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1 Available online at Procedia Engineering (010) Procedia Engineering Procedia Engineering 4 (010) ISAB Copared cost evaluation aong traditional versus innovative strait crossing solutions G. Martire, B. Faggiano, F.M. Mazzolani* DIST Departent of Structural Engineering, University of Naples Federico II, P.le Tecchio 80, 8015, Naples, Italy Received 4 August 010; accepted 4 August 010 Abstract Aong the traditional solutions in use to cross waterways Cable Supported Bridges (CSB), such as suspension and cable stayed ones, nowadays represent one of the ost widely realized. However this structural typology feature several probles, in particular when large spans have to be surpassed. Suberged Floating Tunnel (SFT) is instead an innovative technical solution in the field of waterway crossings, particularly suitable and advantageous in case of long waterway crossings. As a atter of fact, thanks to its odularity, the SFT structural perforance is not greatly affected by the crossing length; also, its cost varies linearly with its length, differently fro Cable Supported Bridges. At the stage of selection of the structural solutions to be adopted a decisive role is played by econoic aspects. In particular, the building cost of each available solution has to be assessed in order to coe to the final choice. Therefore siple procedures for the evaluation of the realization cost of the crossing solutions are needed; such a kind of procedures, considering only the costs related to quantity of steel, are already available for traditional Cable Supported Bridges. The scope of the present study is to provide a siilar procedure for Suberged Floating Tunnels, thus allowing for quickly coparing the cost-effectiveness of the latter solutions with the one of traditional CSB solutions. Soe crossing exaples are also provided, highlighting the conditions where the SFT innovative solution proves to be ore econoically copetitive than traditional ones (CSB). 010 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Keywords: suberged floating tunnel; cable supportde bridge; cost assessent 1. Introduction Waterway crossings represent one of the ost iportant issues of the odern civil engineering, as new and longer crossings are deanded in several places all over the world. Cable Supported Bridges, such as suspension and cable stayed ones, nowadays are the ost suitable solution to cross large distances. However, in case of waterway crossings the presence of water can represent a circustance to take advantage of, instead of considering it just as an obstacle to get over; this is the ain idea which led to a new concept of cable supported bridge: the Suberged Floating Tunnel (SFT). * Corresponding author. Tel.: ; fax: E-aiddress: f@unina.it c 010 Elsevier Ltd. Open access under CC BY-NC-ND license. doi: /j.proeng

2 94 G. Martire et al. // Procedia Engineering (010) 4 (010) SFT fundaentally consists in a tubular structure floating at an iersion depth, fixed in position through anchorage systes ade up of groups of cables or tethers connected to the seabed (Fig. 1(a)). The tunnel is ade up of prefabricated odules assebled in situ; each odule is provided with at least one anchoring group. The SFT is peranently subjected to a positive upward residual buoyancy, providing the necessary pre-tensioning to the anchorage syste [1, ]. (a) (b) Fig. 1. (a) Longitudinal views of the Jintang Strait (China) SFT crossing [1]; (b) Qualitative cost trend curves for SFTs and SBs The SFT solution presents severadvantages with respect to traditional Cable Supported Bridges (CSB), the ost iportant being the reduced environental ipact, both fro the visuand air pollution points of view, perfect suitability for very large crossings and constant cost per unit length due to its odularity []. In particular, the latter aspects are particularly interesting when coparing the effectiveness of this innovative structural typology with classical solutions such as Suspension Bridges (SB). In fact, whereas the cost of a SFT increases linearly, the cost of a SB rises up way ore rapidly as the crossing length increases, tending to becoe infinite as the ain span length tends to a liit value for which the suspension cable syste is not even able to carry its own weight. Fig. 1(b) illustrates qualitatively the trends of cost previously described. Econoic aspects represent an issue of exceptional iportance when the final structural solution to be realized in a waterway crossing has to be selected. Therefore it is fundaental to properly estiate the building cost of each available solution. Thus siple procedures for the assessent of the cost of the crossing systes, such as he ones already developed by Gising [3] for Cable Supported Bridges, have to be provided. In this paper a siilar procedure for Suberged Floating Tunnels is presented, which thus allows for quickly evaluate the cost of such an innovative crossing solution and for coparing its cost-effectiveness with the one of other traditional solutions, such as CSBs. In this context, soe applications are also provided, highlighting the conditions where the Suberged Floating Tunnel proves to be ore econoically copetitive than traditional Cable Supported Bridges. Noenclature cb C sb C SFT f cbd lateral slope of cables of W-shaped anchorage groups of a SFT cable steel specific weight total cost of the cable syste and pylons of a suspension bridge total cost of the cable syste of a SFT cable steel design strength

