Iranian Journal of science and Technology (B: Engineering)
Volume:32 Issue: 5, Oct 2008

This paper explains an experimental research program on the use of GFRP (Glass Fiber- Reinforced Polymer) for retrofitting small-scale slender R/C columns to enhance seismic performance. An important deficiency in many existing nonductile reinforced concrete frames is the inability of the columns to undergo significant deformations while maintaining their load-carrying capacity. As a result, relatively brittle modes of column failure, accompanied by soft storey structural failure mechanisms, are possible. Providing additional confinement to the columns allows them to behave in a more ductile manner.In this study, six 1/2-scale columns were constructed and tested under cyclic loading to examine the effectiveness of the retrofitting technique for improving the seismic resistance of slender concrete columns. Three columns were tested after being retrofitted with GFRP wraps at the potential plastic hinge zone, while three others were tested in the “as-built” condition. One of the “as-built” specimens was constructed according to ACI (318-02) detailing and five others were built in accordance with ACI (pre.1971). In general, the common mode of failure for the “as-built” samples was a brittle failure due to bond deterioration of the lap-spliced longitudinal reinforcement. Test results suggest that GFRP wraps can significantly increase the flexural strength and ductility of slender rectangular reinforced concrete columns.

This paper presents the results of nonlinear finite element analysis of reinforced concrete (RC) frame structures, including the complete behavior of these systems from zero load to the ultimate load. Presented also are the details and results of a corroborative test program involving a large-scale frame model. The effects of the finite element size and tension-stiffening on the nonlinear responses of three RC frames are investigated. In addition, the program is used to carry out a "plastic" analysis of the frame to define the mechanism of failure, the plastic hinge rotations, and the yielding and the plastic hinge lengths. The capability and accuracy of the nonlinear finite element analysis program in predicting the nonlinear response of RC frame structures is verified, along with a comparison between the analytical and the corresponding experimental results. The different behavioral aspects including cracking, yielding and ultimate loads, load-displacement and load-strain characteristics for concrete and reinforcing steel and plastic hinge deformations are studied. The analytical and experimental results indicate that the computed response of RC frame structures is strongly influenced by the finite element size. The finer meshes give lower values of the ultimate load and vice versa for the coarser meshes. With an increase in the number of elements, the structure is slightly more flexible than for the case for the coarse mesh idealization, and the frame tends to be less ductile. An empirical equation has been proposed to eliminate this drawback. The calculated plastic hinge rotations show good agreement with the experimental results. For example, the maximum deviation between the analytical and the experimental values of the plastic hinge rotations is approximately 13%, while the minimum deviation is 4%.

Nowadays, seismic design of structures performed by any seismic code is based on resisting previous natural earthquakes. Therefore, the critical excitation method has been proposed in recent years to consider probable future earthquakes that may be more destructive. Non-stationary critical excitation for a structure is found under specified constraints to resonate the structure. In this paper, the objective function of non-stationary critical excitation is taken as the maximization of all the inter-storey drifts at different times, separately. The power (area under power spectral density function) and the intensity (magnitude of PSD function) are limited, and critical excitation is found according to these constraints. Three techniques of finding non-stationary critical excitation are proposed, optimum line, simple and modified techniques. Then, the proposed techniques are used in many MDOF models and the results are investigated.

The ultimate load capacities of conical and pyramidal shell foundations on unreinforced and reinforced sand were determined by laboratory model tests and numerical analysis. The results were compared with those for circular and square flat foundations. Eight foundation models on unreinforced and reinforced sand were tested in which the influence of shell configuration on ultimate load capacity was investigated. Both the experimental and numerical studies indicated that, if shell foundation thickness increases, the behavior of the shell foundation on either reinforced sand or unreinforced sand gets closer to that of flat foundations. A new factor was also defined to present a unique relation between the ultimate load capacity of shell and flat foundations.

Gravel-Clay mixtures are abundant materials in nature and are frequently used in certain civil engineering projects such as earth dams, levees and landfills. The advantage of using these soils lies in their low permeability owing to clay fraction along with the high shear strength due to the presence of the non-cohesive granular part. Karkheh dam is an example where these soils were used as the impervious core of the embankment. To date, little research has been carried out to investigate the performance of these soils, and therefore, their behavior under cyclic loading is still not well known. In order to investigate the cyclic behavior of gravel-clay mixtures, 51 cyclic and monotonic triaxial tests were performed on specimens with 11 different mixtures and under various confining pressures. Two different types of gravel, i.e. angular and round grains, were utilized to prepare specimens with the same gravel content in order to investigate the effect of the granule shape on the cyclic behavior of the mixtures. All the specimens were prepared with a constant compaction effort. Normalized shear modulus and damping ratio data showed differences in the behavior of mixtures with different kinds of gravel. Based on samples of macro structure performance under cyclic loading, two different mechanisms for the behavior of angular and round granules at contact level were hypothesized. The importance of the sampling method and specimen size for intermediate soils was also noticed.

