Civil Engineering Reference
In-Depth Information
end of its design life. This approach enables the selection of the key
durability parameters, such as chloride diffusivity and cover depth.
• Temporary prestressing for segmental concrete tunnels: Codes can be
applied, but the criteria to be achieved in design are not well codified.
Guidance is given later in this chapter.
• Seismic design of joints: This is not covered by design codes. Guidance
is given later in this chapter.
LOAD COMBINATIONS
Load combinations and partial load factors defined in the design codes
tend to cater to conventional structures such as bridges. An immersed tun-
nel has a different set of loadings and risks, however, and an assessment
of the probability and criticality of a set of loads combining is needed for
the various limit states that should be considered. For example, the impor-
tance of hydrostatic loads and earth pressure loading is much greater for an
immersed tunnel than for a bridge. Also, live loading is far less critical for
an immersed tunnel than it would be for a bridge structure. Therefore, it is
appropriate to reconsider the load factors. There are also some very specific
loads that apply to an immersed tunnel—particularly, the accidental and
extreme loads—and the method of combining these with the normal loads
such as self-weight and earth pressures needs some particular thought.
The ITA issued some guidelines on the load factors for specific loadings
that are particular to immersed tunnels in their State of the Art Report in
1993. However, the methodology for applying load factors according to
the limit state design principles has developed since that time and hence the
guidelines are now out of date.
Eurocodes and AASHTO are probably the most widely used design
codes around the world, and so, the load combinations and factors applied
for both of these are summarized below.
U.S. load combinations for AASHTO for LRFD design
The FHWA have published recommendations in their design manual for
road tunnels, which contains tables for load combinations. It offers a good
start for designers who can carry out this exercise and determine an appro-
priate set of combinations and factors. They are prepared for use with
AASHTO LRFD design codes. Only the limit state load combinations that
are relevant to an immersed tunnel structure are provided. These are
• Strength I: Basic load combination for strength and stability.
• Service I: Load combination used to check for stresses, delection, and
flexural crack control.
