P. Van den Broeck

Wave propagation in layered dry, saturated and unsaturated poroelastic media

Abstract An exact stiffness formalism is presented to study harmonic and transient wave propagation in multilayered dry, saturated and unsaturated isotropic poroelastic media. Smeulders’ extension of Biot’s poroelastic theory is used to incorporate unsaturated porous media with a small gas fraction. A wide range of problems in geophysical and civil engineering can be treated, ranging from amplification of plane harmonic waves, dispersion and attenuation of surface waves to transient wave propagation due to a forced excitation. The effect of full or partial saturation on wave propagation in a poroelastic layered halfspace is demonstrated in a numerical example, in which the layering is caused by a moving ground water table.

Application of a direct stiffness method to wave propagation in multiphase poroelastic media

In this paper, it is demonstrated how a direct stiffness method for wavepropagation in multilayered saturated poroelastic media, based on integraltransform techniques, can be modified to account for a small amount of gasin the pores. Unsaturated media with small gas fractions can be treatedusing Smeulders‘ extension of Biot‘s poroelastic theory. The effect of thepresence of gas bubbles on the fluid bulk modulus and the dispersioncharacteristics of a water-saturated sand of Mol is demonstrated. Thedirect stiffness method is illustrated with a numerical example wheretransient wave propagation in a dry, saturated and unsaturated halfspaceis considered.

OMAX testing of a steel bowstring footbridge

A recent development in operational modal analysis (OMA) is the possibility of using measured, artificial loads in addition to the unmeasured, ambient excitation, while the ratio between forced and ambient excitation can be low compared to classical experimental modal analysis (EMA). Most of these so-called OMAX algorithms lack the intuitiveness of their EMA and OMA counterparts, since they fit a system model that takes both the measured and the operational excitation into account directly to the measured signals. A more physically intuitive subspace algorithm for OMAX, that starts with an accurate decomposition of the measured joint response in a forced and an ambient part, was recently introduced. In this paper, the performance of this algorithm, which is called CSI-ic/ref, is assessed by means of a case study, where a two-span steel arch footbridge is tested in operational conditions, with and without using additional actuators. From a comparison of the modal parameters with results from a finite elementmodel, an OMA algorithm, and an alternative OMAX algorithm, it can be concluded that CSI-ic/ref yields accurate modal parameter estimates.

Human-structure interaction effects on the maximum dynamic response based on an equivalent spectral model for pedestrian-induced loading

This paper investigates the effects of the human-structure interaction (HSI) on the dynamic response based on a spectral model for vertical pedestrian-induced forces. The spectral load model proposed in literature can be applied for the vibration serviceability analysis of footbridges subjected to unrestricted pedestrian traffic as well as in crowded conditions, however, in absence of HSI phenomena. To allow for a more accurate prediction of the maximum structural response, the present study in addition accounts for the vertical mechanical interaction between pedestrians, represented by simple lumped parameter models, and the supporting structure. By applying the classic methods of linear random dynamics, the maximum dynamic response is evaluated based on the analytical expression of the spectral model of the loading and the frequency response function (FRF) of the coupled system. The most significant HSI-effect is in the increase of the effective damping ratio of the coupled system that leads to a reduction of the structural response. However, in some cases the effect of the change in the frequency of the coupled system is more significant, whereby this results into a higher structural response when the HSI-effects are accounted for.

Measurement and prediction of the pedestrian-induced vibrations of a footbridge

Vibration serviceability has become an important issue in the design of modern slender footbridges with large spans. This paper presents the measurements and the numerical predictions of the footfall-induced vibrations of a pedestrian bridge. A series of experiments were carried out with different-sized groups crossing the bridge, varying in number from 10 up to 50 participants and recording the vertical and lateral accelerations at different locations on the bridge. Two types of tests were performed: free walking and synchronized walking by means of a metronome signal, recorded on a tape recorder carried by the group of students. The effect of the test type is analyzed and shows a magnitude in difference between the vertical accelerations caused by the free and the synchronized walking. The increasing trend of the acceleration levels with increasing group size is also clearly observed. A numerical prediction method is used to simulate the synchronized walking experiments based on an updated finite element model of the bridge and a single pedestrian load model. It is shown that the predicted acceleration level is sensitive to the assumptions made regarding the level of synchronization between the pedestrians and the magnitude of the dynamic load generated by each pedestrian. Taking into account these specific measurement conditions, a fair agreement is obtained between the predicted and the observed vertical acceleration levels at seven positions along the length of the footbridge.