Characterization and modelling of

seismic action

Report of WG2: Earthquake resistance

Dan Lungu*, Aurel Stratan** & Radu Vacareanu*

* Technical University of Civil Engineering of Bucharest, Romania

** "Politehnica" University of Timisoara, Romania

COST C26: Urban Habitat Constructions under Catastrophic Events

Final Conference, 16-18 September 2010, Naples, Italy

State of the art

Ground motion produced at a

site is characterised by the

two orthogonal horizontal

components and the

vertical component

Ground motion representation

for structural design may be in

the form of:

– Time histories

– Power spectral density

– Response spectra (elastic or inelastic)

Ground motion modelling

Main characteristics of the ground motion time history:

– Peak ground acceleration (PGA), peak ground velocity (PGV) and

peak ground displacement (PGD);

– Motion duration;

– Frequency content.

PGA and PGV

– Very simple measures of severity of the ground shaking

– Random variables

– Prediction of the peak ground parameters at a site is the target of

the probabilistic seismic hazard analysis (PSHA)

Motion duration

– Time interval between two specified fractions of

the total cumulative energy of accelerogram

– It represents the time interval over which the

motion power is almost constant and near its maximum

E [a(t)] dttot

2

0

t d

Ground motion modelling

Frequency content - concept crucial for understanding

the structural damage potential of ground motion.

Frequency content can be described:

– Directly, by the power spectral density function (PSD), obtained

from stochastic modelling of the acceleration process;

– Indirectly, by the response spectra.

Stochastic measures of frequency content:

– The dimensionless indicators (Cartwright & Longuet - Higgins)

and q (Vanmarcke);

– The f10, f50 and f90 fractile frequencies of the total cumulative

power of PSD and the frequencies f1, f2 and f3 corresponding to

the highest 1, 2, 3 peaks of the PSD.

Deterministic measures of frequency content are the

control frequencies and corresponding control periods

TB, TC, TD.

Modelling of response spectra

EPA and EPV. Elastic response spectrum.

Effective peak acceleration (EPA)

and effective peak velocity (EPV)

characterize the intensity of a

ground motion by averaging the

effects of shaking on the

structures most exposed to that spectral content

The elastic response spectrum is given as smoothed

acceleration spectrum (for 5% damping) having a

specified probability to be exceeded; 0.5 median, 0.1, etc.

Seismic input motions must be

compatible with local soil condition,

intensity of shaking, seismic source

mechanism and hypocentral distance

0.1 0.5

2.5

smean of SAEPA

0.8 1.2

2.5

smean of SVEPV

Design spectrum

Acceleration-displacement response spectra

Design spectra:

– Behaviour factor q used to

reduce spectral accelerations,

accounting for inelastic

structural response (ductility,

redundancy and overstrength)

– Lower bound ag

The acceleration-

displacement response

spectra (ADRS)

– An alternative representation of

response spectra

– Periods are represented by a

series of radial lines extending

from the origin of the plot.

Bucharest'86 - 16 free-field motions

0

50

100

150

200

250

300

0 1 2 3 4 5 6 7 8 9

SD , cm

SA

, cm

/s2

Probabilistic seismic hazard assessment (PSHA)

A cornerstone position for the prediction of the strong

ground motion likely to occur at a particular site

The general PSHA is based on the following

methodology:

– Identification of independent sources of seismic activity and

determination of recurrence relationships;

– Fitting the attenuation relationship on a ground motion

parameter;

– Calculating the peak ground motion

parameter at the site with a specified

probability of non-exceedance during

structure lifetime;

– Delineation of isoseismal maps;

– Construction of uniform hazard

response spectra for design.

