Synthesis of nanosized SiC by low-temperature magnesiothermal reduction of nanocomposites of functionalized carbon nanotubes with MCM-48

Silicon carbide synthesis by a magnesiothermal method was investigated using MCM-48 as the silica source mechanically mixed with carbon nanotubes (CNTs) as the carbon source, and nanocomposites of MCM-48/functionalized CNTs (CNTF). SiC syntheses were carried out with different molar ratios of MCM-48, carbon and magnesium at 700?°C in argon. The MCM-48 and carbon nanotube starting materials and the SiC products were characterized by BET, XRD, FESEM, EDX and TEM. The effect of the carbon content and the type of CNTs (either functionalized or unfunctionalized) on the SiC synthesis was studied. The results show that an improved yield of SiC is obtained when the carbon nanotubes are functionalized, producing a better contact with the MCM-48. This improved contact between the reactants ensures a good degree of reaction in a stoichiometric mixture of silicon and carbon, with no improvement in product formation being achieved by the use of additional carbon. These findings suggest that the degree of contact between reactants is an important factor in the magnesiothermal synthesis of SiC. The SiC products from magnesiothermal synthesis of the functionalized nanocomposite precursors were shown by TEM and FESEM to have unusual nanofiber morphologies mimicking the morphology of the CNTF nanotubes.     

Incremental dynamic analysis of steel braced frames designed based on the first,second and third editions of the Iranian seismic code (standard no. 2800)

Incremental dynamic analysis (IDA) is an emerging method in structural analysis which allows evaluation of seismic capacity and demand of structures through a series of nonlinear dynamic analyses using multiple scaled ground motion records. Seismic behaviour of concentrically braced frames designed based on the first, second and third revisions of the Iranian seismic code, standard no. 2800, has been evaluated through IDA in the present paper. Besides, a brief comparison is made between seismic behaviour of these frames, frames with different heights and different bracing types. Seismic capacity and limit states of such structures have been reviewed through the paper. The IDA results imply that frames designed with the first edition are seriously vulnerable and fail before reaching the acceleration levels predicted in the code. On the other hand, frames designed with the second and third editions, although behaving better, need partial reinforcement in some cases. Other results of this study show that chevron braced frames behave seismically better than X‐braced ones. Copyright © 2009 John Wiley & Sons, Ltd.     

Performance evaluation of different types of steel moment resisting frames subjected to strong ground motion through incremental dynamic analysis

Comparative studies on seismic performance for various types of steel moment resisting frames subjected to near field and far field earthquakes are performed through Incremental dynamic analysis (IDA) method in this study. Near field earthquake has a pulse like effects on the structures. It imports immediate force in very short duration to buildings. Therefore, destructive effects of surge energy are not negligible. Four intensity indices are used, namely, peak acceleration (PGA), spectral acceleration at the structure??s first-mode period (S_a(T₁, 5%)), spectral acceleration at the structure??s nth effective-mode period (S_a(T_n, 5%)) and the Spectral velocity at the structure??s first-mode period (S_v(T₁, 5%)). Numerical results illustrate that the intensity measure parameters related to ground velocity and the higher mode-related parameters present better correlation with the seismic responses of near source ground motion for given systems. The higher mode-related parameters are more suitable for tall systems subjected to near field earthquakes. Moreover, the chosen parameters S_a(T_n, 5%) and S_v(T₁, 5%) of near-fault impulsive ground motions enhance the performance of intensity measure of corresponding conventional parameters, i.e. S_a(T₁, 5%). A comparison for the special and intermediate steel moment resisting frames is made as regard to performance using IDA method. A more efficient performance is observed for the special moment resisting frames compare to intermediate ones.     

Performance‐based evaluation of tall buildings using advanced intensity measures (case study: 30‐story steel structure with framed‐tube system)

Performance assessment of high‐rise buildings has attracted peculiar attention among engineers. Care should be taken once higher‐mode effects are to be incorporated into analyses and designs. Recently, performance‐based evaluations have been widely used by designers to meet the required target capacities of engineering projects. A common tool to perform such studies is incremental dynamic analysis (IDA), which has been utilized for first‐mode‐dominant ordinary structures, whereas taller buildings demand other considerations to be made so that a thorough assessment of the structural response can be achieved. In this paper, performance‐based studies have been carried out for a sample 30‐story tall building, which takes advantage of tubular frame as lateral‐load‐resisting system. IDA is performed subsequently to quantify the structural response against a wide‐range of seismic loadings. Advanced intensity measures (IMs) are applied to optimize the capacity assessments resulting from multitude of non‐linear time‐history analyses. Finally, performance‐based evaluations have been carried out to provide a thorough assessment of target capacities that are normally advised by widely accepted codes. Results are also compared with regular short buildings where higher‐mode effects do not contribute significantly to structural response. Copyright © 2012 John Wiley & Sons, Ltd.     

