حمید محمدی گرفمی

From the past until now, monitoring the health of structures and identifying different locations of damage to their elements have always been one of the basic requirements of maintaining structures and ensuring the safety of residents. Damage is defined as any modification to the shape or material characteristics of a structure that could impair its overall performance. Irreversible damages to structures can be avoided by promptly identifying, repairing, and replacing any damaged components in early stages. Damage to columns, one of the most vital elements of building structures and bridges, can pose serious concerns about the overall health of the structures compared to damage to other elements. This study proposes a detection index based on the slope and curvature of the mode and explores its usefulness in identifying distinct sites of column damage under axial load. The preliminary findings of this research revealed that as the axial load increases, the value of the natural frequency of all modes drops in both healthy and damaged conditions. Considering that the given columns may buckle and become unstable under the impact of axial load less than the design axial load due to damage, this underlines the necessity of addressing the problem of damage detection in columns under this load. The results also demonstrate that the frequency values of healthy and damaged conditions are different due to damage. Damage detection using the suggested index indicates that it is sensitive to the position of the damage, and that by establishing relative maxima in the damaged area, it is possible to recognize the location of the damage with an error less than one percent. In addition, the findings demonstrated that while the amount of axial load affects frequency values, it does not have any effect on the values of the damage detection index.

Structural control and heath monitoring is of particular importance. Columns are one of the most important structural elements, and compared to other elements, the spread of local damage, however small, can lead to premature destruction of the structure. A damaged column may have buckling instability due to an axial force less than the critical load of the healthy condition of the column; therefore, the column damage detection under the effect of axial load is important. In this paper, the column in healthy conditions and damage due to different ratios of base critical load were frequency analyzed in ABAQUS software and modal responses were extracted. Studies have shown that with increasing load, the frequency values decrease. Additionally, at the same load ratios, the healthy state frequency is higher than the damaged state frequency. Then, the curvature of the primary and secondary shapes of the vibration modes was calculated. Based on computational primary and secondary curvatures, DI1 and DI2 damage detection indices were defined. The damage detection results of curvature indices showed the optimal performance of these indices in detecting damage locations, with the difference that the DI1 index shows possible damage locations and the detection is associated with uncertainty. The results also showed that the curvature index of damage detection in one damage location is not affected by changes in damage severity in other damage locations. Additionally, the axial load has no effect on the curvature index of damage detection and this index is affected by support conditions, section area and damage severity.

Damage to structures is inevitable. Besides, the accumulation and spread of local damage can be a source of global damage; hence, the significance and necessity of monitoring the health of the structure. Wavelet analysis of structural responses is a method of damage detection with optimal performance in the time-frequency domain, which yields more information from the analyzed response of the structure in both time and frequency domains. As beams and columns are among the most fundamental structural elements and, expectably, the last elements to be damaged, it bears greater importance to identify damage in them than in other structural elements. This paper draws on modal analysis data of steel beams modeled in Abacus and a proposal of the wavelet analytical method to track different positions of damage along the beam, where the damages are defined as a decrease in the elastic modal. A decreasing shift in frequency values occurred in all modes due to damage. To detect the damage, the mode shape signal was defined as the damage mode shape and the differentiation between the healthy and the damage mode shapes as the wavelet transform input signal. The output signals from the wavelet analysis of the input signal indicated maximum and minimum relative fluctuations in different situations of damage so that the centers of damage were identified with zero error. The results also revealed that with increasing the severity of the damage, only the irregularity of the wavelet coefficients of that damage position increases.

T-shaped joints are one type of beam-column connections in steel structures. Ease of construction, transportation, and installation, as well as suitable ductility, economy, and durability, are the advantages of this type of connection. However, bolting of the T-shaped flange to tubular columns is very challenging due to limited access. In this research, by using through bolts for connecting the T-shaped flanges and the column, the transfer of forces and the rigidity of the panel zone have been provided. This is especially effective in concrete filled tubular (CFT) columns where the passage of concrete through the panel zone is important. On the other hand, the direct transfer of stresses to the column, the stretch of through bolts under cyclic loads, which reduces their performance level, as well as accumulation of bolts, which is caused by large diameter, and four-way connection, especially at a long opening that interferes with the passage of concrete through the panel zone are controversial. Therefore, to provide a suitable solution, in an innovative action, the external diaphragm of the column with through bolts was used simultaneously and the performance of the connection was examined.
In this study, using ABAQUS software, one connection sample with a through bolt and another with an external diaphragm were investigated. The results show that the overall performance of both kinds of connection is quite good.

Due to the abundant advantages, such as ease of implementation, economic efficiency, and appropriate ductility T-shaped elements are widely used in many beam-column connections.  In this research, the through bolts were utilized to connect the Tee profile to the column flange. The direct transfer of stresses to the body of the column increases the length of the intermediate bolts under cyclic loads. Therefore, in an innovative effort, the external diaphragm of the column was used together with the through bolt. Finally, the joint performance of them was examined.
 In this study, based on the validated models in Abacus software, two samples were designed and tested in the laboratory. One of these samples contained a connection with through bolts and, another one had an external diaphragm. The overall performance of both joints was quite good and, they passed the acceptance criteria of rigid joints in the especial bending frames according to AISC and FEMA codes. Both laboratory samples achieved a relative rotation of 0.06 radians without declining resistance. The final failure in the sample without the external diaphragm occurred in the first cycle relative rotation of 0.07 radians and with the separation of the nut from the through bolts, while this issue in the sample with an external diaphragm is ductile and deformable in the flange and the web.