Page 191 - Dynamic Loading and Design of Structures

Page 166

Capacity design generally dominates the response of structures that heavily rely on the
development of inelastic deformations to ensure a satisfactory seismic performance, while
structures that are designed for relatively high seismic forces, hence are not required to
develop significant inelastic deformations, are much less controlled by capacity design
considerations. This interrelationship between the required ductility (and, inversely, the level
of design force) and the degree to which capacity design affects a structure are also
recognized by most codes.

4.4.4 Passive and active control
Although the concepts of inelastic spectra and behaviour factors, coupled with capacity design
principles, clearly dominate current seismic codes, it has to be emphasized that they do not
represent the only conceptual framework available for seismic design. Furthermore, an
engineer should fully realize that designing a structure on the basis of these concepts means
that under earthquakes of an intensity equal to or exceeding that of the design event, damage
to the structure could be both substantial and extending into a large part of the structure.
Perhaps more importantly, formation of a favourable mechanism does not guarantee that
interstorey drifts and/or floor accelerations will be low enough to prevent extensive damage to
the non-structural elements and the content of the building. These and other concerns have led
to the development of alternative conceptual frameworks for seismic design, currently
referred to as ‘passive’ and ‘active’ control of the seismic response of the structure. By far the
most practical approach is passive control that incorporates the fundamental ideas of seismic
isolation and provision of supplemental damping. These will be discussed in the remainder of
this section, followed by a brief reference to the idea of active control.

Seismic isolation and passive control
Isolating a structure from the shaking ground is a rather old concept, but it is only since the
1970s that practical isolation systems have been developed and used for earthquake protection
of buildings and bridges. The concept was initially referred to as base isolation but at present
the term seismic isolation prevails, in view of the fact that the isolating devices do not have to
be always located at the base of the structure.
There are two interrelated ideas behind developing a seismic isolation system: the first one
is to make the structure much more flexible than it is, by altering the way it rests on the
ground, hence shift it to the long period range of the response spectrum that is typically
characterized by reduced accelerations and consequently reduced inertial forces; the second is
to introduce some kind of ‘fuse’ between the structure and the ground, whereby the amount of
base shear to be transferred from the shaking ground to the structure is controlled by the
strength of the fuse. By making the structure more flexible, one might achieve lower seismic
forces, but displacements tend to increase. It is therefore essential to also control the