3 G. Martire G. Martire et al. et al. / Procedia / Engineering 4 (010) g h pl h i k k a l unifor peranent load of a suspension bridge pylon height height of the i th cable group of a SFT suspension cable sag in the ain span suspension cable sag in the side spans ain span length p Q c Q ca Q h Q ha rb u cb u pl side spans length unifor live load of a suspension bridge steel quantity of the suspension cables in the ain span of a suspension bridge steel quantity of the suspension cables in the side spans of a suspension bridge steel quantity of the hangers in the ain span of a suspension bridge steel quantity of the hangers in the side spans of a suspension bridge residual buoyancy of a SFT unitary average cost of cable steel unitary average cost of pylon steel. Siple procedures for the assessent of crossing solution structural cost.1. Cost evaluation procedure In order to deterine the optial configuration of a cable supported bridge it is ainly necessary to estiate, beside of the structural perforance, the structural cost, this being ainly due to the cost of the supporting syste C s and of the deck/tunnel C d, as shown in Eq. (1): CTOT Css Cd (1) A procedure to assess the cost of the supporting syste of Suspension and Cable-stayed Bridges has already been developed by Gising [3]. The costs relative to foundations, shore connections and anchor blocks are not considered in this siplified procedure. In this paper, due to the lack of space, attention is focused on Suberged Floating Tunnels and on Suspension Bridges (SBs), this being the bridge typology holding the record of the longest span in the world... Suspension bridges(sb) cost evaluation procedure A syetrical three-span suspension bridge, subjected to unifor dead load (g) and live load (p), is considered. The cost of the supporting syste, ade up of the cable syste and pylons, can be evaluated through the procedure conceived by Gising [3]. The overall cable steel quantity is due to the quantities related to the ain cable and the

4 496 G. Martire et al. // Procedia Engineering (010) 4 (010) hangers in both the centrand side spans. Reference is ade to the geoetric quantities illustrated in Fig.. The iniu cable steel quantity, needed to carry the assued dead and live loads, for the ain cable and the hangers in the central span (Q c and Q h, respectively) and in the side spans (Q ca and Q ha, respectively) are given by: k+ba k k + j k + ja ka le hpl Mean level of lower hanger sockets k + b= distance fro the pylon top to the lower end of the ain cable part connected to the hangers (side span) k + j= distance fro the pylon top to the hanger sockets (ain span) k + j= distance fro the pylon top to the hanger sockets (side spans) le= length of the suspension cable not connected to the hangers (side spans) l Fig.. Geoetrical configuration of a syetrical three-span suspension bridge Q k 116 cb l 8 k c gpl 1 f cbd 3 l k cb k 8 l 116 l fcbd l () cb k Qh gp j l f 3 (3) Q ca cbd cb ( g p) l Q c k ka b a 8 k a 1 k b a l e la fcbd 8 k l la la 3 la la la cb 4 Qha gpk ka ja ba la (5) f 3 cbd (4) Assuing a constant stress equal to the design strength f pld throughout the pylon, Eq. (6) for the necessary aount of steel for each pylon Q pl can be derived [3]: Q pl pl gpl h 4 pl Q c k ka ba l f pld 4 e 1 (6) 8 k la Therefore the total cost C s of the supporting syste of a syetrical three span suspension bridge is given by: C Q Q Q Q u Q u (7) ss c ca h ha cb pl pl where u cb and u pre the unitary average prices for erected and protected suspension cable steend pylon steel. It is worth noticing that the axiu length of a suspension bridge, previously entioned in section 1, can be calculated as the root of the denoinator of Eq. (). This theoretical liit length gives rise to the verticasyptote depicted in Fig. 1(b). Assuing that the aterial quantity per unit length g related to the stiffening girder is constant, the cost of the

