Construction Imperfections Research Articles

Pile Behavior—Consequences of Geological and Construction Imperfections

This paper identifies some of the causes of imperfections in real pile foundations and then examines their possible effects on pile behavior. The imperfections considered arise from two main sources, natural imperfections caused by the geological circumstances and imperfections related to the construction of the piles. In each case, single piles are considered first, and then pile groups are addressed separately. Case histories are presented to illustrate the effects on pile performance which are predicted by theoretical analyses. The analyses with which these examinations are carried out will still inevitably involve some degree of idealization, but at least attempt to incorporate the key “nonideal” aspects. The paper also discusses the particular problems involved in investigating and analyzing underperforming pile foundations and reviews some methods by which the consequences of imperfections may be overcome or ameliorated.

Modeling of Induction Machines Including Stator and Rotor Slot Effects

Three-phase as well as single-phase induction machines form the backbone of a substantial portion of the adjustable-speed drives' applications. Among the current estimation techniques available for speed-sensorless induction motor drives, speed measurement based on rotor or principal slot harmonics (RSH or PSH) detection in machine line current happens to be a prominent one. In practice, two predominant RSH can be seen. The present paper attempts to investigate the effect of slot permeance on the production of RSH. Simulation studies on a small motor but with a popular stator, rotor slot, and pole combination suggest that their effect is less significant compared to rotor space harmonics effect. Also, out of the two aforementioned RSH, the cause of the less significant slot harmonic is shown to be the negative-sequence-current-induced reverse rotating field. Voltage unbalances and constructional imperfections can cause negative-sequence current even in a healthy machine. Thus, although the less significant RSH is absent in the simulated line current of an idealized machine running from a balanced supply, it can be detected in an actual machine.

PARAMETRIC STUDY ON STEEL TOWER SEISMIC RESPONSE OF CABLE-STAYED BRIDGES UNDER GREAT EARTHQUAKE GROUND MOTION

Analytical parametric study on dynamic characteristics of steel tower of cable-stayed bridges is performed to investigate the individual influence of different design aspects, such as damping mechanism, input ground motion, allowable initial construction imperfections, energy dissipation and tower modal shapes. The results show that the horizontal beam height and length and the low yield energy dissipation system significantly affect tower structural behavior. The initial imperfections within design range have slight detrimental effects on the tower seismic response but these effects grow rapidly beyond the design range. Mass proportional damping leads to overestimate tower axial forces and acceleration response.

Online condition monitoring of induction motors

Condition monitoring of induction motors is a process that may be used to great advantage in mining and other industrial applications. The early detection of motor winding deterioration prior to a complete failure provides an opportunity for maintenance to be performed on a scheduled routine without the loss of production time. Presented in this paper is a theoretical and experimental analysis of a voltage mismatch technique that may be used in operating situations to monitor the health of induction motor windings. It extends previous work in this area by demonstrating the robust nature of the monitoring process not only under conditions of power supply unbalance but also in situations where motor construction imperfections exist and mechanical loads are unpredictable. A suggested procedure for application of this condition monitoring process in industrial situations is also included.

Navier's Straitjacket

The theory of structural design was crystallized in 1826, the year of publication of Navier's Leçons. Navier showed how the internal forces in an elastic hyperstatic structure could be calculated, the most critical section identified, and sections assigned so that the corresponding stresses did not exceed a proportion of their limiting values. It was only over 100 years later that tests on full-scale structures showed that “actual” stresses were very different from those predicted by elastic theory, and this has led directly to plastic methods of design. Paradoxically, those methods lead to the conclusion that the “actual” working state of a structure is essentially unknowable, since it depends crucially on very small imperfections of manufacture and construction. While these imperfections can have no effect on ductile collapse of the structure, some estimate of the working state must be made in order to design against possible instabilities. This aspect of structural design remains largely unexplored.

Integrity of membrane elements, vessels and systems

Membrane filtration may be a powerful tool in disinfection of surface water for drinking water production. Especially ultra-, nano- and hyperfiltration are in principle absolute barriers. However, due to imperfections in module construction and/or damage during operation, leakages may occur. Methods are presented that are suitable for checking and monitoring membrane integrity. For combined systems a detailed monitoring protocol has to be worked out.

Modeling of Collapse Mechanisms of Thin Reinforced Concrete Shells

A numerical model based on an elasto-plastic–fracturing formulation is presented to predict the ultimate strength as well as the related mode of failure of thin reinforced concrete (RC) shells. The model defines a safe domain within which the shell would not collapse under the applied load. It predicts two possible modes of failure, which may be broadly classified into bending-type failure and buckling-type failure. Important issues are considered in the model: the effects of holes, reinforcing members, and concentrated loads; and shape deviations due to construction or modeling imperfections. By comparison with analytical solutions as well as experimental observations, the model is shown to lead to reasonably accurate predictions of the main events leading to the failure of RC shells, including the loading path relating the downward vertical displacement to the applied load. From detailed investigations, the relations between the actual mode of failure and the level of the bending force are explained. To provide a shell with the maximum ultimate strength can be dangerous if an inadequately safe domain results.

