سال:1388 | دوره: | شماره: |تعداد مقالات:7

Tension stiffening, is attributed to the fact that concrete does not crack suddenly and completely but undergoes progressive micro cracking (strain softening). Immediately after first cracking, the intact concrete between adjucent primary cracks carries considerable tensile force due to the bond between the steel and the concrete. The bending stiffness of the member is considerably greater than that based on a fully cracked section, where concrete in tension is assumed to carry zero stress. This tension stiffening effect may be significant in the service load performance of beams and slabs. This phenomenon is also effective in long term behavior of concrete members due to the creep and shrinkage of the concrete. Various methods have been proposed to account for tension stiffening in the analysis of concrete structures. An approach for modelling tension stiffening is to assume that an area of concrete located at the tensile steel level is effective in providing stiffening. In this paper a simple formulation is proposed for study of short-term and long-term behavior of reinforced concrete beams and one-way slabs considering tension stiffening effect. The proposed formula is validated with experimental results and some numerical examples are worked out.

Laterally loaded reinforced concrete beams may fail in shear before their full flexural strengths are attained if they are not adequately designed for shear. Unlike flexural failures, shear failures are very sudden and unexpected. A thorough knowledge of the different modes of shear failure and the mechanisms involved is necessary to prevent them. Shear strength of concrete is an important consideration in the design of RC members. In situations, where use of steel stirrups becomes impractical or difficult to provide such as RC slab, the intrinsic shear strength of the concrete becomes all important. For concrete of low shear strength, quite often the concrete cross-section size needs to be enhanced only to provide for the high shear demand. It is, therefore, pertinent to study the shear strength of concrete. The present describe the experimental investigation on shear strength of concrete beams reinforced with longitudinal tension steel. The primary design variables were the shear span-to-depth ratio in terms of depth of beam and amount of longitudinal reinforcement. The study aims to investigate the effect of increasing percentage of longitudinal reinforcement and depth of beams in terms of shear span-to-depth ratio on shear strength of concrete. Test results show that, the depth of beam in terms of shear span-to-depth ratio is highly influencing parameter of shear strength of concrete in addition to the amount of longitudinal reinforcement and concrete compressive strength. This aspect is not reflected in the Indian Standard [4].

In this paper an efficient method is developed for the formation of null bases of finite element models comprised of four-node quadrilateral plane stress and plane strain elements corresponding to highly sparse and banded flexibility matrices. This is achieved by associating special graphs to the finite element models and using an element with new equilibrium tractions and stress field for the formation of localized self-equilibrating stress systems. The efficiency of the present method is illustrated through some examples.

The environment prevalent in ocean necessitates the piles supporting offshore structures to be designed against lateral cyclic loading initiated by wave action. Such quasi-static load reversal induces deterioration in the strength and stiffness of the soil-pile system introducing progressive reduction in the bearing capacity associated with increased settlement of the pile foundation. To understand the effect of lateral cyclic load on axial response of single pile in soft clay, a numerical model was previously developed by the author. Using the methodology, further analysis has been carried out to investigate how the variation in relative pile-soil stiffness and eccentricity affects the degradation of axial pile capacity due to the effect of lateral cyclic load. This paper presents a brief description of the methodology, analysis and interpretations of the theoretical results obtained and the relevant conclusions drawn there from.

This paper proposes a refined version of particle swarm optimization technique for the optimum design of steel structures. Swarm is composed of a number of particles and each particle in the swarm represents a candidate solution of the optimum design problem. Design constraints in accordance with ASD-AISC (Allowable Stress Design Code of American Institute of Steel Institution) are imposed by the particle swarm optimization based optimum design algorithm developed. A constraint handling method called the 'penalty function method' is introduced to maintain acceptable solutions. The refined version of the particle swarm optimization algorithm proposed in this paper is easy to implement and the results and convergence performance are better than the simple particle swarm optimization algorithm and some other meta-heuristic optimization techniques. The effect of different inertia weight parameters in finding the optimum design is also tested in two numerical examples.

Now a days high strength and high performance concrete are being widely used all over the world. Most applications of high strength concrete have been in high rise buildings, long span bridges and in some special applications in structures. In developed countries, using high strength concrete in structures today would result in both technical and economical advantage. In high strength concrete, it is necessary to reduce the water/cement ratio and which in general increases the cement content. To overcome low workability problem, different kinds of pozzolanic mineral admixtures (fly ash, rice husk ash, metakaoline, etc.and chemical admixtures are used to achieve the required workability.In the present experimental investigation, the mechanical properties of high-strength concrete of grades M40 and M50, at 28 days characteristic strength with different replacement levels of cement with silica fume or micro silica of grade 920-D are considered. Standard cubes (150mm x 150mm x 150mm), standard cylinders (150mm dia x 300mm height) and standard prisms (100mm x 100mm x500mm) were considered in the investigation. In all, 144 specimens were cast with and without silica fume. The mechanical properties viz., compressive strength, flexural strength and splitting tensile strength, and stress-strain characteristics of high strength concrete with various replacement of silica fume viz., 3%, 6%, 9%, 12% and 15%, has been considered. The investigations revealed that the use of waste material like silica fume improved the mechanical properties of high strength concrete witch is otherwise hazardous to the environment and thus may be used as a partial replacement of cement.

Near-field earthquakes are characterized by short duration pulses of long periods with large peak ground velocities and accelerations. Recent studies have shown that the performance of passive energy dissipation systems depends significantly on the characteristics of near-field ground motion pulses. This paper is focused on the viscoelastic (VE) dampers to be used as energy-absorbing devices in buildings. Detailed and systematic investigation on the performance of passive energy dissipation systems during near-field ground motions has been carried. The analytical studies of the model structures exhibiting the structural response reduction due to these VE devices are presented. In order to exhibit the benefits of VE dampers, a nonlinear time history analysis is carried out under strong ground motion records from near-field and far-field earthquakes for all case studies: (a) a 5-story, (b) a 10-story and (c) a 15-story reinforced concrete building. The top story relative displacements as well as the top story absolute accelerations and also the base shear values obtained indicate that these VE dampers when incorporated into the super-structure reduce the earthquake response significantly in proportion to the amount of damping supplied in these devices. Reduction response of structure in 5 and 15 story building is harmonic but in 10 story building, there is no harmony in response of structure. Overall, the highest reduction has been achieved in 5 story building with an average reduction of 100%. Additional viscous damping is suggested as a way to control very large displacements. In order to be effective for mitigating the effects of large near fault motions, large damping values would be required.