Incremental dynamic analysis of steel frames equipped with NiTi shape memory alloy braces

This paper determines the seismic performance of four‐storey concentrically braced frames equipped with either steel buckling‐restrained braces or buckling‐restrained superelastic shape memory alloy (SMA) braces through incremental dynamic analysis. The incremental dynamic analysis technique is used to examine the behaviour of four‐storey braced frames with four different bracing configurations (including diagonal, split‐X, chevron‐V and inverted‐V) under 20 different ground motion records. The study reveals a satisfactory performance at the design intensity level for both types of braced frames. The results show that the SMA braces lead to a uniform distribution of inelastic response over the height of the buildings, as well as mitigating seismic response in terms of maximum inter‐storey drift and residual roof displacement. By comparing the responses of SMA and buckling‐restrained braced frames under higher intensities of earthquake loading, it is found that the SMA braces can be more beneficial especially under severe ground motion excitations. Copyright © 2014 John Wiley & Sons, Ltd.     

On the impact evaluation of various chemical treatments of support on the photocatalytic properties and hydrogen evolution of sonochemically synthesized TiO2/Clinoptilolite

The water splitting process for hydrogen production was investigated over a series of TiO₂-deposited Clinoptilolite photocatalysts assessing the individual and synergistic effects of acid/alkaline and ion exchange treatments of Clinoptilolite as a support on the physicochemical properties and photocatalytic performance. The as-synthesized photocatalysts were characterized by XRD, BET, FESEM, EDX, FTIR, PL, and UV–vis techniques. Based on the FESEM, EDX and BET techniques, higher dispersion of Ti species and an increased accessibility of the micro-channels could be achieved using modified Clinoptilolite supports due to their better textural properties. The fine dispersion of TiO₂ particles reflects higher surface density of active sites and separation efficiency of electron–hole pairs, which accounts for their better photocatalytic performance. The characterization results generally indicated the remarkable synergetic effect of alkaline treatment on the surface morphology and TiO₂ dispersion, especially when it is coupled with NH₄NO₃ treatment. The photocatalytic tests illustrated that employing the treated Clinoptilolite supports increases the photocatalytic activity up to 12–57%. It was found that TiO₂ supported on the alkaline Clinoptilolite followed by NH₄NO₃ treatment effectively splitted water to hydrogen by $896\text{.}8\text{μmol}{\text{g}}_{{\text{TiO}}_2}^{\mathrm{?}1}{\text{h}}^{\mathrm{?}1}$ hydrogen production rate. The results indicate that the ion exchange-alkaline treatment of Clinoptilolite could provide strong basic sites and mesopore structure (average pore diameter of 21.65 nm) with high surface area (73.43 m² g^?1) and well dispersion ability to improve the photocatalytic activity of TiO₂/Clinoptilolite composite for hydrogen evolution.     

Seismic response of steel braced frames with shape memory alloy braces

In this paper, the seismic performance of steel frames equipped with superelastic SMA braces was investigated. To do so, buildings with various stories and different bracing configurations including diagonal, split X, chevron (V and inverted V) bracings were considered. Nonlinear time history analyses of steel braced frames equipped with SMA subjected to three ground motion records have been performed using OpenSees software. To evaluate the possibility of adopting this innovative bracing system and its efficiency, the dynamic responses of frames with SMA braces were compared to the ones with buckling restrained braces. After comparing the results, one can conclude that using an SMA element is an effective way to improve the dynamic response of structures subjected to earthquake excitations. Implementing the SMA braces can lead to a reduction in residual roof displacement and peak inter-story drift compare to the buckling restrained braced frames.     

A simple hybrid damping device with energy‐dissipating and re‐centering characteristics for special structures

The usage of special materials with unique properties in seismic resistant structures has increased since the 1994 Northridge earthquake to overcome limited energy dissipation and ductility. Among them, shape memory alloy (SMA) is a unique metallic alloy that has the ability to undergo large deformations and revert back to its original un‐deformed shape. Thus, a simple and practical hybrid damping device equipped with SMA that provides both energy‐dissipating and re‐centering (strain‐recovering) capabilities is developed and evaluated in the present paper. Quasi‐static loading tests on Nitinol bars are conducted to obtain their mechanical properties in tension and compression. The optimum proportion of SMA and steel in the device is achieved through analyzing various models. Placing the proposed device in semi‐rigid bracing members of special structures will localize the energy‐dissipating and ductility while providing the brace with strain‐recovering capability. Copyright © 2012 John Wiley & Sons, Ltd.     

A comparison of dynamic nonlinear behavior of ordinary and buckling restrained braced frames subjected to strong ground motion

In this paper nonlinear dynamic analyses of 4‐ to 12‐story frames with two types of bracing behavior are performed. In the first series of analyses, the bracing post‐buckling behavior is modeled using a Jain‐phenomenological model. In this model bracing buckled against compressive loading and yielded when subjected to tensile loading. In the second series, buckling restrained brace frames subjected to strong ground motion have been analyzed using elements with a bilinear behavior. The results in terms of story drifts, story shears, story shear versus drift hysteresis behavior and plastic hinge locations were compared. By comparing the response of the frame with the buckling restrained brace and the ordinary brace, better performance was observed for the buckling restrained braced frames. Copyright © 2007 John Wiley & Sons, Ltd.