5 G. Martire G. Martire et al. et al. / Procedia / Engineering 4 (010) deck C d is equal to (u d is the unitary average prices for erected girder steel): C d glu d (8).3. Suberged Floating Tunnel (SFT) cost evaluation procedure The cable syste of a Suberged Floating Tunnel is ade up of single groups of cables, generally lying in the plane of the tunnel cross-section, located along the tunnexis with a fixed inter-axis. Therefore the total cost of the cable syste C cb,sft is siply given by the su of the costs of each cable group C cg,i. The SFT is considered to be subjected only to the peranent residual buoyancy rb, as the live loads reduce the cable tension forces. The geoetrical configuration of the cable syste is depicted in Fig. 3: the geoetricarrangeent of the cable groups is the W-shaped one, it being the ost effective one [4]. i i i i i i i i i hi hi e i L Fig. 3. Geoetrical configuration of a Suberged Floating Tunnel The cost of the SFT cable syste can be thus assessed through the siple following equation: C sft rb i h h sin sin u cb (9) i n cb i i i 1 f cbd e The tunnel cost C d can be evaluated analogously to (8), as the tunnel cost per unit length is constant. 3. Cost coparison between Suspension Bridges and SFT 3.1. The case studies of the Messina Strait and Akashi Strait The Akashi Strait, crossed by the suspension bridge featuring the largest ain span in the world (1991 ), and the Strait of Messina, where a suspension bridge having a ain span of 3300 is planned to be built, are selected as case studies, in order to perfor a cost coparison between the ost advanced Suspension Bridge (SB) designs up to now and SFT preliinary proposals, assued to be built in the sae locations. The SFT proposed for the Strait of Messina crossing features a steel shell-concrete circular cross-section whereas the one considered for the Akashi Strait have a steel-concrete rectangular cross-section, with lateral steel keels having a hydrodynaic shape (Fig. 4). The cable syste is ade up of W-shaped cable groups in both cases. The proposed SFT solutions were designed considering the stresses induced by the residual buoyancy and the hydrodynaic actions due to extree wave and currents foreseen in the considered location and assuing a flat seabed profile, having a depth equal to the average one (00 for the Strait of Messina, 80 for the Akashi Strait). The quantity of aterials involved in the construction of the SBs in the Strait of Messina and Akashi Strait can be found in literature [5]. It is thus possible to estiate the total cost of the considered SFTs and suspension bridges, once the cost per 3 of each aterial is defined, according to the indications given in [3]. The assued unitary costs,

6 698 G. Martire et al. // Procedia Engineering (010) 4 (010) being diensionless as they are divided by the unitary cost of steel S460, are given in Table ,0 External steel sheet (t=30 ) Three lanes otorways Steel keel Reinforced concrete Three lanes otorways External steel sheet (t=6 ) 4.9 3,4 5,0 11, 5,4 Reinforced concrete 1,6 5,0 1,6 6,0 10,1 Railways (a) 10,3 3,4 5,9 10,3 (b) Fig. 4. SFT cross-section proposals: (a) Strait of Messina; (b) Akashi Strait Table 1. Diensionless cost/ 3 of constructionaterials used for SBs and SFTs in the Messina and Akashi Strait Cost/3 [-] Concrete C5/30 Steel S355 Steel S40 Steel S460 Steel S690 steel (SB) steel (SFT) Ballast Fig. 5 shows the coparison of the total cost of the SB and SFT solutions for both case studies, also highlighting the different contributions in ters of structural eleents (cable syste, pylons and deck/tunnel) and aterials involved (r.c., different grades of steel, steel for cables, ballast). The presented costs are divided by the total cost of the SB solution, in both case studies, thus being diensionless. It can be iediately noted that the SFTs total cost is largely lower, it being 5.6% (Messina) and 36.3% (Akashi) of the cost of the relative SB, basically due to the huge reduction of the cost of the supporting syste. In fact SFT cable syste cost is the.7% (Akashi) and 8.6% (Messina) of the cost of the SB supporting syste (cable plus pylons). Furtherore, the cost of the tunnel structure is pretty uch the sae one of the SB deck for the Strait of Messina crossing, whereas it is the 6.9% of the cost of the SB deck for the Akashi Strait case % 90.0% 80.0% 70.0% 60.0% 50.0% 40.0% 30.0% 0.0% 10.0% 0.0% BridgeDeck/Tunnel S460 S40 S355 STRAITOFMESSINA R.C. S355 yste+pylons S460 (pyl) Totalcost SB SFT SB SFT SB SFT Total Ballast R.C. SteelS460(pyl) SteelS460 SteelS40 SteelS % 90.0% 80.0% 70.0% 60.0% 50.0% 40.0% 30.0% 0.0% 10.0% 0.0% AKASHISTRAIT Totalcost Total Ballast BridgeDeck/Tunnel R.C. SteelS460(pyl) yste+pylons SteelS690 SteelS355 Ballast S690 R.C. S355 S460 (pyl) SB SFT SB SFT SB SFT Fig. 5. Coparison of the cost of SFT and SB solutions Finally, it is worth underlining that the previous cost coparison is ade between copletely designed SBs and preliinary designs of SFTs. Therefore the proposed results ay slightly differ fro actuand definitive ones; however, due to the large scatter between the cost of SFTs and SBs, SFT would still prove to be largely cheaper.

