نشریه مهندسی عمران مدرس
سال بیستم شماره 5 (آذر و دی 1399)

Structures designed to resist moderate and frequently occurring earthquakes must have sufficient stiffness and strength to control deflection and prevent any collapse. Since stiffness and ductility are generally two opposing properties; it is desirable to devise a structural system that combines these properties in the most effective manner without an excessive increase in the cost. Steel structural systems including moment resisting and concentrically braced frames have been widely used to resist earthquake loads. Concentrically Braced Frames (CBFs) have high stiffness, and due to the probable buckling of their diagonal members, are not ductile enough. Versus, Moment-Resisting Frames (MRFs) have adequate ductility as their beam sections can undergo inelastic deformations. However, due to the low stiffness of moment frames, the construction costs will be increased. In recent decades, steel shear panels are utilized as one of the lateral resistant systems, in Steel Plate Shear Walls (SPSWs), and the link beam of steel frames with eccentric bracing to achieve the aim of shear performance and keep the adjacent members in the elastic range. The Tubular frame is one of the common lateral resistant systems in which the columns are placed in close spaces and connected through deep MRF beams around the building perimeters. Based on the new design codes, the minimum limit of span-to-depth ratio (7 for moderate moment-resisting frames and 5 for special moment-resisting frames) is not satisfied at tubular system. So the idea of Shear Resisting Frames (SRFs) with non-prismatic beams connected by a shear fuse in the middle of the span was proposed as one of the alternatives. Using SRFs remove these limitations and increase the energy dissipation capability. In this new concept, the shear force in the beam is considered as the displacement-controlled component of the system. Similar to eccentrically braced frames (EBFs), the link is tuned as a sacrificial component so that the seismic energy is dissipated by shear yielding in a small segment in the middle of the beam. According to the stiffeners layout, lateral loading capacity in SRFs usually is achieved through buckling strengths or post- buckling capacity resulted from tension field action or load carrying capacity from the yielding of the web plates. So stiffeners play a crucial role in the lateral loading capacity of shear resisting frames and have a significant effect on the energy dissipation capability. Following this issue, the effect of transverse stiffeners with different layouts and placements (various spaces and two or one-sided arrangement) on the seismic performance parameters (response modification factor, overstrength factor and rotation capacity of link beam) of steel shear frames with different link length ratios where all of them are controlled with shear behavior, are evaluated by finite element cyclic and pushover analysis. At the end, an optimum space is proposed for different link length ratios and the response modification factors and overstrength factor of multi-story shear resisting frames including 3, 5, 7, 9, 10, 15, and 20-story for a specific link length ratio are presented. Also for facilitating the modeling process of multi-story SRFs in SAP2000 software, modeling parameters and acceptance criteria were extracted from cyclic and monotonic curves. Finally, pushover curves from SAP2000 were compared to ABAQUS to validate these parameters. At the end, a 25-story building with two different lateral resisting systems including tubular frame and SRFs were compared.

Pervious concrete is a particular type of concrete with high porosity and often without fine aggregates or with little sand. The sufficient amount of cement paste covers and glues aggregates in a porous matrix together, resulting in a fast drainage system. On the other hand, high porosity and low amount of cement paste lessen the compressive strength of this type of concrete in comparison with normal concretes. Its main application is in the construction of pavements and management of stormwater. Pervious concrete has many economic and environmental benefits. Reducing the costs of surface water drainage, protecting roads from floods during major storms, preventing contamination of rainwater, recharging groundwater resources, and preventing road surface damage from freezing are some of its advantages. In this study, compressive strength, permeability, and freeze-thaw resistance of pervious concrete specimens containing micro silica have been investigated. Fifteen mix designs containing different amounts and sizes of aggregates, and water to cement ratios, and incorporating highly reactive, amorphous micro silica were prepared. The specimens were demolded after 24 hours and were cured in water with a temperature of 23ºC until the test day. The compressive strength of the specimens was evaluated. Then, permeability and freezethaw resistance tests were conducted on selected mixes. According to the results, the compressive strength of the specimens after 28 days of curing was in the range of 101 to 404 kgf/cm2 . Mix designs containing higher cement and micro silica content, and with lower water to cement ratio exhibited high compressive strength. The flow rate of specimens evaluated in the range of 0 to 111 cc/s, and the corresponding permeate velocity was between 0 and 3.43 mm/s. In mix number 11 with the highest cement and micro silica content, the flow rate was equal to zero due to the filling of concrete pores with cement paste, which happened in bottom layers during vibration. It is worth mentioning that the higher content of sand in the mix number 11 was also effective in the sedimentation of cement paste during vibration. Pervious concrete specimens, due to the permeability, are more durable in freezing and thawing cycles than normal concrete. After 10 cycles of freeze-thaw tests, micro cracks appeared on the surface of normal concrete specimens, and they were utterly destroyed after 16 cycles through fracture of cement paste. However, there were no visible cracks in pervious concrete specimens even after 23 cycles. Weight loss of pervious concrete specimens in freezing and thawing cycles was evaluated and compared in different mix designs. Mix number 10 containing medium gravel aggregates displayed better freeze-thaw resistance than mixes numbers 11 to 13, which contain fine gravel aggregates. This can be attributed to the more porous structure of the former specimen in comparison with the latter mentioned ones. In the case of mix number 11, as mentioned earlier, because of low permeability despite high compressive strength, maximum fracture occurred due to expansion in freezing cycles. Some of the studied mixes are appropriate in pavement construction, such as parking lots or sidewalks, including mix number 9, which contains micro silica at an amount of 7 percent of cement weight with compressive strength of 283 kg/cm2 and a flow rate of 111 cc/s.

