mohammad ghazi

In this research, ZnO/ ZnFe2O4 nanocomposites with a weight ratio (1:1) were made using hydrothermal method and annealing temperatures of 600°C and 700°C. Structural, optical, and magnetic properties of the synthesized powders were characterized using X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), ultraviolet-visible spectrometer, and vibrating sample magnetometer (VSM). The results obtained from the XRD diffraction patterns confirmed the formation of the mixed phases, hexagonal zinc oxide and cubic spinel phases of pure zinc ferrite. FESEM images showed that with the increase of annealing temperature, the samples have a cohesive and agglomerated structure. The measurement of the absorption spectra of the synthesized samples showed that the absorption in the visible region increases with increase in annealing temperature. The optical band gap value for the ZnO/ZnFe2O4 composites were in the range of 2.1-2.2 eV which is between the band gap values of ZnO (3.37 eV) and zinc ferrite (1.8 eV). The hysteresis loops measured with VSM indicated a soft and weak ferromagnetic magnetic behavior for both samples.

Fiber Reinforced Polymers (FRP) are often used for structural repairs to improve the strength and ductility of the structures. Previous studies have shown that utilizing active confinement in concrete structures can improve the seismic behavior of concrete columns under pressure. In this study, the behavior of concrete columns actively confined by AFRP fibers was investigated under the combined effects of axial compressive and cyclic lateral loads. Two concrete columns were actively confined by AFRP strips. In addition, a non-confined column (SCR) was utilized as a control specimen. Then all specimens were tested under axial and lateral cyclic loading. Experimental results showed that the ultimate compressive strength and axial strain of specimens with active confinement are improved compared to the SCR specimen. Also, due to the increased number of small cracks in the specimen, a higher extent of energy was absorbed under the applied loading. The loading protocol caused no rupture in the AFRP strips, and no shear crack and brittle failure in the specimens were observed.

Concentrically braced frames are among the prevalent seismic force-resisting systems used in the construction of steel structures. This type of system provides a suitable level of stiffness for structures under low and intermediate seismic oscillations. However, under strong motions, it has noticeable deficiencies such as stiffness loss under compressive force, the unacceptable difference between the tensile and compressive strength of the brace, low energy-dissipating capacity, and overall poor cyclic behavior. To overcome these deficiencies, the idea of the Buckling Restrained Brace (BRB) was proposed a few decades ago.  Since the invention of BRB, extensive studies have been carried out to optimize the new brace system. These studies have resulted in the emergence of different generations of buckling restrained braces. In the first generation of BRB, a concrete-filled sheath had been used around the inner core of the brace. To upgrade that heavy brace, the researchers developed an all-steel brace system that was considerably lighter in weight, faster to build, and easier to inspect its yielded core after an earthquake. Later on, the idea of reducing the length of the core, as well as the sheath, was proposed which led to an even lighter brace, while keeping all the major advantages of the traditional BRB. In this paper, twelve all-steel BRB samples, based on a reduced fuse length, have been investigated numerically. Each brace sample is composed of three boxes, which include the main box, the outer sheath, and the inner box. The outer sheath and the inner box are used to prevent the local buckling of the core in the fuse zone. The outer sheath and the inner box are connected to the brace core at one end only. In this study, the cross-sectional area of the brace core in the fuse zone was considered to be less than half the total cross-sectional area of the original brace section. The samples were loaded by the quasi-static cyclic loading protocol of AISC. The numerical analysis showed that the proposed brace withstood an axial strain level of around 4%. The numerical modeling of the proposed brace was verified by the data reported for an earlier experiment that had been carried out in the laboratory of the Housing and Urban Development Research Center (BHRC). In the numerical study, the effect of influential parameters of the proposed brace on its cyclic behavior was investigated. These parameters included the ratio of the fuse length to the total brace length, the gap between the core and the inner/outer boxes, the inner/outer box thickness, and the friction coefficient between the core and the contact surfaces of the boxes. Using the hysteretic curves of the brace, obtained from the numerical analyses, the ductility parameters, and the amount of dissipating energy were evaluated. The results showed that the obtained amount of the relative lateral displacement of the proposed brace is acceptable according to the code regulations. Moreover, the cumulative inelastic deformation of the proposed brace surpasses the minimum requirement of the code for the predefined loading protocol. The studied samples were stable and had relatively symmetric cyclic behavior in the compression and tension zones. The study showed that the proposed bracing is suitable for the rehabilitation of buildings.