steel structures

In the seismic active areas, strong ground motions usually consist of the numerous successive shocks (Foreshock-mainshock or mainshock-aftershock), which have the significant potential to increase the structural response and cumulative damage. This phenomenon (as called seismic sequence) can affect on the behavior of structures, control the seismic performace of buildings. Multiple earthquakes which have been recorded in all parts of the world are proven that the structures located in the mentioned areas are not only experienced a single event, but also they withstand a series of shocks. Due to the high importance of consecutive earthquakes, application of buckling restrained braces (BRB) and shape memory alloy (SMA) materials as smart materials in engineering sciences in the past decades, this paper tries to evaluate the seismic performance of steel frames equipped with buckling restrained brace by determination of the optimal percentage of shape memory alloy under successive earthquakes. Because SMA has unique advantages and characteristics such as no need to replace after an earthquake, high resistance to corrosion and fatigue, the ability to absorb high energy, the ability to return to the original state by applying temperature, tolerating strain up to about 10% without leaving residual strain after an earthquake, and tolerating multiple cycles of loading and unloading, various applications can be found separately and combined in controlling the behavior of structures. It should be noted that despite the high damage potential of successive earthquakes, they are neglected in the seismic codes and design earthquake is still proposed without successive shocks. Hence, the acceptance of new methods for improving the seismic performance of structures under consecutive shocks seems necessary by the engineering community. Therefore, in this regard, 4 and 7 story steel frames with diagonal buckling restrained braces representing short and mid rise structures were designed based on Iranian codes in ETABS software and then implemented in OpenSees software. After selecting the reference model, the performance of the studied models is verified for the linear and non-linear region through comparison of periods and pushover curve of reference and implemented model. In the following, different percentages of shape memory alloys including 20, 40, 60, 80 and 100% for the 4 story steel frames and 5, 10, 15, 20 and 25% for 7 story steel structure has been considered. The studied models are analyzed with/without shape memory alloys under seismic scenarios with and without seismic sequence in Opensees software. For this purpose, critical successive shocks are selected based on effective peak acceleration (EPA) from PEER center. For compatibility aspects between the seismic analysis and seismic design, the selected records should be scaled by designing spectrum for each fundamental period of studied structure in order to have identical spectral acceleration. The results of nonlinear dynamic analysis show that with the increase in the percentage of shape memory alloy in the 4 story steel frame, the response ratio of steel frames under single and consecutive earthquakes increased, but in the 7 story steel frame, it almost decreased, and this reduction is better felt in the higher stories under the single earthquake. Finally, the optimal percentage of shape memory alloy among the selected percentages in the present study is suggested to be 20% for 4 story steel frame and 15% for 7 story steel frame.

tuned mass dampers are a combination of mass, spring and dampers. tuned mass dampers are set on a specific frequency range, when the structure is subjected to external force in the desired frequency range, TMD reduce the responses of the structure by creating a force against the movement direction of the structure. The most important limitation of TMD is providing a sufficient mass ratio in order to optimally control the structure. The inerter element is an element with two terminals that can produce a force proportional to the acceleration difference created in its two terminals. One of the distinguishing features of this element compared to other structural control tools is the possibility of changing the mass matrix of the structure. Nowadays, extensive research is being done in order to combine the inerter element with structural control systems. Adding an inerter element to conventional tuned mass dampers significantly increases the efficiency of TMD and removes the limitation of TMD to provide a suitable mass ratio. The placement position of the inerter element in the control system is very important. If the inerter element is placed between two inappropriate levels in the system, it will disrupt the function of absorbing dynamic vibrations and cause the responses to intensify. In this research, the optimization of the parameters of the inerter element combination with tuned mass dampers is done using the particle optimization algorithm and with the ability to generalize to all structures

In the past years, the use of steel structures with Saddle (khorjini) beam to column connections was one of the common construction methods in Iran. But the behavior of steel buildings with this type of connection in earthquakes such as Manjil 1369 and Bam 1382 showed the failure of connections before the failure of beams and columns. Considering the advantages of this connection, if new details and proper performance are provided, The Saddle connection can be used as a suitable connection in modern steel structures. For this purpose, in this paper, a new rigid saddle connection with reduced beam sections (RBS) for earthquake-resistant steel structures is presented. In the rigid saddle connection, two beams pass continuously on both sides of the column and are connected to the column by vertical plates. The RBS method has disadvantages such as local buckling of the web and lateral torsional buckling, to avoid these weaknesses and improve the performance of the connection, diagonal stiffeners were used in the beam web. Three laboratory samples were made and tested using IPE 140 profiles. According to the results obtained from these experimental tests, it was found that the sample with a reduced cross-section can withstand a rotation of 0.06 radians and the sample with stiffeners can withstand a rotation of 0.07 radians without any significant drop in the strength, which shows that both connections have an acceptable performance considering the Requirements of the AISC standard, and are placed in the category of rigid connections that can be used in the special moment frame systems. Also, the use of diagonal stiffeners increases energy dissipation and plastic rotation capacity of the connection.

