نشریه مهندسی سازه و ساخت
سال هفتم شماره 1 (پیاپی 31، بهار 1399)

In this paper, the natural frequency of the cracked reinforced concrete (RC) beams in the first and second modes are investigated using the finite element method. In this research, the modelling of the crack is made based on continuity conditions, the correction of the moment of inertia and considering the stress intensity factor (SIF) at the crack point. In this study, the crack in opening mode is corresponded to a rotational spring. The stiffness factor of the spring is derived as a function in terms of the stress intensity factor and geometric and material characteristics of a cracked cross-section. In the present technique, the stiffness and mass matrices of a cracked element are enriched using convert matrices obtained by applying continuity conditions in the crack point. Natural frequencies of the cracked reinforced concrete Euler-Bernoulli beam are determined by inserting enriched stiffness and mass matrices in the eigenvalue equation. The effect of different depths and positions of crack and various boundary conditions are studied on the first and second vibration modes. A comparison between the results of the present work with experimental results and fully simulation in Abaqus clearly demonstrates the accuracy of the proposed technique in the determination of the natural frequency of the cracked reinforced concrete beams.

Hybrid masonry system is a new structural system, in which the masonry panels link to the frame with steel plate connections. Hybrid masonry uses masonry infill for lateral stiffness and strength within frames in addition to supporting out-of-plane (flexural) loads. There are many primary reasons for the development of hybrid masonry systems. Most important ones are to simplify the construction of framed buildings with masonry infill and to provide structural redundancy, which can be utilized for limiting progressive collapse.There are three hybrid wall types including, ordinary hybrid shear wall, intermediate hybrid shear wall and special hybrid shear wall. Hybrid masonry systems can be applied to either concrete or steel-framed structures. This research focuses on steel frames. In this paper, seismic behavior of ordinary, intermediate and special hybrid shear walls are investigated using nonlinear static analysis. To this end, 15 steel frames including, bare frames, infill frames, ordinary hybrid shear wall, intermediate hybrid shear wall and special hybrid shear wall are modeled using ABAQUS software. The stress contours and capacity curves of models are estimated using nonlinear static analysis. Results indicate that failure capacity, and ductility of special hybrid shear walls are more than ordinary and intermediate ones. Moreover, the increase in moment of inertia of frame sections, increases the stiffness and energy dissipation capacity for all three hybrid shear walls

Construction is one of the most important issues in a society economically, socially, culturally and so on. But the issue of quality in the construction projects requires discussion takes more scientific. The objective of this study is to investigate the optimization of the quality of construction projects and construction costs based on system reliability theory. Also, a real construction project is presented to evaluate the efficiency of the proposed model. In order to achieve this goal, a four-step algorithm is defined and proposed model using two methods M3AS and classic ant colony algorithm is solved. The results of both methods are compared. Parzin five-story residential building located on Zaferanieh Street is studied. The results indicated that M3AS innovative approach through local search is more capable than the classical ant colony algorithm for optimization. Most previous studies have not considered the importance of the quality of project. In this study, quality in the form of reliability at the time of completion of the work is quantified. The proposed model has been so overcome lack of expression and effectively using reliability theory. This model can automatically produce reliability and optimal cost of system with regard to structure function of reliability and non-linear function of cost-reliability. In this study, multi-objective optimization model based on reliability theory extends to decision-makers to determine and choose between cost and quality for construction projects. Since a major part of the annual budget allocated to construction projects, these projects are the most important component of the country's development that must be managed properly in terms of cost and quality and be completed according to the schedule. iT is clear that the status of the construction industry in terms of quality and cost should be much improved.