As a passive control system, braces have an effective role in creating structural resistance to lateral forces such as earthquakes and winds. One of the ways to make the braces more economical is to use their inelastic capacity. Ordinary braces perform well in tension; however they buckle under pressure and exhibit undesirable behavior. This problem can reduce dissipated energy due to lack of plasticity, which plays an important role in cyclic loading such as earthquakes. For this reason, buckling-restrained brace (BRB) have become increasingly popular in different countries. BRBs include yielding steel core and an outer steel hollow section. Although the yielding steel core has a low compressive capacity, its capacity in pressure can be increased by limiting its buckling due to the outer steel hollow section. So far BRBs introduced as mentioned have a single yielding core, however In this paper, in order to improve the seismic behavior of BRBs, buckling-restrained brace with three parallel cores with different yield stress have been suggested and introduced. The buckling braces were made in one and three steel core with the same tensile and compressive capacity. These braces were subjected to cyclic tensile and compressive loads in the laboratory under the ATC-24 loading protocol. Hysteresis cyclic performances of each brace were obtained and examined. The experimental results show that: 1) the hysteresis loop of the 3-core brace is thicker and higher than the 1-core brace, 2) indicating that the three core brace has 16.3% and 8.8% higher energy absorption and damping capacity, respectively compared to that of the single core brace. Furthermore, it has better seismic performance.

In recent years, structures for economic and aesthetic reasons have been growing larger and thinner and this subject leading to non - linear behavior of structures. Therefore, the application of different methods for considering the effects of second order analysis in evaluation of the behavior of structures has always been considered by the designers. in this study to investigate the P-Δ effects, analyze the nonlinear behavior of multi - storey space structures. in order to estimate the effects of second order analysis in this study, methods of Stiffness Matrix, Stability Functions and Assume lateral load method have been used. in present study, in addition to comparing the accuracy of the proposed methods, examine the effect of the parameter height on the accuracy of the second order analysis. the accuracy of the second order analysis methods is investigated by comparing the results of the existing research. In the first part of the present study, after presenting the formulation of three methods of second-order analysis of stiffness matrix, stability functions and assume lateral load method, for beam-column element of a portal frame, a two-story regular structure and finally an irregular six-story structure, that studied by previous researchers, the most accurate second-order analysis is introduced by comparing the load-deformation curves. In the second part, in order to investigate the effect of height of structure on the accuracy of second-order analysis methods, similar structures of three, five, and seven floors were analyzed in two longitudinal and transverse directions and the final load coefficients obtained from the second-order analysis with The first-order analysis of the structures is compared. The analysis of three, five and seven-story structures was performed in two assume lateral load and stiffness matrix methods in OpenSees software as well as stability function analysis using Paap software.the accuracy of second-order analysis methods has decreased and this reduction is more pronounced in the transverse direction of the structure. As the height of the structure increases, the accuracy of the stiffness matrix methods, stability functions and the Assume lateral load method in the second-order analysis of the transverse direction are reduced by 15.5%, 10% and 7.2%, respectively. It is observed in more than three storeys, the accuracy of the Assume lateral load method in estimating the P - Δ effects is greatly reduced and the highest correlation with the height of the structure is observed in this method. also among the second - order analysis methods in tall structures, the highest accuracy is related to the method of Stiffness Matrix.

Reinforcement corrosion in concrete structures is considered as one of the main important issues that degrade the durability of RC structures. One of the most important effects of reinforcement corrosion is to reduce the bond between concrete and rebar. Bond strength enables the force transfer from reinforcing steel bar into concrete and guarantees the composite manner of reinforced concrete structures. Many empirical models have been developed to estimate bond strength during the corrosion propagation period. The experimental results are different depending on the test conditions and how to prepare the samples. Models presented by different researchers, even for the same basic assumptions, have fundamental differences in the predicted bond strength, which causes uncertainty in the choice of model and results. In this paper by regression of existing empirical results, some different models are presented for each bar diameter. The reinforcement diameter and corrosion current density are two basic variables in bond degradation models that have been considered for investigation of uncertainty in proposed models. Based on the results, the bond reduction during the corrosion propagation is exponential and the effect of the uncertainty of the corrosion current density is greater than the bar diameter. The effect of uncertainties on the coefficient of variation of the results is more than the effect on the mean. The bond reduction for the smaller diameter bars was lower than the larger bars such that for 10 and 16 mm diameters at 15% corrosion, the bond to primary bond ratio was 0.46 and 0.28, respectively.

