نشریه مهندسی سازه و ساخت
سال ششم شماره 1 (پیاپی 23، بهار 1398)

After failure of a column in structure by an abnormal loading, undesirable performance and damage of connections of the beams connected to the top of this column, can lead to the local failure of floor and in turn result in progressive collapse in the structure. Accordingly, the best method for rehabilitation of the structure against this incident is the strengthening of the connections. The strength and flexibility of the connections during the large rotations result in spreading the axial forces in beams and forming the catenary action. This action can greatly prevent structural failure due to the column removal. In this paper, the performance of prevalent bolted and welded top and seat angle steel connections is evaluated numerically. All parts of the connections are modeled in finite element software. The modeled structure is analyzed nonlinearly under the vertical displacement in the location of removed column, and its results have been verified with experimental results. In addition, the behavior, failure modes, how the catenary action develops and the effects of different parameters on connection behavior are investigated. The results indicated that bolted connections have better performance than welded connections.Also, in welded connections, increasing the thickness of the angles and increasing the length of the connected angle leg to column, improve the catenary action and the resistance of the connections against progressive collapse. But adding the stiffener plate in the middle of the seat angle does not increase the connection capacity in all cases.

This paper quantifies the uncertainty emanated from modeling steel structures using a Timoshenko beam. Using continuous beams to model building structures is a conventional approach in structural dynamic analyses. The use of Timoshenko beams, as a class of continuous beam models, in lieu of finite element models significantly expedites the computations. In turn, the reduction in the computational cost facilitates probabilistic, online identification and rapid damage detection of building structures in the aftermath of a seismic event. The prerequisite for structural identification using this method is the quantification of uncertainties that arise as a result of approximating the structure with a continuous beam, which is the primary objective of this research. To this end, an automated system is developed and subsequently employed to design 1000 finite element models of building structures followed by eigen-analyses to determine their natural frequencies and mode shapes. Thereafter, the residual of the eigen-equation of the Timoshenko beam given the frequency and mode shape from the exact finite element model is computed for each structure. Statistical analysis of these residuals for various structures leads to the distribution type, mean, and standard deviation for the probability distribution of the model error. The results show that the error is nearly insensitive to the height and dimensions of the plan, but is heavily dependent on the structural system. The results also show that the error standard deviation for steel moment-resisting frame systems is 3.5 times smaller than that of the steel concentrically-braced systems. This indicates that the Timoshenko beam model class provides a better estimation of steel moment-resisting frame systems.

Among various active parts of construction industry, irrigation and drainage projects (such as dams, spillways and peripheral parts such as canals) are one of the biggest consumers of cement. Various empirical reports show that the concrete made of Ardabil’s pozzolanic cement does not meet the requirement for irrigation and drainage projects in case of mechanical characteristics which signifies the importance of further research in this area. The objective of this study is to investigate the suitability of replacing Ardabil’s pozzolanic cement with type 2 Soufian cement from the viewpoint of compression strenght at irrigation and drainage projecta. This study focuses on experimental methods by performing standard compression strength tests on base and improved speimens. The initial results showed that solely utilization of Ardabil’s pozzolanic cement in structural parts is not feasible and there should be some other approaches to reach this end. In this regard, 4 additive materials were used instead of cement including: stone powder, fined part of sand, silica fume and fly ash. The results indicated that utilization of fly ash increases the compressive strength of the cement in long term application; however, it causes a reduction in compressive strength in short term application. Furthermore, replacemet of silica fume in lower percentages causes a reduction in compression strength; however, higher percentage of silica fume replacement increases compression strength. The finer parts of the sand slightly decreased the compression strength and the replacement of stone powder yields to an increase in long term compression strength.

This article investigates the effect of the roof cover and the stressed skin action on the optimal design of curved portal frames. According to stressed skin effect, diaphragm is carrying part of the horizontal force of structure and the remaining part is carried out by the frames. Therefore, interior frames are designed for lower forces which, in turn, results in a lighter structure. Stressed skin effect is modelled using the equivalent spring theory. A program which links Sap2000 and genetic algorithm was developed that provides the best sections which result in the minimum weight of structure. Designs are performed by using the allowable stress design and stress skin effect. Seven curved portal frames with different span dimensions, lengths, number of frames and column heights are considered and optimally designed for roof cover with thickness of 0.5 mm and 0.7mm. The objective function is defined as the weight of the structure and the problem consists of eight design variables. The results show that consideration of the stressed skin effect results in a reduction of weight by more than 20% and that increasing the height of column results in 10% reduction of the weight. Moreover, it was observed that increasing the length of the span results in a 5% reduction in weight. Effects of increasing the number of frames and changing the length of the structure are negligible and are only about 1 or 2 percent of reduction in weight. Increase in the thickness of the roof cover sheet about 0.2 mm results in a decrease of the weight by about 3%.