7 G. Martire G. Martire et al. et al. / Procedia / Engineering 4 (010) Application of the cost evaluation procedure for suspension bridges and suberged floating tunnels A nuericapplication of the relationships previously introduced is carried out, with the purpose of providing usefubaci for the selection of the ost efficient structural solutions for strait crossings, given the crossing length and water depth In particular, a coparison between the structural costs of a SFT and a Suspension Bridge (SB) crossing waterways, with variable lengths L and a flat seabed profile, is considered. The SB is assued to have a ain span length l equal to the waterway length L, siilarly to the configuration of the Strait of Messina Bridge (Fig. 6). Two set of geoetric and echanical data of the SB are considered, naely C1 and C, the first one leading to the highest structural cost whereas the second one leads to the lowest one (Table ). L=l ka k h0 SB d f: free water allowance i D: tunnel diaeter SFT Fig. 6. Geoetrical configuration of the considered suspension bridge and suberged floating tunnel Table. Geoetricand echanical data D [] f [] h 0 [] l k cb [MN/ 3 ] pl [MN/ 3 ] f cb,d [MN/ ] f pl,d [MN/ ] C L 0.5l 0.08l C L 0.5l 0.1l The design strength of the pylon steel f pld is reduced to 60-80% of the aterial strength to take into account the stress induced in the pylons by the out of plane wind actions. The SB peranent loads g are assued to be equal to 0.4 MN/, which is a coon value for a SB featuring a slender aerodynaic deck. For the live loads p, a value of 0.16 MN/, corresponding to a otorway plus railway crossing, is considered. The cost of the tunnel structure for SFTs is set equal to the cost of the SB deck, as it was found previously for the Messina Strait case study. The crossing length L, the seabed depth d and the residual buoyancy rb acting peranently on the SFT are considered as variables, in order to assess their influence and iportance on the cost of the three crossing typologies considered. The unitary cost u pl of the steel used for the pylons is set equal to 1 and all the other unitary costs are defined proportionally to it. Table 3 illustrates the assued values for the unitary costs. Figs. 7 and 8 show the coparison of the cost trend curves of SFTs and SBs as the crossing length and the seabed depth vary. The curves are diensionless, as the actual costs are divided by a reference cost, assued equal to the one of the Messina Strait suspension bridge. In particular, the curves in Fig. 7(a) are referred to a variable crossing length, assuing d=00 and rb k =0.7 MN/, while curves in Fig. 7(b) consider a variable seabed depth and a fixed length L equal to 3000 ; clearly SFTs feature a lower liit for the seabed depth (d in,sft ) allowing for their realization, it corresponding to the condition where the SFT cables would feature a null length. It can be noticed that the SFT solution is noticeably cheaper than the SB one, particularly for large values of the crossing length; as a atter of fact, Suspension Bridges are econoically copetitive with SFTs only for crossing lengths being lower

8 8300 G. Martire et al. // Procedia Engineering (010) 4 (010) than 500. Furtherore, also for very large values of the seabed depth the SFT is largely less expensive than suspension bridges. It is worth noticing that the cost of SBs does not increase as the seabed depth increases, the pylon height not being influenced by it for the assued geoetrical configuration (Fig. 6). The coparison between SFTs and SBs cost curves becoes less heavy for the SBs if larger values of rb k and d, and lower values of L, are considered, as shown in Fig. 8(a) (rb k =1.5 MN/, d=800 ) and Fig. 8(b) (rb k =1.5 MN/, L=500 ). As a atter of fact, the SB solution turns out to be