A submerged floating tunnel (SFT) must have enough strength to resist to various external loadings such\ud
as bending, torsion, tension, and compression. The expected main deformation of SFT is caused by bending moment.\ud
And this bending moment makes tensile stress and compression stress on the wall of SFT. Thus, bending moment is a\ud
main affecting factor on the safety of SFT. Until now, a reinforced concrete tunnel was suggested for SFT by other\ud
researchers. In this study, an internally confined hollow reinforced concrete tunnel and a double skinned composite\ud
tubular tunnel were proposed. And their bending strengths were studied and compared with that of a reinforced concrete\ud
tunnel. The analysis results showed the proposed SFT structures had enough strength to guarantee safety

Han, S.H.

Han, T.H.

Park, W.S.

Won, D.

Yum, K.D.

Hasanuddin University Repository

 822 
 
 
Proceedings of the 7th International Conference on Asian and Pacific Coasts  
(APAC 2013) Bali, Indonesia, September 24-26, 2013 
 
 
 
BENDING STRENGTH ANALYSIS OF STEEL-COMPOSITE  
SUBMERGED FLOATING TUNNELS 
 
T.H. Han 
1
, D. Won 
1
, S.H. Han 
1
, W.S. Park 
1
, and K.D. Yum 
1
 
 
 
ABSTRACT: A submerged floating tunnel (SFT) must have enough strength to resist to various external loadings such 
as bending, torsion, tension, and compression. The expected main deformation of SFT is caused by bending moment. 
And this bending moment makes tensile stress and compression stress on the wall of SFT. Thus, bending moment is a 
main affecting factor on the safety of SFT. Until now, a reinforced concrete tunnel was suggested for SFT by other 
researchers. In this study, an internally confined hollow reinforced concrete tunnel and a double skinned composite 
tubular tunnel were proposed. And their bending strengths were studied and compared with that of a reinforced concrete 
tunnel. The analysis results showed the proposed SFT structures had enough strength to guarantee safety. 
 
Keywords: SFT, tunnel, composite, bending strength, submerged. 
 
 
                                                 
1 Coastal Development & Ocean Energy Research Division , Korea Institute of Ocean Science & Technology , 787 Haeanlo, Ansan 
426-744, KOREA 
INTRODUCTION 
A submerged floating tunnel (SFT) has been 
considered as an interesting structure to connect the 
banks separated by a narrow and deep sea. Reinforced 
concrete (RC) tunnels have been suggested as SFT 
structures. Because the governing load of a SFT is 
bending moment, a structure with enhanced stiffness and 
ductility will be useful for a SFT. Therefore, two new-
type structures which have superior stiffness and 
ductility were proposed for the SFT.  
 
 
Fig. 1 ICH RC structure (Han et al., 2008) 
 
 
Fig. 2 DSCT structure (Han et al., 2010) 
 
 
 
 
One is internally confined hollow RC structure which 
is hollow (ICH) RC structure with a steel or fiber 
reinforced polymer (FRP) tube as shown in Fig. 1. The 
other is double–skinned composite tubular (DSCT) 
structure which is composed of two concentric steel or 
FRP tubes and concrete between them as shown in Fig. 2. 
In this study, the application of the ICH RC and DSCT 
structures to SFTs was investigated by the analyzing 
their bending strengths. The dimensions of ICH RC SFT 
and DSCT SFT were determined referring the RC SFT 
proposed in the guidebook published by SFT Research 
Group of Japan (Kanie, 2010). The bending analyses 
were carried out by using an exclusive program with the 
considerations of material nonlinearity and confining 
effect of concrete (Han et al., 2010; Han et al., 2013). 
And also the analysis results were compared with those 
of the RC SFT with the reference design.  
 
BENDING STRENGTH ANALYSIS  
 
Bending Strength of RC SFT 
SFT Research Group of Japan proposed a RC SFT 
section of the middle part of the SFT (not the connection 
part) as Fig. 3. The RC SFT has the 23,000mm of the 
outer diameter and 21,000mm of the inner diameter. The 
applied material properties are summarized in Table. 1. 
 
 
 
T.H. Han , et al. 
 
