Advances in Seismic Design Methodologies - Earthquake Engineering for Structural Design - page 419

Advances in Seismic Design Methodologies 419

When the ground motions move the base of the structure, its masses experience
forces of inertia as the structure attempts to follow the ground motions. The resultant
movements of the structure depend in a complex way on the characteristics of ground
motions, the dynamic properties of the structure, the characteristics of the structural
materials and the type of foundations. Earthquake forces are generated by the inertia of
buildings when they dynamically respond to ground motions.
The dynamic nature of the response makes earthquake loading different from other
building static loads. As a consequence, the need to consider the dynamic behavior of
buildings sometimes generates discomfort within the designers. The designer
temptation to consider earthquakes as static loads is a trap which must be avoided since
the dynamic characteristics of the building are fundamental for evaluating the structural
response. Nevertheless, the effective earthquake design methodologies can be, and
usually are, simplified without detracting from the effectiveness of the design. Indeed,
the high level of uncertainty about the ground motions generated by earthquakes
seldom justifies the use of complex and sophisticated methodologies. Instead of an
intricate methodology, it is preferable to use a simplified one, but considering some
specific features of seismic design. In this way, a good seismic design is the one where
the designer takes control of the building, by dictating how the building has to respond
(King, 2000).

9.5.2MethodsofAnalysis

The analysis of a structural system to evaluate both deformations and internal forces
induced by applied loads or ground excitation is an essential step in the design of a
structure to resist earthquakes. A structural analysis procedure requires:
- Representation of earthquake ground motions.
- Model of structure.
- Method of analysis.
For the representation of earthquake ground motions, there are the following
procedures:
- Response spectrum is a simple plot of the peak of steady-state response

(displacement, velocity or acceleration) of a series of oscillations with varying
natural frequency or period, which are forced into motion by the same base
vibration or shock (see Section 9.4).

- Lateral load distribution in function of ground and structure natural periods, based

on the assumptions that the structure response is controlled by the vibration modes.

- Representation of time-history acceleration in function of site-recorded ground

motions or artificial accelerogram compatible with the design response spectrum.
For modeling the structures, there are the following approaches:

- 2-D modeling of structure, when only the plain structural response is considered,

for regular structures.