Many codes and regulations for designing of steel structures the same as 10th part of Iranian national buildings code don’t offer any solution link-to-column connection in eccentric braced frames. In eccentrically braced frames (EBF), required ductility and a major part of absorption and energy depreciation is done by link to column connection. The ability of link in absorb and depreciate energy is depended on specification and geometric details. So design of link-to-column connection in resistant frames to earthquake with eccentrically braced frames remains an unsolved problem. This paper is discussing about the effect of adding cover plates on improving the seismic behavior of not reinforced moment connection of the link beam and preparing the ductility demand of connection by finite elements method. In this paper the model fracture recognition is introduced with adopting the experimental results with the maximum plastic strain index. Welded link-to-column connections have the potential to fracture in the wing of link beam, before adequate rotation. So three types of cover plates are assessed to delay the fracture of connection. The result was that the triangular cover plates have the best performance.The result was that the triangular cover plates have the best performance.The result was that the triangular cover plates have the best performance.

Steel moment frame and bracing panels compound structures are suitable systems for medium-height resistant skeletons due to their desirable ductility along with high rigidity when facing large dynamic forces caused by earthquakes. In this research, the seismic behavior of two 10-story buildings with compound systems of moment frame/eccentrically braced frames as well as moment frame/eccentrically braced frame with zipper elements are evaluated. The evaluation and changes of the seismic response parameters of the studied models are based on performing nonlinear time history analyses. The studied models are designed according to the fourth edition of seismic design provisions in Code 2800, plus the 6th and 10th issues of the Iranian National Building Regulations. Following the results of this research, it has been recognized that the existence of large coherent pulses in the time history of powerful earthquakes affects the response parameters of the building to a great extent. The results of this research includes the analytical process in the graphs related to maximum push of the drift, floors acceleration and velocity of the studied structural models and also the time history of story’s drift and axial force of the columns. The results show that seismic responses and demands under near-field records have a bigger appearance than the corresponding effects obtained exposed to far-field records.

The effect of seismic vertical components on the seismic behavior of structural systems can be considerable. The vertical component of ground motion records can cause the brittle collapse of columns resulting in global collapse of structures. This effect is more crucial for near fault ground motions with larger vertical accelerations. Nevertheless, the effect of seismic vertical excitations is neglected in time-history analysis procedures on common structural systems. This may results in unconservative results in dynamic analyses. Therefore, it is very important to evaluate the damaging effects of vertical component of seismic excitations. In this paper, the effect of vertical excitations on the seismic response of steel moment frames with regular and irregular configurations along the height. Incremental dynamic analysis is employed to consider the effect of seismic excitations with various intensities. In IDA analysis, the seismic loads are applied to the structures with increasing intensities. In this paper, the seismic loads are applied to the structures in two manners, once considering only the horizontal component and secondly considering vertical and horizontal components simultaneously. The IDA curves are compared for two cases to find the effect of vertical excitations on global seismic demands. Also, the evaluations are made separately for regular and irregular frames to find the effect of vertical irregularity. The observed results revealed that the vertical components may increase the nonlinear displacements, especially for large intensities. The investigations demonstrate that the increase of column axial forces causes brittle buckling of columns and expedites the formation of plastic hinges leading to global collapse of structure. In irregular frames with soft stories, the buckling of slender columns is more crucial than regular structures. Therefore, it is suggested that the vertical components are considered for analyzing steel moment frame structures against near fault ground motions

Concrete performance is of very high importance in civil engineering projects. One of the most common ways to measure the performance of concrete, is the slump test. To save time, money and materials, it is better to use intelligent methods in predicting the slump. Therefore, in this study a method based on soft computing is used, so without the need to perform arduous physical experiments, one can obtain an estimate of the slump.In this study, an adaptive neuro-fuzzy model which has the benefits of both neural network and fuzzy inference system, is used to predict the concrete slump. In order to train the algorithm for future use, comprehensive experimental data is essential .So by collecting data related to 44 concrete slump experimental tests, variables such as water-cement ratio, sand, gravel, silica fume and super plasticizer which are the principal components of concrete, are considered as input variables and the amount of slump is considered as the output variable in the proposed model.In order to evaluate the performance of the proposed model and accuracy of the results, the results of the adaptive neuro-fuzzy model is compared to that of artificial neural network model, which is obtained in a parallel research done by author, by statistical parameters such as correlation coefficient and root mean square error. By averaging the results of ten different classifications of experimental input data, the correlation coefficient is approximately equal between adaptive neuro-fuzzy and neural network slump. While the root mean square error obtained by using adaptive neuro-fuzzy model is 0/4477 which is less than 0/6964 by neural network model. The difference in the output error of the two models are due to different learning algorithms used in two models and unknown number of hidden layers and neurons in the desirable artificial neural network model.

