نشریه مهندسی عمران مدرس
سال بیست و یکم شماره 1 (فروردین و اردیبهشت 1400)

At about 8.00 am of 20th December 2015, a fire started in a shop in Plasco building, located in center of Tehran, which resulted in tragic collapse of the building and claimed the lives of 22 fire fighters and other people. According to the findings of the investigations, the main reason of ignition was non-permitted extension of cablings in the shop of origin of fire; for use of electrical heating devices. This incident took the attention of Iranian society, Engineers and authorities to the need of improvement of fire safety of buildings. Plasco was collapsed after about three and a half hours of the ignition. Therefore the question was arisen how the scenario of the fire growth and temperature rise in the building was. The place of the building and depot of debris were visited and examined by special teams. Steel, concrete and other materials, which some of them were molten due to high temperature, was taken from the debris for further experimental works. Different tests were carried out on the samples. Heat release rate of textiles was discussed in another article. In this paper; the results of physical, mechanical and XRD/XRF tests and mineralogy/petrography examinations have been presented. The most tests carried out on the concrete samples taken from the ceiling system of the building. For comparison purposes, samples were also taken from the apparently non exposed ceilings. A number of these un-exposed samples were tested in a fire resistance furnace and their changes were utilized as a reference for comparison with samples of burnt sections of the building. The results revealed that the concrete cores taken from the fire exposed ceilings had experienced temperatures between 500-700 ᵒC. Taking the thermal and physical properties of concrete into account; this revealed that the gas temperature of fire in Plasco was much higher than these amounts. This was in agreement with findings of FDS modeling, which was presented in another paper and showed temperatures of about 1000-1100 ᵒC occurred in some parts of the building in fire. On the other hand, molten blends of different materials were seen in the debris, comprising molten metal, glass and cementitious materials. It showed that temperatures about 1400 ᵒC could be occurred in the incident. But these temperatures are not usually seen in enclosure fires; rather it must be happened under the debris. Fifteen stories were collapsed at the end of tragedy. In that time, the massive amounts of textiles and other combustibles were burning in five upper stories, while there were still large quantities of textiles and clothes in the lower stories. So, after the collapse of the building, all combustible materials and even gasoil fuel stored in tanks in the basement could be burnt under the debris, approved by observations in next days during the operation of extinguishment and removal of debris. The failed construction systems comprised large panels and steel elements; therefore air flow was possible under the debris for supporting the combustion. On the other hand, large amounts of debris made a relative thermally insulated condition, especially in lower parts, which could produce a temperature rise considerably higher than recorded in enclosure fires. The observation of molten blends of cementitious materials supported this assumption and it was also in agreement with possible maximum temperatures in adiabatic combustions of hydrocarbons, which are far higher than these figures.

Damage occurrence is always inevitable in structures. So far, many examples of damage types in engineering structures have been recorded with many losses of human and financial. For this reason, the detecting of structural damages during its exploitation to provide safety with the lowest cost has been the subject of many researchers in the last two decades. In this regard, the wavelet transform is a powerful mathematical tool for signal processing, has attracted the attention of many researchers in the field of health monitoring of structures. Wavelets are a combination of a family of basic functions that are capable of detecting signals in the time (or location) and frequency (or scale) range. In fact, wavelet transform is based on the principle that any signal can be transformed into a set of local functions called wavelets. Any local feature of a signal can be analyzed using the corresponding wavelet functions. The wavelet transforms to the singularities points in the signals are sensitive and can be used to detect abrupt changes in modes, which often indicate damage. In this study, free vibrations of a four-story building with specified boundary conditions have been investigated and monitored the health of the building basis on experimental results using the continuous wavelet analytical method are studied and the damage that may occur in these structures has been evaluated and analyzed. Building modeling is done in finite element software using the sandwich model. In this four-story building, eight-layer sandwich panel (polystyrene, concrete, steel, concrete) is used symmetrically. The fourteen natural frequencies of the sandwich structure were compared with the experimental model. and the main modes of the structure obtained to influence the health of the structure. An error of less than 2.5% reveals a good match between the results of the two models. Changes in the values of natural frequencies and also the inconsistency of the modes shape، based on Modal Assurance Criterion (MAC) and the angle between modes of shape confirm the damage of the structure. Precast panel health monitoring results show that based on the experimental results, the damage location using the coif5 function with scale parameter 8 has been successfully identified and shows a higher perturbation of the coefficients at the damage locations than the other functions. Thus, the relative maximum and minimum jumps in the wavelet coefficients occurred at the location of the damage and considering the maximum or minimum wavelet coefficients generated at the damage location as the center of damage, the damage center can be identified with an error of less than 8%. The disturbance of the wavelet coefficients of each of the damage locations are independent of the other damage locations with different intensities. Also, the effect of higher modes is more pronounced in the damage intensity index as in the torsional modes of the structure, the maximum wavelet coefficients are greater and the intensity of the damage is increased. In addition, in the process of reducing the structural stiffness, the first and second stories play a more important role, and around the openings are the critical points of the structure.

