Page 243 - Forensic Structural Engineering Handbook

LOADS AND HAZARDS: THEIR NATURE, MAGNITUDE, AND CONSEQUENCES 7.27

E. A soil profile with v less than or equal to 600 ft/s (180 m/s), or average field standard
2
penetration test results less than 1000 lb/in , or soft clay
F. Soils that are vulnerable to failure or collapse (liquefiable, quick and highly sensitive,
weakly cemented), peat and organic clays, very high plasticity clays, and very thick
soft/medium stiff clays
The NEHRP Recommended Provisions for Seismic Regulations for New Buildings and
23
Other Structures contains a procedure to determine the effective soil classification when
more than one of these materials are present in the top 100 ft (30 m). Further, NEHRP con-
tains tables of factors that can be used to modify response spectra to represent different
excitation characteristics for each soil condition. Therefore, by using these factors, spectra
developed for records at one site can be modified for soil conditions to estimate response
spectra for another nearby site.
Conversions between sites should reference motions at bedrock. Spectra at the record-
ing site should be normalized to bedrock using factors appropriate for the soil conditions at
that site, and then site-specific spectra can be generated for the site of interest. NEHRP pro-
vides two factors for different frequency ranges of interest. Therefore, judgment is needed
when using these factors to adjust ground motion data. In most cases, appropriate consul-
tants with expertise in the extrapolation of seismic motion data should be consulted.

Analysis Procedures
Time-History Analyses. Perhaps the most accurate analyses that can be performed are
nonlinear, time-history analyses. These studies require actual time histories of ground
motion at the site of interest, or representative artificial time histories generated from
response spectra. The nonlinear response characteristics of the lateral load-resisting system
of the building must be established accurately. Finally, the analysis needs to be performed
in piecewise fashion, accounting for the formation of plastic hinges, element failures when
appropriate, and P-delta behavior.
Empirical Analyses. Simplified approaches are available in several resources. 1,23
Specifically, NEHRP allows an analytical approach that approximates earthquake response
by a series of static lateral loads applied at the floor levels of the structure. This approach
can be used accurately for structures that are relatively regular in configuration and that are
up to 240 ft (73 m) tall, and for structures that have plan or vertical irregularities and are
less than 100 ft (30.5 m) tall (although the NEHRP criterion for determining applicability
is based on frequency). Plan irregularities include shapes that are likely to induce signifi-
cant torsion, diaphragm discontinuities, and nonparallel elements in the lateral force-
resisting system, among others. Vertical irregularities include soft stories, nonuniform
weight distributions, and discontinuities in the vertical lateral force-resisting system,
among others.
With this empirical approach, P-delta effects are neglected if the stability coefficient θ
is less than 0.10:
P Δ
θ = x
Vh C d
xsx
where P = building weight above building level of interest
x
Δ= calculated story drift
V = calculated seismic shear at level of interest
x
h = story height below level of interest
sx
C = deflection amplification factor that is a function of type of seismic force-
d
resisting system
If this limit is not satisfied, rational analysis must be used to evaluate P-delta effects.