Nonlinear time history analysis (NL-THA) is the most accurate method to estimate the seismic demand of structures and predict their failure. To that end, extensive efforts have been made to develop fast and convenient methods to carry out nonlinear static analyses. In recent years, pushover methods have been widely used as a suitable tool to evalute the seismic performance of structures. Also, various advanced pushover procedures have been proposed to take into account the effect of higher modes and the change in the dynamic properties of structures in the nonlinear phase. Therefore, different pushover procedures have been further developed for this purpose. The nonlinear static analysis  has been widely employed to evaluate the nonlinear behavior of structures. The pushover analysis was first expanded in a number of studies to investigate buildings. Not many studies have been conducted on the seismic demand of latticed space structures. In the present work, therefore, an optimization procedure has been employed to refine the performance of the pushover analysis in estimating the seismic response of double-layer barrel vault roofs with vertical double-layer walls. In the  method proposed herein, the coefficients of the modal load combinations of the studied structures have been optimized using the simplex algorithm to find the optimum load pattern. Fifteen models with various rise-to-span and height-to-span ratios were considered to assess the accuracy of the proposed method in predicting the seismic demand of these structures. The models were analyzed using the OpenSees software. In order to model the buckling behavior of the members, each member was divided into two nonlinear beam-column elements with an initial imperfection of 0.1% at its mid-node.  The models were designed with the dead, snow, temperature, and earthquake loads having been considered. All of the mentioned loads, with the exception of snow load, were applied to the structures as concentrated nodal loads. The snow load, by contrast, was applied to the structures in two symmetric and asymmetric patterns in accordance with the sixth volume of the Iranian national code of buildings. For earthquake loads, the 4th edition of the Iranian code of practice for seismic resistant design of buildings was used. It should be noted that the seismic mass of the roof of each model was calculated by considering the entirety of the dead load in addition to 40% of the snow load. In the design process of each model, the dead, snow, temperature, and earthquake load combinations were formulated based on the AISC-ASD89 standard. The sections of the members of the structures were chosen from hollow tubular sections, with  their slenderness ratios limited to 100. Afterwards, pushover analyses were performed using the optimized load pattern. The obtained results were compared to those of the incremental dynamic analyses (IDA) and two other well-known pushover methods, namely the MPA and the conventional first-mode pushover analysis. The results revealed that the proposed pushover method can provide a good estimation of the base shear and intial stiffness of the structures when compared to dynamic analyses. In addition, an increase in the rise-to-span ratio of the roof causes an improvement in the accuracy of the proposed pushover method. Also, in comparison with the  MPA and conventional pushover procedures, the responses produced by the proposed method are closer to those generated by dynamic analyses. In addition, a comparison of the obtained drift patterns reveals that the results of both the pushover and incremental dynamic analyses along the longitudinal direction of the wall are quite close to each other. Another advantage of the proposed pushover method is that it also produces acceptable results on the nodes on the roof of the space structure. Also, along the transverse direction, the proposed method yields better results.

The load type imposed on the structures is one of the important issues of the modal identification Experimental methods. Generally the loads applied to a structure for dynamic testing are divided into two categories: artificial stimulation and ambient loads. Applying artificial loads to large structures such as bridges and tall buildings is difficult, costly and in some cases impossible. For this reason, modal identification of such structures is generally done by ambient vibration tests. However this experimental methods, also include problems such as large noise amplitude relative to the measured responses that this causes errors in the results and in some cases leads to unrealistic modes. As a solution, modal information can be calculated from several different methods and compared with each other to ensure the accuracy of the results. In this paper, a new scheme for natural frequencies extraction of structures from their ambient vibration is presented. For this purpose, the combination of two mathematical techniques of random decrement (RD) and proper orthogonal decomposition (POD) methods were used. The reason for using these two methods, is their ability to reduce the noise effects. In other words, combining of these two methods can lead to a very powerful tool for extracting structural frequencies from its ambient vibration under high amplitude noise conditions. The proposed algorithm consists of three steps: In the first step, after measuring the acceleration response of the structure at the appropriate points, the effects of random vibration are eliminated from the response by RD method and only dynamic properties of the structure remain in the acceleration records. Secondly, the acceleration records are separated into several structural modes using the proper orthogonal decomposition technique and finally, at the last step, the proceeded responses are transformed by the fast Fourier transform into the frequency domain to extract the natural frequencies of the structure. The strength of the proposed method is its robustness to the use of very high amplitude noise data, which is one of the challenges in the ambient vibration experiments. The accuracy of the proposed algorithm was evaluated by numerical modeling and experimental study. To investigate the efficiency of the new method, the numerical and experimental results were compared with the frequencies obtained from commonly modal identification methods such as extended frequency domain decomposition (EFDD) and stochastic subspace identification (SSI). A very good agreement was observed between the results of methods. Furthermore, Studying the effect of noise on the new algorithm results shows that increasing the ratio of noise to acceleration amplitude up to 250, did not affect the results precision and the main frequencies of the structure can be obtained with good accuracy. In this study, the effect of the number of sensors used in the ambient vibration test also was investigated on the accuracy of the new algorithm results. It was concluded that the minimum number of sensors (even one number) and repetition of the experiment can be used to extract structural frequencies from its ambient vibration with high accuracy. The results of this study showed that the new method can be used as a suitable tool to determine the natural frequencies of structures from its ambient vibration under severe noise conditions and to control the results obtained from other methods.