Regarding the importance of bridges as one of the most critical infrastructures, their maintenance, and health monitoring is of high priority. Interaction between the moving vehicles and bridges is amongst the fields of study that have been investigated in depth by numerous researchers in the field of bridge engineering. Among different proposed methods of structural health monitoring of bridges, the indirect methods that do not need the healthy structure response are of high interest because of their ease and low maintenance costs.
The response of a moving mass passing through a bridge can be analyzed for the indirect prediction of the beamchr(chr('39')39chr('39'))s mechanical properties. This can lead to the detection of possible damages or degradations in the structure. By mounting high precision accelerometers on the moving vehicle and recording the corresponding signals, it is possible to capture the sudden change of mechanical properties pertaining to the existence of damage in the bridge.
In the current study, an FE code is developed in order to analyze the moving vehicle response. In this code, the bridge is modeled as an Euler-Bernoulli beam, and a complete model comprising stiffness and damping of the suspension system of moving vehicle is built. In order to verify the results of the code, comparisons are made with the outcomes of modal analysis. The sensitivity of the FE results with respect to the number of elements is examined. These comparisons clearly show that both methods reach the same values for a sufficiently high number of elements for the moving vehicle response.
Following verification of the code, a brief review of the concepts underlying the variational mode decomposition (VMD) method is given for a self-contained representation. The VMD can be used to decompose a signal into a number of signals with limited bandwidth. Although it has found many applications in different signal processing cases (e.g. in the field of electronics, mechanical vibrations of machines, or even in the analysis of economic and financial time series), extending its application to the field of structural health monitoring is entirely a recent and ongoing topic of research.
After the introduction of the VMD, damage in the beams is implemented by using fracture mechanics concepts. Different damage scenarios are applied in order to check the reliability and robustness of using VMD as a damage detection method. These include different damage locations (single, dual) and damage severity represented in terms of crack depth. By having a reliable means for the analysis, the novel variational mode decomposition (VMD) is applied to analyze the signals recorded from the vehiclechr(chr('39')39chr('39'))s back axel in search of any possible irregularity in the signal properties. By monitoring results attained for several damage cases, the following conclusions can be given:• The variational mode decomposition (VMD) can highlight the presence of irregularities in mechanical properties that can be reached directly from decomposed signals.
• The location of these signal irregularities coincides with the presumed location(s) of the crack(s).
• The severity of the signal irregularity and corresponding instantaneous energy is proportional to the degree of damage imposed on the beam. 
• The moving vehiclechr(chr('39')39chr('39'))s natural frequency plays an essential role in the bridgeschr(chr('39')39chr('39')) structural health monitoring. The signal processing results exhibit amplified abrupt changes for the vehicles with the natural frequencies close to the beamchr(chr('39')39chr('39'))s fundamental frequencies.  
Regarding the above conclusions, analyzing moving mass response with the VMD can be a reliable damage detection technique.

The high ductile of steel moment-resisting frames (SMRFs) during earthquakes has been challenged due to the brittle fractures of their welded (rigid) beam to column connections. Consequently, SMRFs have suffered severe damages and have produced collapse in main structural members (such as beams and columns). During previous years, energy dissipative devices in connections have been developed by researchers to resolve the ductility problem in rigid beam to column connections of SMRFs. Slit steel damper (SSD) as one of these devices contains a plate or a standard section with a number of slits in its web. The damper can dissipate the seismic input energy with inelastic deformation absorption and also prevent seismic energy transmission to the main structural members (such as beam and column). Due to the uniform strut width of SSD, stress concentration at the end parts of the damper struts is produced and unbalanced distribution of von-Mises stresses along the struts is shown. Furthermore, slit dampers are commonly fractured in the end parts of its struts. The low participation of the middle parts of struts in the energy dissipation is caused. Henec, finding the best shape of slits has been attracted by researchers. In this study, new geometry shape of SSD was proposed for improving rigid beam to column connections of steel structures. For investigating the performance of the proposed damper, the behavior of a rigid connection with the common and proposed SSD was assessed subjected to monotonic and cyclic loads in ABAQUS software. The proposed SSD has the same weight in comparison with that of the common SSD. The results of assessment was shown that in the proposed SSD reducing the width of damper slits in two its ends and increasing its middle parts improved its seismic performance in comparison with that of the common SSD. The proposed damper in comparison with common one subjected to shear load can effectively contribute to about 41% of the total dissipated energy. Furthermore, using the proposed damper in a rigid beam to column connection subjected to cyclic loading can effectively contribute to about 51.8% of the total dissipated energy. The performance of the proposed SSD shows that first, the middle part of strip treat like fuse and the suitable ductility provide. Then, the maximum stresses transfers to the top and bottom of strips. Due to the distribution of stresses in more area, the strength of the proposed damper increases. Therefore, withstanding a large number of loading cycles until the failure in this proposed damper, it can be used instead of welded connection in SMRFs.