The steel shear wall is an appropriate resistant system against lateral loads that is been utilized in modern high-rise buildings and upgrading existing buildings because of numerous benefits. This article examined the operation usage of this system in compound reinforced concrete moment frame using suggested connections (additional stirrup). According to this, a steel shear wall three-story reinforced concrete single bay frame (suggested connections) modelled and the plastic behavior and its behavior factor with the reinforced concrete frame, stud connections, and bare reinforced concrete moment frame compared. The result showed that the behavior factor system comparing to bare reinforced concrete frame and reinforced concrete frame contains the steel shear wall with stud connections have increased and about 6.5 (the extent) can be considered.The steel shear wall is an appropriate resistant system against lateral loads that is been utilized in modern high-rise buildings and upgrading existing buildings because of numerous benefits. This article examined the operation usage of this system in compound reinforced concrete moment frame using suggested connections (additional stirrup). According to this, a steel shear wall three-story reinforced concrete single bay frame (suggested connections) modelled and the plastic behavior and its behavior factor with the reinforced concrete frame, stud connections, and bare reinforced concrete moment frame compared. The result showed that the behavior factor system comparing to bare reinforced concrete frame and reinforced concrete frame contains the steel shear wall with stud connections have increased and about 6.5 (the extent) can be considered.

Factors such as improper performance of gates, their failure, and inappropriate design of gate dimensions are likely to be followed by dangerous dam overtopping. In this article, the nonlinear equations expressing the flow rate through the dam bottom gate or intake as well as the water head inside the reservoir were converted into linear equations in the state space. Based on these linear equations, the inlet and outlet hydrographs of the dam were plotted and the equilibrium point of the diagrams duly determined. Upon adjusting the equilibrium point between the inlet and outlet hydrographs and the height (head) and volume of the reservoir water, the bottom gate or intake dimensions were calculated. The inflow and outflow flood hydrographs fully overlap in case the smart prediction and flood control system along with the pulse method is used for routing of the flood to the reservoir, such that the difference between the two is negligible. Therefore, the proposed smart system offers sufficient accuracy. Then, the digital controllers and other electronic devices were built using microcontrollers, sensors, ultrasonic distance meters, and radio wave transmitters and receivers. Different programming languages were employed in the design building of systems. The proposed flood prediction and control system is equipped with alarm systems to inform the operators in the case of emergencies. After their design and building, the systems were repeatedly used and tested in the laboratory and open channels. The results were favorable and of high accuracy. This is a simple, useful, and reliable method, and can be a suitable substitute for previous ones.

Natural hazards and human activities bring about overall or local ground subsidence. Predicting the effects of large differential settlement on response of structural frames, especially in congested urban areas, is a main issue in design process as inadvertence can lead to irrecoverable human and financial losses. In order to better understand the consequences of the subsidence phenomenon on the superstructure, a three dimensional reinforced concrete (RC) moment frame designed according to the current Iranian codes of practice is analyzed due to six separate nonlinear analysis cases considering three locations for the increasing differential settlements (under corner, exterior, and interior columns) as well as two support conditions (fixed and flexible); Various results including change in pattern of bridging beam bending moments, axial force redistribution in columns, and also maximum tolerable elastic and inelastic settlements are evaluated. Based on the findings of the numerical models, although cases with the settlement imposed under an interior column have the least vertical downward displacement at formation of first plastic hinge in a beam compared to corresponding cases, the highest capacity to bear differential settlement occurs due to one of the cases of a corner column location. Moreover, formation of plastic hinges and redistribution of axial forces in first-story columns are highly affected by the modeled support conditions.

Compressive strength and flexural properties of the cement mortar, and Water-cement ratio in the mix design, the ratio of aggregates to cement and cement strength class are the main parameters determining the properties of cement mortar. The optimization of the concrete mix design technology plays an important role, recently statistical method that has been considered by researchers to optimize the response surface methodology (RSM). In this article to achieve an optimal mix of factors affecting the quality of the mortar properties of this method has been used. In this regard, 12 mix design has been evaluated. The samples in the vicinity of solutions containing 0, 5 and 10% sodium chloride are located. The result of the paper represents the possibility of using this method is the continuous improvement of concrete mix design. Which can serve as a useful tool in concrete production units and production units used prefabricated concrete. Search optimization results with response surface method for compressive strength and flexural show that optimal mix design to achieve maximum compressive and flexural strength a mix design which the percentage of chlorine used at a rate of 5%, cement strength class is highest (42.5 MPa) and water to cement ratio is the minimum amount.

The redundancy factor is applied to increase the redundancy of structures that not had a sufficient degree of static indeterminacy in many international standards. The redundancy factor is different in the design standards. In the fourth edition of the standard No. 2800, the redundancy factor is proposed to improve the seismic performance of buildings that are considered with two values 1 and 1.2. Therefore, the base shear can be increased to a maximum of 20%, according to the standard No. 2800. In this paper, the effect of the redundancy factor on the seismic performance of the moment resisting concrete structures is investigated using nonlinear static analysis. Also, the numerical evaluation of the value provided by the standard No. 2800 for the redundancy factor is assessed. The studied models have 4, 6 and 8 stories with a story height h=3.2 m where 1, 2, 3 or 4 bays have to be located. ETABS software used for modeling and analysis. Then, the performance of the structures is evaluated by applying once a redundancy factor 1 and again the redundancy factor 1.2. Based on the results of this research, it can be concluded that it is justified to account directly structural redundancy in the design by using a redundancy factor, as proposed in some international building codes.