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One of the most common types of steel frames is Concentrically Braced Frame (CBF). A certain type of CBF called Chevron braced frame has always been interesting to designers due to architectural advantages. When lateral earthquake-induced displacements are applied to these frames, their nonlinear behavior starts with buckling of compression brace. The buckled brace experiences a considerable reduction in resistance. On the other hand, the tension brace tends to increase its internal force. The unsymmetrical behavior of braces in tension and compression leads to application of a force known as unbalanced force to the beam. The unbalanced force can yield the chevron beam and result in failure of this kind of CBF configuration. In this paper the seismic behavior of ordinary chevron braced frames, super-X braced frames, zipper braced frames and suspended zipper braced frames are investigated. In order to achieve this goal, six types of chevron braced structures are considered. These structures are symmetrical and are braced in one bay of the perimeter frames. The braces of the structures are selected so that that they comply with the seismic provisions of Chapter 10 of Iranian National Building Code. By changing the section properties of chevron beams, six types of chevron braced frames are obtained. Then, five types of these frames are converted to super-X frames, six of them to zipper-braced frames and one of them to suspended zipper frame. The performance of these structures is assessed by non-linear static analysis. For each structure, the pushover curves are plotted and the capacity demand ratios (DCRs) of different elements for Life Safety performance level are determined.  The performance of different elements is investigated by interpreting the outputs. It is observed that in ordinary chevron frames slight strengthening of the beam is not helpful unless it is fully strengthened so that it can resist the unbalanced force linearly. Otherwise, strengthening the beam may exacerbate the global performance of the frame. It is also observed that the conversion of inverted-V configuration to super-X configuration improves the behaviors of these frames. The non-linear analyses show that adding the zipper column to the frame can improve the seismic behavior of the frame only when the structure has the characteristics of suspended-zipper frames.    

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Reinforced concrete structures are considered as bare frames in analysis and design process, including the main structural members such as beams, columns and shear walls. However, in the urban areas, structural frames are filled with masonry walls as acoustic and thermal insulation in the middle or peripheral areas of the buildings therefore they have not the same behavior as the empty frames. This research aims at investigating the effect of unreinforced masonry infill panels on the response of RC frames against the progressive collapse. For this purpose a micro based modeling approach is adopted for numerical simulation of the masonry infill panels and through a parametric study, the effect of influencing parameters is numerically investigated. Also an equivalent compressive strut model is proposed for macro modeling of masonry infill panels. The comparison between numerical and available experimental results confirms the reliability of the developed model. Finally, the developed macro model is used to investigate the influence of masonry infill panels on the progressive collapse resistance of reinforced concrete frames with different ductility capacity. It is shown that, under the progressive collapse phenomenon, the ductility of RC frame is the main effective parameter and masonry infill panel play an important and non-ignorable role in structural response

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Abstract: Steel frames with Khorjeeni connections have been widely used in the traditional construction of buildings in Iran during the past decades. In the traditional form of Khorjeeni connections, double section beams are not cut at the intersection with columns, rather they are connected to the column by means of two angles placed over the top and under bottom of the beam flanges. This type of connection offers advantages for frames, which carry gravity loads, but it has deficiencies when the frames are subjected to lateral loads. Like other structural frames, there are masonry infills in many frames with Khorjeeni connections. The behavior of composite frames subject to lateral loads differs from that of bare steel frames. In this paper, positive and negative effects of masonry infills, when strengthed by reinforced shotcrete, are studied on the behavior of steel frames with Khorjeeni connections. Finite element method was used to carry out nonlinear static analysis of subassemblages of this type of frames. Initially, the results of some experiments were utilized to verify the details of the model. Then numerical models of two span or four span frames with different configuration of bracings and masonry infills and characteristics of shotcrete were studied. The results showed that infills increase the stiffness and strength of frames in the absence of bracing considerably. Even when bracings are present, the increase in stiffness and strength is significant. When the infills are strengthened, their effects on stiffness and strength of composite frames increase. However if the steel frames are not strong enough, their strength limit the effects of infills. The masonry infills, however, have some negative effects on the behavior of Khorjeeni frames. Parts of the column in the vicinity of connections are prone to plastic damage, particularly when the infills are relatively strong. The Khorjeeni connections are subjected to vertical forces and tortional moments. Due to limited vertical strength of these connections, top stories of this type of frames may suffer when compressive action of strut is mobilized. For other bays, this action introduces additional moments, which may damage the connections. Therefore, considering these negative effects of infills is very important when seismic behavior of the existing frames is assessed. When the infills are strenghted by shotcrete, these negative effects become even more important and the inter story drift rations need to be limited in order to avoid failure in conections.

