نشریه آنالیز سازه - زلزله
سال بیستم شماره 4 (زمستان 1402)

Advanced analysis refers to a method in which the strength and stability of the system and structural members are recognized in an integrated manner and there is no need to separately check the capacity of the structural members. This approach is a suitable method for evaluating the real behavior of structures and makes structural designers better understand the main characteristics affecting the actual behavior of structures. Undoubtedly, one of the most widely used structural systems in the construction industry is Concentrically braced frames (CBFs). Mainly, these kinds of frames collapse because of a soft story formation in one or more stories in which excessive brace buckling occurs. Using second order inelastic analysis, this study provides an intuitive understanding of the collapse mechanism of CBFs with 6 and 18 stories subjected to mainshock-aftershock sequences. Such understanding will support development of design methods that preclude low-capacity collapse modes specially under multi-shock excitations. This paper assesses the collapse mechanism as a stage in which the imposed seismic energy fails to dissipate and eventually leads to uncontrolled kinetic energy in structure. The investigation focuses on the role and distribution of the various energy measures and different dissipating mechanisms throughout the structures. Collapse mechanism is identified for various combinations of the utilized 32 mainshock-aftershock pairs that are gradually scaled following the incremental dynamic analysis (IDA) process. The distribution of input and dissipated energies along various stories reveals the role of upper stories in damping the imposed energy. Furthermore, the similarity between the height profile of the residual drifts and the story imposed energies highlights the characteristics of the structures in adapting their drift response to a mode with the highest energy absorption.

Reinforcement of structures is not cost-effective despite plastic deformation in the main members. Therefore, this defect can be solved by adding dampers. The function of the damper is in such a way that before the bracing member, it surrenders and prevents the creation of a plastic joint in it. Although the existence of a damper improves the seismic behavior, it does not affect the reversibility of the structure, and their repair after an earthquake is sometimes accompanied by problems due to permanent changes in the entire structure. Shape memory alloys (SMA) as smart materials compensate for many shortcomings of current energy consuming systems. The effect of shape memory system, elastic behavior, inherent damping and high strength are the most important characteristics of these alloys. In this research, a new type of slit damper (under the title of butterfly slit damper) has been analyzed in four groups with different dimensional ratios h1/h and b1/b, with and without SMA, by adding bar-type SMA and placing it in such a way that in loads Compressive and tensile force created in the brace, there is always a tensile force in a number of SMAs. In the proposed combination, the diagonal element under the effect of tensile and compressive force causes cutting in the slit damper and tension in the SMA. The results showed that the use of SMA, in addition to increasing the hardness and resistance of the system, creates the ability to accept and eliminate the phenomenon of buckling of the brace under pressure.

Pervious concrete is a special type of lightweight concrete with low or even zero slump, which consists of cement, coarse aggregate, a limited percentage of fine aggregate (or no fine aggregate), various chemical and pozzolanic additives. In this article, structural LECA with a volumetric weight of 750 kg/m3 and a fixed water-to-cement ratio (W/C = 0.3) was used to make lightweight pervious concrete and the effect of different ratios of lightweight aggregate to cement (A/C) including 1.5, 1.8, 2.1, 2.4, 2.7, 3, total porosity and volume percentage of cement paste on compressive strength , specific electrical resistance and percentage of water absorption of lightweight pervious concrete aged 28 days were investigated. With the increase of A/C ratio from 1.5 to 3, the volume of cement paste decreased from 30.873% to 15.436% in the samples and the total porosity increased from 21.64% to 38.08%, which led to a decrease Specific electrical resistance decreased from 11.45 to 6.841 , compressive strength decreased from 13.27 MPa to 4.37 MPa, and water absorption increased from 11.185% to 12.695% in lightweight pervious concrete samples. The results of this research showed the improvement of physical properties and the decrease of mechanical properties and durability of lightweight pervious concrete containing LECA.

