Civil Engineering Reference
In-Depth Information
The top key of the secondary end is initially omitted, allowing the tunnel
element being immersed to locate its primary end shear key onto the lower
secondary end key. The top key is then installed later on, in accordance
with the designer's assumptions on settlement.
As a three part key is needed, the overall depth of an individual steel
key can be no more than approximately one-third of the tunnel total
height. In fact, the height is a little less than a third of the total height
because of the space needed for immersion joint components in the top
and bottom slabs. The combination of shallow depth and high eccentric
loading means that a high level of tension arises at the extreme fiber of
the shear key due to the moments applied. This tension needs to be trans-
ferred back into the tunnel element structure and can require a significant
quantity of horizontal tensile reinforcement and high tensile bars, such
as Macalloy bars, to fix the base plate of the shear key to the concrete
structure (Figure 9.17).
Lateral eccentric loading is an important consideration in the design of the
keys. If there is a horizontal misalignment between the ends of the tunnel
elements or if there is any unevenness or out of tolerance on the bearing sur-
faces, this could cause eccentric loading. The shear keys should be designed
for an element of torsion to account for the possibility of these effects. The
use of bearings in the shear key assembly will mitigate uneven loading. The
type of bearings selected needs to be free of any long-term maintenance
requirements. Access for inspection and possible replacement has to be con-
sidered carefully in the design. It is possible for the temporary condition to
design on the basis of there being no bearings, but to use packing or shim
plates between the keys to get even bearing pressures on the contact surfaces.
Bearings should be introduced for the permanent arrangement, however.
Bearing
Shear key
Anchor
bars
Immersion joint
Figure 9.17
Steel shear key arrangement at immersion joint.