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By increasing demand for oil in recent years, explorations from deep offshore fields are feasible. In such deep waters, even fixed offshore structures may have considerable movements under design loads, while having less displacements of the platform is often requested.  Many innovative concepts have been proposed to minimize responses of structures under environmental loads in recent decades. In a tension leg platform, the buoyancy force causes tension in the tendons, which is changed by platform movement and produces a lateral stiffness to reverse the platform into its initial position. The amount of generated additional stiffness depends on the platform displacement and buoyancy forces. Fixed submerged tanks may be used in design of a compliant platform in deep water to reduce transfer weight of the structure to the support and to decrease the effects of legs buckling. However, the tanks should be located in an appropriate water depth to minimize the effect of wave forces. In order to decrease the response of fixed offshore platforms in deep waters, an innovation concept is presented. In this concept, a submerged tank is tied up to the platform in an appropriate location acting a buoyancy force to the system. This force adds tension force to the legs which may reduce the required chord diameters and/or eliminate some braces. However, the added mass of the tank due to wave action has considerable effect on dynamic behavior of the system. In addition, the vertical buoyancy force of the tank generates a resistance moment in the system when the tank oscillates. This resistant moment depends on the location of the tank and time. In this paper, considering the effects of the tank on the platform responses, solutions for reducing platform displacement are investigated.  Analyses have been carried out by taking into account the large deflection and nonlinear geometry effects for which a MATLAB program has been developed featuring the following capabilities: Calculation of wave forces based on the Morison equation for jacket members and the Froude-Krylov method for the tank. Taking into account waves and structure interaction. Non-linear analysis of the structure considering large deformations effects. Dynamic analyses results showed that the tank acts as a weight damper under wave actions. In this case, the added mass has also contribution on the inertia force. So, there is an optimum stiffness for each mass. For dual mass damper and buoyancy functioning of the tank, the stiffness should be defined in such a way that the performance of the tank would be appropriated in both consequences. Results of analyses on a case study platform show that the performance of the tank on reducing the platform responses is much better for the dual mass damper and buoyancy functioning comparing to only the mass damper functioning.

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Explosion is considered as the most hazardously event in petrochemical facilities and offshore structures. In these facilities, pressure vessels are very important because their explosion may result in damage to other modules. In practical design, external blast load is applied to one side of pressure vessels as uniform load.In this paper we try to propose more realistic distribution to conform experimental results. This paper includes validation of Eulerian domain capability in finite element program ABAQUS to carryout uncoupled Eulerian Lagrangian analysis .The results show good agreement between Eulerian capability and experimental results in locations that do not have high turbulence effect, but in points where turbulence effects and vortexes are increased, error in numerical model is larger. Also, this paper shows that the method which is usually used to apply blast loads to cylindrical materials has a great error in comparison with numerical simulation and experimental results. Thus, in this paper is presented a blast load distribution which can be used in future research and industrial designs for vertical shape or horizontal shape of cylindrical materials with a variety of different diameters.

