Design Features

Generally, the tunnel cross section is designed so that the buoyancy covers the structural weight and the tunnel is then subjected to an upward force, which is exerted by a fluid [Won et al., 2019]. However, in some of the latest design where pontoons are used the tunnel is subjected to a downward force. The balance of buoyancy weight ratio plays an important role to control the dynamic behaviour of SFTs when tension leg is used to stabi-lize a structure under water.

There are different types of tube cross sections, like circular, elliptical, polygonal and rect-angular. The configuration depends not only on structural purposes, but also on the facilities and the traffic lanes considered, in some cases it is included a railway or a pedestrian walk.

The driving criterion for the design of the concrete tube is the water tightness criterion in SLS, this requirement is common in all the designs of SFTs. Therefore, the membrane forces in the concrete shall be always of compression and cracks should be avoided. This

criterion governs the amount of prestressing and post tensioning level in the concrete tubes.

In the design of the Bjørnafjord’s SFTB it has been chosen a twin tube cross section, which has several advantages. In case of foreseen scenarios in one tube, the other can be used as escape route by the users. The twin tube cross section has by test shown a stable behaviour under current and wave action, eliminating the need for extra design remedies to eliminate uncontrolled motion [Olsen et al., 2016]. Moreover wind-tunnel tests underline twin tube as a preferable cross section rather than a rectangular box.

In most of the projects the cross section is made by concrete and steel reinforcement, while in the Prototype in Qindao Lake the cross section is a steel-concrete-aluminium sandwich [Mazzolani et al., 2008]. The external aluminium layer, which is corrosion-resistant, works as an energy absorber in case of external impact, due to its alveolate shape. However, in this case the length of the tunnel is 100m, while in the Bjørnafjord project it is over 5000 meters.

The external diameter, on the other hand, has the prominent influence on the ratio between

Figure 2.1:Twin tube configuration ,[Olsen et al., 2016]

the water buoyancy and the tunnel weight (BWR), which is expected to be larger than one.

It was detected that the increase of the BWR ratio, that usually ranges between 1.25 to 1.4, can lead to impressive improvements of the SFT response to extremely severe sea states [Perotti et al., 2013]. The BWR is controlled by the water ballast, which is important dur-ing both the installation procedures and functiondur-ing of the structure. The size of the ballast is fundamentally different between the pontoon solution and the tether solutions that are described afterwar.

Generally, cylindrical shapes are preferred because they minimize the drag force due to current, and the vortex shedding induced vibrations. In addition, adopting a round shape for the tunnel prevent rotational moment due to fluid forces. Moreover, it is the best shape for uniform inner/outer pressure. The only drawback is that they are more expensive in the fabrication.

2.1 Design Features The elliptical cross section has a greater impact resistance due to explosion to that of rect-angular and circular cross section. The analysis of the impact acceleration of pipe sections with different cross sections, shows that the impact acceleration peak values of the rectan-gular cross section are the biggest in both flow direction, followed by those of the circular and then the elliptical shapes, [Gang et al., 2020].

Another type of cross section is the triple deck, Fig. 2.2. The advantages for this proposal

Figure 2.2:Triple deck cross section, [Xiang et al., 2017]

are the efficient shape for primary load carrying of vertical and horizontal loads, and the re-duced volume concrete compared to the twin tube solution. However, it is less efficient for secondary load carrying (plate bending), and it is subjected to high drag forces and vortex shedding. This solution is suitable only for single span tunnel, [Xiang et al., 2017].

Figure 2.3:Funka Bay SBT crossing in Japan, [Kanie, 2010]

In japan the first feasibility design of a SFT is referred to the Funka Bay crossing, Fig. 2.3, which has a total length of 30Km and a maximum depth of 120m, [Kanie, 2010].

The tunnel consists of a single cylinder with a steel skin plate for the perimeter of the tunnel and light weight concrete for the body, the buoyancy weight ratio is 1.5. In this design,

special attention is given to arrange the legs in the cross section not to cause rotational torque by restoring force with horizontal and vertical displacements. For that purpose, it is introduced the idea of a flexible leg installed around the tunnel through the frictionless sheath to tie the left and right tethers. Then the forces acting in the legs are automatically kept in equilibrium.

Due to large reaction forces in the landafalls of the SFTs, it is a common design strategy to increase the cross-section dimensions in these regions.

2.1.1 Horizontal Bracing-Twin Tube SFTB

A rigid connection between the two tubes is achieved by diagonal bracings. Some bracings with regular spacing are adopted to secure escape routes, control rooms and other facili-ties. Horizontal bracing between the main cross tubes is required to limit the lateral wave induced flexural response to an acceptable level.

A comparative study is presented in [Olsen et al., 2016]. It outlines the truss model as

Figure 2.4:Comparison horizontal bracing system, [Olsen et al., 2016]

the best option compared to a Vierendeel model, Fig. 2.4. Whereas the truss and Vieren-deel configuration exhibit similar global response in terms of tube axial forces, their local response differs significantly. The Vierendeel frame rotates under pure shear, consequently secondary bending moment in the main tubes are much higher.