was studied. Compressive, and splitting tensile strengths were used to investigate the mechanical properties of concrete containing ground granulated blast furnace slag (GGBFS) at 28 days of age. GGBFS was used in the form of weight percentages instead of cement (0, 30, 40, and 50%). The results of the experiments were used to simulate the properties of concrete materials in ABAQUS software. According to the results, the compressive and splitting tensile strengths of the specimen with 30% GGBFS were 10% and 5.02% higher than those of the control specimen. Then 12 concrete beams were simulated in ABAQUS software. The effect of bar type (steel and GFRP), the ratio of longitudinal reinforcement, and the mechanical properties of concrete with and without GGBFS were investigated. The results showed that increasing the ratio of GFRP longitudinal reinforcement in beams without GGBFS caused a significant increase in the load-bearing capacity up to 21.89% and a decrease in the central displacement of the beams up to 15.10%. Also, the use of steel bars with the same ratio of longitudinal reinforcement as GFRP bars achieved better results (an increase of 29.38% in bearing capacity and a decrease of 35.15% in the central displacement of the specimens). It should be noted that the values of the bearing capacity and central displacement of the simulated specimens were compared with the corresponding values in the relations provided in the ACI 440.1R-15 guide.

As steel fibers are important reinforcement materials in concrete, in this study, the behavior of hook-shaped steel fibers from concrete is predicted through the use of artificial neural networks. In the absence of comprehensive laboratory data, data obtained from finite element analysis was used for modeling. The simulations are carried out using ABAQUS software's finite element method in 3D. Using the concept of the transition zone of the interface, whose parameters were obtained by inverse finite element analysis and experimental tests conducted on a sample of fibers, this model has been developed to simulate the interaction between fibers and concrete. On the basis of the results of the numerical model validated against the experimental results, the effective parameters of the fibers were extracted, and a neural network was then constructed based on the results. A multilayer forward perceptron artificial neural network and back-propagation training algorithm are used to predict pull-out force, with Marquardt-Lonberg optimization applied. The results demonstrate that the neural network model presented in this research is an effective method for predicting the pull-out force of fibers from concrete, in part because it allows the use of more variables in modeling, as well as delivering more accurate results.

The Dirac delta function provides a simple and effective tool for representing point loads and singularities in structural problems, often leading to closed-form solutions. In this study, buckling of simple double-ended columns with one and two cracks has been analyzed analytically. Although in recent years this issue has attracted the attention of many researchers, the methods presented to solve the problem usually have a significant computational load. Therefore, in this study, a new approach has been used to solve the problem using the property of the Dirac's delta function. This approach simplifies the problem-solving process and significantly reduces the computational cost. Based on this, the crack is modeled with a bilateral behaviour via Dirac delta function. This model takes into account the crack closure effect on buckling behaviour of column by introducing a suitable switching criterion, which allows each crack to be open or closed depending on the sign of the axial strain at the crack centre. The proposed method was used to finding the buckling load, determining the effects of crack stiffness for both one and two-crack scenarios, and accounting for the effect(s) of crack opening and closing on the buckling load. For validation purposes, the finite element software SAP2000 was utilized.

Repairing and Strengthening of concrete structures is of special importance and the mechanical properties of repair mortars and their compatibility with the base concrete are significant aspects in the field of repairing of damaged concrete structures. The bond strength parameter of materials is one of the important properties in the selection of repair mortars. In the current Experimental studies, the bond strength of 7 types of cement base mortars consisting Nano-Silica (NS), Micro-Silica (SF) and Polyvinyl Alcohol (PVA) fibers have been tested on base concrete according to ASTM C882 standard. The base concrete is made of fiber concrete with a target compressive strength of 45 MPa and consisting macrosynthetic fibers. The workability of fiber concrete was 120 mm, and the method of curing the samples was done as the wet method. The obtained results indicate that all 7 types of repair materials of this research were compatible with the base concrete and the sample with the combination of PVA fibers and Nano-Silica in cement-based mortars has increased the bond strength of the samples by 85% compared to the samples without fibers. The highest bond strength among the samples was related to cement based mortar containing PVA fibers and Nano-silica (PVA0.75NS6) with bond strength equal to 21.83 MPa.