Internal Axial Load Research Articles

A study of the axial load behaviour of helical piles in sand by frustum confining vessel

Helical piles as screwed piles are a popular solution for removable or recyclable foundations that support road or offshore structures. In this study, the frustum confining vessel developed at the Amirkabir University of Technology (FCV-AUT) is used to study the axial behaviour of helical piles as the physical modelling of their apparatus. Three models of helical piles are made of hollow galvanised iron pipes (diameter of 34 mm and length of 85 cm), to which single-, double- and triple-helical plates (diameter of 70 mm) are welded. The influences of the number of helices and the geometry of the piles on failure mechanism are studied at two different densities of Babolsar sand by conducting 12 tensile and compressive loading tests in FCV-AUT. The piles are instrumented using strain gauges in order to study internal axial load distribution during pile loading. Two static analysis methods are used to verify the experimental results. The results have shown that the FCV has good performance in modelling the behaviour of helical piles under compression tests. It is also known that the shear resistance parameters of the surrounding soil are more affected by three- or multi-helix piles with higher relative densities.

Effect of internal axial loads on the stability of sway-permitted prismatic columns

The stability of prismatic columns subjected to internal axial loads is a common issue during engineering analysis and design. Because of the big difference between the effects of internal loads and end loads, the traditional effective length method is not applicable. Current codes do not provide design method for this issue and engineers require the ability to analysis and design this kind of columns efficiently. Using the concept of negative stiffness, a relationship between end loads and internal axial loads applied on sway-permitted prismatic columns is established so that the internal loads can be considered equivalent to end loads and the critical value can be obtained easily by Euler formula. A practical method to determine the second-order effect is also developed. Eigenvalue buckling and elastic nonlinear analyses are performed to examine the proposed method and the comparison results indicate it has high accuracy.

Stability of Sway-Inhibited Prismatic Columns Subjected to Internal Axial Loads

AbstractPrismatic members subjected to internal axial loads exist widely in engineering applications. Because the effects of internal loads and end loads on stability are quite different, the traditional effective length method is not applicable. Engineers require an efficient and convenient method for solving the stability problem of such members. A two-bar model is proposed to illustrate the concept that the effect of compressive axial loads on the stability of sway-inhibited columns can be understood as a negative lateral stiffness. After obtaining the expression of negative lateral stiffness, a relationship between end loads and internal loads can be established so that the internal loads can be considered equivalent to end loads. Then, the column subjected to internal loads can be analyzed as a normal column (without internal axial loads), and the critical buckling load can be obtained easily with the Euler formula. Eigenvalue buckling analyses were performed to examine the proposed method, and the c...

Fundamental frequency evolution in slender beams subjected to imposed axial displacements

The influence of applied axial loads on the fundamental vibration frequency is strictly connected with the stability analysis of elastic slender beams. For this reason, the correct evaluation of the fundamental frequency is of primary importance in designing new structures and components, as well as in monitoring existing ones. At the same time, if an internal axial load arises in a slender element as the consequence of an imposed (static) axial end displacement, then a different dynamic structural response is encountered respect to the case in which a beam end is free to slide, during transverse vibration, and a (constant) axial load is applied externally. This difference is due to the change in the axial boundary condition. Moreover, the presence of an initial curvature of the beam axis may significantly affect the aforesaid response. The experimental study proposed in the present paper investigates the dependence of the fundamental frequency on the axial load in slender beams subjected to imposed axial end displacements. The considered specimens presented different geometrical imperfections (initial curvatures), and were tested in two different constraint conditions (hinged–hinged and hinged–clamped). In addition, the behaviors observed during the experiments were reproduced by numerical simulations offering a valid confirmation for test results and contributing to understand the evolution of the fundamental frequency in the analyzed slender elements subjected to imposed axial end displacements.

Stability Analysis of Two-Segment Stepped Columns with Different End Conditions and Internal Axial Loads

Members with varying geometrical and/or material properties are commonly used in many engineering applications. Stepped columns with internal axial loads constitute a special case of such nonuniform columns. Crane columns in industrial buildings or structural columns supporting intermediate floors are important applications of stepped members in civil engineering. Since neither axial load nor stiffness is constant along the column height, the stability analysis of a stepped column is usually more complicated than that of a uniform column. Determination of exact buckling loads for stepped columns with different end conditions is not always practical. This paper shows that variational iteration method (VIM), a kind of analytical technique recently proposed for solution of nonlinear differential equations, can satisfactorily be used to obtain approximate solutions for buckling loads of stepped columns with internal axial loads. VIM solutions perfectly match with the exact solutions available in the literature for some special cases of two-segment stepped columns. For many other cases, that is, for various values of three design parameters, namely, (i) load ratio, (ii) stiffness ratio, and (iii) length ratio, approximate buckling loads for two-segment stepped columns are determined using VIM and presented in tabular form which can easily be used by design engineers.

