Civil Engineering Reference
In-Depth Information
Stiffness based on
combination of stiffness
of concrete and free opening
Stiffness based on
stiffness of concrete
No rotational
stiffness—free to rotate
Compression
Compression
Compression
Joint open
Joint fully compressed
Joint partially compressed
Figure 9.16
Segment joint rotational stiffness for a longitudinal analytical model.
the gradual addition of concrete ballast can take place. Initially, this is
usually just the keel concrete placed prior to launching the tunnel element.
The condition when the keel concrete is placed and the tunnel element is
set on its launching sleds should also be analyzed. Launching loads need to
be applied to the element modeled as a beam. Dynamic loading as well as
static loads must be assessed in this instance.
The construction sequence from here on is important to the remaining
analysis work required. There may be further concrete ballast added prior
to towing the tunnel element to the site, or the element may be towed high
in the water. If this is not known, the designer may have to undertake a
number of analyses to ensure either can be accommodated or he may have
to restrict the contractor to build the elements in a prescribed manner. For
the tow condition, wave loading should be applied to the shell acting as a
beam to assess the risk of buckling or overstressing. Again, stiffening may
need to be designed to cater to the loading during tow. If the steel shells are
transported using semi-submersible barges, then the tow loads clearly do
not occur, but some handling loads may need to be considered as the shell
is transferred on and off the barge.
The outfitting of the steel shell will impose a number of further loading
scenarios. The inner structural concrete is placed as the element is floating,
which gradually reduces the freeboard of the element. Each concrete place-
ment will change the load distribution along the tunnel element and the ele-
ment needs to be analyzed both longitudinally and transversely. In the
transverse condition, it is important to look at the sections adjacent to the
last concreted section as they may be drawn lower into the water and have
increased hydrostatic pressure acting on the external plate, but with no
concrete yet behind the plate to provide stiffening. The designer will need
to define the sequence of concreting based on the analysis, and provide this
to the contractor to ensure the steel shell is not overstressed. The designer
