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Abstract Low height beams in concrete moment frames, decrease the ability of beams in controlling lateral displacement of buildings. Because of that, in the sixth subject from Iranian Regulations of Buildings and its following, in the 3rd edition of Standard 2800, the height of buildings with low height beams has been limited to 3 floors or 10 meters. According to that, in this study, concrete buildings with different amount of stories and moment frames, with medium ductility and the height of beams in 30 centimeters, have been analyzed (with linear equivalent static and spectral dynamic analysis Methods)and designed on the basis of seismic principles in the 2nd and 3rd edition of Standard 2800. Finally, studies continued with nonlinear static analysis and the performance point of structures determined (with capacity spectrum and displacement coefficient methods) in 4 levels of different seismic risks. Studies show that only one or two story buildings that designed with 2nd edition of Standard 2800 have enough safety in design base earthquake level (DBE) and also higher buildings, should go under rehabilitation.

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Abstract Due to its simplicity, the structural engineering profession uses the nonlinear static analysis or pushover analysis. Modeling of such analysis requires the determination of the nonlinear properties of each component in the structure, quantified by strength and deformation capacities depending on the modeling assumptions. Pushover analysis is carried out for either user-defined nonlinear hinge properties or default-hinge properties, available in some programs based on the FEMA guidelines. While such documents provide the hinge properties for several ranges of detailing, the programs may implement averaged values. The user needs to be careful; becduse the misuse of default-hinge properties may lead to unreasonable displacement capacities for existing structures. This paper studies the possible differences in the results of push-over analysis due to default and user-defined nonlinear component properties. Four- and eight-story buildings were considered to represent low- and mediumrise buildings for this study. Plastic hinge length and transverse reinforcement spacing were assumed as effective parameters in the user-defined hinge properties. The observations showed that plastic hinge length and transverse reinforcement spacing have no influence on the base shear capacity, while these parameters have considerable effects on the displacement capacity of the frames. The comparisons pointed out that an increase in the amount of transverse reinforcement improves the Frames displacement capacity. Also Our findings clearly showed that the user-defined hinge model is better than the default-hinge model in reflecting the nonlinear behavior compatible with the element properties. However, although the default-hinge model is preferred due to its simplicity, the user should be aware of the circumstances provided in the program and thus should avoid the misuse of default-hinge properties.

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In recent years using steel plate shear wall system, because of its advantages in comparison with other earthquake resistant systems, has been a matter of attention. Some of its advantages relative to other systems include abundance advantage, high ductility, good hysteretic behavior and energy absorption capacity, high stiffness and economic advantages. Regarding that in Iran there is high seismicity risk and the need to strengthen old and unsafe urban textures and buildings, using this system as a lateral load resistant system seems appropriate and economical. In the present research strengthening of x-braced steel frames with steel plate shear walls is evaluated. Addition of bracing to unbraced frame spans, substituting braces with thin steel shear wall panels and adding thin steel shear wall panels to unbraced spans which do not have architectural requirements are considered as retrofitting strategies. The focus is on the methods in which retrofitting is only done by adding steel plate shear wall elements to braced frames. Some of these methods have many economic and practical advantages. Others are only proper for some special cases. In this study a number of x-braced steel frames designed by the first edition of Iranian code of practice for seismic resistant design of buildings (Iranian Standard No. 2800) are taken as basic frames which need to be retrofitted. These basic frames are retrofitted by adding steel panels with different methods. Then nonlinear static analysis (pushover analysis) with displacement control pattern has been done on both basic and retrofitted finite element frame models and the capacity curves (diagram of story displacements against base shear) of basic frames and retrofitted frames are compared. Considering the results of the pushover analysis of models in which seismic retrofitting is done by replacing x-bracing earthquake resistant system with steel plate shear walls and the results of other methods of strengthening, it is seen that seismic behavior of retrofitted frames is more desirable in terms of overstrength factor (Ω) and overall ductility of structure ( ). The failure and fracture mode in most of the medium-rise frames was ductile but in the short-rise frames the fracture was brittle. Thus, replacing the braces in short-rise structure with thin steel shear walls seems irrational and unjustified economically. But it is to be mentioned that strengthening and increasing the moment of inertia of the adjacent columns of steel shear wall panels in structures with brittle fracture mode could result the change from brittle to ductile fracture. The results of this research show that in the case of steel braced frames with regard to some scientific, technical and practical points; replacing concentric steel bracing earthquake resistant system by steel plate shear walls can be used as a suitable method for retrofitting a wide range of existing steel structures in Iran.