Computing free vibration properties such as natural frequencies and mode shapes of fluid-structure interaction (FSI) systems leads to a special type of asymmetric eigen-problems. Standard methods for solving symmetric eigenvalue problems cannot be applied directly for solving these asymmetric problems and should be modified. The pseudo symmetric subspace iteration method is a well-known method in this field which uses symmetric matrices instead of original asymmetric ones. However, this method is not so efficient in computing high number eigenpairs of the fluid structure systems (say > 40). Accelerated pseudo symmetric subspace iteration method increases the efficiency of the basic method utilizing constant size subspace and shifting technique. However, this method uses a very conservative shifting value, which is always smaller than last converged eigenvalue. In this study, an aggressive shifting technique which selects shifting value larger than converged eigenvalues and near unconverged eigenvalues, is proposed to solve the asymmetric eigen-problems. This technique improves efficiency of the accelerated pseudo symmetric subspace iteration method by 30 to 40 percent. Also, a computable error bound is proposed as convergence criterion for the asymmetric eigen-problems. This error bound, on the one hand, guarantees the accuracy of the converged eigen values and, on the other hand, gives an approximate range for unconverged values. This error bound is necessary to select the shifting value in the aggressive technique. In this paper, previous methods were studied first and then the proposed method is investigated and examined by several practical examples.

The aim of this study was to investigate the length of the gusset-plate hinge zone proposed by Iranian National Building Code (Part 10) for Special concentrically braced frames (SCBF). For this purpose, numerical studies were done on the seismic behavior of this frames using ABAQUS software. Numerical modelling procedure is validated using the reproduction of the results of an experimental study. A five story building with Special diagonal concentrically braced frames was modeled in ETABS software and a portal frame from the first story of the building was considered for investigation. The detailing of the frame was designed for tension and compression based on Iranian code. Seismic behavior of single and double tapered gusset plates with various connection angles and dimensions as well as various length of the hinge zones including -3t, -2t, -t, 0t, t, 2t and 3t were investigated using nonlinear cyclic deformation loading to the ATC-24 test protocol.Seismic behavior of the connections were studied based on ductility, number of cycles before failure, drift before failure and tension and compression strength.Evaluation of the failure limit of the models was carried out using the concept of equivalent plastic strain. Based on the results of the investigated models it can be said that: It is not recommended to use double gusset plates instead of single ones for SCBFs. Gusset plates with a 15 degree angle of inclination have higher ductility capacity and better seismic behavior than ones with a 25 degree angle of inclination. For single gusset plates with a 15 degree angle of inclination, the appropriate length of the hinge zone is -2t, which is not consistent with Iranian code. For single gusset plates with a 25 degree angle of inclination, the appropriate length of the hinge zone is 2t, which is consistent with the Iranian code.

The most common method of strengthening columns is confining them with FRP composites, which increases their ultimate axial and lateral load capacity. The main objective of this research is to investigate square and rectangular RC columns strengthened with FRP composites. For this purpose, an existing laboratory sample was initially modeled in Abaqus software. As a result, the accuracy of modeling was proven as the laboratory test results and the results generated by the software were compared and showed to be close. Then, with regard to variables such as cross-sectional shape and dimensions of the column and the number of composite layers, 18 analytical samples were determined and modeled in Abaqus software. The samples were divided into two groups of 9: square cross-sections and rectangular cross-sections. Each group included three non-reinforced samples, three samples reinforced with one layer and three samples reinforced with two layers of CFRP. The results obtained from the finite element analysis of these samples showed that the cross-sectional shape of the column did not affect the ultimate axial load capacity, but affected the ultimate lateral load capacity. Also, adding CFRP layers had a profound effect on the ultimate lateral load capacity of the columns and unlike the ultimate axial load capacity, the presence of the second layer of CFRP was very effective. Moreover, as the length-to-width ratio in rectangular samples and the dimensions in square samples increased, the effect of the number of layers on the ultimate axial and lateral load capacity and the energy absorbed by the column decreased.

The prediction of nonlinear behavior of shear walls under lateral loads requires simple and precise analytical models representing the nonlinear behavior of the shear walls as well as the experimental results. In recent decades, various analytical models have been proposed by researchers to predict the nonlinear behavior of reinforced concrete shear walls in order to consider the most important behavioral properties of the wall. The model used for analysis and design of the wall should be simple and accurate enough to predict the hysterical behavior. Using the method of fiber section elements can be considered as one of the most accurate methods in wall modeling. But due to the complexity of the modeling and high duration of the analysis, the use of other methods is suggested. Considering the recent progress in modeling nonlinear behavior of materials to provide more accurate behavior against cyclic and seismic loads, these material models can be directly used in modeling of structural systems. In this study, in addition to using the fiber section model, a method based on axial elements is also used. It is implemented by the concept of equivalent spring’s behavior and using the advanced material models in the platform of OpenSees. The results show that the use of this simplified model compared to the fiber method, in addition to reducing the computational cost in both aspects of the duration of modeling and analysis, also satisfies the desired accuracy.