Fixing integer ambiguities is a non-trivial problem, especially if we aim at computational efficiency and high performance (or success rate). For this reason it has been a rich source of Global Positioning System (GPS) research over the last decade. A brief review of ambiguity resolution using the method of Least-Squares AMBiguity De-correlation Adjustment (LAMBDA) and rapid GPS ambiguity resolution for short and long baseline (KTH method) is presented in this article. It continues with some numerical comparisons between two methods with real (float) and simulated GPS data. Finally, we end the paper with conclusions and some remarks. This comparison shows that the results of ambiguity resolution for short baselines are exactly the same using the KTH, LAMBDA and Trimble Total Control (TM) software. Also, for very long baselines these methods and software were not successful in solving ambiguities. However, the success rate of Trimble Total Control software was lower than for the others. This research also shows the exact effect of ionosphere in ambiguity resolution techniques. Any improvement in this area can improve the quality of ambiguity resolution significantly. More research with extra GPS observations in different conditions must be made for better results in the future.

In this investigation, characteristics of vertical drops with subcritical flow at the upstream channel and sloping aprons at the downstream channel have been studied experimentally. Flow characteristics such as pool depth, downstream depth, and energy loss have been measured. Two physical models of 0.41 and 0.21 m height were built and the inverse slope was set at 5°, 10° and 15° degrees. Experimental data were compared with the previous investigators’ results for horizontal downstream channels. A method is presented to estimate flow characteristics. It was shown that the values of the relative pool depth for sloping aprons are larger than those with a horizontal downstream channel. It was found that the values of the relative downstream depth for sloping aprons are slightly larger than those with a horizontal downstream channel. Experimental results show that the relative energy loss increases by increasing the angle of the invert and the maximum increase is for the 5° degree slope. Predictions for the pool and downstream depths agree with the experimental data, but the differences between the predictions and the experimental results of the relative energy loss are significant.

A developed model was applied to displacement vectors acquired from a project that monitored the vertical movements of a building in the main campus of Nigde University. The estimated subsidence and its corresponding standard deviation errors by the model are -4.89 mm ± 0.53 mm for block 1, -1.23 mm ± 0.35 mm for block 2, and -3.65 mm ± 0.54 mm for block 3, respectively. The tilt angle is estimated and its corresponding standard errors are 16”.61 ± 2”.39, -18”.93 ± 1”.95, and -13”.87 ± 4”.64 arc second for Block 1, Block 2, and Block 3 respectively over a period of one year. Results showed that the model produced in this paper can be used to estimate the vertical movements of a building with a Mat foundation.

Laboratory and field methods that are used to determine the soil water characteristic curve (h-θ function) are expensive and time consuming. A physical model for predicting the h-θ function based on the soil particle-size distribution curve and soil bulk density with different scale parameters (α, i.e., constant, linear, and logistic models) has been proposed in the literature. Unfortunately, many databases do not contain the full particle-size distribution, but instead contain only the sand, silt, and clay mass fractions. A method for estimating the particle-size distribution from clay, silt, and fine plus very fine sand mass fraction (particle radii, between 25 and 125 μm) has been presented in the literature and is improved by using all sand particles (radii between 25 and 999 μm, modified model). The objectives of the present study were to evaluate the predicted soil water characteristic curve for 16 soil samples with different textures based on clay, silt, and sand fractions, and soil bulk density using the improved method for the prediction of particle-size distribution (modified model), different values for the scale parameter as constant, being obtained from linear and logistic models. The results indicated that for clay and silt loam soils using a radius of 999 μm (the modified model), the particle-size distribution, and consequently soil moisture characteristic curves, was predicted more accurately than those obtained by using a radius of 125 μm for the largest particles of the soil. This is specifically shown for a determined by the logistic procedure. The values of α based on the logistic model were best suited for the clay, silt loam, and loam soils. The values of α based on the linear model were appropriate for the clay and silt loam soils. Further, the values of constant α were best suited for the clay soils. However, these results are considered indicative rather than conclusive due to the small size of the data set for clay, silt loam and sandy loam soils. Finally, it is proposed to test the modified model for a wider data set.

The explicit solutions of the Green and Ampt infiltration equation, by developing a modified Adomian Decomposition Method (MADM) and Variational Iteration Method (VIM) solution with an exact solution, are compared. These solutions were compared with the exact implicit solution and a good trend between our approaches and the exact solution was found. The best result is gained by only the first three terms in the series solutions of MADM. As the number of terms increases, the error in the lower bound of the computational domain grows and the upper bound converges after the first three terms of MADM. On the other hand, variational iteration converges after the first two terms. Both methods addressing this phenomenon seem quite promising, and by considering only a few terms the results are accurate, making both methods fast and efficient.

In the present paper, simulations of tidal currents in the Persian Gulf using depth average flow solver version of NASIR (Numerical Analyzer for Scientific and Industrial Requirements) software are verified. The hydrodynamic equations utilized in this work consist of depth integrated equations of continuity and motion in two dimensional horizontal planes (SWE). The effects of rainfall-evaporation are considered in the continuity equation and the effects of bed slope and friction, as well as the Coriolis effects are considered in two equations of motion. The vertex base finite volume method is applied for solving the governing equations on triangular unstructured meshes. Due to the complexity of the coastal boundaries and the existence of irregularly shaped islands, turbulent flow circulations may play an effective role in the formation of flow field parameters. Therefore, the effect of turbulent modelling on the accuracy of the depth averaged circulating flow simulation is investigated. The performance of the computer model to simulate tidal flow in the Persian Gulf domain is examined by imposing tidal fluctuations to the main flow boundary during a limited period of time. In addition, in order to illustrate the computed tidal flow characteristics in the Persian Gulf, an S2 tidal constituents chart is presented.