Concrete is the most widely made construction material in the structural engineering world. Advantages such as high compressive strength, availability of raw materials, and low preparation cost make concrete one of the most important used construction materials. Under harsh environmental conditions, aggressive agents such as sulfates and chlorides penetrate the concrete through these cracks to damage the concrete. While concrete cracks are not only expensive to repair, they are often hard to detect as well. It is now identified that the strength of concrete alone is not sufficient, the degree of harshness of the environmental condition to which concrete is exposed over its entire life is very important. Self-healing concrete is a type of concrete that has the ability to repair itself without the need for an external agent during cracking. Concrete containing microorganisms has self-healing properties. The self-healing agent contains a specific concentration of bacteria with a nutrient in the concrete that produces calcium carbonate while the water and environmental conditions are suitable for the concrete. In this research, four different specimens of concrete have been made. Concrete containing microorganisms is made in two different concentrations of bacterium bacillus pasteurization (107,109 cells/ml) and calcium lactate nutrients and is compared with concrete containing silica fume, concrete containing latex and control concrete. In all four specimens, the same mix design was used with a water/cement ratio of 0.48 and containing silica fume, latex polymer, and calcium lactate, which replaced cement in different percentages. Specimens were subjected to compressive, flexural, and tensile strength tests at 7 and 28 days of operation, and the results were compared. The results showed that the best performance among all specimens for concrete containing silica fume and self-repair agent (bacteria and brain material) increased compressive strength and reduced tensile and flexural strengths compared to the controlled specimens. The use of a self-healing agent in concrete increases the compressive strength of concrete, but this increase is not as great as the increase in silica fume. Bacteria with a higher concentration have a negative effect on the compressive strength of concrete so that more use of bacteria in concrete increases the compressive strength to such an extent that it even reduces the compressive strength compared to the concrete strength of the control specimens. The self-healing agent reduces the flexural and tensile strength of concrete, as opposed to silica fume but they are better than latex and produce better results.

A large main shock may consist of numerous aftershocks with a short period. The aftershocks induced by a large main shock can cause the collapse of a structure that has been already damaged by the preceding main shock. These aftershocks are important factors in structural damages. Furthermore, despite what is often assumed in seismic design codes, earthquakes do not usually occur as a single event, but as a series of strong aftershocks and even fore shocks. In other word, structures that are located in seismically active regions often may be subjected to successive earthquakes which occurred with significant PGA in a short time after each other. For this reason, this paper investigates the effect and potential of consecutive earthquakes on the response and behavior of reinforced concrete structures. For this purpose, the response modification factor (R factor) which is one of the significant parameters in the structural design of buildings and decreases the lateral forces induced by earthquakes, is calculated and estimated for reinforced concrete moment frames under critical single and successive earthquakes. Thus, three reinforced concrete moment frames with 5, 7, and 12 stories are designed according to Iranian seismic codes (standard No. 2800) and modeled in Opensees software. After the design of the samples, critical seismic scenarios with/without successive shocks are selected from “PEER” center. Consecutive earthquakes not only occurred in the similar directions and same stations, but also their real time gaps between successive shocks are less than 10 days. In the following, R factors of RC moment frames are calculated from the results of incremental dynamic analysis (IDA(, time history and nonlinear static analysis (pushover). The results show about 20% reduction in the R factor and, also increment of damages under successive earthquakes comparing to the individual one. Finally, the idealized multilayer artificial neural networks, with the least value of Mean Square Error (MSE) and maximum value of regression (R) between outputs and targets were then employed to estimate the R factors. Theses artificial neural networks are designed based on the features of frame properties, successive earthquakes. Comparison of predicted R factors with real values indicates the adequate ability of networks in estimation of the results. So that, the average error for the artificial neural network model for predicting the calculated results from IDA, Pushover and Linear Analysis is less than 4%. To be more specific, more than 73% and 93% of the simulated R factors are within ±5% and ±10% of the real values for artificial neural network model. This is an indication that the networks have learned to generalize the unseen information well and reflects good precision in simulating. Moreover, it can be seen that the values simulated by the artificial neural network model spread around the 45 degree line which implies neither over-estimation nor under-estimation.

