Jia-Bao Yan

Anzhen Chen

Tao Wang
email: wangtao@iem.ac.cn

Compressive behaviours of steel-UHPC-steel sandwich composite walls using novel EC connectors

Keywords: UHPC wall, composite wall, sandwich structures, connectors, compression tests, steel-concrete composite

This paper reports the developments of steel-UHPC-steel sandwich composite walls with novel EC connectors (SUSSCW-ECs). The ECs in SUSSCW-ECs directly linked the two faceplates that enhance the composite action of the composite wall. Axial compression tests on 11 SUSSCW-ECs to investigate their compressive behaviours, and the selected key influencing parameters in the testing program included layout of C-channels, steel faceplate thickness, horizontal and vertical spacing of ECs, and strength of core. Under compression, the SUSSCW-ECs exhibited an elastic-plastic-recession three-stage working mechanism. Tests results show that major local buckling of faceplates ended the elastic stage, and UHPC crushing occurred at the peak compression resistance of SUSSCW-ECs. Installing the ECs with horizontal webs exhibited larger axial compression resistance of SUSSCW-ECs than those with V-web ECs. Increasing the steel faceplate thickness improved compressive behaviours of SUSSCW-ECs. Increasing horizontal and vertical spacing of EC connectors resulted in reduced ultimate compressive resistance of SUSSCW-ECs. Using UHPC improves both peak resistance and elastic stiffness of sandwich composite walls. Finally, this paper developed theoretical models to predict ultimate compressive resistance of SUSSCW-ECs, and validations against 11 test results showed their reasonable estimations.

Introduction

Steel-concrete-steel (SCS) sandwich composite walls (SCSSCWs) becomes popular recently in high performance infrastructures, e.g., shielding walls in new generation nuclear power plant [START_REF] Varma | Steel-plate composite (SC) walls for safety related nuclear facilities: Design for in-plane forces and out-of-plane moments[END_REF], immersed tunnels in Hongkong-Zhuhai-Macau Bridge [START_REF] Lin | The deployable element, a new closure joint construction method for immersed tunnel[END_REF],

shear walls [START_REF] Ji | Cyclic in-plane shear behavior of double-skin composite walls in high-rise buildings[END_REF][START_REF] Nie | Experimental study on seismic behaviour of highstrength concrete filled double-steel-plate composite walls[END_REF][START_REF] Yan | Seismic behaviour of double skin composite shear walls with overlapped headed studs[END_REF][START_REF] Yan | Seismic behaviours of SCS sandwich shear walls using J-hook connectors[END_REF], and protective structures (e.g., ice-walls for offshore platforms) [START_REF] Yan | Design and behavior of steel-concrete-steel sandwich plates subject to concentrated loads[END_REF][START_REF] Yan | Punching shear resistance of steel-concrete-steel sandwich composite shell structure[END_REF]. Typical components in SCSSCWs include two external steel faceplates, a sandwich concrete core, and faceplate-core bonding measures. The extensive advantages of SCSSCWs over conventional RC structures include savings on fabrication costing through reducing mould and manpower for concrete casting, reducing fabrication details of reinforcements, improved efficiency for structure construction, and maintaining structural integrity under impact and blast loads [START_REF] Remennikov | The development and ballistic performance of protective steelconcrete composite barriers against hypervelocity impacts by explosively formed projectiles[END_REF][START_REF] Liew | Impact Tests on Steel-Concrete-Steel Sandwich Beams with Lightweight Concrete Core[END_REF][START_REF] Bruhl | Design of composite SC walls to prevent perforation from missile impact[END_REF].

With such advantages, the applications of SCSSCWs are extended as oil containers, shield tunnels, bridge decks, offshore decks, and protective structures.

Previous extensive studies have been made to improve the structural performances of SCS sandwich structures (SCSSSs) through improving the steel-concrete interfacial bonding and core materials. Different steel-concrete interfacial bonding have been proposed in SCSSSs. Epoxy type of adhesive materials have been adopted as the bonding measures for SCSSSs by Solomon et al. [START_REF] Solomon | Flexural tests of steel-concrete-steel sandwiches[END_REF] and Marshall et al. [START_REF] Marshall | Bond Enhancement for Sandwich Shell Ice Wall[END_REF].