On way to understand the behavior of rivers is to study of flow structures and bedforms. Most rivers have rough beds, that are called bedforms. These shapes have different types depending on the hydraulic conditions that cause the resistance to flow, so sufficient knowledge of the hydraulic resistance of the flow is necessary to calculate discharge, depth, water velocity, flood prediction and sediment transport to reduce the damages that are caused by rivers. Despite years of research and experimentation on bedforms, there is still no adequate and accurate equation to predict the geometry of bedforms and their interaction with flow. The RANS turbulence model is not sufficiently accurate in detecting flow separation in high-altitude and high-angel lee side dunes, but it is more appropriate in other dunes. The LES turbulence model can be more appropriate in detecting flow separation in the vicinity of the bed. But in the layers away from the dune, it produces relatively severe turbulences. The DES turbulence model is more accurate in detecting the flow separation in large-scale dunes and has less execution time than the LES, but this model produces irregular vortices in smaller dunes near the bed. In this research, with the aim of investigating the effect of dune geometry on flow structure, numerical simulation of flow motion on dunes in open channel duct was investigated. In this regard, 29 simulations were performed to study the effect of the geometry of five types of dune with different angles and heights in different hydraulic conditions and different bed roughnesses with RANS and DES turbulence models. The STAR-CCM+  was used in order to simulate the numerical model in this research. This software provides highly realistic results by providing a seamless environment with high network production capability and extensive simulation tools, which helps professionals in the process of working with fluids.In this research, VOF method is used to calculate free surface area, and the solution method is Implicit Unsteady. In order to sensitize the numerical model to the number of computational cells as well as to optimize the runtime and output accuracy, several blocks of lattice dimensionality change in data points and dune locations were used. To select the appropriate networking dimensions, 8 models were run to optimize the time and accuracy of the model. To evaluate the accuracy of the simulations, the numerical model results were compared with those of previous researchers. The results of comparison of the numerical model and the experimental model showed that only about 9.5%, 15.5%, 14.5%, 9.4%, 12.2% and 7.4% were different. This error rate indicates good numerical model accuracy. Also the results of numerical model + STAR-CCM were compared with SSIIM numerical model. then, by using the dimensional analysis, effective factors on the interaction of dune geometry and flow structure were investigated and finally, a formula was developed to predict this interaction. The results of the evaluation of the obtained formula for investigating the effect of dune geometry on the hydraulic flow showed that the presented formula with error of 11.25% and 0.86 R2, due to the completely random nature of bedform formation, is very accurate.

Fibers incorporated in concrete, have different shapes and sizes. The main reasons for incorporating fibers in concrete, are to prevent cracking, disintegration of concrete and to increase its resistance to dynamic loading and explosion. As for ordinary concrete, pozzolanic materials can be used along with the fibers. In this paper, the effect of the incorporation of polypropylene and glass fibers in concrete is investigated using the newly developed “Twist-off” method. The “Twist-off” method developed by Naderi is a partially destructive method with high accuracy and very low cost and negligible surface damage. In this method, a metallic disc with 25mm height and 40mm diameter is attached to the surface of concrete under test, using epoxy adhesive. After the setting and hardening of the glue, an ordinary torque-meter is used to apply a torque until the metallic disc is separated from the surface of the concrete. Since the adhesional strength of the epoxy resin to concrete surface is much higher than the strength of the concrete, the failure is bound to happen at the concrete surface. Therefore, the failure measured torque is used to estimate the concrete strength. In order to estimate the compressive strength of the concrete by the method of “Twist-off”, a calibration graph is prepared by employing concrete cubes with different strength. The “Twist-off” strength of these cubes were measured and they are related to their compressive strengths, using ordinary compressive testing machine. Therefore, compressive strength of pozzolanic concrete containing glass and polypropylene fibers were also measured, using 150 mm concrete cubes. The surface strength of the fiber concrete cubes was measured using the “Twistoff” method. The results obtained in this research, show that, addition of glass and polypropylene fibers to pozzolanic concrete increases the surface strength, compressive strength, modulus of rapture of concrete and the load bearing capacity of concrete beams. The random distribution of fibers in concrete beams, increases its ductility and at the failure load, pull out of the fibers are predominant compared to their breakage. The results tend to show that the addition of two percent fibers tend to increase the strength, ductility and the modulus of the elasticity of the pozzolanic concrete. It was also observed that while the addition of glass fibers increases the surface strength of the low strength concrete (40 MPa or less), its addition to the concrete reduces its strength. Compared with the effect of the polypropylene fibers, it was seen that, the increase in the surface strength of concrete with glass fibers is more pronounced. Examination of the results presented in this paper tend to indicate that the “Twist-off “ method can be used for the determination of the surface strength of concrete as well as its compressive strength, with acceptable accuracy and very little surface damage to tested area. Compared with other in-situe methods for concrete strength assessment, the ”Twist-off” method is much cheaper and needs no expert operators. Statistical analysis of the comparative results of the “Twist-off” and the ordinary compressive testing method indicates that the torque obtained in the “Twist-off” method, can be directly related to the compressive strength of the concrete without the need to calculate stress intensities. The relation between the “Twist-off” and compressive strength tests is seen to be linear.