Current research is to use artificial intelligence computation to provide algorithms that meet all Mabhas-6, Mabhas-10, and Standard-2800 regulations of national building codes while minimizing the weight of structures. In previous studies, the control of boundary conditions and optimal design of steel moment frames was done in 2D and only with older algorithms. One of the most important tasks of this research is to perfect new constraint rules to control all constraints and to optimally design the common short and medium length steel structures. All the necessary regulations and controls for the three main types of steel structures widely used in the country: 1) braced frames and 2) frames with shear walls, and 3) a dual system of moment frames and shear walls are implemented. Finally, the results of this algorithm were validated in the Tabriz hospital project. One of the valuable results of this work is that it facilitates a safe design and control process for steel buildings by providing a graphical panel of model inputs and displaying all the key results of the structure in the form of text and diagrams. The work process is intelligently programmed to automatically run by simply inputting a standard ETABS file into the optimal seismic design process algorithm. In the ETABS file, each group sections are defined as a list to allowing artificial intelligence algorithms to choose the best placement. The hospital project demonstrated design safety as well as the ability to significantly reduce the total weight of the steel.

Using a Tuned Mass Damper (TMD) in a structure, is a reasonable solution for absorbing its movements caused by external forces. However, when designing a TMD on the grounds of passive control, it is a challenging task as this device can be tuned once and for a specified range of frequency. Employing more than one TMD is another option; although this will lead to higher cost and might increase the base shear of the structure. In this paper, to provide a wide range of frequency and mode shapes in the analysis, nine types of steel structures are designed, having the story number of 4, 8, and 12, respectively, and then subjected to 22 acceleration records of FEMA-P695; These records, are a suitable choice for generating statistical results as they provide a wide range of magnitudes. Three of these structures are uncontrolled, and the remaining are equipped with a TMD on their roof, being of 0.5% and 1% mass ratio and considering the first mode frequency for the TMD design. The design of the TMDs is carried out via Den Hartog's formula.Using incremental dynamic analysis (IDA), fragility curves are created with constant 0.1g steps for PGA intensity measure. In addition, for considering the uncertainties in the performance of the TMD and the structure due to the changes in frequency, a 10% error is applied for the first mode frequency in the nonlinear design of the structures. The maximum drift ratio is used as a damage measure due to its simplicity and comprehensive coverage. Multiple earthquake recordings and their statistical characteristics, such as mean, median, 16%, and 84% of the recorded amplitudes and their more robust components, are utilized to examine the IDA curves to eliminate any ambiguity about structure response. This paper presents its novelty by applying a statistical method for choosing the mass ratio of TMD, considering the possible real-world quantities for this parameter and a wide range of frequencies for the excitations; therefore, limiting the TMD stroke. Subsequently, verifying the linear and non-linear behavior of the model used in this paper is carried out by modeling a 40-story steel structure equipped with a TMD situated on its roof and tuned based on its first mode under the Kobe Earthquake. Furthermore, the displacement response of the 4, 8, and 12-story structures, being equipped with a single TMD of 0.5% and 1% mass ratio, respectively, are compared to their uncontrolled state by exposing them to the Landers earthquake.Results show that using TMD reduces the maximum drift ratio of the structures. Considering the first 16% of the acceleration records, as expected, a 12-story steel structure equipped with a TMD of 1% mass ratio on the roof, presents the best results of maximum drift improvement ratio of 2.54%. Moreover, for reducing computational effort, another alternative is applying a limited number of earthquakes to the structure. By using the median for the duration and PGAs of all FEMA-P695 data to estimate this earthquake record, the maximum drift improvement ratio is then 1.61% for the twelve-story structure resulting in decent numbers compared to the first method. Moreover, all types of the 4, 8, and 12-story structures (uncontrolled, controlled with a TMD of 0.5% mass ratio, and controlled with a TMD of 1% mass ratio) were subjected to the Kobe earthquake, and their average roof displacements were compared. Among these three types of structures, the 12-story structure was recorded to have the highest rate of maximum roof displacement compared to its uncontrolled state, being 3.47%.