However, adhesive materials offered insufficient transverse shear resistance for the cross section of SCSSSs. Malek et al. [START_REF] Malek | Steel-concrete sandwich members without shear reinforcement[END_REF] chose the angle connectors as the bonding methods for SCSSSs, and their test results showed that the shallowly-embedded angles all failed in breakout mode and the structure could not achieve full flexural failure. Headed studs with the tying bars or overlapped headed studs have been used in the SCSSSs as shown in Fig. 1(a), especially the SCSSCWs developed by Zhang et al. [START_REF] Zhang | Effect of shear connectors on local buckling and composite action insteel concrete composite walls[END_REF], Choi et al. [START_REF] Choi | Strength and behavior of steel plate-concrete wall structures using ordinary and eco-oriented cement concrete under axial compression[END_REF], and Yan et al. [START_REF] Yan | Compressive behaviour of normal weight concrete confined by the steel faceplates in SCS sandwich wall[END_REF]. However, the steel-concrete bonding by both headed studs and angles all depend on their anchorages in the concrete core.

These indirect bonding measures become worse on maintaining the structural integrity as they were subjected to impact or blast loads [START_REF] Liew | Impact Tests on Steel-Concrete-Steel Sandwich Beams with Lightweight Concrete Core[END_REF]. Through bolts [see Fig. 1(b)] were used as the steel-concrete bonding in SCSSCWs by Ji et al. [START_REF] Ji | Seismic behavior of steel tube-double steel plate-concrete composite walls: Experimental tests[END_REF]. However, the openings in the two faceplates for bolts compromised their integrity and brought fabrication difficulties. Friction-welded bars [see Fig. 1(c)] offered strong direct connections on the two external faceplates for SCSSCWs. Moreover, these through friction-welded bars provided strong cross-sectional shear resistance. However, the friction-welding equipment for SCSSCWs limits their depth within 0.2~0.8 m, which maybe not suitable and economic enough for the shear walls. Continuous double J-hook connectors [START_REF] Yan | Seismic behaviours of SCS sandwich shear walls using J-hook connectors[END_REF][START_REF] Yan | Seismic behaviours of novel SCS sandwich shear walls with CFST boundary columns[END_REF] make up this limitation on the depth of SCSSCWs [see Fig.

1(d).

Previous tests [START_REF] Yan | Seismic behaviours of SCS sandwich shear walls using J-hook connectors[END_REF][START_REF] Yan | Seismic behaviours of novel SCS sandwich shear walls with CFST boundary columns[END_REF] confirm the excellent seismic performances of SCSSCWs with J-hooks. However, this type of connectors tends to result in a high risk of interlocking problems for SCSSCWs with a great amount pairs of J-hooks. Thus, to develop new connectors for SCSSCWs becomes necessary, which requires balancing the low costing, easy fabrication, and strong steel-concrete bonding. From this point of view, the authors developed the SCSSCWs with enhanced C-channel connectors (ECs) [see Fig. 1(e)~(f)]. This new ECs offers relative high steel-concrete interfacial bonding [START_REF] Yan | Shear behaviour of novel enhanced C-channel connectors in steel-concrete-steel sandwich composite structures[END_REF]. Thus, to study the axial compressive behaviours of SCSSCW with ECs (SCSSCW-ECs) becomes necessary.