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The most important characteristic of brace frames is their significant and appropriate stiffness as well as their compression strength against earthquake forces. Built-up special concentrically braced frames (SCBFs), which contain double angle braces, are among the common steel structural systems resisting lateral loads. Along the built-up brace length, the stitch and connector distances make significant role in cyclic and ductility behavior of braced frames due to possibility of out of plane buckling.The results of experimental studies of built-up double angle braces illustrate that setting the stitches closer to each other can improve the post buckling behavior of systems, resulting in increasing the final compression strength, close to box-shaped brace strength. In addition, an individual member buckling is possible by increasing the stitch distances along built-up braces. According to AISC seismic provisions regarding built-up SCBFs, the slenderness ratio of individual elements between the connectors should not exceed 0.4 times the governing slenderness ratio of the built-up member. Also, connecting built-up members by the use of welding is not permitted within the middle one-fourth of the clear brace length. In fact, AISC seismic provision has prohibited the use of stitches and connectors in the protected zones of built-up specially concentrically braced frames such as the center one-fourth of the clear brace length and a zone adjacent to each connection equal to the brace depth in the plane of buckling.
In this research, seismic provisions related to built-up diagonal and X-braced SCBFs are numerically investigated under cyclic loading using a single-bay single-story frame. The numerical study is performed on models, which contains parameters such as back-to-back and face-to-face connection types of built-up members. Seismic behavior of these braces are investigated from the view points of cyclic and failure behavior. This investigation is performed on both types of diagonal and X-braced steel frames. The cyclic behavior of systems is studied based on post buckling capacity, structure initial stiffness, and final compression strength. Failure behavior of systems is investigated with regard to failure cycle and ductility capacity. In order to evaluate of seismic behavior and ultimate ductility of the numerical models, regarding to proximity of initiation and propagation of steel cracks, the concept of plastic equivalent strain is used to predict system failure.
The results of this study show that increasing the number of stitches or decreasing their distances along the length of the built-up members may not necessarily improve behavior of braced systems.That means inelastic deformation consent will probably occur in individual elements between stitches resulting in earlier failure of braces. Therefore, current seismic provisions such as not exceeding the slenderness ratio of individual elements between stitches from 0.4 times of the governing slenderness ratio of the built-up member for compression sections, are conservative in SCBFs and can be changed according to the type of braces. In addition, Failure of double angle back-to-back diagonal braces occurs sooner in comparison to face-to-face braces. Also, in X-braced frames, cyclic and failure behavior of built-up face-to-face braces are more desirable than the similar back-to-back braces in general.

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Application of composite sections in structures is increasing in recent years. This type of structure utilizes the strength and ductility of steel as well as strength and low cost of concrete. Composite action is developed when the two load carrying structural members such as concrete slab and supporting steel beam are integrally connected and deflect as a single unit. The extent to which composite action is developed depends on the provisions of shear connectors between steel and concrete. The horizontal shear that develops between the concrete slab and the steel beam during loading must be resisted so that the slip will be restrained. A fully composite section will have no slip at the concrete-steel interface. As we know, the shear connectors provide the interaction necessary for the concrete slab and steel beam parallel to the beam. One of the most important objectives in design and construction of steel moment resisting frames, is the ability of high energy dissipation due to yieldings and plastic deformations in beams in such a way that formation of plastic hinges in beams occurs prior to that of the columns. This procedure leads to fulfilling the strong column-weak beam relation. On the other hand in steel frame structures, it is common to use concrete slab in order to construct floor diaphragms. However in design codes, specially National Building Code of Iran, it is not clearly mentioned how to consider the effects of the concrete slab on connection behavior and the strong column-weak beam relation control. In this study, the behavior of bare and composite beams in steel moment frames under monotonic and cyclic loading has been investigated through numerical modeling in ABAQUS finite element software. The requirements of National Building Code of Iran regarding the ratio of bending moment strength of columns to beams ,α, have been studied. In this regard a direct connection with uniform beam section is simulated. Rotation of beams and steel columns in different sections for formation of plastic hinges are considered as a criterion to measure these relations. In order to determine the effect of the number of shear studs (percentages of composite actions) and their location specially in protected zone within a fixed length of the girder, reduced beam sections (RBS) are modeled. The results indicated that decreasing the initial distance of shear studs arrangement from the column face leads to increasing the composite action. Analytical results also showed that the effective width of concrete slab depends on the load transfer and the force distribution. Based on the results of this study, it is suggested that in calculation of strength ratio of girders and columns, the effect of the floor slabs should be taken into account to ensure that the requirement of strong column-weak beam is fulfilled, otherwise column failure may occure before girder failure. It must also be mentioned that the relative strength of columns to girders can affect the panel zone behavior in such a way that different values of α, require different shear strength of panel zone.

