Geology Reference
In-Depth Information
software such as SHAKE or through reference to published ground
motion spectra for particular ground pro
les. Generally, thick soft
soil pro
cation of longer period waves.
The design ground motion then needs to be applied to the structure.
Structures have their own dynamic characteristics and if the incoming
frequencies match the natural response frequencies of the structure,
then movements may be magni
les may lead to relative ampli
ed (
Figure 6.32)
. Structural engineers
will take the incoming design earthquake characteristics and calculate
the response of the structure. For more frequent, smaller earthquakes,
the structural engineer will design the structure as far as possible to
behave elastically (no permanent displacement). In the event of an
extremely large and less probable event, a structure can be designed to
be fail-safe. Redundant elements such as additional steel beams can be
included that yield under extreme loads but also change the funda-
mental frequency of the building, damping the response to the shak-
ing. Other options are to put a building on springs of some kind or to
include hydraulic actuators or pendulums that again reduce the struc-
tural shaking. An example of an innovative aseismic design is the
foundations for the Rion
Antirion Bridge constructed in Greece in
2005. The cable-stayed bridge, with
-
five main spans extending 2.25 km
across a fault zone, was designed to withstand horizontal accelerations
of 0.5g at ground level and up to 2m offsets between adjacent towers.
Underlying each tower is thick soil and the depth of sea is up to 65m.
The towers were founded on 90m diameter cellular structures placed
on a 3.6m layer of gravel placed on the natural soil, which was
reinforced by up to 200×2m diameter tubular piles to depths of 30m.
The foundation structure is not attached to the piles; the gravel acts
as a fuse, limiting the transfer of load to the superstructure. The
piles in the underlying soil are there to prevent rotational bearing
failure. Details of the design of the foundations are given by
Combault
et al
. (2000) and further references are given at the web
page for the bridge.
Two recent earthquakes, however, show that even with good
design practice, earthquakes can cause damage to a level that is not
anticipated. As a result of the February 2011, Christchurch, NZ,
earthquake, many small one-and two-storey buildings were destroyed
or badly damaged, as one might expect near the epicentre of an
earthquake with magnitude exceeding 6.0, where the ground
motion might be expected to be dominated by high frequencies.
Widespread liquefaction was also a major contributor to the
damage of these smaller buildings. However, for this earthquake,
because of its shallow nature and possibly other factors that served
to concentrate and amplify the ground motion, unexpectedly large
accelerations and forces were generated. In the case of the March
2011 earthquake that struck NE Japan (east of Honshu), most engi-
neered buildings on mainland Japan withstood the very strong