Geopolymers have attracted the attention of many researchers in recent years as a non-cement concrete. Instead of cement, various material such as metakaoline, zeolite, rice husk ash, etc are used in Geopolymers. In order to complete the geopolymerization process, a binder is used along with the selected material from the chemical solutions. Preparation of chemical solutions usually liberates a lot of heat. In this research, steel slag was used as a substitute for cement. In addition, solutions of sodium silicate and potassium hydroxide used as chemical solutions. This research evaluates the effects of chemicals and the mixing temperature on properties of geopolymer mortar. For this purpose, 20 different mixing ratios were made. Setting time, mixture temperature, elasticity module and compressive strength of sample were measured. The results showed that the higher the amount of chemicals solution creates mores mixing temperature and compressive strength and less setting time. Between solutions of sodium silicate and potassium hydroxide, the effects of increasing of the sodium silicate solution are greater in strength growth, temperature growth and setting time reduction. The electron microscopic images of the samples with the highest and the least strength showed that the higher amount of chemicals and the temperature of the construction caused the formation of more hardened geopolymer gel and the width of the cracks observed in the mix design with less chemical solutions It is nearly 3 times more. In addition, X-ray radiation testing for the samples with the highest and lowest strenght showed that the intensity of reflection in the sample with lower strenght was higher; indicating that the hardened geopolymer gel was less due to the reduction of chemical concentration.

Performance-Based Plastic Design (PBPD) method has been recently developed to achieve the enhanced response of structures to earthquake. To reach this goal, several ways have been suggested. In this paper, the schemes of calculating base shear in the (PBPD) procedure will be studied. Two techniques of determining base shear will be used. The base shear of two steel moment frames will be found by these two processes. By using the difference between the input energy and the elastic strain energy to obtain the plastic energy, the plastic design is then performed to detail the frame members in order to achieve the intended yield mechanism and behavior. The inelastic seismic behaviors of the four frames were studied through nonlinear static and dynamic analyses. These two suggested techniques and their comparisons have not been proposed yet. This kind of plastic design has three main parts. Calculating base shear is the first and important portion. The second part consists of the member plastic design for flexible behavior. To design elastic members by using column trees is the third and last part. Throughout this study is devoted to present two methods for calculating base shear. The moment frames will be investigated in this article.

Structural fuses are some points in structure which are exposed of destruction due to vast internal forces and plastic joints are summed up in them and possible destruction would start in these points, so the best way is substituting well-featured materials such as HPFRCC rather than conventional concrete against earthquake with rigidity behaviour of strain under tension and capability of high energy absorption before failure. The formation and features of plastic joints in HPFRCC beams and frames are investigated in this paper considering 12 concrete and HPFRCC beams and 12 columns with variables such as compressive strength, lateral load type, axial load in column. Results showed that in HPFRCC beams, increasing compressive strength leaded into increasing force and displacement, curve, plastic joint length. HPFRCC beams under concentrated loading had the more displacement and energy absorption, and beam under uniform loading had the maximum force and the beams under two-point loading had longer plastic hinge rather than beams under uniform loading. HPFRCC frames had lateral force and displacement of respectively 7% and 18% more than their relevant concrete frames and curvature, plastic joint length were increased up to 1.18 and 1.30 times of RC frames, respectively. Therefore the HPFRCC material is effective and appropriate for new concrete structures subjected cyclic loading.