Including the innovations on the steel-concrete bonding, different core materials were used in SCSSCWs to improve their structural performances. Normal weight concrete (NWC) was extensively adopted as the core materials for SCSSCWs [START_REF] Varma | Steel-plate composite (SC) walls for safety related nuclear facilities: Design for in-plane forces and out-of-plane moments[END_REF][START_REF] Lin | The deployable element, a new closure joint construction method for immersed tunnel[END_REF][START_REF] Ji | Cyclic in-plane shear behavior of double-skin composite walls in high-rise buildings[END_REF][START_REF] Nie | Experimental study on seismic behaviour of highstrength concrete filled double-steel-plate composite walls[END_REF][START_REF] Yan | Seismic behaviour of double skin composite shear walls with overlapped headed studs[END_REF][START_REF] Yan | Seismic behaviours of SCS sandwich shear walls using J-hook connectors[END_REF][START_REF] Yan | Design and behavior of steel-concrete-steel sandwich plates subject to concentrated loads[END_REF][START_REF] Yan | Punching shear resistance of steel-concrete-steel sandwich composite shell structure[END_REF][START_REF] Remennikov | The development and ballistic performance of protective steelconcrete composite barriers against hypervelocity impacts by explosively formed projectiles[END_REF][START_REF] Liew | Impact Tests on Steel-Concrete-Steel Sandwich Beams with Lightweight Concrete Core[END_REF][START_REF] Bruhl | Design of composite SC walls to prevent perforation from missile impact[END_REF][START_REF] Solomon | Flexural tests of steel-concrete-steel sandwiches[END_REF]. Eltayeb et al. [START_REF] Eltayeb | Structural performance of composite panels made of profiled steel skins and foam rubberised concrete under axial compressive loads[END_REF] used the lightweight foam concrete in SCSSCWs. Ultra-light weight concrete was used in the SCSSCW for offshore constructions [START_REF] Yan | Design and behavior of steel-concrete-steel sandwich plates subject to concentrated loads[END_REF][START_REF] Yan | Punching shear behavior of steel-concrete-steel sandwich composite plate under patch loads[END_REF]. More recently, ultra-high performance concrete (UHPC) was used in the high performance steel-concrete composite structures. Wang et al. [START_REF] Wang | Experimental and numerical investigation of the interfacial properties of non-steam-cured UHPC-steel composite beams[END_REF], Chen et al. [START_REF] Chen | Experimental study on fatigue performance of UHPC-orthotropic steel composite deck[END_REF], and Cao and Shao [START_REF] Cao | Finite element analysis of headed studs embedded in thin UHPC[END_REF] developed steel-UHPC composite slabs. Zhu et al. [START_REF] Zhu | Shear behaviour of steel-UHPC composite beams in waffle bridge deck[END_REF] developed the steel-UHPC composite bridge deck. Xiong et al. [START_REF] Xiong | Flexural performance of concrete filled tubes with high tensile steel and ultra-high strength concrete[END_REF] applied the UHPC in composite columns. More recently, Lin et al. [START_REF] Lin | Ultimate strength behaviour of S-UHPC-S and SCS sandwich beams under shear loads[END_REF] applied the UHPC in SCSSSs to improve their ultimate strength behaviours. This paper aims to further improve the ultimate axial compressive behaviour of SCSSCWs through using UHPC. Previous studies have experimentally and numerically studied flexural behaviours of SCSSSCWs [START_REF] Yan | Numerical studies on steel-UHPC-steel sandwich beams with novel enhanced C-channels[END_REF][START_REF] Yan | Steel-UHPC-steel sandwich composite beams with novel enhanced C-channel connectors: tests and analysis[END_REF].

This paper is a continuous study on axial compression behaviours of steel-UHPC-steel sandwich composite walls with ECs (SUSSCW-ECs).

This paper firstly reported a testing program with 11 SUSSCW-ECs and one SCSSCW-ECs under axial compression. The studied parameters were layout of C-channels, faceplate thickness (ts), vertical spacing of ECs (Sva), horizontal spacing of ECs (Sha), and concrete core strength (fc). Through these 12 axial compression tests, the axial compressive behaviours as well as their influencing key parameters were revealed and discussed. Including these tests and analysis, this paper also developed analytical models and modified code equations for ultimate compressive resistance of SUSSCW-ECs. Followed validations were performed to check these models.

Compressive tests on SUSSW-ECs

Details of SUSSCW-ECs

The typical fabrication procedures for SUSSCW-ECs are as shown in Fig. 2, which comprises (1) preparing the two faceplates with the reserved bolt holes and welded C-channels, (2) forming the steel skeleton through assembling the two faceplates, and