In this study, the optimization of prestressed cables in steel structures was evaluated. The type of optimization is considered as locating the position of the cables in the floors. Optimizing structures by considering the existing conditions and constraints or the same constraint functions has always been one of the goals of engineers and designers. So far, a lot of research has been done on optimization and various methods have been proposed to solve it. In general, optimization methods can be divided into three categories: mathematical methods, optimization criteria and meta-exploratory methods. In this research, genetic algorithm, which is one of the meta-heuristic methods, has been used. It is necessary to use the genetic algorithm method to have appropriate objective functions and constraints relative to the design variables. In this research, the weight or area of the structure as the objective function and the stress and deformation constraints at the same time are considered as constraints of the optimization problem. The optimal location, diameter and amount of prestressing force of the cables are considered as problem variables. OpenSees software and nonlinear static method were used to analyze the structures. To implement this issue, three- eight- and twelve-story structures with different span ratios in two modes of cross-bracing on top of each other as well as cross-bracing in different spans are examined and it is observed that in 60% of the models two The middle spans and only 40% of the two side spans are braced as the optimal spans, it is also observed that the coefficient of behavior for 3-story structures with one bracing span in the range of 4.61 to 5.88, in 8-story structures and 12-story with two bracing openings varied in the range of 3.51 to 4.9 and 4.04 to 5.34 respectively. It can also be said that the prestressing force for 3-story structures in the range of 0.12 to 0.18, for 8-storey structures in the range of 0.1 to 0.28 and in 12-storey structures in the range of 0.1 to 0.32 the final cable tension varied.

Viscous dampers are one of the passive control systems that reduce the seismic demand of the structural elements of a building and minimize the building damage. However, the proper arrangement of viscous dampers greatly affects their increased performance, and it is not economical to use viscous dampers in all floors. In this study, the viscous dampers are designed according to four distribution methods based on FEMA 356: uniform damping coefficient distribution, distribution proportional to shear force of floors, distribution based on shear strain energy of floors and distribution based on shear strain energy of effective floors. In order to find the optimal distribution method, the seismic responses of the structure including the maximum drift ratio, maximum relative acceleration of floors, and strain energy are extracted and the results obtained from the rehabilitation in the four methods are compared with the proposed performance indices. Considering the effect of each method of damping coefficient distribution on the structural performance indices, it can be argued that the damping coefficient distribution based on the effective floors has a more appropriate performance compared to other methods of damping coefficient distribution. The results of time history analysis show that the viscous dampers used in the benchmark buildings cause a 50% improvement in the performance index of mid- and high-rise structures and also a 40% improvement in the force index of mid- and high-rise structures under the far-field acceleration.

Passive control method, which is one of the solutions for strengthening and designing structures under earthquake loadings, has different sub-branches. One of these methods is to improve the existing weak buildings by using different types of damper. The seismic behaviour of the strengthened structure with different type of dampers is one of the main issues in choosing the strengthening method. Due to the energy absorption property, dampers have many applications in both construction and retrofitting. During an earthquake, these devices are activated and convert the earthquake input energy into heat energy, in other words, it absorbs the incoming energy. In previous studies, the use of this damper in various structures has been evaluated, but there is no specific category to investigate its effect on various types of steel structures in terms of height and type of damper. In this paper, the performance of this type of dampers in steel structures with different heights is discussed. The effect of using this type of dampers on the response of the structure such as the story drifts and the base shear is investigated and its performance is compared in steel structures with different heights. In the study of displacement, base shear and energy absorption parameters for three structures of 3, 6 and 12 floors, it was observed that viscous dampers had the best response to friction dampers and ADAS.

In this study, a wireless piezoelectric sensor is fabricated for measurement of dynamic vibrations of steel structure. In contrast to widespread usage of piezoelectric sensors in the aerospace engineering, the application of these sensors in usual civil engineering structures is very rare. This study aims to evaluate the performance of wireless piezoelectric sensors for applications in the structural health monitoring of steel structures. To reach this aim, a wireless electrical network is firstly designed and fabricated for capturing the output voltage of the piezoelectric element. Then, the piezoelectric element and associated wireless network were installed on a prepared steel profile by using epoxy. Different mechanical loads are applied to the steel sample and the voltage induced in the piezoelectric element is saved on a computer. The mathematical relation between the amplitude of applied load and output voltage is determined. The results of experimental tests show that the fabricated piezoelectric sensor can predict the pattern of applied dynamic loads with enough accuracy.