Lots of ecosystems including soil and water in the world is contaminated by the arsenic every year. The emission of arsenic (As) to the surface and groundwater by human activities such as mining, agricultural and industrial activities is considered a global threat to the ecosystem and human health. Arsenite and arsenate are the two dominant arsenic species in contaminated soils that are highly toxic to the human health and ecosystems. Thus, the As elimination from aqueous solution is considered as crucial issue. Among the different removal methods, adsorption is the low cost, and high efficient technique for the As elimination from aqueous phase. In the adsorption process, the adsorbent type is the one of the main factors of successful removal process. Application of nano-adsrobent may lead to produce less secondary waste in the adsorption process. Moreover, bimetal nano-adsorbent due to the some properties including increasing As removal in the early time was selected as adsorbent to remove As from aqueous solution. Many researches believe that Jacobsite nanoparticles (MnFe2O4) are an effective absorbent for the removal of organic and inorganic materials. Due to the special properties of nanoparticles such as high reactivity, Jacobsite nanoparticles were selected for the adsorption of arsenic from water and prepared based on co-precipitation method. The prepared nanoparticles were characterized through the X-ray fluorescence (XRF), X-ray diffraction XRD, scanning electron microscopy methods (SEM), and pHpzc. According to the XRD, the obtained peaks for the synthesized adsorbent were followed by the previous researches indicated the production of Jacobsite. Based on the XRF, the Mn-oxide and Fe-oxide was 27.8% and 65.5%, respectively. Overall, results of XRD and XRF proved that the synthesized nanoparticles were Jacobsite. Moreover, based on the Fe-SEM, the nanoparticle size was less than 100 nm with mean size of 33.8 nm. Moreover, the he pH of zero point of the nanoparticle (pHpzc) of the synthesized adsorbent was 7.2.  In the presnet study, Response Surface Methodology (RSM) was used to model and optimize the adsorption process of arsenic from solution with Jacobsite nanoparticles. Four factors of pH (3 to 11), concentration of arsenic in solution (1000 to 4000 μg/l), amount of nanoparticles (1 to 5 g/l) and time (15 to 195 min) were selected as independent factors affecting the adsorption efficiency of arsenic. The central composite design (CCD) was used to design of the experiment and optimize the model parameters. Variance analysis indicated that prediction of adsorption of arsenic from the nano-adsorbent by the CCD model was well performed (p <0.0001) with the high accuracy (R2 of 96.24%). The results showed that the effect of four factors pH, nanoparticles, initial arsenic concentration and time was significant. According to the optimization objectives, the results showed that the optimum pH, amount of nanoparticles, time and initial concentration of arsenic were 3, 2 g / l, 48 min and 3250 μg/l, respectively. The arsenic removal from the solution at optimum values ​​calculated for the factors was estimated to be 79.7%. However, 94.77% of As was removed in the adsorption experiments.

