Encased Columns Research Articles

Geosynthetic-Encased Sand Column Behavior under Triaxial Test Simulation

Encasing a cylindrical granular column with tensile resistant material is proven an effective practice to improve granular column strength. Laboratory triaxial compression tests were conducted on encased granular columns to study the tensile resistant material reinforcing effects. This study explores the behavior of geosynthetic-encased sand columns using a converted approach to reduce laborious efforts. The encased column behavior is simulated by applying persistently increased chamber pressure to unreinforced sand column specimens. Three column specimen diameters, 5, 7 and 10 cm, were employed to study the scale effect. The test results indicate that the converted approach simulates larger diameter encased column well.

Calculating Temperature Distribution in Concrete Encased H-Section Steel Column in Fires Using TFIELD

Steel–concrete composite columns are used extensively in modern buildings in recent decades due to the benefits of combining two construction materials: reinforced concrete is inexpensive, massive, and stiff, while steel members are strong, lightweight, and easy to assemble. For concrete-encased composite structural members, an additional advantage is that the concrete used for encasing a structural steel not only increases its stiffness, but also protects it from fire damage and local buckling failure. Traditionally, the fire resistance of composite structural members has been determined in standard fire tests, with the temperature-time curves representing more severe heating conditions compared to that which occurs in many typical natural fire compartments. To design a concrete encased H-section steel structure safely and economically, it is necessary to calculate temperature distribution in composite steel-concrete columns under natural fire. In this paper, the program TFIELD written by first author is used to calculate the temperature distribution in a concrete encased H-section steel column under natural fire and ISO 834 fire. The calculating results under the standard ISO 834 fire and a natural fire have been compared which exhibit obvious differences.

Design-Oriented Strength Model for FRP-Confined Concrete Members

This study concerns assessing 20 existing models for predicting the compressive strength of concrete uniformly confined externally with composite materials. An extended database of 471 experiments on cylindrical concrete columns is utilized for the comparisons. The research classifies the experimental data in three distinguished subcategories according to the information available for the mechanical properties of the FRPs to investigate their effect on the divergences of the models. Apart from the use of the tensile strength obtained from coupon tests, the use of properties from the manufacturer data on FRP fibers can lead to minimum error of predicted strength of concrete externally wrapped or encased in FRP tubes. The study results in the proposal of an upgraded empirical model that indirectly encompasses the unique deformational characteristics of different Young’s moduli of FRP sheets and tubes. The model utilizes the strong linear dependence of the product of the varying effective strain at failure of the confining material (εje) times the varying confinement effectiveness coefficient (k1) on the Young’s modulus of the reinforcing fibers (Ef). Thus, there is no need to estimate average empirical constant values for the εje or k1, and a more rational approach comes up that further restricts errors in strength modeling. The proposed model provides a prediction with absolute average error of 8.6% for wrapped columns and 6.3% for FRP-tube encased columns.

Performance of quasilinear elastic constitutive models in simulation of geosynthetic encased columns

Past numerical simulations of geosynthetic encased columns (GECs) using different versions of the quasilinear elastic hyperbolic model for the encased granular material have, in certain cases, yielded unrealistic results. In this paper the cause of such results is investigated by performing three-dimensional finite element analyses of GECs in soft clay, utilizing three common functional forms of the hyperbolic model for the encased granular material. Results indicate that one form of the hyperbolic model can predict an unrealistic lateral response for GECs during application of load to the column. In addition, the inability of hyperbolic models to properly account for soil behavior near failure compromises their ability to realistically capture the behavior of encased granular soil in GECs. Modeling the behavior of soil near failure is essential for properly simulating the behavior of GECs, as soil shear failure is necessary to mobilize the tensile stresses in the encasement and improve the stress–displacement response of the GEC. Although this type of hyperbolic model behavior was demonstrated for the specific case of encased soil in a GEC, the limitations of the hyperbolic model described herein apply equally to other geotechnical problems in which some portion of the soil mass is at or near failure.

Numerical study of the effect of geosynthetic encasement on the behaviour of granular columns

ABSTRACT: In very soft soils, the use of granular columns can be restricted due to the lack of adequate lateral confining pressure. In these conditions, the columns can be encased by a suitable geosynthetic to provide the required confining pressure and to increase their bearing capacity. Using a high-strength geosynthetic for confinement not only increases the strength of a granular column, but also prevents lateral displacement of the column into the very soft surrounding soil. This paper describes three-dimensional finite element analyses carried out to simulate the behaviour of a single granular column with and without encasement in a very soft clay using the computer program ABAQUS. Comprehensive numerical analyses were performed to study the influence of the geosynthetic stiffness, the friction and dilation angle of the column material, the length of geosynthetic encasement, the diameter of the column, the length of the column and the coefficient of in situ lateral earth pressure. Model results show that the stress–settlement behaviour of granular columns can be significantly improved by encasing them. The stiffness of the encasement was found to have a major effect on the stress–settlement response of encased columns and their associated load-carrying capacity. For partially encased columns, the optimum length of encasement was found to be a function of the stress that is applied to the column.