During the recent two decades, many structures have become in need for seismic retrofit due to changes in either requirements of relevant seismic codes or the building usage. Viscous dampers are increasingly employed for seismically retrofitting various structures thanks to their easy installation and minimum intervention into the existing structure while significantly improving the seismic performance of the building. The main objective of this paper is to present a new design procedure for the evaluation of probabilistic seismicity of steel structures equipped with nonlinear viscous dampers. For this purpose, numerical investigation was conducted on, as benchmark structures, three steel-made moment-resistant frames with 3, 9, and 20 stories. Due to significant differences in their dynamic characteristics, these three structures represented low-rise, mid-rise, and high-rise buildings, respectively. Nonlinear time history analysis was performed using two near-field and two far-field accelerograms at various intensities (making up a total of 10 accelerograms). Moreover, nondimensional analysis was done to evaluate the uncertainties associated with velocity exponent of the nonlinear viscous dampers. Next, probabilistic estimation of damages occurred to the benchmark structures was performed and the effect of the uncertainty associated with the dimensionless power intensity of the damper on the seismic response of the structure. Investigating the results of the seismic analysis for viscous dampers at power intensities of 0.2, 0.3, and 0.4, it was found that although the seismic retrofit had significantly improved the seismic performance in all model structures but the proposed design procedure was more effective in the retrofitted high-rise structures at the power intensity of 0.4.

In this study, the seismic behavior of an all-steel buckling-restrained (AB) steel plate shear wall (SPSW) with incline slits under fire and cyclic loading was investigated. ABSPSW was composed of two thin steel infill plates with a narrow distance from each other, which were embedded with incline slits on each plate. These slits were in opposite directions to each other. The finite element (FE) numerical model was validated with three test specimens and after ensuring the modeling strategy, the parametric study was performed by considering variables such as wall plate thickness, slit width, strip width between two slits, and degree of temperature. A total of 256 FE numerical models were subjected to coupled temperature-displacement analysis. The results of the analysis showed that the high temperature reduced the seismic performance of the ABSPSW so that at 917oC, the load-bearing capacity was reduced by 92%. In addition, with the increase in the temperature, the yield point of the infill plate and frame occurred in a small displacement. The average decrease in shear strength at 458°C, 642°C, and 917°C was 18%, 46%, and 92%, respectively, compared to the shear strength at 20°C. Also, with increasing the temperature to 917°C, ductility increased by an average of 75%.

Concentric bracings (CB) are one of the most prevalent lateral load bearing systems in steel structures. These bracings have a remarkable lateral stiffness and strength, but their compressive buckling prevents them from being ductile and absorbing optimal energy. Consequently, in recent decades, researchers have conducted extensive studies to improve the concentric bracing behavior, which resulted in the development of different design and execution methods for concentric bracings. In this paper, by using numerical and experimental studies, a new method is proposed to improve the behavior of concentric bracings. In this method, a local fuse (LF) is used along the brace. This fuse is restrained by auxiliary elements (AE) to prevent its local buckling under compressive load. This makes the brace behaves in a similar manner in both tensile and compressive cyclic loads, resulting in ductile behavior and high-energy absorption. In this study, by using numerical results, an investigation is done for proper position of the fuse along the braces and its optimal shape and length. In addition, an analytical study has been performed comparing the structural behavior of concentric braces with LF-AE braces. The results have been demonstrated that LF-AE braces have better performance than concentric braces.