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Usually, progressive collapse is defined as the progress of a primary local damage within the structures that, like a chain chemical reaction, causes to partial or total collapse of the structures. Although, many researches on progressive collapse under blast load have been made, it can be seen that rupture aroused in structures during strong earthquake events will not happen suddenly, but because of failure in structural design or performance, the weak elements will destroy easier. Subsequently, energy redistribution will occur that may disconnect the adjacent members. Further, the progressive collapse phenomenon will take place and subsequently cause to collapse all the structures. In the recent years, the incidence of catastrophic events such as September 11 has attracted a lot of attention to the issue of the progressive collapse and lead to be considered in the design of new structures. In order to prevent damages by reducing the progressive collapse, different strategies for designing against the progressive collapse have been presented in the government documents of USA, such as UFC and GSA. Although, many researches have been made on the progressive collapse in recent years, but the structures deck effect on the progressive collapse has not been considered sufficiently.
Nowadays, due to the increase of the speed of construction and lightweight construction, the usage of new systems has been increased. Among these systems, Bubble Deck system is notable. This structural system functions as a two-way slab and a lightweight structural member. In Bubble Deck system, the plastic spherical hollow core (PSHC) is used instead of the concrete situated in the central zone of the cross sections around the slab’s mid-span, where the shear stress is relatively small, compared to the supports. PSHC creates a hollow space in the slab. The Bubble Deck technology uses spheres, made of recycled industrial plastic, to create air voids, while providing the strength through the arch action.
The objective of this study is to evaluate the progressive collapse of reinforced concrete structures with Bubble Deck floor system. The behavior of structures, the amount, and the mode of the collapse distribution can be studied by various methods. SAP2000 software is used to model, design and analyze the structures. DoD2013 statement has been selected as the reference criteria, and based on that, all the uploading and collapse measurements has been determined. The equivalent nonlinear shell layered element is used to define the slab sections in numerical modeling. 48 structures with the same plans (3 spans on each direction) were modeled. Moment resisting system is chosen as lateral resisting systems. The models were in 4, 6 and 8 stories. Story height of all structures is 3.5 m, and also three types of span length to story height ratios including 1.5, 2.5 and 3.5 are investigated.
The results show by increasing the number of floors, the structure’s performance against progressive collapse will decrease and the middle-rise structures (6 stories) have shown the best performance rather than others. Also, by increasing the ratio of span length to floor height, progressive collapse resistance is reduced. The most observed damage in the Bubble Deck floors is less than 25 percent of floor area which affected by progressive collapse in the middle-column removal scenario. This value of damage satisfy DoD2013 targets.
 

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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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The moment-resisting steel frame building is highly used due to their advantages such as, high speed construction coupled with appropriate strength and ductility. The main advantage of this system is related to architectural considerations and the possibility of creating openings within all spans. Connections play an outstanding role in the seismic responses of this structural system. The connections are generally assumed to have a rigid behavior in analyzing and designing of the moment-resisting steel buildings. Studying of the previous investigations indicates that the assumption of rigid behavior for the beam-to-column connections is not always correct and can bring about a significant error in the responses. In this study, behavior factor of moment-resisting steel frames considering joint flexibility is evaluated. To do so, some intermediate moment-resisting steel frames with various number of stories and bays including 1-bay, 1- and 2-story frames, 2-bay, 2-, 4-, 6-, 8-, and 10-story frames, 3-bay, 3-, 6-, 9-, and 12-story frames and 4-story, 2-, 6-, 10-, and 14-story frames are designed regarding Iranian seismic code and Iranian national building code for designing steel structures. After that, the capacity curves of these frames are achieved using pushover analysis once considering rigid connections and again taking joint flexibility into consideration using OpenSees software. To model the nonlinear behavior of connections, one zero-length rotational spring is assigned to each end of beam members. Then, the behavior factor of each frame is calculated using the recommended procedure of FEMA-P695. The outcomes show that for the frames with rigid connections, the acquired behavior factors are almost close to 5 (which is the prescribed behavior factor in Iranian seismic code for the intermediate moment-resisting steel frames). Furthermore, for the frames with semi-rigid connections (60%), the behavior factors are close to 5 as well. For 10-story 2-bay, 12-story 3-bay, and 14-story 4-bay frames the prescribed behavior factor in Iranian seismic code does not meet. For these frames, the ratio of height to total-span that is known as the slenderness coefficient of the frame is higher than others, so these frames fall into slender frames. Results show that for the frames with semi-rigid connections (60%), despite of decreasing the over-strength factors in some cases, their ductility increased, therefore, the behavior factors are achieved higher than those of the frames with rigid connections. All in all, it is observed that the nonlinear behavior of connections can significantly affect the seismic behavior of the moment-resisting steel frames. Comparing the behavior factors calculated in this investigation with the prescribed value of this factor in code 2800 showed that for the frames with rigid connections, 80% of the obtained behavior factors are higher than 5. For frames with semi-rigid connections (80% and 60%), 0%, and 66% of the behavior factors meet the proposed value of code 2800, respectively. Regarding the observations, it is recommended that the influence of joint flexibility be considered in assigning a value of behavior factor to design the moment-resisting steel frames.    
Keywords: rigid connections, semi-rigid connections, moment-resisting steel frame, nonlinear static analysis, distributed and concentrated plasticity models

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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.