation of the flow and scour. In flow pattern simulation, the errors and differences are greater under moving bed conditions than a rigid bed.

Magnetorheological elastomer (MRE) materials are widely used in the development of smart isolators and absorbers due to their stiffness and damping adaptability. This study investigates the performance of MRE isolators and elastomeric isolators from near-field and far-field earthquakes in benchmark base isolation buildings. All earthquakes are simultaneously applied in two horizontal directions to the horizontal plan. Vertical earthquakes are not attended in the dynamical analysis of benchmark base isolation buildings. For making an isolator model, the effect of bilateral interaction has been considered. The behaviors of MRE isolators and MR dampers are compared. To this end, three control systems including adaptive isolator, passive isolator and semi-active MR damper are considered. The results show that the MRE isolator has a better performance in near-field earthquakes due to its variable stiffness and damping, as compared to the elastomeric isolator. The semi-active MR damper for both far-field and near-field earthquakes has a better control to reduce base displacement, but causes to increase floor accelerations, story drifts and story shear. According to the results of this study, it can be observed that MRE isolator can be used instead of MR damper. The MRE isolator can reduce the base displacement without increasing other responses.

To reduce the financial pressure on local governments and improve operational efficiency, the Chinese government has introduced the Public-Private Partnership (PPP) model for the development of infrastructure. However, the government credit risk generated by PPP projects may damage private and public interests. Therefore, this study evaluates the government credit risks of non-profit PPP projects by using a systematic and rounded model that employs interval value hesitant fuzzy sets and intuitionistic fuzzy hierarchical analysis (IVHFS-IFAHP). This study employs the perspective of the private sector to establish risk index systems using the dimensions of Wuli-Shili-Renli (WSR) methodology. IFAHP is used to collect expert opinions for the comparison of the importance of the indexes and establish an intuitionistic fuzzy judgment matrix in order to calculate the weight of each index. IVHFSs are adopted to obtain the scores of the indexes coupled with the results acquired by IFAHP to produce comprehensive scores of the indexes. The feasibility and practicability of this method were verified empirically. This study provides an effective method for the private sector to evaluate the government credit risks of non-profit PPP projects and provides a reference for participation in PPP projects.

In this paper, strong ground mot (SGM) parameters are calculated using discrete wavelet transform (DWT) in different kinds of soils with different magnitudes. The main earthquake record (MER) is divided into approximation and detailed signals using wavelet transform with denoising. The high and low frequencies of MER are separated from each other. Previous studies showed that the approximation signal has the greatest effect on dynamic response and it is very similar to the main signal. Then SGM parameters of the new signal are calculated by DWT decomposition. This process continues over five levels and, in each level, SGM parameters are calculated and compared with the MER and its error percentage is presented. In DWT with the denoising method, the curve becomes softer such that the calculation time reduces. Results show that the error percentage in the first two levels is less than 1% and for the third level, this index is less than 3%. In addition, the reduction percentage of calculation time is 1%, 4%, and 8%, respectively, in the first to third levels. The best result is relative to the third decomposition level in which error value as well as computational time reduction is nearly 3% and 8%.

Eccentrically braced frames (EBF) are lateral resisting systems with appropriate ductility and strength against earthquakes. An important kind of arranging such systems, recommended by Popov and also presented in AISC, is eccentrically split-X bracing. The axial force applied to the beam outside link beam is reduced causing the improvement of the behaviour of this type of bracing. In this research, for the first time, ductility factor, overstrength factor and response modification factor of eccentrically split-X braces are investigated through nonlinear static and incremental dynamic analyses and fragility curves are presented for different ratios of link beam length to span length. For this purpose, three buildings, 2-, 6- and 10-storey structures with the ratios of link beam length to span length (e/L) of 0.05, 0.1, 0.15 and 0.2 are considered. Ductility factor of R𝛍=3.55, overstrength factor of Rs = 2.31 and response modification factor of RLRFD =8.06 are calculated under 10 earthquake records. It is concluded that the most appropriate values of e/L ratio in the eccentrically split-X bracing are 0.1 for tall structures and 0.05 for small ones. According to the log-normal distribution, the fragility curves are also plotted considering collapse prevention (CP) and immediate occupancy (IO) performance levels.

This paper presents the results of experimental and numerical study on a proposed exterior semi-precast moment-resisting concrete connection. Steel linkage element connects precast concrete beam to column in two cases of bolted and welded connections. These connections were compared to monolithic connection based on stiffness, strength, energy dissipation capacity and ductility factor. An accurate 3D nonlinear finite element model has been simulated to study the behavior of these connections. A good agreement was observed between numerical results and experimental ones in behavior and damage mode. Although, all samples satisfied all the code criteria, but the trend of failure in bolted and welded connection was observed on the weld and bolts area, at the connection zone between beam and steel linkage. However, in monolithic connection, the yielding was observed in rebars and crushing of concrete at the end of beam. The initial stiffness of bolted and welded connection specimen was less than that of monolithic connection specimen. But, the beam moment capacity factors of these samples were a little more than monolithic sample; so, the ductility factors of them were a little less than that of monolithic sample.