(3) casting of the UHPC. Fig. 3 shows the site casting and fabrication of the SUSSCW-ECs. It can be found that the formworks and manpower for site casting are greatly saved. This experimental study prepared 12 specimens that include 11 SUSSCW-ECs with UHPC and specimen with NWC. The selected key influencing parameters on ultimate compressive behaviour of SUSSCW-ECs were layout of C-channels, faceplate thickness (ts), vertical spacing of ECs (Sva), horizontal spacing of ECs (Sha), and strength and type of core (fc). Three specimens UDW1-1, UDW1-2, and UDW1-3, were designed with controlling parameters of ts =3.00 mm, Sva=90 mm, Sha=115 mm, and UHPC core. Since the C-channels can be installed with different layout in SUSSCWs, C-channels in UDW1 and UDW2 were installed with their web horizontally and vertically, i.e., H-web and V-web, respectively. To study the influence of faceplate thickness, SUSSCW-ECs UDW1, UDW3, and UDW4 were fabricated with faceplates in different ts values of 3.0, 4.8, and 5.8 mm, respectively.

To study the effect of Sva on axial compression behaviour of SUSSCW-ECs, UDW1, UDW5, UDW6, and UDW7 were prepared in different Sva values of 90, 130, 170, and 240 mm, respectively. To investigate the effect of Sha on axial compression behaviour of SUSSCW-ECs, UDW1, UDW8 and UDW9 were prepared in different Sha values of 115, 175, and 265 mm, respectively. H-web and V-web, respectively. It can be observed that all the SUSSCW-ECs are with the same width, height, and depth in concrete of 600, 660, and 120 mm, respectively. The rest geometric details, e.g., horizontal and vertical spacing of ECs, faceplate thickness, and type of the core materials are given in Table 1.

M8.8 high strength bolts (φ=14mm) were used in SUSSCWs, and their yield strength and elastic modulus are 640 MPa and 202 GPa, respectively. Q235 mild steel plates with nominal thickness of 3.0, 4.5, and 6.0 mm were used in the fabrication of SUSSCW-ECs. Q235B C-channels were also adopted to fabricate the ECs in SUSSCWs, and their geometries are shown in Fig. 2. Standard tensile tests on coupons cut from these plates and C-channels following Chines code GB/T228.1 [START_REF]Metallic Materials: Tensile testing[END_REF] to obtain their mechanical properties. Their tensile stress-strain curves are plotted in Fig. 5. More details of the mechanical properties, e.g., elastic modulus, yield and ultimate strengths, are tabulated in Table 1.

UHPC was selected as the core materials for UDW1~11, and its mix proportions are 1.000: 0.225: 0.250: 1.100: 0.300: 0.019 for cement: water: silica fume: Quartz sand: Quartz powder: superplasticizer, respectively. To improve mechanical properties of UHPC, three percent (in volume) φ0.12 mm and 8 mm long steel fibres with smooth surface and yield strength over 1600 MPa were used in the mix proportions of UHPC. Fig. 6 shows the stress-strain curves of UHPC under both uniaxial tension and compression. Specimen UDW10 adopts C50 NWC, and its compressive strength equals to 54.4 MPa.

Setup and measurements

Fig. 6 depicts the compression-test setup for SUSSCW-ECs. All the axial compression tests were performed by a 1500-ton SHIDAI SHIJIN multi-functional testing machine in Concrete Structural Laboratory, Tianjin University. To measure the axial shortening of SUSSCW-ECs, 12 LVDTs, with six LVDTs on each end plate, were installed as shown in Fig. 6. To measure the strains of steel faceplates and assist the judgement of their local buckling, linear strain gauges were also installed on the two external faceplates as shown in Fig. 7. The reaction forces during the tests were automatically measured by the testing machine. These measurements of shortening, strains, and reaction forces were recorded by a data logger.

Test results

Failure modes

Fig. 8 shows the observed failure modes of representative specimens with investigated parameters. It shows that local buckling of faceplates and crushing/splitting of UHPC core were the major failure modes occurred to the SUSSCW-ECs. 3) occurred at the loading level of 4100 kN, which is about 40% ultimate load carrying capacity (Pu). Furthermore, as the loading proceeded, separations between the faceplates and concrete core were observed at positions (1), (2), and (5) as the reaction force (P) equals to 5040 kN. After that as P increases to 5870 kN, the local buckling of faceplate at position (5) becomes more severe. As P increases to 8550 kN, the whole horizontal strip of faceplate between two neighbouring rows of ECs at position (5) locally buckled. During this process, the local buckling of faceplates at positions ( 3) and ( 4) become more severe. At the loading level of 10650 kN, local buckling occurred to the faceplate at position [START_REF] Yan | Seismic behaviours of SCS sandwich shear walls using J-hook connectors[END_REF].

