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本帖最后由 plot 于 2011-5-31 12:31 编辑
CHRISTCHURCH SEISMIC DESIGN LOAD LEVELS
INTERIM ADVICE
1 BACKGROUND
With the earthquakes that have occurred, there is a significant risk that the Canterbury area
will be subject to a period of increased seismicity. This should be considered as a possible
series of triggered events, rather than simply an aftershock sequence. There may be a period
of up to 50 years or more, during which the seismic hazard due to smaller events near
Christchurch is significantly increased.
This effect is similar to what has been experienced in the Nelson/Buller area, commencing
with the 1929 Murchison earthquake. There are a number of minor faults or fault systems
that may be suffering increased stress from the recent earthquakes, some of which are already
showing signs of increased activity.
A likely scenario from one of these faults is another earthquake in the Magnitude 6-6.5 range,
possibly again close to the CBD. This has the potential for similar actions to the Lyttelton
earthquake, that is, a short duration event, with high ground accelerations.
The assessed risk of another earthquake near Christchurch aggregates to approximately 6% on
an annual basis. Prior to the earthquakes, this probability might have been assessed as less
than 1/100th of this value.
Consequently, the Design Basis Earthquake (DBE) for Christchurch and its surrounds needs
to be increased to cover this.
Conversely, there is no perceived increase in hazard due to the Alpine fault. This has
generally been considered the major seismic hazard for Christchurch and is not significantly
affected by the recent activity. The Alpine fault may fail in an earthquake of up to Magnitude
8, but the distance to the Christchurch CBD is such that the ground accelerations will be
highly attenuated. In this case, Christchurch may expect a long duration event with relatively
low ground accelerations.
As a consequence of the above, it is considered that the MCE earthquake is relatively
unchanged; assuming that the Alpine fault is the controlling fault for Christchurch.
2 PROPOSED SEISMIC DESIGN LOAD REVISIONS
Taking into account the notes above regarding seismicity, SESOC has recommended to the
Department of Building and Housing that the hazard factor in the Canterbury region is
increased to a minimum of:
Z = 0.3, for all periods below T = 1.5sec
Further, it is recommended that the risk factor for serviceability limit states is increased to:
R = 0.33
The revised z factor is intended only for use for the design and assessment of buildings and
structures, pending further research.
Structures designed to NZS3604 need separate consideration. If designers are working to
NZS3604:1999, the seismic load provisions will not comply. It is recommended that the
bracing provisions of NZS3604:2011 are used instead.
All structures with periods in excess of 1.5 seconds should be subject to special study,
pending further research.
Questions still to be resolved are:
• What is the impact on the MCE?
• How do we progress the design and evaluation of IL3 and IL4 buildings? If it is
assumed that the MCE is unchanged, that would imply that the effective R factor for
IL4 may be reduced. However it would be prudent to continue to use the same R
factors as before, pending the research outcomes.
John Hare
President
Structural Engineering Society |
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