Conventional construction cementitious composites typically contains cement, fine and gravel aggregate and additive in a specific ratio along with water. It is extensively used to bind the structural elements together like the bricks, stones, and concrete blocks, or end connection of the column and beam, and to develop a sufficient bond with the substrate as a repair cementitious composite, due to its several advantages, such as low cost, appropriate compressive strength, and easy access. However, some weaknesses of the cementitious mortar influence its performance under different conditions. For example, low tensile strength, brittle behavior, unacceptable performance against shrinkage cracks, and lack of resistance against stress concentration are some of these critical properties of the mortar, if not modified, the structures will be deteriorated in a short time. These deficiencies emerge from extravagance water, bleeding, plastic settlement, shrinkage stress and strain concentrations due to external limitations. When loads are applied and further increased, type of cracks grow and reach a critical condition, and catastrophic failure is precipitated. In this situation, the mortar will be exposed to severe damaging factors such as premature saturation, disadvantage of freeze-thaw, scaling, and corrosion of steel. In recent years, researchers in the field of concrete technology have focused on the using of a variety of fibers such as carbon, steel, glass, polypropylene and basalt fiber into the cementitious composites to improve their mechanical properties (especially their ductility behavior) and to some extent their durability. Accordingly, in the present study, the hybrid effect of different percentages of basalt and polypropylene fibers on the workability, mechanical behavior and durability properties of cementitious mortar was investigated. Polypropylene fibers are known in the field of reinforced concrete, but basalt fibers are a new potential additive in this field. In recent decades, researchers have studied more about basalt fibers because of their potential reinforcement properties in composite materials. The basalt fibers are an appropriate replacement for another fibers, including glass, steel, and carbon fibers in plenty of applications due to their excellent properties such as high mechanical properties specially tensile strength and flexural strength, good resistance to low and high temperature, low cost, durability, vibration resistance, high elasticity modules, great failure strain, acceptable persistence to chemical assault, impact load and fire with less toxic materials. Samples containing a hybrid composition of 0.05 and 0.125 percent (weight percent of total cement and aggregate) of the basalt and polypropylene fibers have been used in order to evaluate the effect of fibers so that a total of 4 types of mixed designs containing hybrids of basalt and polypropylene fibers were made and its results have been compared with a control sample. As expected, after analyzing the results, the fibers had no significant effect on the compressive strength of the cementitious composite, while the results of this study reported a favorable and remarkable performance of these fibers in increasing flexural and splitting tensile strength, as well as the water absorption of cementitious mortar is favorably decreased by the fiber. The sample containing 0.125% basalt and polypropylene fibers increased flexural and splitting tensile strengths by 28 and 23%, respectively. Also, the sample containing 0.125% basalt fiber and 0.05% polypropylene fibers resulted in 9.3% increase in compressive strength, 24% decrease in sorptivity and 15% water absorption. The results of the current study have shown that the simultaneous use of basalt and polypropylene fibers can improve significantly the mechanical behavior and durability properties of cementitious mortar, whereas according to the previous studies if each of these fibers is used separately, such a desirable result will not be obtained.

Precipitation affects quantity and quality of water resources and agricultural production. Therefore, the estimation and analysis of its spatial-temporal variations is of great importance. In many regions of Iran, limited spatial-temporal information is available due to sparse distribution of monitoring stations and short observational records. On the other hand, dependency of rain-fed and irrigated production systems on precipitation increases the importance of the analysis of spatiotemporal variations of this weather variable. One way to address this limitation is to use regional/global gridded datasets. In this study, monthly precipitation data were obtained from the CRU dataset (developed principally by the UKchr('39')s Natural Environment Research Council (NERC) and the US Department of Energy) and used to investigate temporal trends in annual, seasonal and monthly precipitations in 675 grid cells (0.5°×0.5°) across Iran over two periods, 1957-1986 and 1987-2016. The results of the previous studies showed that the CRU gridded dataset offers quality data in Iran, especially for trend analysis. Also, the accuracy of the CRU dataset was validated in 14 selected stations regarding monthly precipitations and temporal trends over two different periods, pre-1987 and post-1987. The significance of temporal trends was assessed using a modified version of the rank-based nonparametric Mann-Kendall (MK) test. Trend magnitudes (i.e. slope) were estimated with the Theil-Sen approach and the Trend Free Pre-whitening (TFPW) procedure was applied to remove the effect of serial correlation. The results confirm the acceptable accuracy of the CRU dataset for trend analysis purposes, especially over the last three decades, except in the northern strip of the country (RMSE=10.71mm, R2=0.84). Two 30-year periods (1957-1986 and 1987-2016) were compared in terms of spatial patterns and temporal trends. Annual precipitation over the last three decades (1987-2016) has decreased as compare to the previous 30-year period (1957-1986) in most parts of the country. Over the last three decades, around 42% and 50% of the country’s total area experienced significant and insignificant decreasing trends in annual precipitation, respectively. National average annual precipitation has decreased by 15.78 mm/decade over the same period. Three important regions regarding agricultural production experienced the most significant reductions in annual precipitation: (1) Ardebil, East Azerbaijan, Kurdistan, Kermanshah, Ilam, Lorestan, Zanjan, Hamadan, and parts of West Azerbaijan, Markazi and Gilan (in the west and northwest), (2) Sistan and Baluchestan, Kerman, and southern parts of South Khorasan (in the south and south east), and (3) North Khorasan, northern parts of Razavi Khorasan and east of Golestan (in the east and north east). Reduced annual precipitation was mainly attributed to the reduction in seasonal precipitations in winter and spring, which have critical role in agricultural production and domestic water supply. Temporal trends were also analysed at the monthly scale. January, February, March and December revealed the largest number of grid cells with significant decreasing trends over 1987-2016 while November is the only month with significant number of grid cells experiencing significant increasing trends. The results of this study show that the monthly time series of the CRU TS 4.01 dataset, which has an almost complete spatial and temporal coverage in Iran over the last 60 years, are promising alternatives to weather station observations especially in data-scarce regions of Iran. Analysis of variations and the seasonal and monthly scales help understand the recent climate change and target the most crucial features of it when it comes to formulating adaptation strategies.