Finally, as the P achieved its ultimate value of 11207 kN, crushing occurred to the UHPC core and cracking can be observed in the core. 5) as the P value achieved 290, 880, 1960, 2640, and 3280 kN, respectively. As P increased to 4650 kN, separation between the faceplate and concrete core was observed at position [START_REF] Yan | Seismic behaviours of SCS sandwich shear walls using J-hook connectors[END_REF]. As P increased to 5530 and 7740 kN, local buckling occurred to the faceplates at position ( 6) and [START_REF] Yan | Design and behavior of steel-concrete-steel sandwich plates subject to concentrated loads[END_REF]. Finally, the P achieved its ultimate value of 8297 kN and crushing occurred to the UHPC core.

Load-shortening curves

Fig. 9(a)~(e) depicts the load-shortening (P-Δ) curves of SUSSCW-ECs. Fig. 9(f) shows the general P-Δ curves of SUSSCW-ECs. These figures show that the P-Δ behaviours of SUSSCW-ECs under axial compression can be divided into three working stages, i.e., SI (elastic working stage), SII (nonlinear working stage), and SIII (recession stage). However, the general P-Δ behaviours of SUSSCW-ECs with UHPC are different from those with NWC especially at their third working stages. The SUSSCW-ECs with UHPC exhibit an immediate drop after the peak load capacity.

Meanwhile, the SUSSCW-ECs with NWC exhibited a much flatter recession branch in the P-Δ curves. This is due to the more brittle failure of the UHPC under compression than the NWC. The SI starts from the loading and ends at the minor concrete crushing and major local buckling of faceplates, i.e., curves OA' or OA in Fig. 9(f). During this stage, slight minor local buckling occurred to the faceplates at several point locations (see section 3.1), but these slight minor local buckling of faceplates show very limited influence on the initial stiffness (Ke) of the SUSSCW-ECs. This is because that the contribution of the faceplates on Ke in SUSSCW-ECs is smaller than that of UHPC, and these buckling points of faceplates do not affect Ke of the cross section. During the second stage SII, the P-Δ behaviours of SUSSCW-ECs with UHPC exhibit nonlinear behaviour due to the developments of major local buckling in the two faceplates. It can be also observed from Fig. 9(e)~(f) that these nonlinear behaviours in SUSSCW-ECs with UHPC are much less significant than that in composite walls with NWC. Moreover, the specimens with thicker faceplates and larger spacing of ECs exhibit faster reductions in their stiffness of P-Δ curves before achieving their peak compressive resistances. This is because that the major local buckling and nonlinearities of UHPC core govern the nonlinear P-Δ behaviours in SII stage. Since the UHPC exhibits less significant nonlinear compressive behaviours (see Fig. 6), the nonlinear behaviour of P-Δ curves in SII is governed by those parameters influencing the local buckling of faceplates and compression nonlinearities of core materials in the composite walls. Finally, at the end point of SII [i.e., B or B' point in Fig. 9(f)], crushing of the UHPC and more severe local buckling occurred to the SUSSCW-ECs. After that, in the recession branch of P-Δ curves [i.e., curves of BC or B'C' in stage SIII as shown in Fig. 9(f)], all the SUSSCW-ECs with UHPC exhibit fast drop in their load carrying capacities due to the brittle compressive behaviour of UHPC after crushing. However, specimen UDW10 with NWC exhibits a more flat recession branch than those with UHPC due to the more ductile post-peak recession compressive behaviour of NWC than UHPC.

During this recession stage SIII, major local buckling occurred to more positions of the two external faceplates. Meanwhile, splitting and crushing of the UHPC were observed in the core. Finally, all the specimens failed in crushing of UHPC and faceplate local buckling.