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One of the effective ways to mitigate earthquake damage in structures is passive control of structures. Yielding metallic dampers economic passive control devices. Not only yielding metallic dampers are easy to erect, but they can also be used as a passive control systems easily. In this paper, the aim is to develop a design procedure for steel structures equipped with a combination of yielding metallic dampers so that, dampers will experience specific nonlinear behavior when subjected to various seismic hazard levels. For this purpose, the first step is providing seismic hazard spectra with different return periods for the intended site of construction. In this research, this step has been taken by using the Tehran probabilistic analysis hazard project data and then plotting uniform hazard spectra with 75-year, 475-year, 975-year and 2475-year return periods. After determination of uniform hazard spectra with mentioned return periods, behaviors of structures equipped with yielding metallic dampers have been investigated in the form of one-storey one-span, one-storey two-span and multi storey multi span frames. Required equations for behavior of these structures under monotonic loading is developed. In the beginning of design process, the performance criteria for the structure and the damper is set and by using the derived equations, design of single degree of freedom frames based on performance criteria has been carried out. These single degree of freedom structures have different periods and strength reduction factors. After designing the single degree freedom structures, nonlinear static analysis results have been compared with result of nonlinear time history analysis. For this purpose, 7 earthquake records have been chosen and scaled based on Iranian code of practice for seismic resistant design of buildings and used for dynamic analysis. Results showed that all performance criteria of 75-year and 475-year hazard levels have been satisfied but for 975-year and 2475-year hazard levels, six cases have not satisfied the desired critera with 6 percent error. In order to verify the presented numerical analysis of multi degree of freedom structures, an experimental study has been chosen and the results of these two works have been compared. This verification showed that the presented analysis can model the structures and dampers with acceptable accuracy. Performance criteria of multi degree freedom structures have also been proposed. Three, 3-storey, 6-storey and 9-storey buildings equipped with dampers have been designed and based on proposed method and the desired performance of dampers have been achieved. Time history analysis have been carried out for each return period. For these analyises, 7 earthquake records were chosen and scaled based on Iranian code of practice for seismic resistant design of buildings. Comparison of performance point displacement levels and the displacements obtained from 28 nonlinear analyses, showed up to 13 percent error. Meanwhile, the displacement levels of each set of dampers for 75-year, 475-year, 975-year and 2475-year return periods, confirmed efficiency of proposed design method and all dampers met the mentioned performance criteria. The results also showed that when hazard level increased, the difference between the results of nonlinear time history analyses and static nanlinear analyses have also increased.

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Current design philosophy for conventional lateral resisting systems is that the frames should not collapse during major earthquakes, however significant structural damage in elements such as beams, braces and sometimes columns may occur. The presence of residual drift due to inelastic deformations may hinder building occupancy or functionality after major earthquakes, and may increase associated repair costs significantly. During last two decades, practicing engineers and researchers have tried to develop seismic resisting systems that can minimize and potentially eliminate residual drift due to earthquakes. Proposed structural systems utilize the so-called “self-centering” systems that can improve the seismic behavior, provide higher resiliency and overcome the significant residual drift of conventional systems. Self-centering (SC) seismic resistant systems, introduced in the literature are developed for both steel and concrete structures. For the steel structures, they may be categorized into three primary groups: SC moment frames, SC rocking systems and SC braced frames. The most important similarity between self-centering systems is that the lateral load resistance of the system has a flag-shaped hysteretic loop. That is the characteristic of systems that self-center after large lateral displacements. Considering the normal practices of construction industry in Iran, it is more feasible and favorable to use metal yielding dampers instead of viscous or friction dampers. Also considering the economic issues, self-centering mechanisms which use pretension tendons are more feasible compared to shape memory alloys. A yielding metallic damper called comb-teeth damper (CTD) provides energy dissipation mechanism. CTD is made of steel plates and includes a number of teeth that dissipate energy through in-plane flexural yielding. The new self centering brace (SCB) can substitute the conventional braces to provide desired seismic performance and to reduce residual deformations and repair costs. The proposed brace can be easily disassembled in the field which provides the possibility of inspection of the core after a large earthquake. Parameters of this system should be selected so that they can provide appropriate stiffness, strength and energy dissipating capacity. In this paper, initially the overall mechanical behavior of the device has been defined in terms of its internal components, based on an analytical approach. The mechanical equations for the SCB were decomposed into two portions, which are the pretension tendons that cause the self-centering behavior and the CTD links that support the energy dissipation mechanism. Also finite element analysis has been conducted to verify the hysteretic responses and mechanics of the proposed SCB. Based on the results, the characteristics of finite element responses have good similarity with the analytical results and show that either of the approaches are reasonable to predict the SCB behavior. Then a parametric finite element analysis has been conducted by varying the mechanical properties of steel elements to optimize the properties of the system. The results show that the desired self centering and energy dissipating capacities would be achieved using the new SCB. Lastly, nonlinear time history analyses have been performed to investigate the characteristics of some 6 story steel buildings equipped with the new SCBs. The results confirm the feasibility of using the new SCBs in braced frame structures.