Construction of geogrid encased stone columns: A new proposal based on laboratory testing

Geogrid encasement has recently been investigated to provide an alternative and perhaps stiffer option to the now established method of geotextile encased columns (GECs). To construct geogrid encasement, the geogrid is typically rolled into a sleeve and welded using a specialized welding frame. However, the process is unlikely to be economical for site construction and therefore an alternative method of encasement construction was investigated in this paper. The technique comprises overlapping the geogrid encasement by a nominal amount and relying on interlock between the stone aggregate and section of overlap to provide a level of fixity similar to welding. A series of small-scale tests were initially used to investigate the technique, followed by medium-scale compression tests using different geogrids and typical stone column aggregates. The results of testing indicate that the “method of overlap” provides a simple and effective method of encasement construction, providing a level of fixity similar to welding. A full circumference of overlap should generally be adopted to achieve adequate fixity. Biaxial geogrids are best suited to the technique, with increased encasement stiffness resulting in increased column capacity and column stiffness. Higher strength geogrids are also more robust, providing a greater resistance to cutting from pieces of angular crushed rock. Site trials are recommended for final confirmation of the technique.

Improvement of soft soils using geogrid encased stone columns

In recent years, geotextile encasement has been used to extend the use of stone columns to extremely soft soils. Although the technique is now well established, little research has been undertaken on the use of other encasement materials such as geogrid. This paper discusses the results of a series of small-scale model column tests that were undertaken to investigate the behaviour of geogrid encased columns. The tests focused on studying the effect of varying the length of encasement and investigating whether a column that was partially encased with geogrid would behave similarly to a fully-encased column. In addition, isolated column behaviour was compared to group column behaviour. The results of partially encased column tests indicated a steady reduction in vertical strain with increasing encased length for both isolated columns and group columns. Bulging of the column was observed to occur directly beneath the base of the encasement. A significant increase in column stiffness and further reduction in column strain was observed for fully-encased columns, with strain reductions in the order of 80%. This range of performance may lend the techniques of partial and full geogrid encasement to a series of potential site applications.

Geosynthetic-encased stone columns: Numerical evaluation

Stone columns (or granular piles) are increasingly being used for ground improvement, particularly for flexible structures such as road embankments, oil storage tanks, etc. When the stone columns are installed in extremely soft soils, the lateral confinement offered by the surrounding soil may not be adequate to form the stone column. Consequently, the stone columns installed in such soils will not be able to develop the required load-bearing capacity. In such soils, the required lateral confinement can be induced by encasing the stone columns with a suitable geosynthetic. The encasement, besides increasing the strength and stiffness of the stone column, prevents the lateral squeezing of stones when the column is installed even in extremely soft soils, thus enabling quicker and more economical installation. This paper investigates the qualitative and quantitative improvement in load capacity of the stone column by encasement through a comprehensive parametric study using the finite element analysis. It is found from the analyses that the encased stone columns have much higher load carrying capacities and undergo lesser compressions and lesser lateral bulging as compared to conventional stone columns. The results have shown that the lateral confining stresses developed in the stone columns are higher with encasement. The encasement at the top portion of the stone column up to twice the diameter of the column is found to be adequate in improving its load carrying capacity. As the stiffness of the encasement increases, the lateral stresses transferred to the surrounding soil are found to decrease. This phenomenon makes the load capacity of encased columns less dependent on the strength of the surrounding soil as compared to the ordinary stone columns.

State of the art report on steel–concrete composite columns

Steel–concrete composite columns are used extensively in modern buildings. Extensive research on composite columns in which structural steel section are encased in concrete have been carried out. In-filled composite columns, however have received limited attention compared to encased columns. In this paper, a review of the research carried out on composite columns is given with emphasis on experimental and analytical work. Experimental data has been collected and compiled in a comprehensive format listing parameters involved in the study. The review also includes research work that has been carried out to date accounting for the effects of local buckling, bond strength, seismic loading, confinement of concrete and secondary stresses on the behaviour of steel–concrete composite columns.

Novel form of actin-based motility transports bacteria on the surfaces of infected cells.