823 
 
 
Fig. 3 Dimensions of RC SFT for Funka Bay (Kanie, 
2010) 
 
Table 1 Material properties 
Material 
Compressive or 
Yield Strength 
Modulus of 
Elasticity 
Concrete 39.2Mpa 30.4GPa 
Mild Steel (SS400) 400MPa 206GPa 
Re-bar (SD345) 345MPa 206GPa 
 
The behavior of this RC SFT depends on the 
arrangement of re-bars. In the reference RC SFT, 25mm-
diameter re-bars were arranged with the spacing of 
200mm in the longitudinal and transverse directions.  
For the analysis of a DSCT and ICH RC SFTs, the mild 
steel in Table 1 was applied to the tubes.  Fig. 4 shows 
the axial load-bending moment (P-M) interaction curve 
of the RC SFT. The peak axial strength and bending 
moment of the RC SFT were 2,660MN and 9,062MN-m, 
respectively. 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 4 P-M interaction curve of RC SFT 
 
Bending Strength of ICH RC SFT 
In the ICH RC SFT, the outer re-bars are equally 
arranged to the RC SFT.  But an inner steel tube is 
settled inside of the SFT instead of the inner re-bars and 
inner cover concrete. Therefore the inner diameter is 
larger than that of the RC SFT by double thickness of the 
inner cover concrete (inner diameter=21,300mm). The 
thickness of the inner steel tube is 6.7mm which is the 
minimum thickness not to yield and not to be buckled by 
the passive pressure from the Poison effect of the 
concrete. The cross section of the tunnel wall is shown in 
Fig. 5. Considering the thickness of the inner steel tube, 
the ICH RC SFT has lager cross sectional space by 
9.52m2 than the RC SFT.  
Fig. 6 shows the P-M interaction curve of the ICH 
RC SFT. Its peak axial strength and bending moment of 
are 2,582MN and 9,656MN-m, respectively. Its axial 
strength is 2.93% smaller than that of the RC SFT. But 
its bending strength, the dominant factor, is 6.55% larger 
than that of the RC SFT although it has thinner tunnel 
wall. Therefore the ICH RC structure is supposed to 
have enough safety as a SFT structure. 
 
 
Fig. 5 Tunnel wall of ICH RC SFT 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 6 P-M interaction curve of ICH RC SFT 
 
Bending Strength of DSCT SFT 
The DSCT structure is composed of inner and outer 
steel tubes and concrete is filled between them. 
Therefore, the cover concrete is not necessary for the 
DSCT structure and the tunnel wall thickness is thinner 
than the RC structure by the double times of the 
thickness of the concrete cover. The inner diameter of 
the confined concrete and the outer diameter of the 
DSCT SFT are 21,300mm and 22,700mm, respectively.  
 
 
Bending Strength Analysis of Steel-Composite Submerged Floating Tunnels 
824 
 
 
Fig. 7 Tunnel wall of DSCT SFT 
 
The thicknesses of the outer steel tube and inner steel 
tubes are 20mm and 34.4mm, respectively. The 
thicknesses were determined considering the yield 
strengths and local buckling strengths of the inner and 
outer tubes. Fig. 7 shows the cross sectional dimension 
of the DSCT SFT wall. Fig. 8 shows the P-M interaction 
curve of the DSCT SFT. Its peak axial strength and 
bending moment of are 3,006MN and 9,233MN-m, 
respectively. Its axial strength and bending strength are 
is 13.01% larger and 1.89% larger than those of the RC 
SFT. Considering the smaller wall thickness of the 
DSCT SFT, the DSCT SFT has enhanced strength and 
enough safety.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 6 P-M interaction curve of DSCT SFT 
 
Required Quantity of Materials  
The unit required quantities of the necessary 
materials to build the three types of the SFTs were 
compared. Because the quotations of steel are very 
sensitive by the market circumstances, only material 
quantities per unit length were compared. And the 
quantities of the re-bar and the steel tube were 
categorized into a same group (steel). Fig. 7 and Fig. 8 
show the unit required quantities of concrete and steel to 
build the RC SFT, the ICH RC SFT, and the DSCT SFT, 
respectively. Fig. 8 shows the required concrete 
quantities of the ICH RC SFT and DSCT SFT reduced. 
 