Nowadays, the major concerns to meeting the growing demand for water and the sustainable development goals in the nation are these facts that a relatively large area of the nation is in arid and semi-arid regions and the annual aridness continuously increases. Generally, however, the solution to this concern could be tackled by further research on the river engineering issues and providing appropriate solutions for better use of the river water resources. One of these solutions is to minimize the sediment while diverting water away from its natural path into an intake, as the sediment transport through the intake is a serious problem. However, 3D flow conditions in river divisions makes it difficult to characterizing the flow of water and sediment into the intakes. The results of the most recent studies suggest application some patterns, including Submerged Vanes, Sills and spur dikes, to minimize the sediment transport into the intakes. However, it has been shown that the cut-off of sediment flow into the intakes may not completely occur by applying the above-mentioned technologies, individually.
This study aims to assess the influence of these structures application on the amount of sediment flowing into the intakes, individually and as various layouts, under different hydraulic conditions. Three different variables including ratio of bed sediment transport into intake, volume fraction of sediment deposited within intake and dividing stream-plane were studied to better analyze the sediment volume entering the intake. In this regard, the parameters associated with the  dividing near-surface stream Index, dividing near-bed stream Index” and “the ratio of the width of dividing near-bed stream to dividing near- surface width Index” were studied at the up-stream of the main channel, under different hydraulic conditions and sediment control systems.
The results indicate that the simultaneous use of “sill-spur dike”, as well as “sill-spur dike-submerged vanes” has high efficiency in sediment removal at the intake. Furthermore, dividing stream-plane can be significantly influenced by all the sediment control structures used herein, although the effect of spur dike and submerged dikes was  noted to be more profound. Generally, it can be concluded that spur dike causes an important decrease in “dividing near-surface stream Index”, “dividing near-bed stream Index” and “the ratio of the width of dividing near-bed stream to dividing near- surface width Index”, thus resulting in significant reduction in the sediment entering the intake. This observation may attribute to the induced flow width reduction in the main channel usually associated with converting the flow direction from the straight line into a curve, thus creating a secondary circulation flow. On the other hand, submerged vanes considerably minimize the amount of sediment flowing into the intake through increasing “dividing near-surface stream Index” and decreasing “dividing near-bed stream Index” and “the ratio of the width of dividing near-bed stream to dividing near- surface width Index”. It may be linked to the powerful secondary flow developed at the intake-entrance.
The results also show that the change in the length and height of the spur dikes and sills respectively can induce significant changes in dividing stream-plane, which in turn causes dramatic changes in the sediment transport into the intake. Additionally, the parameters of dividing stream-plane, ratio of bed sediment transport into intake, and volume fraction of sediment deposited within intake are strongly determined by the amount of discharge rate, in a way that the rise in discharge rate leads to increase of “dividing near-surface stream Index” and “dividing near-bed stream Index”. However, a decreasing trend in dividing flow ratio was observed, in some systems and discharge ratios.
As opposed to the case, where the sediment control systems were not used, a reduction of 80% and a growth of more than 100% were observed in dividing near-bed stream Index and dividing stream-plane, respectively, when some systems were installed in the main channel. It, in turn, decreased the entering sediment down to 90%. In the same manner, a reduction of 50% was found in the ratio of the width of dividing near-bed stream to dividing near- surface width Index when the sediment control structures were mounted, resulting in a decrease of  100% in ratio of bed sediment transport into intake approximately.