The local boundedness of the perturbed motions of an imperfect gyroscope in gimbals with dissipative and accelerating forces

An unbalanced dynamically symmetrical gyroscope in gimbals with constructive imperfections is considered in a central Newtonian field of forces. It is assumed that there is a moment of forces of viscous friction acting on the axis of rotation of one of the rings of the suspension and an accelerating (electromagnetic) moment applied to the axis of rotation of another ring. The equations of motion have a partial solution for which the basic plane of the frame is perpendicular to the direction from the specified fixed point of the frame to the centre of gravitation, the basic plane of the mantle is parallel to this direction and the rotor rotates with an arbitrary constant angular velocity. The equations of perturbed motions of the reduced system with two degrees of freedom are obtained to within third-order terms at the corresponding position of equilibrium. In the domain of admissible values of the parameters F o the characteristic equation of the system is considered and its coefficients are written down. A domain in F o is specified in which complex conjugate pairs of the eigenvalues have small moduli of the real parts but the absolute values of the second- to fourth-order off-resonance mistuning between the imaginary parts are not small. For an imperfect gyroscope in gimbals with dissipative and accelerating forces the sufficient conditions of the local uniform boundedness of motions perturbed with respect to the specified partial solution are obtained in this domain. The conditions found provide the local uniform boundedness of solutions irrespective of the forms of higher than the third order in the equations of perturbed motions. These conditions are obtained in the form of constraints for the coefficients of the normal form and, finally, for the original parameters of the system and the real and imaginary parts of the eigenvalues. To provide a clear interpretation of the results, special cases when all but two parameters are fixed are analysed. The domains of local uniform boundedness are constructed in the two-dimensional domains F o using a personal computer.

Ultimate Strength of RC Wall Panels with Openings

Test results of 12 reinforced concrete (RC) wall panels with openings are presented. The panels have been subjected to in‐plane vertical loads applied at an eccentricity to represent possible accidental eccentricity that occurs in practice due to constructional imperfections. The 12 specimens consist of two identical groups of six panels each. One group of panels is tested in one‐way in‐plane action (i.e., supported at top and bottom edges against lateral displacement). The second group of panels is tested in two‐way in‐plane action (i.e., supported on all the four edges against lateral displacement). Openings in the panels represent typical door and window openings. Cracking loads, ultimate loads, crack patterns, and lateral deflections of the panels are studied. Empirical methods have been developed for the prediction of ultimate load. Also, lateral deflections, cracking loads, and ultimate loads of identical loads tested under one‐way and two‐way action are compared.

Ultimate Strength of R.C. Wall Panels in Two‐Way In‐Plane Action

Test results of 24 reinforced concrete wall panels in two-way action (i.e., supported on all the four sides) and subjected to in-plane vertical load are presented. The load is applied at an eccentricity to represent possible accidental eccentricity that occurs in practice due to constructional imperfections. Influences of aspect ratio, thinness ratio, slendemess ratio, vertical steel, and horizontal steel on the ultimate load are studied. Two equations are proposed to predict the ultimate load carried by the panels. The first equation is empirical and is arrived at from trial and error fitting with test data. The second equation is semi-empirical and is developed from a modification of the buckling strength of thin rectangular plates. Both the equations are formulated so as to give a safe prediction of a large portion of ultimate strength test results. Also, ultimate load cracking load and lateral deflections of identical panels in two-way action (all four sides supported) and oneway action (top and bottom sides only supported) are compared.

First-order analysis of thermal velocities for non-axisymmetric display-tube electron beams

Abstract Thermionic emission velocities play an important part in formation of the electron beam in a cathode-ray tube (CRT), and contribute significantly to spot size. The first-order thermal approximation provides an efficient method of incorporating the emission statistics into computer simulation of the beam. A three-dimensional formulation of the theory has been implemented in a computer program for application in CRT design. This will permit study of the effects of tube misalignments, beam deflection, and astigmatic focusing systems. An initial application of the program suggests that astigmatism, due to geometrical imperfections in tube construction, may be an important cause of spot degradation in some cases.

Buckling of Reinforced and Prestressed Concrete Members - Simplified Calculation Method

S tructural instability considerations provide a special group of design problems for reinforced and prestressed concrete members wherein axial or in-plane forces play an important role in structural performance. The nonlinear behavior of a member due to the actual stressstrain relationship, effects of cracking, creep and current deformations must be duly accounted for in order to carry out realistic strength predictions. On the other hand, simplified methods are needed which are practical to use in actual design situations and which can allow for the nonlinear effects in a sufficiently accurate manner. This paper describes the basic features of structural instability in reinforced and prestressed concrete members such as columns, plates and shells. Simplified methods are discussed, which are based on the tangent modulus theory and ultimate limit state considerations. Several studies have been made by comparing theoretical results with buckling tests on plate and shell specimens. In general, the design methods proposed show good agreement with available experimental results. Emphasis is put on the significance and adequate representation of construction imperfections, specifically geometry and materials. A simplified but powerful design method is given, by which geometric out-of-shape imperfections can

