Page 164 - Dynamic Loading and Design of Structures

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Table 4.4a Seismic coefficients C a of the 1997 UBC.

Soil type v (m/sec) Z=0.075 Z=0.15 Z=0.2 Z=0.3 Z=0.4
s
S A (hard rock) > 1,500 0.06 0.12 0.16 0.24 0.32N a
S B (rock) 760–1,500 0.08 0.15 0.20 0.30 0.40N a

Sc (very dense soil) 360–760 0.09 0.18 0.24 0.33 0.40N a
S (stiff soil) 180–360 0.12 0.22 0.28 0.36 0.40N a
D
S (soft soil) <180 0.19 0.30 0.34 0.36 0.36N a
E
1
S (special ) See footnote 1 below Table 4.4b
F

Table 4.4b Seismic coefficients C of the I997UBC
v

Soil type v s (m/sec) Z=0.075 Z=0.15 Z=0.2 Z=0.3 Z=0.4
S (hard rock) >1,500 0.06 0.12 0.16 0.24 0.32Nv
A
S (rock) 760–1,500 0.08 0.15 0.20 0.30 0.40N v
B
Sc (very dense soil) 360–760 0.13 0.25 0.32 0.45 0.56N v
S D (stiff soil) 180–360 0.18 0.32 0.40 0.54 0.64Nv

S E (soft soil) <180 0.26 0.50 0.64 0.84 0.96N v
1
S F (special ) See footnote 1
1
Soil with v s < 180 and large thickness (S E has limited thickness); requires site specific geotechnical
investigation.

4.3.4 Inelastic spectra and design spectra

For the vast majority of engineering structures it is not economically feasible to design them
to withstand the seismic actions corresponding to a return period of about 500 years (the
design earthquake in many modern codes, see Section 4.3.2) without developing inelastic
deformations. This has long been recognized (Newmark and Hall, 1982), but the
complications arising from the need to account in a simple and practical way for the inelastic
response of a structure to the design earthquake without carrying out a proper non-linear
analysis, are still a matter of controversy, as well as the subject of current research. The
powerful modal analysis procedures, although strictly applicable to elastically responding
structures only, are nevertheless used for analysing structures expected to develop significant
amounts of inelastic deformation when subjected to the design earthquake. It is clear that such
a procedure is not really rigorous, and there are situations (particularly in bridge design) that a
full inelastic dynamic analysis is required by codes (see Section 4.3.7); however, due to its
relative simplicity, this ‘equivalent’ modal analysis still forms the basis of most current code
procedures. The basis of this type of analysis is the inelastic spectrum derived for nonlinear
SDOF systems,