Many studies have been conducted on the absorption and protection of electromagnetic waves to reduce the harmful effects of electromagnetic radiation on the environment. High conductivity shields are used to prevent the penetration of electromagnetic waves. A convenient and useful method of obtaining electromagnetic shielding materials is the addition of conductive carbon materials including carbon fibers, carbon filaments, and carbon nanotubes. Carbon nanotubes can easily form a conductive network within a material field due to their two-dimensional tubular structures and high conductivity, which results in a high electrical permeability ambience. Therefore, the increase in dielectric losses results in reflection losses of electromagnetic waves. Thus, the presence of carbon nanotubes in the adsorbent improves the absorption properties of electromagnetic waves. In this study, the electromagnetic wave absorbing properties of multi-walled carbon nanotubes (MWCNT) functionalization with carboxyl (-COOH) group /cement composites with different shapes; chiral, zigzag and armchair were studied by short circuit of the waveguide and matched load methods. The influence of the MWCNT shape and sample thickness on the electromagnetic wave absorbing properties were discussed and analyzed in the frequency range of 8–12 GHz by a short circuit of the waveguide and matched load methods. The samples were prepared in two thicknesses of 3 and 6 mm, and the amount of nanotubes added was 0.1%wt. The addition of 0.1 wt.% MWCNT greatly enhances the absorption performance of the cement mortar in the frequency range of  8–10 GHz. With the increase of thickness from 3 mm to 6 mm, the frequency bandwidths of the reflection loss for MWCNT/cement composites increases but the number of peaks decreases.  By comparing the results of electromagnetic wave absorbing of the samples with two different methods are deduced that in the samples with a thickness of 3 mm, the absorbing of waves by matched load method is better than short circuit method without matched load. However, in samples with a thickness of 6 mm, there is not much different. Also, the electromagnetic wave absorbing of the composite samples with a thickness of 3 mm performed better by short circuit method at frequencies below 10.5 GHz, while the composite samples had better absorbing in the matched load method at frequencies greater than 10.5 GHz. In addition, the electromagnetic wave absorbing of the composite samples with a thickness of 6 mm show better results by short circuit method at lower frequencies and by matched load method at higher frequencies. Moreover, the absorption behavior of the chiral sample with thickness 6 mm differs from the other two samples because the chiral nanotubes are asymmetric and zigzag and armchair nanotubes are symmetric. Furthermore, the structural analysis and surface morphology of MWCNT/cement composites with different shapes have been explored using the scanning electron microscope (SEM) technique. Scanning electron microscope images of MWCNT/cement composites show dispersion of nanotubes in composite. Connecting of nanotubes and cement leads to reduction of porosity and formation of regional conductive network. As a result, the electrical conductivity is increased and the electromagnetic field in the network is attenuated.

Following Northridge earthquake, wide spread brittle cracking had been observed in steel moment connection, and this, was in contrast with the philosophy of designing steel moment frames which accounts for dissipating energy by forming plastic hinges at beams. This situation led to the development of improved connections to make them less prone to brittle fracture. However, studies have shown that these new connections, typically known as post-Northridge connections, can still have the tendency to fracture but in a ductile manner when subjected to ultra low cycle fatigue loading. Ultra low cycle fatigue loading consists of limited cycles of loading with large amplitude which induce strains that are several times greater than yielding. Searching the literature, varied methods have been proposed to predict cracking in ductile steel for both monotonic and cyclic loading. In this research, a micro mechanical model called cyclic void growth model has been applied to predict the instance and location of cracking in the steel structure. For the purpose of predicting the low cycle fatigue failure, finger shaped steel moment type connections with top and bottom cover plates which their experimental data were available, used as a benchmark study. A micro mechanical model is integrated into the ABAQUS finite element program in order to simulate crack initiations in the cover plate welded beam to column connection. For this purpose, a Fortran code is linked with the ABAQUS software for simulating the crack and specifically to predict when and where the crack initiates. By understanding the crack initiation and the location of this crack, a trend line for low cycle fatigue under various constant drift angels are put together. The trend line provides a number of cycles for the crack to initiate by applying the specific drift angle. Therefore, a finite element model of a cover plate welded moment connection was developed and was used in order to simulate cracking in the connection model. Thus, each crack location and the number of cycles to initiate the crack were detected. Utilizing cyclic void micro mechanical model of growth analysis, which is a technique to predict fracture in a ductile material, different cover plate connections were modeled in the steel moment frame, and then their critical points to trigger the crack were identified. Finally, for the finger shaped cover plate moment connection, considering different loading curves data and in order to present the low cycle fatigue life prediction, displacement versus the number of half cycles diagram is produced. Finite element results demonstrated acceptable agreement with the experimental data. Furthermore, the low cycle fatigue life of connections under loading with constant amplitude is estimated, and S-N curves are proposed. These curves can be applicable for engineering purposes, such as performance based design. Also it is demonstrated that the finger plate joint revealed a good performance against soft cracking in low cycle fatigue compared to a number of previously tested joints. The results of the S-N curve for a constant displacement loading averaged 73% of the lifetime of the initial cracking. Sensitivity analysis with 20% tolerance on the intrinsic parameters of the micro mechanical model showed a maximum change of 15% in the responses.