Enteropathogenic Escherichia coli (EPEC) attach to cells (attachment) lining the intestine and induce a decrease in the number of the cells' microvilli (effacement). This attachment and effacement is followed by diarrhea, which may be explained, at least in part, to the loss of microvilli and the decreased ability of the infected cells to absorb fluids. EPEC also attach to the surfaces of a number of cultured cells including CaCo-2, LLC-PK, and PtK2 cells. The extracellular, attached EPEC induce filaments of actin to form in the cytoplasm just underneath the EPEC surface attachment sites. Beneath some of the attached EPEC, the actin filaments become organized into membrane encased columns that extend up to 6 micrometers above the cell surface creating "pedestals" on which the EPEC rest. The raised pedestals can be readily observed in stereo pairs taken using the Intermediate Voltage Electron Microscope. The concentration of non-muscle isoforms of myosin II and tropomyosin near the base of the pedestals suggests a similarity of these structures to brush border microvilli. Video microscopy indicates that these EPEC pedestals can bend and undulate, alternately growing longer and shorter while remaining tethered in place on the cell surface. Some of the attached EPEC also translocate along the cell surface, reaching speeds up to 0.07 micrometers/sec. Both types of movement are inhibited by cytochalasin D, indicating that actin polymerization in the pedestals is required for the motility of EPEC on the host cell surface. In this respect, EPEC motility on host cells resembles the intracellular motility of Listeria, but there are differences in the actin filament bundles induced by the two different bacteria. The most obvious one is the interposition of the cell membrane between EPEC and the actin filaments in the pedestal in contrast to the close apposition of actin filaments to Listeria. The intensity of fluorescence of rhodamine phalloidin is nearly uniform along most of the length of the pedestals indicating a constant number of actin filaments, whereas the fluorescence intensity decreases along the length of Listeria tails reflecting the disassembly that occurs all along the tails. Epec's movements may be a hybrid of Listeria filopodia and Aplysia inductopodia movements. This paper is the first report of a microbe attached to the extracellular surface of an infected cell propelled by an intracellular actin polymerization-dependent mechanism.

Seismic Repair and Strengthening of a Severely Damaged Concrete Frame

A repair technique designed to strengthen and repair reinforced concrete frames with heavily damaged columns was tested experimentally. A two-thirds scale model of two bays and two stories of an exterior moment-resisting frame with heavily damaged columns was repaired and strengthened by completely encasing the damaged columns. The damage was characterized by heavy shear cracking and spalling of the concrete in the window space between the spandrel beams. In addition to repairing the damage to the columns, the new columns were designed to increase the lateral capacity of the frame and to shift the mode of failure from shear in the columns to flexural hinging in the beams. Test results of the frame was successfully shifted from column shear failure to flexural hinging of the beams. The results also showed that the encased columns behaved monolithically, and that the lateral strength of the frame was substantially increased

建物内を伝播するせん断波速度による実在建物の構造特性の評価

In this study, we have placed special emphasis on the velocity of propagation of shear waves through the building segment to evaluate individually the characteristics of the buildings. The study consists of the following 3 steps : first, to investigate the way of impact delivery and the properties of propagated waves with regard to a structurally simple building; secondly, to measure the shear wave velocity in several buildings of different structure and examine the resulting data; finally, to assess, in theoretical analysis, the applicability of this method by comparing the theoretical with the measured values. In the experiment constituting the first and second steps of the study, a horizontal impact force, exerted by about 15 persons, was imposed on the top of the building. Propagation of this wave motion was observed on each floor by vibration sensors. Shear wave velocities (sVs) measured for representative buildings of 3 different structural types are as follows : Steel frame : sVs=200〜300m/sec. Concrete encased column as part of steel frame : sVs=370m/sec. Concrete encased steel frame : sVs=450〜700m/sec. Buildings of the same structural type differ as to the above values principally depending on whether or not they are constructed with earthquake resisting walls or braces. In the case of a structurally simple building, theoretical examination ensured that the above measured values agree well with the theoretical values.

Restrained composite columns in uniaxial bending

Abstract The strength and behaviour of about 250 encased I-section columns, braced against sidesway and under uniaxial bending, have been presented. The cases studied cover the practical range of end eccentricity, stiffness of rotational end restrains (assumed elastic), slenderness and concrete cover, and include symetrical as well as asymmetrical arrangements of eccentricities and restraints, and major and minor axis bending cases. The columns are analysed by computer using a new iterative technique based on Newmark's integration procedure. Typical curves are presented showing the variation with load of the support and maximum moments, central deflection and strains at critical section. In almost all cases the peak load is reached when material failure occurs. The failure loads for columns under asymmetrical bending and with large effective end eccentricity are found to increase with slenderness, contrary to expectation. Because of material non-linearity, support moments in slender columns become negative at failure and moments throughout the length of certain short columns reverse. The paper also discusses the likely error in the computed strength due to the assumption of reversibility of the materials stress-strain curves.