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This paper investigates the behavior of a steel moment frame system with long spans subjected to compartment fires and progressive collapse scenarios due to girder drop and column removal. In this study, initially, a typical 15 – story building with moment frame system with long spans and story height 3.2 (m) was designed using relevant chapters of national building code of Iran for conventional gravity and lateral loads. In order to perform thermal analyses, the most critical frame of this structure is modelled in finite element software OpenSees. Then the nonlinear behavior of the frame is studied at elevated temperatures under different fire scenarios and progressive collapse Scenarios due to girder drop and column removal. In this analyses, the structure is subjected to both gravity and thermal loading simultaneously. Also for performing thermal analyses, nonlinear analyses and standard fire curve (ISO 834) are used.
Results of this study indicate that beams do not deform significantly until approximately 400°C, but after that, vertical displacements of beams increase significantly due to degrading mechanical properties of steel. So beams deform and collapse at about 500°C to 600°C. Also heating the beams of structure, initially causes the axial force in the beams due to thermal expansion restraint. So Demand to Capacity Ratios of beams increase from early stages of fire and the most increase in DCR_nom occurs at about 350°C to 400°C. Also by one story girder drop, columns survive to 500°C. But at higher temperatures (about 600°C to 800°C), these heated columns lose their strength and buckle. In column removal scenarios in first and 7^th story, where beams have lost their strength under effect of gravity loads and at about 400°C respectively, more damage is observed compared to girder drop scenarios.
 

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   This paper investigates the effects of various parameters, including support conditions, the Demand to Capacity Ratio (DCR) of the member under gravitational loads, the section factor (the ratio of perimeter to area), and the fire insulation coating thickness on the fire resistance duration of steel columns under fire effects. To this end, four steel H-shaped columns and four steel tube columns with the height of 4 meters, are subjected to the standard fire curve (ASTM E119) from four sides, and the effects of different parameters are studied. Initially, a heat transfer analysis is carried out on the 2D cross-section of columns with its fire insulation coating in Abaqus software. Then, a nonlinear general static analysis is performed on a 3D steel column model subjected to gravity (concentrated axial) and thermal loading simultaneously.
Results of this study indicate that the columns only expand but do not deform significantly until approximately 250^oC. After that, a decrease in the steel strength and stiffness and as a result, a decrease in fire resistance and bearing capacity of the steel column occurs. This is accompanied by an increase in the mid-span horizontal displacement of the column and an increase in the effect of the P-δ bending moment, which results in the column failure at about 500^oC to 650^oC. The results also show that the fixed or pin support condition on the bottom end of the column does not significantly affect the column failure time under fire effects. In square box columns, the increase in the section thickness increases the fire resistance duration of the steel column. However, increasing the section width does not significantly affect the column failure time. In H-shaped columns, the increase in the flange thickness and the decrease in the column web height increases the column fire resistance duration. On the other hand, the results indicate that the section factor, the initial load level of the member due to gravitational loads, and the fire insulation coating thickness have a significant effect on column failure time to the extent that with the increase in DCR of the member from 0.3 to 0.7, the failure time of the column decreases by about 25 to 35 minutes.
Based on the results of this study, two formulae have been presented to calculate the failure time of protected columns by CAFCO300. The results of these formulae have also been compared to a relationship proposed in Chapter 10 of the Iranian National Building Regulations. It is found that the results of these formulae are fairly similar, when the initial DCR equals 0.7. Therefore, the relationships of the present study provide a more optimal and accurate design of the fire insulation coating thickness, because this load level can only occur in structures that are not designed for lateral loads and are designed only under gravitational loads.