Civil Engineering Reference
In-Depth Information
(i) Seismo - tectonic environment of the area;
(ii) Topography and soil condition of the site;
(iii) Dynamic characteristics of the structure;
(iv) Structural system, ductility and material used;
(v)
Importance of the structure.
Whereas (iii) and (iv) above are clearly linked, they are treated separately in codes, with a degree of
justifi cation. The fi ve parameters listed above are considered in different ways in seismic codes. A brief
description of each parameter is given below:
(i) The 'Zone Factor' accounts for the anticipated seismic activity at the construction site. In this
factor, the peak ground acceleration, obtained from seismic hazard studies, is given either
directly (as a percentage of acceleration of gravity, g) or implicitly, as illustrated in Section
3.4.5 .
(ii) The 'Site Factor' represents the effect of the different foundation materials on the strong- motion
characteristics and the probability of high amplifi cation or resonance due to the proximity of
the period(s) of vibration of the site and the structure, as discussed in Section 1.3.2. The topol-
ogy of the site is taken into account in only a small number of codes.
(iii) The ' Response Modifi cation or Behaviour Factor' refl ects the relative seismic performance of
different structural systems, in terms of local/global ductility, redundancy and redistribution
capability, and the predicted mode of failure (brittle or ductile). It is also referred to as the
response reduction factor or, wrongly, the ductility factor. Different relationships between force
reduction factor and ductility are reviewed in Section 3.4.4 .
(iv) The ' Material Factor ' refl ects the ability of the structural material to dissipate energy and
respond in a ductile manner. For instance, masonry is inherently less ductile than steel. In almost
all cases, this factor is implicit in the behaviour factor described above.
(v) The 'Importance Factor' accounts for the importance of the building by decreasing the probabil-
ity of damage or collapse for important, environmentally sensitive or exceptionally heavily
populated structures; i.e. it depends on the occupancy of the structure (potential fatalities), its
use (importance) and the consequence of its damage (environmental). Implicit in this parameter
is the defi nition of an acceptable probability of being exceeded attached to the design ground
acceleration. Higher probability is associated with less important structures, as stated in Sections
4.4 and 4.7. The design values of PGA specifi ed in international building codes correspond
frequently either to 10% of probability of being exceeded in 50 years (return period of about
475 years) or to 2% in 50 years (return period of about 2,475 years), especially in North America.
However, more recently, acceleration response spectra are also provided for different return
period and probability of exceedance corresponding to the various limit states to comply
with.
(vi) The ' Design Spectrum ' , defi ned in Section 3.4.5, accounts for the coupling between structural
periods of vibration and earthquake characteristics, as well as travel path such as attenuation
and long- period amplifi cation. The latter effects are expressed, in seismic codes and guidelines,
by the corner periods
T
1
and
T
2
used to defi ne the elastic response spectrum and the long- period
exponent utilized to characterize the decay in the long-period range, as shown in Figure 4.39 .
In most codes, the spectrum is ' fl attened' for periods shorter than
T
1
, to account for softening
of short-period structures, which may lead to an increase in applied loads.
The above list is not exhaustive; each national code uses its own philosophy and notation and adopts
a different format for the above-mentioned parameters. It provides, however, the fundamental ingredi-
ents to estimate the design base shear
V
B
from equation (4.34) .