An automated, energy-based, finite-difference procedure for the elastic collapse of rectangular plates and panels

A FORTRAN IV, large capacity, computer program has been developed to determine collapse loads and bifurcation loads for linear and nonlinear prebuckling behavior for fiber-reinforced, laminated, rectangular plates and panels under general loading systems and boundary conditions. The program is based on the principle of total potential energy and uses finite-differences in the discretization process. Whole-station spacing has been used to calculate the strain energy associated with an area-element and an orthogonal finite-difference grid that provides for variable spacings in perpendicular directions is incorporated. Numerical results are presented that compare favorably with results obtained via the general computer program STAGS. Other numerical results are presented that illustrate the types of boundary conditions, applied loads, cut-outs and initial geometric imperfections that can be handled by the present program. A brief study of the effect of panel construction and initial geometric imperfections on the buckling behavior of fiber-reinforced panels is presented.

Phase Step Beam Waveguide

Experimental tests on a new type of beam waveguide constituted by a series of equispaced dielectric frames are described. Field-pattern and power-loss measurements have been performed on short-circuited sections of the waveguide working at 10 GHz and on the 37-GHz prototype constituted by Teflon square frames. In comparison with the more common iris and lens beam waveguides, it presents advantages concerning diffraction, reflection, and dissipation losses. In addition, it is lightweight and compact. The experimental results confirm these advantages, along with a low sensitivity to assembling and constructive imperfections. Design criteria are suggested as a result of the optimization for the lowest losses obtained through numerical computations performed on the equivalent open resonator.

Spectroscopy by Grating Synthesis

Spectral information obtained by synthesis of two or more diffraction gratings is studied. Also the case of two gratings with relative motion and the case of N fixed gratings are discussed. We show that, using grating synthesis, it is possible to obtain transfer functions that increase the total obtainable information with respect to the single grating. The possibility of building a synthesis spectroscope is presented, taking into account both the gratings aberration and some construction imperfections. Finally we show that this synthesis method, with a proper numerical elaboration, gives practically the same information as we can obtain with a single equivalent grating.

An investigation of the factors determining maximum resolution in a quadrupole mass spectrometer

Describes a series of experiments designed to investigate the factors which limit the maximum resolution obtainable with a conventional quadrupole mass spectrometer under normal operating conditions. Included in the investigation were the effects of imperfections in mechanical construction and electrical supplies and also operation with contaminated electrode surfaces. In normal operation and with reasonable care taken in the mechanical construction and assembly, the finite length of the quadrupole rods, in this case 150 mm, was the principle factor setting an upper limit to the resolution. All the results suggested that the ions did not spend a sufficient time in the quadrupole to reach an equilibrium oscillation in the transverse AC field. Under all operating conditions resolution was found to be dependent, to a first approximation, on the number of cycles of AC field N experienced by the ions. The dependence of maximum resolution Rm on N was found to follow the simple empirical relationship, Rm=KN1.7. This formula applied for both low and high energy ions (2-32 eV) and for the mass range H2+ to Xe+.

Phase detection electromagnetic flowmeter-design and use.

Two types of gated sine-wave electromagnetic flowmeter are examined critically, and found to be sensitive to phase shift in the transducer signal. This phase shift is always found in a sine-wave flow transducer because of imperfection in construction, particularly in those used for physiology and medicine. A flowmeter design that discards amplitude modulation and is sensitive to phase change only is described. Advantages over other types of electromagnetic flowmeter are discussed. Results obtained with the phase detection flowmeter are illustrated.

DESIGN AND PERFORMANCE OF A FUSED SILICA MICROBALANCE

The construction and the use of a robust microbalance with a fused silica beam are described. The design is such that the microbalance is undisturbed by convection even in surroundings where it would be impossible to use an ordinary microbalance; nevertheless the instrument is very convenient to operate. A comprehensive theoretical treatment of the silica torsion balance and of the effects of constructional imperfections is given and shown to be in good agreement with experiment. The performance of the microbalance is illustrated by means of an example from the microchemical work of B. G. Harvey, where four samples of americium oxide, of masses between two and six micrograms, were weighed on 100 mgm. scale pans, with a proven accuracy better than 0.3%. The precautions that were taken to minimize the harmful effects of adsorption and dust are described. A commercial version of this microbalance is now available.

The Resolving Power of the Magnetic Electron Microscope

The chief factors limiting the performance of electron microscopes are discussed: spherical and chromatic aberration, diffraction, imperfections in lens construction. Methods of approach to the correction of aberrations are reviewed. It appears preferable to proceed from a consideration not of the final aberration equations, but from the fundamental equations of electron motion, with the use of relaxation methods. It is shown that, even if the mechanical difficulties in lens construction are overcome, a very great reduction in spherical aberration is required to improve the resolution to below 10 A. Further reduction in chromatic aberration is unnecessary in present circumstances.