Bearing capacity (BC) test results are presented for bounded and unbounded twin circular footings on unreinforced and geocell-reinforced (GCR) sand. Analysis of the results demonstrate material, scale and size effects on the BC for a given combination in materials (sand-GCR), footing (single-twin) and the problem geometric dimensions. The significance of these combinations on BC and settlements is used to arrive at suitably modified BC factors for design that could be generalized. Plots given relative to reference cases for which BC design solutions are available provide correction factors to modify classical BC equations. Values of the BC and BC factors represent the lumped effect of all or separate problem variables including scale and any experimental limitations. Compared with previous works, these results give deeper critical depths for twin footings on unreinforced and GCR sand and BC higher than 4 times the reference case.

One of the most prevalent ground motion Intensity Measures (IMs) is the spectral acceleration at the fundamental period of the structure. Previous research has shown that vectorizing scalar IMs leads to a more reliable structural response, particularly in the nonlinear region and near collapse. Furthermore, nonlinear behavior of ductile structures results in an elongation in the "effective period" of the structure. Therefore, this paper proposes a new approach for selecting ground motion records considering the effect of spectral shape and period elongation. This method contains two disaggregation analyses at the fundamental and elongated period of the structure. Nonlinear dynamic analysis is conducted on a set of reinforced concrete moment resisting frames designed based on ACI 318-05 as representatives of modern structures. Results show a considerable decrease in the median collapse prediction, margin against collapse and dispersion of the structural response. The presented approach can give a better prediction for the vulnerability of the structure toward collapse.

This study attempts to reliably quantify the seismic response parameters of the steel diagrid structural systems. In that regard, in addition to the conventional static pushover analysis (SPA), the dynamic pushover analysis (DPA) based on the incremental dynamic analysis (IDA) technique is also considered. FEMA P-695 recommends a methodology for establishing seismic performance factors (SPFs). The present study tries to propose a simpler framework for estimating and validating SPFs while applying the concepts of FEMA P-695 guideline. The results show that the R-factors obtained with the SPA procedure for steel diagrid systems are conservative and the IDA-based probabilistic method gives a more rational value for the R coefficient. Furthermore, the proposed simplified method has good agreement with FEMA P-695 in predicting the collapse capacity of diagrid models.

In recent years, researchers have paid specific attention to safety, air pollution, and energy consumption, which are all affected by the vehicle type. The primary objective of this study is to achieve a thorough understanding of the relationships between transportation attitudes, safety attitudes, personality, as well as lifestyle and vehicle type for the first time in Iran, as a developing country. For this purpose, an interview questionnaire was designed, and 740 participants were interviewed in Tehran. Vehicle types are categorized into seven groups: mini, compact, medium, heavy, SUV, pickup, and motorcycle. In the first stage, factor analysis was used to identify the latent variables regarding transportation attitudes, safety attitudes, personality, and lifestyle. In the second step, by using these latent variables, individuals are classified, and a two-level clustering is formed. The correlation analysis performed in the third step shows a significant relationship between each category of people and the type of car. Likewise, the analyses carried out in this study reveal the significant relations between the vehicle type (classified based on fuel economy level, production country, vehicle emission, and vintage) and the attitudes of its owner. This information is beneficial to both vehicle manufacturers and transportation planners.

Soil deposits may experience different hydraulic states in terms of suction (ψ) and degree of saturation (Sr) while subjected to large strain caused by dynamic loading during earthquakes. This phenomenon is investigated by considering the effects of initial ψ, initial Sr, sample preparation method, drainage condition, and cycle number (N) on dynamic properties. Shear modulus (G) and damping ratio (D) are specifically studied for an unsaturated sand using a modified cyclic simple shear device for all zones of soil water retention curve (SWRC). Results revealed the relative significance of both ψ and Sr in relation to SWRC on observed trends in G and D. Through analyzing some results from the literature, it was shown that most of the data follow a sigmoid function in the plane of normalized stiffness (G/Gsat) versus suction normalized to the air entry value. It was also revealed that minimum damping is not in conjugation with maximum stiffness due to the contribution of other mechanisms in damping like inter-particle water lubrication. Comparison between results of two testing methods indicated a notable influence of initial fabric on G. Furthermore, the overall trends in G and D against N are marginally influenced by initial ψ and Sr.

In transportation literature, User Equilibrium (UE) has been widely studied since early 1950’s, many studies of which define equilibrium flow of traffic for uncapacitated networks based on Wardrop’s first principle, implying also a Nash Equilibrium (NE). Although, in general, the two equilibria (UE and NE) are not explicitly the same, they are shown to be equivalent under special conditions, for uncapacitated UE, when volume-delay functions are separable, continuous, non-decreasing and non-negative. A good deal of research is devoted to explain UE in capacitated networks based on Wardrop’s first principle and the concept of generalized costs. However, UE for capacitated networks, even under the defined special conditions, is not equivalent to NE. This paper extends Wardrop’s first principle to explain UE in capacitated networks, which, under the same special conditions of uncapacitated networks, would represent an NE as well. Moreover, a complementarity equilibrium model is proposed for UE, based on an extension of Wardrop’s principle.

In this paper, symmetry of graph models (structures) is investigated. All canonical forms previously derived in literature for bilateral symmetry are derived from the formula for rotationally repetitive structures (systems) considering the angle of rotation as 180 degrees. Different nodal numberings result in different patterns for matrices associated with bilaterally symmetric structures. In this study, it is shown that all these forms have the same nature and can be considered as particular forms of circulant matrices associated with rotationally repetitive structures. In order to clarify this point, some numerical examples are investigated using both the classic approach and the canonical forms.