econoically copetitive with the SFT one even for larger crossing lengths, up to 000 and for shorter crossings (L=500 ) a iniu seabed depth approxiately equal to 300 is sufficient to let the SB be cheaper than the SFT. Table 3. Diensionless values assued for the aterials unitary costs u pl u cb,sb u cb,sft u d [-] [-] [-] [-] C C Cost [-] SB SFT SB1 Theoretical liit length 1 SB SFT rb k=0.7 MN/; d=00 SFT1 Crossing length [] Cost [-] din,sft SB SFT SB SB1 rb k=0,7 MN/; L=3000 SFT1 (a) (b) SFT Seabed depth [] Fig. 7. SFT and SB cost curves as a function of the: (a) Crossing length (rb k=0,7 MN/; d= 00 ); (b) Seabed depth (rb k=0,7 MN/; L= 3000 ) Cost [-] SB SFT rb k=1.5 MN/; d=600 SB1 Theoretical liit length 1 SFT1 SB SFT Crossing length [] (a) (b) Cost [-] din,sft SFT1 SFT SB1 SB SB rb k=1.5 MN/; L=500 Seabed depth [] Fig. 8. SFT and SB cost curves as a function of the: (a) Crossing length (rb k=1,5 MN/; d= 800 ); (b) Seabed depth (rb k=1,5 MN/; L= 500 ) 4. Conclusive rearks The siplified procedure presented in this paper, for the assessent of the structural cost of a Cable Supported Bridge, siilar to the one proposed by Gising [3] but here including the innovative typology of Suberged Floating Tunnels, allows to quickly copare the overall cost of the superstructure, therefore constituting an iportant tool to help aking decisions during the early stage of a waterway crossing planning. The cost assessent procedure developed is applied and a coparison between the cost trend curves relative to

9 G. Martire G. Martire et al. et al. / Procedia / Engineering 4 (010) SFTs and SBs is ade, considering different values of the geoetricand echanical paraeters governing the proble. The obtained curves confir that the SFT solution is largely cheaper than traditional SB one, particularly when large distances have to be surpassed. The seabed depth and the residual buoyancy acting on the SFT are the other paraeters that ainly influence the cost coparison between these structural typologies: in fact, as the value of these paraeters increases, SBs becoe ore econoically copetitive with SFTs. However, the SFT solution still proves to be considerably ore econoically effective than the classic SB one. Two noticeable case studies are considered to copare the cost of potential SFT solutions and of actual SB designs: the Messina and Akashi Straits. The obtained results show that in both cases the proposed SFT solutions are considerably less expensive than the corresponding suspension bridges, as the SFTs cost approxiately 1/4 (Messina Strait) and 1/3 (Akashi Strait) of the SBs. In particular, it is the enorous difference in the supporting syste cost that gives rise to such a large scatter between the SFT and SB overall costs. These rearks, together with the other advantages assured by SFTs [], confir that such a revolutionary crossing solution will represent the future in the field of waterway crossings. References [1] Faggiano B, Landolfo R, Mazzolani FM. Design and odelling aspects concerning the suberged floating tunnels: an application to the Messina Strait crossing. Proceedings of the 3rd International Conference on Strait Crossings, Bergen, Norway, 001. [] Faggiano B, Landolfo R, Mazzolani FM. The SFT: an innovative solution for waterway strait crossings. Proceedings of the IABSE Syposiu Structures and Extree Events, Lisbon, Portugal, 005. [3] Gising, Niels J. Cable Supported Bridges: Concept and Design. nd ed. John Wiley & Sons, [4] Martire G, Esposto M, Faggiano B, Mazzolani FM. Perforance evaluation of anchorage cable systes for Suberged Floating Tunnels. Proceedings of the Strait Crossing Syposiu, Trondhei, Norway, 009. [5] Jansen L. Long Span Suspension Brisges for Strait Crossings. Proceedings of the Strait Crossing Syposiu, Trondhei, Norway, 009.