Civil Engineering Reference
In-Depth Information
3m diaphragm
spacing (typical)
Longitudinal
plates
Diaphragm
plate
Shear
connectors
Stiffeners
Figure 3.9
Sandwich construction (developed from SCI publication 132).
in a concrete section is the function of shear reinforcement. To be effective
at resisting shear, these studs have to extend through the full depth of the
section and must be anchored in the compression zone.
The analysis of such a section under varying loads is a specialist subject
and the reader should refer to detailed papers on the subject published by
the Steel Construction Institute in the United Kingdom (SCI Publication
132, 1997). The section is subject to both compression and bending forces
and the composite action is complex. The stiffness of the section varies as
the concrete cracks. This variation is normally neglected in the analysis of
a concrete section but is significant in this double skin composite. A con-
siderable amount of physical model testing has been undertaken on these
sections to validate the analytical methods used for design.
In design terms, such a composite section is an elegant solution to a struc-
tural problem. Both the steel and the concrete are used efficiently and pro-
duce a very strong section. There are, however, difficulties in transforming
this elegant structural concept into a practical immersed tunnel.
The thickness of the walls is less because it is such an efficient struc-
tural section. This leads to an overall tunnel cross section that is lighter
and therefore has a shallower draught that can make towing to the tunnel
site easier. However, this advantage of a thinner section thickness is offset
because the weight of the concrete also serves a ballast function. If it is not
in the structural section as part of the load carrying members, it has to be
applied elsewhere, either as internal ballast or on the roof. To place this