Progressive collapse in the structures is defined failure of the entire or major part of a structure that has initiated from the small part of the building, continued in the other structural elements and in this way, progressive collapse become larger and larger in the structural elements, one after another. Finally, the remaining structural system will not be able to resist the lateral and gravity loads. So, the prediction of safety margin in buildings against the progressive collapse due to the earthquake loads is one of the important issues in the earthquake and structural engineering. In this paper, modeling of the progress and distribution of the collapse in the elements of the structures, one after another, was carried out in moment resisting reinforced concrete short buildings due to the earthquake loads and effects of the column removal have been investigated on the progress and propagation of the collapse. For this purpose, the progressive collapse potential of a 3 story reinforced concrete building with ordinary moment resisting frame has been evaluated due to the column removal in presence of FEMA_P695 two component ground motions. According to the results of the nonlinear time history analyses, collapse distribution is not affected by the earthquake records in the progressive collapse and follows a special pattern. In the early stages of distribution, collapse occurred in the beams surrounding and upper parts of the column removal area, then transferred at the height of the structure from the first ceiling to the upper ceiling, vertically and eventually, distributed horizontally in the stories. Therefore, recommendations and suggestions have been presented for seismic optimization of the reinforced concrete short buildings against the progressive collapse. Also, distribution patterns of the collapse have been presented to predict the collapse propagation due to the column removal in presence of the earthquake loads.

According to increase of population in cities, tall buildings and especially framed tube structures have been become interesting for structural engineers. Framed structure act primarily like cantilevered box beams and since they generally have much larger lateral dimensions than the internal shear wall cores, they are more effective in resisting the overturning moments of the lateral loads. However, due to flexural and shear flexibilities of the frame members, the basic beam bending actions of the framed tubes are complicated by the occurrence of shear lag, which could significantly affect the stress distributions in the frame panels and reduce the lateral stiffnesses of the structures. In this paper, a method has been adapted to study the shear lag coefficients for web and flange panels and for point, uniform distributed and triangular distributed loadings and then, the shear lag coefficients for the web and the flange panels are obtained for the parabolic shape lateral loads. Finally, by analyzing a 40-story framed tube structure under parabolic Shape lateral loading using this approximation method and comparing it with computer analysis, the error value in stress and displacement is estimated. The results of the research indicate that this method is suitable for the initial stages of the analysis and design of these types of structures.

After poor performance and brittle fracture of moment connections in the 1994 earthquake in Northridge, researchers proposed new connections to improve the behavior of steel moment frames. Generally, these modified connections can be classified into two main categories: reduced beam section (RBS) and other than RBS connections such as bolted flange plate, bolted unstiffened and stiffened extended end-plate moment, and welded unreinforced flange-welded web moment connections. In this study the behavior of special steel moment frames using each of these two types of connections was investigated. For this purpose, several steel moment frames made up of these two types of connections were analyzed using Opensees software through nonlinear static procedure. For modeling, the behavior of connections was modeled using Bilin material. Then, using FEMAP696, the seismic parameters of all moment frames were determined and compared. Results indicated that the moment frames of RBS connections have higher seismic performance than the other moment frames. Furthermore, based on the numerical results, regardless of the connection type, it seems that 3 is not an appropriate value for the over-strength factor. Based on these numerical results, values equal to 4 and 5 can be proposed for the over-strength factor for moment frames of RBS and other than RBS connections respectively.

With the rapid population growth, the need for elevators is being increased. The common type of elevator used in the building industry is a cable-lifted elevator which requires an elevator pit, machine room, and also counter-weight. Hydraulic elevators have been used for high loads and a few floors and do not require to machine room on the roof. Screw systems such as power screw used more for industrial lifters, due to its low speed, frictional losses, and heat production at higher speeds and also the possibility of buckling. Ball screw has soft motion, low sound and vibration, as well as very low frictional losses, which can have a higher speed than a power screw, and therefore may be applied for the elevator. For this purpose, the design of a ball screw-driven elevator for carrying of six people in three floors with a speed of 0.6 m/s has been considered. The conceptual design of the elevator and its components, selection of appropriate ball screw and nut, design of the straight bevel gear, and finally, the determination of the motor power and speed in both conditions with and without counter-weight have been presented. Comparison of these systems with the conventional traction system show that at the same speed, this method requires a motor with higher power in the case of no counter-weights, but if the speed is reduced to 0.45 m/s, a motor with less power is needed and among the other benefits such as the elimination of the pit, machine room, and the counter-weight, makes this method to be more preferable comparatively and easier to implement in the residential buildings. If the counter-weight is used, the power of the motor is reduced significantly and the system may be performed utilizing the existed single-phase current in home buildings using a proper inverter.