Reshaping berm breakwaters has a large berm above still water level (SWL) and their seaward profile is reshaped under wave attack. When a quarry is available near the construction site but it is not possible to produce a sufficient quantity of large armore stones for a conventional rubble mound breakwater, a berm breakwater is a feasible solution. According to the conditions of the rock quarries in the coastal regions of Iran, it is difficult to produce large armor stone from them. Thus, the concept of berm breakwater has attracted engineers to develop design and construction of this type breakwater in many projects in Iran.Berm breakwater are normally constructed with only two layers. Due quarry yield of rock quarries, not only producing large armor stone is difficult, but also quarry runs - which are used for core layer - have very small size. So, application of a secondary or filter layer is necessary for using such small size materials as core layer. Thus, most of the Iranian berm breakwaters have been designed and constructed with three layers. The present study aims to investigate the necessity for of filter layer to be applied in reshaping berm breakwater by considering the influence of sea states and structural parameters on the erosion of the core layer materials that includes the number of waves, waves height and period waves, water depth, the gradation coefficient of filter layer and the berm width. In this regard, experimental studies were carried out on two layers and three layers breakwaters under irregular waves with JONSWAP wave energy spectrum. Results showed that a wide berm width can preclude erosion of core material under design wave condition. Hence, berm breakwaters can be constructed without filter layer by using an optimized berm width to protect core materials against erosion. A dimensionless parameter is proposed to evaluate the threshold for eliminating filter layer of core materials in berm breakwaters. Considering the aforementioned conditions for optimum berm width and sea state and structural conditions of the present study, required berm width of a two-layered berm breakwater can be calculated using formula of Shekari and Shafieefar (2013) for recession plus at least 4 times of stone diameter. Due to difficulties of implementing a filter layer, selecting the optimized berm width and removing the filter layer can be very effective in reducing the cost and execution time of this type of structure. Filter is a layer in penetrable structures (mostly in coastal and slope protection structures) that precludes erosion of materials due to waves and currents without increasing pore water pressure in materials of lower layers. Filter, in a sense, may contain granular materials, geotextiles, or a combination of geotextiles and granular materials. Further, filters have different applications including prevention of erosion in corechr('39')s materials which is due to the negative pressure fluctuations and movement of fluid through the structure, reduction of hydrodynamics loadings on outer stone layers, and drainage. Filters are categorized into three types based on their resistance to erosion of bed materials in shore or slope protection structures: Geometrically tight filters, Stable with geometrically open filters and Unstable open filters.

In recent decades, the use of an efficient and cost-effective method to provide soil stability has been a major challenge for civil engineers. With increasing urban population, the need for underground spaces increases and deep excavation is an inevitable affair in civil projects. Deep tunnels and large buildings require deep excavations, which must use some techniques for stabilize it. Soil-nailing (reinforcing soil at the site) due to the fast build, is a good way to provide stability. It can also be described as a top-down construction technique for the improvement of behavioural properties of in-situ soil mass. Soil-nailed system is formed by inserting relatively slender reinforcing bars into the slope. Depending upon the project cost, site accessibility, availability of working space, and the soil and groundwater conditions, soil-nails can be inserted into the ground. Soil-nail is generally known as conventional and injectable nails but nails with screw plates or "helical soil-nails" are also important due to the faster build and no need for groutings. Helical soil-nails are new alternative to the conventional soil nails or tie-backs for stabilization of slopes, excavations and embankments due to ease of installation, minimal site disturbance and immediate loading capability. Helical soil-nails are installed by application of torque without a drill hole and derive its capacity from one or more helical plates attached to the nail. The shear strength-displacement behavior at the interface is an important parameter in design of various geotechnical engineering projects, for example, soil-nails, retaining walls, shallow foundations, pile foundations, etc. In soil-nailing, behavioure of interface between the soil and nail estimated by pull-out test. The behaviour of interface is governed by numerous factors, such as stress conditions, soil properties, method of installation and soil-nail interface boundary conditions. The pull-out resistance is measured as the most important factor in the design of the nailing system, by pull-out test. This study, because of limited learning of helical soil nail, aimed to investigate the pull-out resistance by a 3D finite element modeling with abaqus software and compare its results with laboratory data. A review of the literature for the screw soil-nails as well as a comparison of its performance with conventional soil nails is discussed and numerical results of a series of pull-out tests on a screw soil-nail are presented. And review of  the overburden pressure and plate number and plate distance effect is followed. The results show that in helical soil-nail pull-out a high overburden pressure effect can be seen. A semi-linear relationship between peak pull-out force and overburden pressure is observed for different methods of calculating the helical soil-nail capacity that it is indicating that it satisfies the Mohr-Coulomb failure criteria. Rupture surfaces occur at distances farther than the nail surface, and three times the diameter can be considered the optimal distance of the plates. Using fewer plate distance does not increase resistance, also using more plate with fewer distance does not increase resistance. Comparison of modeling and laboratory results indicates that modeling of pull-out test can model the behavior of helical soil-nail and verify its performance in a field soil slope.