The loading rate and specimen size are two main influential factors which control the tensile and compressive strengths of Quasi-brittle materials such as concrete, ceramic and rock. Most of the studies in the past have been focused on the size effect in the static loading situations i.e. in situations in which the effect of the loading rate and inertia can be ignored. In particular, fracture mechanics size effect have received substantial attention both with respect to the physical testing and the numerical modeling. On the other hand, combined effect of the specimen size and loading rate on the rock strength has received little attention in the literature. Understanding the dynamic size effect of Quasi-brittle materials such as rock is essential for better analysis and design of rock structures. This is particularly the case when rock is subjected to the blasting loads or when it is prone to the strain bursting. Studies on the failure of rock under the coupled effect of specimen size and loading rate are far from sufficient. Due to the limitations of the laboratory test devices, limited research efforts have been conducted on the size effect of materials under dynamic loading. In this study, a 3D hybrid finite-discrete element code called CA3 was used to simulate the Split Hopkinson Pressure Bar test. The Incident and Transmitted bars were modeled by the finite element method while the Brazilian specimen was simulated using a Bonded Particle Model (BPM). The bars were assumed to beave elastically while the simulated specimen could develop micro and macro cracks which eventually could end up to complete disintegration and failure. Brazilian specimens with different sizes were numerically modeled. The specimen contained a vertical notch so that fracture mechanics size effect under high strain loading rate could be studied. The samples were subjected to different loading rate by adjusting the incoming wave in the incident bar. A micromechanical model in which the contact bond strength was allowed to vary in proportion to the relative velocity at the contact point of the involved particles was employed to capture the loading rate effect. The effect of sample size on the dynamic tensile strength of rock was explored and compared with the static size effect. The results were analyzed and discussed using the dimensional analysis approach. The numerical results suggest that the dynamic size effect on tensile strength of rock is different from the static size effect. While for small loading rates, the rock strength reduces as the specimen size increases, this is not the case when high loading rates are involved. For high loading rates, with the increase in the specimen size, the tensile strength initially increases. However, with further increase in the specimen size and the increase in the distance between the notch tips and the impact points, it appears that the inertia and loading rate effects reach to a stable situation, i.e. with further increase in the specimen size, the material strength remains constant. This interesting observation is discussed and compared with the published data in the literature.

The 1994 Northridge earthquake motivated the researchers to overview the conventional design philosophies. At that point, one of the safest lateral load resisting systems was the fully restrained welded steel moment frame and it had been the dominant design choice in the seismic regions. The confidence in the fully restrained frames has been decreased by brittle failures of welded connections. Thus, the researchers introduced the new seismic structural systems; braced frames and hybrid frame.
Hybrid steel frame is a new lateral resistant steel moment frame that is designed based on introducing the new energy dissipating mechanism. In order to enhance frame’s seismic performance, selected rigid connections are replaced with the ductile energy dissipating semi-rigid connections. This concept at the first glance is similar to the eccentrically braced frame. In the eccentrically braced frames, structural fuses are isolated links distributed throughout the frame, while in the hybrid frames fuses are semi-rigid connections placed at the selected locations with particular patterns. The seismic performance of hybrid frame is in such a way that story drift results in the rotation of the semi-rigid connections. Thus, for a properly designed connection that behaves in a ductile manner, the rotation is absorbed by angle or plate yielding without bolt or weld fracture. It would lead to excessive end plate or angle distortion at ultimate rotation that can be retrofitted after earthquake.
In this research, the ductile semi-rigid connection is used in hybrid frames. Finite element modelling of the hybrid frame is conducted in OpenSees computer program. The semi rigid connections are implemented in OpenSees by nonlinear plastic rotation ends. The nonlinear hinges are modelled by using Ibarra Krawinkler deterioration model. The panel zone is also modelled by Krawinkler model proposed in the FEMA 355C. Then, several different patterns and locations of semi-rigid replacements within 3 story benchmark SAC frame are selected. All the frames are subjected to nonlinear static analysis as well as cyclic displacement analysis. For the assessment of the frames subjected to seismic excitations, nonlinear dynamic history analyses are conducted subjected to 40 Los Angeles records. The finite element numerical model of the SAC frame is also verified by comparing the results with the technical literature. Normalized base shear, energy dissipation capacity, and maximum story drift angle under 40 Los Angeles records are obtained for each frame. Finally, based on the mentioned parameters and design criteria the frame with desirable performance is selected.
In general, hybrid frames have less base shear, less energy absorption, and more drift compared to frames with rigid joints (SAC frames). The lower energy absorption of these frames is due to the fact that in beams that are connected to the column in a rigid manner, the plastic joint of the beam is not activated and the semi-rigid connection is responsible for energy absorption. The geometric characteristics and cyclic behavior of semi-rigid joints are such that they absorb less energy than plastic beam joints. The data obtained from different analyzes on hybrid frames are slightly different and this shows that the effect of semi-rigid joints in short-term frames is not significant.

