نشریه سازه و فولاد
سال چهاردهم شماره 28 (تابستان 1399)

The fragility curves are used to indicate the probability of damage to buildings during the earthquakes with different intensities. The main objective of this study is to present a new design procedure for exploring the effectiveness of using viscous dampers in the seismic rehabilitation of steel structures using the seismic fragility analysis. In addition, a straightforward formula is presented to determine the effectiveness of seismic rehabilitation of steel structures with viscous dampers. The inter-story drift as one of the effective parameters for assessing the safety of buildings after an earthquake is used in this study as a damage index. To compare the damage probability of buildings with and without damper, three 3, 9 and 20-story benchmark buildings are used. Furthermore, the earthquake records corresponding to three levels of seismic hazard are used for the nonlinear dynamic analysis. Comparison of the fragility curves between the steel structures controlled and the buildings without the damper shows a significant decrease in the damage probability after the rehabilitation with the viscous dampers. The results of numerical analysis showed that although seismic rehabilitation in all benchmark buildings led to the improved seismic performance, the decrease in the response of the 9-story structure is higher compared to the other buildings.

Transformation induced plasticity (TRIP) steel is one of the advanced high strength steels (AHSS) group with multiphase microstructure that are replacing conventional steels in the automotive industry because of their very good mechanical properties. Fusion welding methods have challenged their application by destroying of microstructure. Therefore, in this paper two type of spot joining, resistance spot welding (semi-fusion) and friction stir spot welding (solid state), investigated. First, thermal history of metal during both of welding obtained using finite element analysis. Then, TRIP steel sheets joined under same condition of finite element models. Based on the temperature data obtained from finite element analysis and the data obtained from experimental tests such as microstructure (FE-SEM and XRD), microhardness, residual austenite value in the welding zone and the lap shear strength of joints, a comparison was made between the two methods. The microstructure of the weld region in both methods was mainly martensite, but in the FSSW method, some retained austenite also identified. Also, the maximum hardness in RSW was 505 VHN and in FSSW was 478 VHN. It can be related to the lower peak temperature and non-melting and non-solidification during the process, and the presence of some austenite in the microstructure of the weld metal. The maximum temperatures for the FSSW and RSW were 940 and 1900 ° C, respectively. The FSSW strength was 9.8 kN, which was higher than the RSW bond strength (7.8 kN). This is due to the presence of a martensitic microstructure with an appropriate hardness and a certain amount of austenite in the final microstructure

In the past decades, although the regulations of American Institute of Steel Construction (AISC) ordained that the link to column in Eccentrically Braced Frames (EBF) had to conform with the moment frames designed specially, unfortunately most of them got destroyed after Northridge earthquake in 1996. From this point, therefore, the regulations of AISC regarding the fore mentioned designed and performed links reconsidered more carefully; the final results have not been included in revised regulations of AISC yet. In recent studies, different ways have been put forward to solve this problem; almost all of them believe in keeping away the critical section of links to columns from common sections. The present research does its utmost to find the most useful dimensional limits for holes made in steel joist; it is going to be done through making use of RBW, holding the plastic joint at a special distance from the critical section and increasing the capacity of plastic joint circulation. To validate the results, calibrated samples were compared with Okazaki and colleagues’ experimental results in 2004 in America. This article shows that applying RBW in plastic joints, modeled in most of the links, causes critical sections to be kept away and plastic joint circulation be also observed.

Wavelet transform is a new and useful method for signal analysis. Wavelet function is a composition of basic functions which can separate a signal in time and frequency. Therefore, wavelet transforms are capable of detecting many of unknown aspects of information which can not be detected through other methods. A wavelet-based nondestructive method for damage detection in structures that is effective and accurate is presented in this text. The proposed method is based on the analysis of the response signal of damaged structures. The main idea is to extract the damage location from the coefficients of the wavelet transform applying on the signals coming from the structural responses. After analysis of the structure by ABAQUS software, displacement response of structure was extracted along its longitudinal axis. Afterwards, the response was used for the wavelet analysis by the wavelet toolbox of the MATLAB software and the detection of damage in the structures was investigated. The results of wavelet analysis showed that the wavelet coefficients plots have considerable rise at or around the damage location. A sudden change or peak on the wavelet coefficients plot can indicate the location of a crack. Based on the obtained simulation results, it appears that this method can provide an advantageous alternative to classical methods for structural damage detection, such as Fourier transform.

The use of prestressed concrete, has begun progress today and on this basis, both experimental and theoretical has strong backing, but this technique is less concerned with steel, and especially when the combination of concrete and steel is used, the lack of instructions becomes more apparent. In this paper, by presenting the proposed increase in load method, the first results of the plastic analysis of a single-span beam were compared with the strain increase method and the experiment and then, the method of analysis of a continuous double beam with combine of stiffness and flexible method, such as a bridge, has been investigated and finally, the variables of cable length, eccentricity of the cable, and the thickness of the slab in terms of performance optimization, are illustrated by the diagram. It has been found that in the combinatorial method analysis , the inversion of smaller matrices is required compared to the stiffness method, and  the results in the optimization section are shown, adding cables will increase the final load bearing in this sample, and a larger eccentricity will cause a larger internal resistance in the beam. Also, although the thickness of the concrete slab strength, increases the final capacity in these specimens, it does not affect the elastic capacity in pre-tension before concrete.

The issue of predicting the residual load-bearing capacity of damaged concrete-filled steel tube (CFST) columns subject eccentricities is not widely assessed. The post-fire behavior of CFST columns is experimentally assessed here by running tests with different cross sections (circular and square), subject to eccentric axial loads. Here, the focus is on the influence of cross-sectional shape, load eccentricity (25 and 50 mm), slenderness and fire protective coating parameters on the post-fire bearing capacity of CFST sections. In this paper, the reduction factors are proposed to estimate the residual capacity of the CFST columns with the load eccentricity based on the previous experimental results. The modified design methods based on Eurocode 4 are applied in predicting the residual load-bearing capacity of the concrete-filled hollow tube columns after being exposed to fire. The assessments reveal that the simplified method of Eurocode 4, in the ambient temperature, subject to both the concentric and eccentric loading conditions and yield safe results, thus, a better prediction method.