Vrancea seismic

subcrustal source

Hazard curve for Iasi City

1.E-04

1.E-03

1.E-02

1.E-01

50 150 250 350 450

PGA , cm/s2

PGA

Contribution to the research development Seismic motion leading to exceptional actions on structures

Seismic action is characterised by high uncertainty and

can be specified in probabilistic terms only

Mistakidis, E., Apostolska, R., Dubina, D., Graf, W.,

Necevska-Cvetanovska, G., Nogueiro, P., Pannier, S.,

Sickert, J.-U., Simões da Silva, L.,

Stratan, A., Terzic, U.

outlined several phenomena that

can lead to exceptional seismic

action, related to near-fault effects

and local site conditions

Near-fault effects:

– Long-period, high-amplitude pulse

in the forward-directivity region

– Vertical component of the seismic

action important

Seismic motion leading to exceptional actions on

structures: local site conditions

Geotechnical conditions - soft

soil layers:

– Amplification of PGA

– Amplification of spectral

accelerations in the long-period

range

"Trapping" of seismic waves

inside basins:

– amplification and

– increase of duration of the

seismic motion

Surface topography -

amplification for irregular

topographies, such as crest,

canyon, and slope

Seismic action in urban habitats

Gioncu and Mazzolani: city-site interaction

City-site interaction influence of densely urbanized cities

on the ground motion

– Superposition of vibrations produced by buildings over soil

vibrations coming from the source gives rise to a modification of

the free-field motion

– The largest effect is produced in the case of a dense constructed

area situated in a soft soil

– Each point on the surface can

have different movements,

explaining the strange and

highly variable damage within

identical building sets

Seismicity of Vrancea seismic source and soil

conditions in Bucharest

Lungu, D., Arion, C., Calarasu, E. and

Lungu, D., Vacareanu, R., Aldea, A.

Informations related to:

– Seismicity of Romania

– Seismic instrumentation

– Available strong

motion records

– New seismic zonation map

from the Romanian

seismic design code

Seismic microzonation

of Bucharest - a tool for

urban planning and

earthquake risk reduction

Selection of time-history records for dynamic

analysis of structures

Stratan and Dubina - overviewed code requirements and

selected references related to selection of time-history

records

Code provisions:

– how are the records obtained (through artificial generation, from

existing recordings of past earthquakes, or through simulation);

– compatibility between earthquake records and the seismic

source, travel path and site characteristics;

– matching between the target response spectrum and the ones of

earthquake records and

– number of records used and implications on result interpretation

Selection of time-history records for dynamic

analysis of structures

Artificial accelerograms are generated

using stochastic algorithms. Can be

improved by accounting for some

seismogenetic features or through

semi-artificial accelerograms. Simple

pulses can be used.

Simulated records are obtained

through physical simulation of source

and travel path mechanisms, and may

account for site effects.

Recorded accelerograms are obtained

from real seismic events in the past

with similar source, travel path and

local site conditions. Scaling usually

necessary.

Simulation of accelerograms for fuzzy analysis of

structures

Fuzzy stochastic tools for structural analysis and

reliability assessment were investigated by Sickert, J.-U.,

Kaliske, M., Graf, W.

The uncertain character of both earthquake records and

structural system behaviour was considered within a

response history procedure.

Using the model fuzzy randomness, earthquake

excitations are described as fuzzy random processes

which represent a fuzzy set of real valued random

processes.

Figure 1. Flowchart of generation algorithm.

Figure 2. Fuzzy intensity function.

Scenarios based earthquake hazard assessment

Romanelli, F., Peresan, A., Vaccari F. & Panza, G.F.

investigated scenario earthquakes, also named

neodeterministic seismic hazard assessment (NDSHA)

NDSHA - a hybrid method consisting of modal summation

and finite difference methods

Artificial seismograms of the vertical, transverse and

radial components

of ground motion

are computed at

a predefined set

of points at

the surface

Recommendations for further development

There seems to be a gap between the existing knowledge

on characterization of seismic action and the code

provisions:

– Lack of code provisions for near-fault effects

– Behaviour factor q independent of response spectrum

characteristics

Time-history records for nonlinear analysis difficult to

obtain:

– Time-histories compatible with the characteristics of the seismic

source, travel path and site effects require expertise in

seismology, that few structural engineers would have.

– Code requirement of matching to code spectra requires scaling of

records which alter the "seismological" compatibility.

– A close collaboration between seismologists and structural

engineers is needed to advance the current state of practice in

structural analysis under seismic action.