As service years increase, the corrosion of steel rebar stands out as a major problem for existing reinforced concrete (RC) structures in corrosion-inducing environment. The mechanism of steel corrosion in concrete is an electro chemical process, which is often accelerated by the ingress of aggressive chemicals, for example chloride ion. The accumulation of corrosion products on steel rebar is able to generate circular stress which could result in cover cracking. Corrosion of steel rebar will degrade the physical appearance and reduce its original cross section. Corrosion often appears to be non-uniform and localized. Corrosion damaged RC elements displayed smaller yield strength, ductility, energy dissipation capacity, etc. Corrosion level of stirrup tends to be higher than longitudinal rebar due to smaller diameter and less cover protection. Stirrup corrosion decreased confinement behaviour on concrete, thus exacerbating the degeneration of the deformation capacity and the ductility of the RC structures. The corrosion of reinforcement steel bars (rebar) is a natural electrochemical reaction RC structures have to face with. It is exacerbated by exposure to corrosion-inducing environment factors, including de-icing salt, marine salty water, carbon dioxide, sulfur dioxide, etc. The chloride from salt (NaCl) could make hazardously chemical attack on steel bar by acting as an efficient catalyst in the corrosion process. The corrosion of steel bar in the existing reinforced concrete structure has raised great concern over its safety and seismic performance among practising engineers, researchers and residents, etc., because steel bar is the most essential element in RC. Corrosion reduces the effective cross-section area of longitudinal and transverse rebars.
Due to a small concrete cover of transverse rebars compared with longitudinal rebars, the corrosion of them becomes earlier and more severe, leading to cracks in concrete, a decrease of confinement, an intensification of reduction in deformation capacity and ductility of reinforced concrete structures. For this purpose, an experimental investigation is carried out on reinforced concrete specimens include spiral and stirrup and the variables include the corrosion percentage, rebar diameter, transverse rebar pitch, and confined core diameter. Results demonstrate that the high degree corrosion has a fewer significant effect on the reduction in confinement strength, and smaller-sized transverse reinforcements are less sensitive to corrosion.

Although fossil fuel resources are declining, their use also pollutes the environment. Also, due to the increase in the average wind speed in the world, the use of wind turbines, which are classified in structures with new and renewable energy, will be very cost-effective. Wind turbine towers can be made of concrete, steel, conical, lattice, wood, or multi material. Given that the investment cost to build a wind farm and connect it to the transmission network is 75 to 85 percent, and the cost of building the structure is 15 to 25 percent of the total cost. Steel lattice towers can reduce the cost of building a wind turbine structure by 30 percent and therefore, a complete and correct model for analyzing these types of structures will be very important and economically noteworthy. Wind turbine lattice towers are usually made and executed with bolt connections. In this case, the number of bolts is very high, which increases the need for cyclical and reciprocal loads. Joint slip in these structures refers to the relative displacement of bolt connection under the influence of force. Therefore, creating a joint slip will be inevitable due to the ease and speed of execution in which the bolt hole is made of a larger bolt diameter. Joint slip increase the displacement of lattice towers So much so that the maximum displacement of the tower is twice as high as that of static methods. And not considering it will destroy the tower and assuming the reliability factor will make it uneconomical. In this type of structure, the tower is often made of angle and single angles are used in cross members and bracing and double angles are mostly used in the bases of lattice towers. With this explanation, in this study, the force curve of the displacement of the of three samples with single angle section in the laboratory and four samples with double angle section in Abaqus software was modeled and were affected by reciprocating loads and then the results of numerical modeling were validated with laboratory samples. In the models modeled in the software, after sensitivity analysis, the type and size of the mesh is precisely minimized the resulting error. In this investigation available data in joint slip develops bolt connections which include angles with equal leg. It offers force-displacement curve of different connections for double angles and their connections damping ratio are calculated likewise and the effectiveness of each variable on the slip of the node is expressed in double angles. The results show that joint slip occurs during service loads and this effect depends on the number of bolt, the diameter of the bolt, the bolt cross-sectional area, the thickness of the angle and the effective cross-sectional area among these, screw diameter is the most important variable for predicting joint behavior. Also, the viscosity damping ratio for single and double angle connections is almost equal and can be assumed to be 42.5. This ratio increases with increasing number and decreasing bolt diameter. This investigation is beneficial for designing wind turbine lattice towers and in it, provides structure behavior to the designer more accurately.

