Environmental Engineering Reference
In-Depth Information
From this estimate pga versus combined annual exceedance probability, allowing for
the three faults and accounting for earthquake magnitude.
12.3.3
Other forms of expression of seismic hazard
The discussion so far has centred on assessing the hazard in peak ground acceleration (on
bedrock) terms.
For more advanced dynamic analysis, it will be necessary to obtain accelerograms of
earthquakes of a similar nature to that expected considering the source faults, scaled to the
expected peak ground accelerations. In some cases, synthetic accelerograms may be used.
More than one accelerogram will usually be needed.
For simplified analysis of concrete gravity dams and other structures response spectra
may be used but with some limitations on the output of the analyses.
12.3.4
Selection of design seismic loading
There are two ways of selecting the design seismic loading:
(a)
Deterministic
- which requires the assessment of an operating basis earthquake OBE,
and Maximum Design Earthquake MDE.
ANCOLD (1998) adopted the following definitions for OBE and MDE:
Operating Basis Earthquake
- the OBE will produce a level of ground motion
which will cause only minor and acceptable damage at the damsite. The dam, appur-
tenant structures and equipment should remain functional and damage from the
occurrence of earthquake shaking not exceeding the OBE should be easily repairable.
Maximum Design Earthquake
- the MDE will produce the maximum level of ground
motion for which the dam should be designed or analysed. It will be required at least that
the impounding capacity of the dam be maintained when subjected to that seismic load.
The OBE is often accepted as a loading which has a 10% chance of being exceeded
in a 50 year period, or an annual probability of exceedance of 1 in 475; and the
Maximum Design Earthquake (MDE), which may be taken as the maximum loading
at the site from MCE on the known faults, or the load which has an annual probabil-
ity of exceedance of say 1 in 10,000.
(b)
Probabilistic
- which assesses the effects on the dam and its foundations of a range of
seismic loads, e.g. from 1 in 100 Annual Exceedance Probability (AEP) to 1 in 10,000
AEP, or 1 in 100,000 AEP, resulting from earthquakes on known faults, or estimated
probabilistically as detailed in Section 12.3.1.
The steps involved in this process are (ANCOLD, 1998):
(i) determine the AEP of earthquake ground motion (P
E
) over the range of earthquake
events which may affect the dam.
Table 12.2
gives an example for AEP vs ground
acceleration;
(ii) determine the conditional probability (P
BC
) that for each of the ground motion
ranges (e.g. 0.125 g to 0.175 g in Table 12.2) the dam will breach. In assessing this
conditional probability all modes of failure should be considered and the proba-
bilities combined, making allowance for interdependence and mutual exclusivity
or otherwise (e.g. for embankment dams, slope instability, piping, liquefaction/
instability, and for concrete gravity dams, overturning, and sliding);
(iii) assess the probability of failure for each range of ground motion by multiplying
the AEP with P
BC
i.e. P
B
P
BC
- see Table 12.2;
(iv) sum the probabilities to give the overall annual probability of failure due to
earthquake.
P
E