Transverse vibration of pre-tensioned nonlocal nanobeams with precise internal axial loads

This paper investigates the transverse vibration of a simply supported nanobeam with an initial axial tension based on the nonlocal stress field theory with a nonlocal size parameter. Considering an axial elongation due to transverse vibration, the internal axial tension is not precisely equal to the external initial tension. A sixth-order nonlinear partial differential equation that governs the transverse vibration for such nonlocal nanobeam is derived. Using a perturbation method, the relation between natural frequency and nonlocal nanoscale parameter is derived and the transverse vibration mode is solved. The external axial tension and nonlocal nanoscale parameter are proven to play significant roles in the nonlinear vibration behavior of nonlocal nanobeams. Such effects enhance the natural frequency and stiffness as compared to the predictions of the classical continuum mechanics models. Additionally, the frequency is higher if the precise internal axial load is considered with respect to that when only the approximate internal axial tension is assumed.

Load Testing and Interpretation of Instrumented Augered Cast-in-Place Piles

AbstractGiven that piles are composed of different materials and installed using a variety of methods, ASTM D 1143/D 1143M-07 Standard Test Methods for Deep Foundations Under Static Axial Compression is justifiably general so that it has as many applications as reasonably possible. There are aspects of the test setup, the test procedures, and data interpretation for instrumented piles (some of which are specific to augered cast-in-place piles) that are either not discussed in detail in this ASTM standard or are not addressed at all. Specifically, the use of variable time hold times can obscure the shape of the conventional load-deflection curve, as well as, influence the interpretation of the ultimate pile capacity. Unload-reload cycles induce additional non-uniform internal axial loads within the pile which complicate interpretation of the strain gage data and increase the potential for errors. Attempts to maintain a constant top load during the hold period can lead to operational difficulties in taking ...

FREE VIBRATIONS OF SELF-STRAINED ASSEMBLIES OF BEAMS

This paper examines the natural frequencies and modes of transverse vibration of two simple redundant systems comprising straight uniform Euler–Bernoulli beams in which there are internal self-balancing axial loads (e.g., loads due to non-uniform thermal strains). The simplest system consists of two parallel beams joined at their ends and the other is a 6-beam rectangular plane frame. Symmetric mode vibration normal to the plane of the frame is studied. Transcendental frequency equations are established for the different systems. Computed frequencies and modes are presented which show the effect of (1) varying the axial loads over a wide range, up to and beyond the values which cause individual members to buckle (2) pinning or fixing the beam joints (3) varying the relative flexural stiffness of the component beams. When the internal axial loads first cause any one of the component beams to buckle, the fundamental frequency of the whole system vanishes. The critical axial loads required for this are determined. A simple criterion has been identified to predict whether a small increase from zero in the axial compressive load in any one member causes the natural frequencies of the whole system to rise or fall. It is shown that this depends on the relative flexural stiffnesses and buckling loads of the different members. Computed modes of vibration show that when the axial modes reach their critical values, the buckled beam(s) distort with large amplitudes while the unbuckled beam(s) move either as rigid bodies or with bending which decays rapidly from the ends to a near-rigid-body movement over the central part of the beam. The modes of the systems with fixed joints change very little (if at all) with changing axial load, except when the load is close to the value which maximizes or minimizes the frequency. In a narrow range around this load the mode changes rapidly. The results provide an explanation for some computed results (as yet unpublished) for the flexural modes and frequencies of flat plates with non-uniform thermal stress distributions.

Triaxial compression data on nuclear explosion shocked, mechanically shocked, and normal granodiorite from the Nevada Test Site

Compression tests were performed at room temperature on solid circular cylinders of medium-grained granodiorite from the Hardhat Event conducted at the Nevada Test Site. Confining pressures ranged from 1 to 7000 bars. A piston-cylinder apparatus equipped with an internal axial load cell and manganin coil was used. The confining fluid was a low-viscosity oil. Pre-detonation specimens were cut 0°, 45°, and 90° to the drill core axis and were taken from 195-, 225-, and 255-meter depths. Post-detonation rock was sampled at 23 and 27.5 meters from ground zero, which was located at a depth of 290 meters and experienced a calculated peak shock pressure of approximately 20 kb. Specimens mechanically shocked in the laboratory (35 to 40 kb) were from the 195-meter depth pre-detonation sample. No orientational effects were observed. The failure of granodiorite under compressional stresses was found to be adequately described by a modified Coulomb maximum shear stress theory. Under uniaxial compression, the mode of failure was brittle shear. Under triaxial compression, a quiet slip failure was observed at low confining pressures and brittle shear at high confining pressures. A possible explanation for the occurrence of slip failure may be the presence of a fluid pore pressure within the specimen. Strain measurements were made on several specimens. At atmospheric pressure, the stress histories of the rock were clearly apparent. Under confining pressure, shock effects rapidly disappeared, and, at 4.5 kb, the behavior was essentially that of normal granodiorite. Young's modulus at atmospheric confining pressure for unshocked rock was found to be 0.64 ± 0.26 × 106 bars, which in the upper limit agrees with the reported dynamic value. The average value for rock shocked by nuclear blast is 0.18 × 106 bars and that for mechanically shocked rock is of the order 0.01 × 106 bars.