International Journal of Civil Engineering
Volume:8 Issue: 3, sep 2010

Neither damage mechanics model nor elastoplastic constitutive law can solely describe the behavior of concrete satisfactorily. In fact, they both fail to represent unloading slopes during cyclic loading which actually define the amount of the damage in the material. To overcome the disadvantages of pure plastic models and pure damage approaches, the combined effects need to be considered. In this regard, various classes of plastic-damage models have been recently proposed. Here the formulation of constitutive and damage evolution equations of a plastic-damage model originally proposed by Lubliner et al and later on modified by Lee and Fenves is first discussed and then its computational aspects in three-dimensional space are emphasized in detail. A part of the implementation in 3-D space needs to be reformulated due to employing a hyperbolic potential function to treat the singularity of the original linear form of plastic flow proposed by Lee and Fenves. The consistent algorithmic tangent stiffness used to accelerate convergence rate in solving the nonlinear equations is also derived. The validation and evaluation of the model to capture the desired behavior are shown with several numerical examples of monotonically and cyclically loaded simple tests. The numerical simulations of single-element tests confirm the robustness, accuracy, and efficiency of the algorithm at the local and global levels. At the end, two structural examples are presented to demonstrate the capabilities of the model in 3-D applications.

Elevated water tanks as one of the main lifeline elements are the structures of high importance. Since they are extremely vulnerable under lateral forces, their serviceability performance during and after strong earthquakes is a matter of concern. As such, in recent years, the seismic behavior of water tanks has been the focus of a significant amount of studies. According to the literature review, it has been realized that although many studies have been investigated the effects dynamic characteristics of structure, only a few studies have been conducted on the seismic behavior of the elevated water tanks. In The current study, three reinforced concrete elevated water tanks, with a capacity of 900 cubic meters and Height of 25, 32 and 39 meters have been utilized and subjected to an ensemble of earthquake records. The behavior of concrete material was assumed to be nonlinear. Seismic demand of the elevated water tanks for a wide range of structural characteristics has been assessed. The obtained result revealed that scattering of responses in the mean minus standard deviation and mean plus standard deviation are approximately 60 to 70 percents Moreover, simultaneous effects of mass increase and stiffness decrease of tank staging led to increase in the base shear, overturning moment, displacement and hydrodynamic pressure Equal to 10 to 20 %, 13 to 32 %, 10 to 15 % and 8 to 9 %, respectively.

In this paper the response of cantilevered RC beams using smart rebars under static lateral loading has been numerically studied, using Finite Element Method. The material used in this study is Shape Memory Alloys (SMAs) which contains nickel and titanium elements. The SMA exhibits little residual strains under cycles of loading and unloading even after passing the yielding zone. Since Young’s Modulus of this material is much lower than that of conventional steel reinforcement, it is not feasible and economical (due to relatively high price of SMAs) to replace the total steel with SMA bars. Therefore, different quantities of steel and smart rebars have been used for reinforcement. During lateral loading, rebars yield or concrete crushes in compression zone in some parts of the beams and also residual deflections are created in the structure. It is found that by using SMA rebars in RC beams, these materials tend to return to the previous state (zero strain), so they reduce the permanent deformations and also in turn create forces known as recovery forces in the structure which lead into closing of concrete cracks in tensile zone. This ability makes special structures to maintain their serviceability after earthquake. Also replacing steel rebars with smart ones reduces lateral stiffness of beam. Moreover, in each model the amount of stiffness reduction and its effects have been studied.

Investigations on vibration behaviors of railway track systems are attempted in this research through a comprehensive field investigation into the free vibration of track systems and response of tracks to train moving loads. In-situ modal analysis was used for the fist time in a railway track field as an efficient method of investigating dynamic properties of railway track systems. Natural frequencies and mode shapes of the track system in different in-situ track conditions are obtained. Validity of the laboratory results is evaluated by comparing the obtained field data with the laboratory results available in the literature. Efficiency of rail welded joints in CWR tracks and the effects of replacing timber sleepers with concrete sleepers on dynamic behavior of the track are investigated. Advantages of concrete sleeper tracks over timber sleeper tracks from serviceability and “passenger riding comforts” aspects are discussed. Rail deflections were calculated by using auto-spectra obtained from vibrations of the track under train loads, leading to the development of a new mathematical expression for the calculation of the track dynamic amplification factor. The results obtained are used to evaluate the accuracy of the track conventional design methods.

From the time that civil engineers have used steel in building structures, they tried to increase its strength so as to produce more economic and lighter structures by using more elegant sections. Increase of steel strength is not always useful for all members of a steel structure. In some members under certain conditions, it is needed to reduce the strength as much as possible to improve the behavior of structure. By using very low strength steel according to the Easy-Going Steel (EGS) concept in this research, it is shown that the performance of diagonal Eccentrically Braced Frames (EBFs) improves substantially. For this purpose, a finite element analysis was used to simulate diagonal eccentrically braced frames. Fifteen diagonal eccentrically braced frames were designed through AISC 2005. By substituting very low strength steel instead of carbon steel with equal strength in the links, their performance improve fundamentally without any global or local instability in their links.

In this paper, the problem of layout optimization for X-bracing of steel frames is studied using the ant colony optimization (ACO). A new design method is employed to share the gravity and the lateral loads between the main frame and the bracings according to the requirements of the IBC2006 code. An algorithm is developed which is called optimum steel designer (OSD). An optimization method based on an approximate analysis is also developed for layout optimization of braced frames. This method is called the approximate optimum steel designer (AOSD) and uses a simple deterministic optimization algorithm leading to the optimum patterns and it is much faster than the OSD. Several numerical examples are treated by the proposed methods. Efficiency and accuracy of the methods are then discussed.

According to the Iranian code of practice for seismic resistant design of buildings, soft storey phenomenon happens in a storey when the lateral stiffness of the storey is lower than 70% of the stiffness of the upper storey, or if it is lower than 80% of the average stiffness of the three upper stories. In the combined structural systems containing moment frames and shear walls, it is possible that the shear walls of the lower stories crack; however, this cracking may not occur in the upper stories. The main objective of this research is to investigate the possibility of having soft storey phenomenon in the storey, which is bellow the uncracked walls. If the tension stresses of shear walls obtained from ultimate load combinations exceed the rupture modulus of concrete, the walls are assumed to be cracked. To estimate rupture modulus from compression strength, 144 compressive specimens and 144 rupture modulus specimens were constructed. For calculating the tension stresses of shear walls in different conditions, 10 concrete structures containing 15 stories were studied. Each of the structures was investigated according to the obligations of Iranian, Canadian, and American concrete building codes. Five different compressive strengths of 30, 40, 50, 60, and 70 MPa were assumed for the concrete of the structures. In other words, 150 computerized analyses were conducted in this research. In each analysis, 5 load combinations were imposed to the models. It means, the tension stresses of the shear walls in each storey, were calculated 750 times. The average wall to total stiffness ratios of the buildings were from 0.49 to 0.95, which was quite a wide range. The final conclusion was that the soft storey phenomenon did not happen in any of the structures investigated in this research.