Georgios Kampas

Size Versus Slenderness: Two Competing Parameters in the Seismic Stability of Free‐Standing Rocking Columns

Abstract When a free‐standing column with a given base becomes taller and taller, there is a competition between the increase in its size (more stable) and the increase in its slenderness (less stable). This article investigates how these two competing phenomena affect the stability of tall, slender, free‐standing columns when subjected to horizontal and vertical ground shaking. The main conclusion of the article is that the outcome of this competition is sensitive to local details of the ground shaking and the dominant frequency of a possible coherent, distinguishable pulse. The often‐observed increase in stability due to increase in height (despite the increase in slenderness) may be further enhanced due to a sudden transition from the lower mode of overturning with impact to the higher mode of overturning without impact. The article proceeds by offering a simple mathematical explanation why the vertical ground acceleration has a marginal effect on the stability of a slender, free‐standing column and concludes that the level of ground shaking that is needed to overturn a tall free‐standing column of any size and any slenderness is a decreasing function of the length scale of the dominant coherent acceleration pulse normalized to the base width of the column.

Seismic protection of structures with supplemental rotational inertia

AbstractThis paper investigates the alternative strategy of suppressing ground-induced vibrations with supplemental rotational inertia. The proposed concept employs a rack-pinion-flywheel system where its resisting force is proportional to the relative acceleration between the vibrating mass and the support of the flywheels. This arrangement, known in the mechanical networks literature as the inerter, complements the traditional supplemental damping and stiffness strategies used for the seismic protection of structures. The paper shows that the seismic protection of structures with supplemental rotational inertia has some unique advantages, particularly in suppressing the spectral displacements of long period structures—a function that is not efficiently achieved with large values of supplemental damping. The paper shows that this happens at the expense of transferring appreciable forces at the support of the flywheels and proceeds by examining to what extent the finite stiffness and damping of the suppor...

Transverse versus Longitudinal Eigenperiods of Multispan Seismically Isolated Bridges

This paper is motivated from the wider need in system identification studies to identify and interpret the eigenvalues of seismically isolated bridges from field measurements. The paper examines the transverse eigenvalues of multispan bridges which are isolated in both transverse and longitudinal directions at all supports including all center piers and end abutments. The paper shows that regardless of the value of the longitudinal isolation period of the deck, the length of the bridge, and the number of spans, the first transverse (isolation) period is always longer than the longitudinal isolation period of the deck. This result cannot be captured with the limiting idealization of a beam on continuously distributed springs (beam on a Winkler foundation) which yields the opposite result of the first transverse period always being shorter than the longitudinal isolation period. This fundamental difference between the response of a flexural beam supported on distinct, equally spaced springs and that of a be...

Modal Analysis of Isolated Bridges with Transverse Restraints at the End Abutments

This paper examines the eigenvalues of multi-span seismically isolated bridges in which the transverse displacement of the deck at the end-abutments is restricted. For moderate long bridges the first natural period of the bridge is the first longitudinal period, while the first transverse period is the second period, given that the flexural rigidity of the deck along the transverse direction shortens the isolation period offered by the bearings in that direction. This paper shows that for isolated bridges longer than a certain critical length, the first transverse period becomes longer than the first longitudinal period despite the presence of the flexural rigidity of the deck. This critical length depends on whether the bridge is isolated on elastomeric bearings or on spherical sliding bearings. On the other hand this result can not be captured with the limiting idealization of a beam on continuous distributed springs (beam on Winkler foundation) – a finding that has practical significance in design and system identification studies. The paper discusses the implications of this finding in design.

Modal identification of seismically isolated bridges with piers having different heights

This paper investigates the modal identification of seismically isolated bridges when the localised nonlinear behaviour from the isolation bearing initiates at different times due to the uneven height of the bridge piers. More specifically, a three-span bridge supported on spherical sliding bearings is examined. Three different states of the same system with different natural periods emerge during an excitation; the linear system (LS), the partially isolated system (PIS) and the fully isolated system (FIS). Firstly, the paper identifies the time intervals that each state performs by using acceleration data. Subsequently, modal identification techniques such as the prediction error method and a time-frequency wavelet analysis are applied on each interval. The LS results are dependable compared to the PIS which is a mildly nonlinear system. The results corresponding to the FIS suggest that it is preferable to apply the modal identification techniques on each interval independently, rather than on the entire response signal.