Pd Greening

Source near-field effects and pulse tests in soil samples

Shear modulus measurement using bender elements in laboratory samples has become very popular. However, the test results are hard to interpret. Simple plane wave test models are too coarse and result in substantial measurement uncertainty: near 100% in G0. A possible refinement of the test model is based on Stokess fundamental solution for an isolated source. Near-field effects appear, and they have been regularly quoted as a major source of uncertainty in tests. There are some criteria in use for avoidance of near-field distortions, but they are signal dependent and not always successful. After some consideration of Stokess fundamental solution the authors propose a new frequency domain signal-independent criterion to avoid near-field effects. Time-domain criteria are also given, but they are shown to be signal dependent. Applying this criterion to some experimental results the authors then also show how near-field induced errors are not responsible for much of the observed signal distortion. Uncertain...

Identification and updating of loading in frameworks using dynamic measurements

Abstract This paper is concerned with the effect of structural loading on dynamic performance. This topic is recognised as being of importance when validating finite element (FE) models with experimental data. A strategy for including axial load effects in a model updating procedure is developed. The method can be used to identify loading in structural frameworks using measured dynamic data. The effectiveness of the new method is demonstrated by means of case studies involving both simulated and experimental data. The theoretical study allows aspects of the sensitivity of the method to realistic levels of experimental noise to be studied as well as the way in which dynamic load identification can be enhanced with static measurements. The experimental case study proves the practical success of the technique. Updated axial load parameters are compared with static measurements of the same quantities.

Effect of experimental pre–stress and residual stress on modal behaviour

Considerable research effort has been expended demonstrating the existence of variability in dynamic response characteristics of nominally identical structures. It is accepted that, however precisely the tolerances are defined for the finished product, the method by which these are achieved vary from one component to the next. It is inevitable that residual stresses within the component will arise and correspond to the severity of the manufacturing process. In addition, poorly defined tolerances will lead to lack of fit in redundant structures, causing geometry and stress–stiffening alterations due to the internal loads. The work presented in this paper considers the development of residual and prestresses in two simple structures under laboratory conditions. The residual stresses are introduced to the material through standard manufacturing processes. By the use of annealing the influences of the local stresses are identified. The source of variability due to the annealing process is compared with inconsistencies introduced by the measuring process. The second example considers the dynamic influence of introducing known loads within a redundant frame structure. The likely effects of imposed pre–stress and work hardening of structures during their working life are discussed.

Mode Traces in Degenerate Eigensystems and Augmented Assurance

Eigenpairs, contextually indicative of the frequencies and oscillatory modes of structural systems, are considered as functions of a single parameter. Undesired permutation of modal designations is noted to arise following events of transitory eigenvalue coalescence with respect to the parameter, resulting in the definition of nonsmooth functions. The tracing of eigenpairs across such events is outlined for the permanently degenerate, general eigenproblem as the general case; the cases of transitorily degenerate and distinct eigenvalues are thus accounted for. The foundation for mode tracing is the assumption of eigenvector consistency across parameter intervals, used as a means of eigenpair reconciliation. The resulting traced modes are smooth and their variations physically pertinent, which is necessary in iterative schemes in which convergence may otherwise be jeopardized. The proposal of an augmented modal assurance routine that potentially augments the assurance of tracing and, therefore, the maximum permissible parameter perturbation, is given; a threefold increase in the insight to consistency entails. The notion of the routine is to forward and backward cast eigenvectors utilizing their derivatives in affine modeling. A numerical example of the modes of a cyclic frame as functions of a cyclic distribution of membrane forces demonstrates the concepts and utility of the proposal.

Orientation Dependence of Earthquake Ground Motion and Structural Response

The strength of horizontal earthquake ground shaking varies with orientation, and yet is typically condensed into a single response spectrum for seismic design. This chapter investigates how this orientation-dependence (“directionality”) should be considered in the specification of seismic demand in codes of practice. The sources of orientation-dependence are identified, and their implications for ground motion selection, scaling and modification are discussed. A series of nonlinear response history analyses are presented, based on simple bridge piers with circular, square and rectangular cross-sections, each subjected to a number of ground motions rotated through non-redundant orientations. It is shown that ground motions with realistic orientation dependence in their elastic spectral response are more demanding than those with elastic orientation independence, and that the variation of response with orientation is not significantly affected by the cross section shape. This suggests that design based on the geometric mean spectrum, as used in several existing codes, is unconservative with respect to real seismic demand.