Today, the embankment dams are considered more prominent than concrete dams because of its formation and lower cost. According to the official site of the International Commission on Large Dams, embankment dams account for about 64 percent of the worldchr('39')s total dams. The results of the statistical analysis of the International Commission on Large Dams showed that the main cause of damage of half of the embankment dam is erosion that usually occurs during the first impounding of the dam reservoir. Therefore, the stability and leakage conditions of the embankment dam should always be examined. Moreover, the analysis showed that drying of the embankment dam due to drought and re-watering the reservoir is similar to the first dam impounding and therefore it is necessary to investigate it. One of the factors affecting the physical and mechanical properties of embankment dams is the effect of drying and wetting cycles of the core soil due to high fluctuations in reservoir water levels during prolonged dry periods and re-watering. In this study, the effects of frequent wetting and drying cycles of Doosti dam clay core were investigated. Investigation of the effect of frequent cycles of wetting and drying on compressive strength of soil showed that by applying drying and wetting cycles on specimens prepared from Doosti dam core borrow area, the compressive strength of specimens decreased. Increasing the number of wetting and drying cycles of the specimens increases the intensity of the compressive strength reduction of the specimens so that the greatest decrease in compressive strength occurred in the third cycle. Application of wetting and drying cycles on the specimens showed that application of such cycles reduced the amount of soil cohesion. The highest decrease in cohesion of the specimens occurred in the first three cycles and in the next four cycles, the intensity of cohesion parameter decreased. The cohesion of the specimens was reduced by 50% by applying six drying and wetting cycles. Results showed that with increasing number of drying – wetting cycles, there is no significant change in internal friction angle. Furthermore, the results of the hydraulic conductivity test showed that the application of six cycles of drying and wetting increased the hydraulic conductivity by 1.9 times. Furthermore, the characteristic curves obtained in this study showed different drying and wetting cycles, indicating that the percentage of volumetric water content in all cycles decreased due to increased suction. The soil-water characteristic curve also shifts downward with increasing number of wetting and drying cycles, decreasing soil water retention capacity and increasing the possibility of internal erosion and dam instability. Finally, Doosti dam numerical modeling was performed using GEOSTUDIO software in end of construction conditions and in two cases without drying and wetting cycle and with six drying and wetting cycles. The results obtained for numerical modeling in general were very consistent with Doosti dam instrument readings. Moreover, the results of the core settlement after the six drying and wetting cycles compared to the non-cycled state in numerical modeling showed a 38.3% increase in the dam core settlement.

Compressive strength (CS) and rapid chloride permeability test (RCPT) are the most important tests in the concrete industry. CS is the most significant characteristic of concrete mechanical properties that can show other mechanical properties like the module of elasticity. Chloride penetration could degradation of concrete durability. In the Persian Gulf, chloride penetration is the most dangerous effect on steel rebar corrosion. Therefore, CS and RCPT are related to mechanical and durability properties and should be studied more carefully. In this research, CS and RCPT are predicted using soft computing. For this purpose, Bayesian inference is used for prediction of them. Bayesian inference is a subset of linear regression but unlike conventional regressions that are deterministic, this type of regression is probabilistic. So, in this research is used of probabilistic analysis replaced deterministic analysis. Gene expression programming (GEP) is used for comparison of their results versus Bayesian inference. For research performing, 100 concrete samples containing metakaolin are considered that 75 samples are selected as training, and 25 samples are selected as testing data. seven input data are considered for prediction of CS and RCPT that contains the age of concrete (day), cement (kg/m3), water (kg/m3), metakaolin (kg/m3), fine aggregate (kg/m3), coarse aggregate (kg/m3) and surface resistance (KΩS). Output parameters are CS (MPa) and RCPT (Coulomb) that for predicting them, independent analysis should be performed. Results show that Bayesian inference in CS prediction has an excellent ability that the R2 coefficient for training and testing is 0.96. These values for GEP were 0.93 and 0.96 respectively. Values of root mean square error (RMSE) and mean absolute error (MAE) in Bayesian inference for training are 2.55 and 1.84 MPa respectively. These values for testing are 2.75 and 2.25 MPa. The values of RMSE and MAE for GEP training are 3.46 and 2.60 MPa and for testing these values are 3.43 and 2.65 MPa respectively. A comparison between evaluation parameters (i.e. R2, RMSE, and MAE) showed that Bayesian inference and GEP have excellent accuracy. In Bayesian inference, R2 coefficients for RCPT training and testing are 0.98 and 0.97 respectively. These values for GEP are 0.96 and 0.97 respectively. RMSE and MAE values in Bayesian inference for training are 223.14 and 161.58 Coulomb and these values for testing are 269.56 and 233.25 Coulomb respectively. RMSE and MAE values for GEP in training are 311.73 and 239.34 Coulomb respectively and these values for testing are 306.92 and 252.67 respectively. Results of CS and RCPT are showed that Bayesian inference is a good method for the prediction of concrete properties. On the other side, Bayesian is linear and has a little time consuming compared to nonlinear methods like GEP. In the next part of this study, first-order reliability method (FORM) is used for reliability analysis of CS and RCPT. Reliability index or beta and probability of failure (Pf) are the most important component in FORM analysis that are calculated in each analysis. For this purpose, mean values of input data are selected as inputs in reliability analysis. Results of reliability analysis indicated that when the CS is considered less than 45 MPa, the probability of failure is not considerable. Reliability analysis of RCPT in concrete samples is indicated that the value of 2000 Coulomb is a threshold value for the probability of failure. Therefore, if the RCPT of concrete samples is less than 2000 Coulomb, the probability of permeability is increased.