It is clear that steel structures are severely vulnerable to fire since the yield strength and the elastic modulus of the steel decease when the temperature increases, causing a reduction in the load-bearing capacity of the steel member. There are different methods for a steel element to reach required fire resistance rating. One of the most current methods in the world is the use of spray-applied mineral fire protective coatings. It is obvious that after a severe earthquake, the probability of the occurrence of the fire is high, e.g., because of the rupture of gas pipes. Unfortunately, there are not enough research pieces and reports on the performance of these coatings during an earthquake. However, we know that during a severe earthquake, the structure of the building will sustain significant deformations and it can cause the damage and delamination of the installed fire protective coating. Therefore, in this research, the post-earthquake fire performance of a mineral based spray-applied fire protection coating was studied. The type of this fire protection coating was cement based. Different application details of this mineral based spray-applied coating including coating thicknesses and reinforcing steel mesh details were investigated. First, four fire protected steel columns were subjected to cyclic lateral load tests. The type of the connection of these columns was a stiffened extended end-plate moment connection. This connection type was used for special steel moment frames. The thickness of the fire protection coating for steel specimens was determined for fire resistance rating of three hours. Then, these columns together with two reference columns were subjected to fire resistance tests to make a comparison between results. By using the qualitative and quantitative results of these tests, the type and amount of seismic damage of fire protection coating were determined in earthquakes with two different intensities. Also, it became clear that reinforcing the fire protective coating with a steel mesh in the region of formation of seismic plastic hinge in a steel element could reduce the seismic damage of the coating sufficiently.

The effect of fatigue on the behavior of steel structures has long been the focus of researchers. However, with the widespread damage of steel structures after the Northridge earthquake, it is important to investigate the issue of low cycle fatigue. The bylaws attempted to address the weaknesses of the connections and introduced prequalified connections. In the present study, the behaviors of two prequalified welded unreinforced flange-welded webs and reduced beam section under low cycle fatigue were investigated. The behavior of high cyclic fatigue was the focus of many researchers, while there is only a limited scope of experimental data available for LCF. In this study, the S-N-curve was used to obtain the Nastar method and available experimental data. This curve was developed using the available experimental results for the low cycle region. Four buildings with different heights were analyzed using linear time history analysis and their behavior under low cycle fatigue was considered. Then, the cumulative fatigue damage was investigated by the Palmgren-Miner fatigue analysis method for the above two connections. Rainflow method was also used for counting cycles. By defining the cumulative fatigue index, for the critical beam of structures, the maximum value of this index was observed at the WUF-W, which was obtained for 0.059 3-storey structure and was increased by 0.24 with the increasing structural height. The index for the critical column in the WUF-W junction in the 15-storey structure was 0.042, while in the other structures, the maximum value was up to 0.197. Also, the fatigue index values for WUF-W binding were higher than those obtained for RBS, indicating that the first connection performance was weaker. Maximum rate of cumulative fatigue index in beams on WUF-W to RBS was 1.29 and for columns, this rate was 1.34. The results indicated the need for greater attention to the effect of low cycle fatigue on connections of steel moment frames, especially on the WUF-W.

A new structural system of diagonally gridded elements, the so called ``diagrid'', has been introduced in recent years in which the frames that are made up of intersected members surround the main structure. This type of system provides excellent shear rigidity and stiffness, in contrast to the similar tubular structures. The triangular configuration offers structural stability for gravity and lateral loads, and the removal of external columns gives the building a unique architectural beauty. Swiss Re (London), Hearst Tower (New York), Cyclone Tower (Asan, South Korea), Capital Gate Tower (Abu Dhabi) and Jinling Tower (Nanjing, China) are prominent cases with diagrid system worldwide. The response modification factor (R-factor) for diagrid systems is not yet explicitly introduced in the existing building codes and few studies have been conducted on the assessment of this parameter. On the other hand, previous researches show that the angle of diagonals play an important role in the structural behavior. In this paper, the effect of the angle change of diagonal elements on R-factor, collapse capacity and collapse fragility curves of steel diagrid structures are investigated. To this end, 6, 8, 10, and 12 story diagrid buildings with different angles for diagonals varying from 580 to 780 were selected and modelled using OpenSees software. The post-buckling behaviour of diagonal members was taken into account in modelling process. The models were then analyzed using nonlinear static analysis and the R-factor of the system was calculated according to the proposed method by ATC-19 guielines. Nonlinear incremental dynamic analysis (IDA) were performed and collapse fragility curves were extracted using 44 far-field records in order to evaluate their seismic performance and to validate the calculated R-factor. Next, the reliability of the applied seismic performance coefficients were controlled through the methodology proposed by FEMA P-695 taking into consideration the uncertainties in modeling, designing, earthquakes and experimental data. Results show that the effect of diagonal angles on the response modification factors depend on the building height. However, it can be stated that, in general, changing the angles of diagonals in steel diagrid systems do not have a significant effect on their R-factor. It is also observed that an increase in the angle of diagonal members will reduce the uncertainty in collapse data.