Dynamics of Structures

Source: Roberto Leon, Department of Civil and Environmental Engineering, Virginia Tech, Blacksburg, VA

It is rare nowadays that a whole year goes by without a major earthquake event wreaking havoc somewhere around the world. In some cases, like the 2005 Banda Ache earthquake in Indonesia, the damage involved large geographic areas and casualties in the six figures. In general, the number and intensity of earthquakes is not increasing, however, the vulnerability of the built environment is rising. With increasing unregulated urbanization around seismically active areas, such as the Circum-Pacific "belt of fire," sea rising in low-laying coastal area, and increasing concentrations of both energy production/distribution and digital/telecommunication network critical nodes in vulnerable areas, it is clear that earthquake-resistant design is key to future community resilience.

Designing structures to resist earthquake damage has progressed greatly in the last 50 years, primarily through work in Japan following the 1964 Niigata Earthquake, and in the United States following the 1971 San Fernando Valley Earthquake. The work has advanced along three parallel tracks: (a) experimental work aimed at developing improved construction techniques to minimize damage and loss of life; (b) analytical studies based on advanced geometrical and non-linear material models; and, (c) synthesis of the results in (a) and (b) into design code provisions that improve the ability of structures to resist unexpected loads.

Seismic testing in a laboratory setting is often difficult and expensive. Testing is primarily carried out using the following three techniques:

1. Quasi-static testing (QST), where parts of a structure are tested using slowly applied and equivalently predetermined lateral deformations with idealized boundary conditions. This technique is particularly useful to assess the effects of structural detailing on the toughness and deformation capacity of particular parts of structures.
2. Pseudo-dynamic testing (PSDT), where loads are also applied slowly, but the dynamic effects are taken into account by solving the equations of motion as the test progresses and by utilizing direct test feedbacks (primarily the instantaneous stiffness) to assess the actual stiffness and damping characteristics of the structure.
3. Shake tables, where scale models of complete structures are subjected to input motions using a hydraulically actuated base or foundation. Shake tables represent a more faithful testing technique, as the structure is not artificially restrained, the input is true ground motion, and the resulting forces are truly inertial ones, as one would expect in a real earthquake. However, the power requirements are enormous, and only a few shake tables capable of working at nearly full-scale exist around the world. Globally, there is only one large shake table capable of carrying out tests on full-scale structures, which is the shake table at the E-Defense facility in Japan, built in the aftermath of the 1985 Kobe earthquake.

In this experiment, we will utilize a small shake table and model structures to study the dynamic behavior characteristics of some structural models. It is these dynamic characteristics, principally the natural frequency and damping, as well as the quality of the structural detailing and construction, which make structures more or less vulnerable to earthquakes.