 
     Fig. 7 Comparison of required steel (ton/m) 
 
 
 
 
 
 
 
 
 
 
Fig. 8 Comparison of required concrete (ton/m) 
 
DESIGN OF DSCT SFT  
Referencing to the data from SFT Research Group of 
Japan (Kanie, 2010), a DSCT SFT section was designed 
for the connection part. In the original design of the RC 
SFT, the outer and inner diameters were 24,000mm and 
21,000mm.  
 
Applied Load  
In the original design, the RC SFT is a continuous 
structure with 70@100m-span. The design considered 
self-weight of the SFT, buoyancy, wave loads and 
seismic loads for the periods of 145-year and 950-year. 
In this case, the maximum moment of the SFT occurs at 
the both end point because it is fixed at there. And the 
wave load makes the maximum bending moment. The 
design moments by the wave loads for the periods of 
145-year and 950-year are 2,095MN-m and 3,048MN-m, 
respectively. The maximum axial load occurs by the 
seismic load. The design axial loads by the seismic loads 
for the periods of 145-year and 950-year are 2.91MN 
and 4.55MN, respectively. 
 
 
 
T.H. Han , et al. 
 
825 
 
DSCT SFT Design 
Based on the calculated axial load and bending 
moment, two DSCT SFTs were designed. The cross 
sectional dimensions are shown in Table 2. The 
compressive strength of the concrete is 29.43MPa. The 
yield and ultimate strengths of the steel are 250MPa and 
392.4Mpa, respectively.  
 
Table 2 Dimension of DSCT SFT 
Item 
Type-1 
(H95) 
Type-2 
(H90) 
Outer diameter (mm) 24,000 24,000 
Inner diameter (mm) 22,800 21,600 
Thickness of outer steel (mm) 24.50 24.50 
Thickness of outer steel (mm) 45.38 42.99 
 
 
DSCT SFT Analysis 
By using the analysis results of the DSCT SFTs, P-M 
interaction curves and moment-curvature curves were 
plotted as shown in Fig. 9 and Fig. 10. As shown in the 
figures, the bending moment strength and the axial 
strength of H90 are 13,227MN-m and 4,020MN, 
respectively. They are larger than the maximum design 
loads and H90 is safe under the load case. The bending 
moment strength and the axial strength of H95 are 
8,726MN-m and 2,730MN, respectively. They are larger 
than the maximum design loads and H95 is safe under 
the load case.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 9 P-M interaction curve of designed DSCT SFT 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 10 Moment-curvature curve of designed DSCT 
SFT 
 
CONCLUSION 
In this study, to replace the RC structure which is 
used for SFTs with a new-typed structure, an ICH RC 
SFT and a DSCT SFT were designed and analyzed. The 
new-type SFTs were verified to have enough axial 
strength and bending strength to build safe SFTs. And 
also they had enhanced ductility. 
 
ACKNOWLEDGEMENTS 
This research was financially supported by Korea 
Institute of Ocean Science & Technology (KIOST), 
project no. PE98941.  
 
REFERENCES 
Han, T.H, Lim, N.H., Han, S.Y., Park, J.S. and Kang, 
Y.J. (2008) Nonlinear concrete model for internally 
confined hollow reinforced concrete column,   
Magazine of Concrete Research, 60(6): 429-440. 
Han, T.H, Stallings, J.M, Cho, S.K. and Kang, Y.J. 
(2010) Behaviour of a hollow RC column with an 
internal tube, Magazine of Concrete Research, 62(1): 
25-38. 
Han, T.H., Won, D.H., Kim, S. and Kang, Y.J. (2013) 
Performance of a DSCT column under lateral 
loading: analysis, Magazine of Concrete Research, 
65(2):121-135. 
Han, T.H., Yoon, K.Y. and Kang, Y.J. (2010) 
Compressive strength of circular hollow reinforced 
concrete confined by an internal steel tube, 
Construction and Building Materials, 24(9):1690-
1699. 
Kanie, S. (2010) Feasibility studies on various SFT in 
Japan and their technological evaluation, Procedia 
Engineering, 4:13-20. 


Bending strength analysis of steel-composite submerged floating tunnels

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Bending strength analysis of steel-composite submerged floating tunnels

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