Thin-walled cylindrical steel silos are one of the major storage structures in most of industrial and agricultural sectors. There are different load cases that should be considered in design of silos, such as, filling and discharge loads, wind load, seismic load and thermal loads. Nevertheless, during the life cycle of a silo, filling and discharge of particulate solids exert the most frequent loads on the silo walls. Due to larger values of discharge pressures as compared with those of filling pressures, discharge loads are considered for structural design of silos. Considering small wall thickness of steel silos, they are susceptible to buckling failure. Under discharge pressures, high meridional (axial) compression and internal pressure form at the base of silos that can lead to elastic-plastic elephant’s foot buckling mode. Therefore, it is deemed as the main buckling failure mode under discharge loads of silos. The wall friction coefficient of silos mainly depends on the wall surface characteristic and type of the ensiled material. This coefficient is a key variable in determination of magnitude and distribution of discharge pressures. To assess the effect of this variable on buckling capacity of steel silos, three example silos with different aspect ratios were considered. Each silo was loaded by the concentric discharge pressures in accordance to Eurocode. Subsequently, 3D linear and non-linear buckling analyses (i.e., LBA and GMNA analyses, respectively) were performed for different amounts of wall friction coefficient that varied between 0.2 and 0.6. Considering the results obtained, LBA analyses predicted an elastic axial compression buckling mode in the upper edge of base strake, where there is a change in shell wall thickness. Also, an elastic-plastic elephant’s foot buckling mode at the base strake of each silo was predicted by the GMNA analyses. Moreover, the load-path curves of example silos extracted from the GMNA analyses showed a bifurcation buckling that was followed by a dramatic reduction in post-buckling resistance. This held true for all three silos and all different values of wall friction coefficient considered in this study. The discharge buckling resistances estimated by the LBA were up to three times larger than those predicted by the GMNA. Therefore, including non-linearity in discharge buckling assessment of silos is urgently required. The effect of wall friction coefficient on buckling capacities of steel silos was significant for the LBA analyses that governed by axial compression. However, the elephant’s foot buckling mode observed under discharge load is affected by the both axial and internal pressures. As a result, adopting more sophisticated analyzing procedure that includes geometrically and materially non-linearity in the calculations (i.e., GMNA analyses) showed quite marginal effects for this coefficient (with the maximum difference of 8% in buckling capacity). As an extra investigation, the Eurocode provisions on stress design of steel silos under meridional compression with coexistent of internal pressure have also been examined. Eurocode recommends a reduction in critical axial buckling stress due to accompanying internal pressure, in terms of the plastic pressurised imperfection reduction factor αpp. As compared with the finite element results, for all the cases considered in this paper, the critical axial membrane stress calculated with respect to the Eurocode provisions yielded satisfactory predictions.

Concrete is the most widely used building material in construction industry worldwide and its constituents are easily accessible everywhere. However, cement industry, as the producer of the primary binder of concrete, is one of the effective sources of environment degradation. Cement production needs extraction of mineral resources and burning fuel and causes extensive greenhouse emission due to disintegration of raw materials. Cement production alone is responsible for 7% of global CO2 emission with estimated annual growth of 4%. Toward environmental sustainability, one way is partially or totally replacing cement by waste or byproducts of other industries such as fly ash, ground granulated blast-furnace slag (GGBS), waste water, metakaolin, and silica fume. Geopolymer is a cementitious material with comparable characteristics to those of ordinary cement produced by alumina- and silica-rich waste materials. Therefore, it does not require energy-intensive and pollutive calcination process. Geopolymerization is formed by reaction of silica-alumina under an alkaline solution which creates three dimensional Si-O-Al-O polymeric chains to attain compressive strength, compared to the ordinary cement which develops calcium silicate hydrates (C-S-H) as the main adhesive. Extensive research has conducted on geopolymer concrete. However, more investigations are needed to better understand characteristics of geopolymer concrete containing additives. Fibers are proved to have a positive effect on mechanical strength of concrete. As well, fillers such as microsilica can improve mechanical and durability of concrete. Moreover, most studies in this area are focused on fly ash-based geopolymers and the investigations on GGBS-based geopolymer are rare in the literature. In this study, mechanical and durability of GGBS-based geopolymer concrete containing CFRP fibers and microsilica is investigated. Different concrete samples with 0-3% CFRP fibers and 0-10% microsilica are prepared and experimentally tested. Sodium Hydroxide (NH) and Sodium Silicate (NS) solutions are used as alkali activators. 8 M NH as well as NS with 14.7 Na2O and 29.4 SiO2are used with the NS/NH ratio of 2.5. Since no standard exists for mix design of geopolymer concrete, proposed mix design by Venkatesan and Pazhani (2015) is used. Alkaline to binder ratio of 0.4 is selected with 430 kg/m3binder.The specimens were tested after 28 days of curing. Next, mechanical and durability tests including compressive strength, tensile strength, ultrasonic pulse velocity, water absorption, RCPT, and acid resistance are conducted on the samples. Also, microstructure of the geopolymer concrete is investigated. Results of experimental tests show that, compressive and tensile strength of geopolymer samples decrease by adding microsilica. However, 5% microsilica is the best value to enhance mechanical properties of geopolymer concrete. On the other hand, microsilica can enhance durability properties of geopolymer concrete so that adding 5% microsilica causes moderate improvement of water absorption and chloride penetration. The greatest impact of microsilica is on acid resistance by which adding 5% microsilica resulted in 67% improvement of compressive strength loss. However, unlike the microsilica, CFRP fibers have detrimental effect on mechanical properties and durability of the geopolymer concrete since adding fibers can yield disruption of concrete integrity .On the other point of view, microstructure study shows that all the specimens exhibit micro cracks that can inversely affect the performance of concrete. Also, SEM images show that there is not a strong bond between CFRP fibers and binder paste which yields lower performance of concrete specimens containing fiber.