Load versus strain of faceplate curves

Fig. 10(a)~(i) depicts the load versus strain of faceplates curves for representative specimens. In these curves, the yielding strain lines were also provided as the judgements for the failure modes of specimens. These figures show that the failure modes of faceplates greatly depend on the layout of ECs on faceplates and strength of core materials. Moreover, the load-strain curves also exhibit a three working-stage mechanism as those P-Δ curves. In the SI, all the load-strain curves exhibit the linear relationship between the reaction force and compressive strain. However, at the end of SI stage, the steel faceplates start to yield (e.g., UDW1~4) or elastic buckling occurred to them (for specimens with large spacing of ECs, e.g., UDW7~8). Thus, in the followed stage SII, major local buckling occurred to faceplates that can be found in the obvious overturning (or stop increasing) changes in the load-strain curves of specimens UDW1~4, which implies plastic buckling occurred to these specimens since their strains are beyond yielding points; meanwhile, for specimens with larger spacing of ECs, e.g., UDW7-8, elastic major buckling occurred to the faceplates.

After the peak load, all the steel faceplates exhibited the relaxation behaviour without taking any loads. However, the steel faceplates in SUSSCW-ECs with NWC (i.e., specimen UDW10) exhibited different failure sequences of faceplates from those with UHPC core. The major local buckling occurs to faceplates as the SUSSCW achieved its peak load, and the strains in the faceplates at ultimate load also exceed the yielding strains that implies the plastic buckling of faceplates. This is because that the strain at ultimate compressive strength of the UHPC (about 3500 με) is much larger than that of the C50 NWC (about 2500 με). Thus, from the optimization view, high strength steel with yielding stress less than 700 MPa may be considered in the further design of such type of composite shear wall.

Elastic stiffness (Ke), ultimate compressive resistances (Pu), and ductility index (DI)

The Ke, Pu, and DI ratios of SCSSCW-ECs can be found in the P-Δ curves of SUSSCW-ECs. The Ke equals to the ratio of 30% ultimate load (P0.3) to its shortening (Δ0.3). Meanwhile, the DI ratio is defined as the ratio of the shortening at 85% Pu (Δ85%) in the recession branch of P-Δ curves of SUSSCW-ECs to the shortening at peak load Δu. Thus, these determined Ke, Pu, and DI ratios of SCSSCW-ECs are tabulated in Table 1.

Discussions

With these reported failure mode, P-Δ and load-strain curves of SUSSCW-ECs, and strength and ductility indexes, the effects of studied parameters were discussed. stronger axial compression behaviours than that with V-web ECs. The ultimate compression resistance (Pu) of SUSSCW-ECs with H-web ECs is 17% higher than that with V-web ECs. This is because the ECs with their webs vertically-installed in the UHPC core result in larger stress concentration than those ECs with their webs horizontally installed as the SUSSCW-ECs subjected to vertical compression as shown in Fig. 11. However, the differences of Ke, Δu, and DI ratio are only 4%, 1%, and 5%, respectively. This is because the layout of ECs only affects the degree of stress concentration in front toe of the C-channels, and the initial stiffness of the SUSSCW is not influenced that is only determined by the moment inertia of cross section. Since DI ratio of the SUSSCW-ECs is more governed by the brittleness of UHPC under compression. This explains the limited influence of layout of ECs on ductility of SUSSCW-ECs. value of SUSSCW-ECs is increased by 8% (10%) and 12% (18%), respectively; meanwhile, the DI ratios of SUSSCW-ECs are both reduced by 6%, respectively. This is due to that the increasing ts value improves the equivalent cross-sectional moment inertia, which results in the increased Ke value. Moreover, with the increase of ts value from 3.0 to 4.8 and 5.8 mm , the cross-sectional area of faceplates was increased by 59% and 93%, but their slenderness ratios (λ=Sv/ts) were reduced from 30 to 18.9 and 15.5, respectively. These increased cross sectional area and reduced slenderness ratio of faceplates increased their compression resistance. However, due to the UHPC was used in the SUSSCW-ECs, the two faceplates only contributed a small portion to the ultimate compression resistance of SUSSCW-ECs. Thus, the increasing ts value has limited influence on Pu and Ke of SUSSCW-ECs. In addition, this also explains the quite limited influences on DI ratio of SUSSCW-ECs with the increase of ts from 3.0 to 5.8 mm. the Pu value is decreased by 12%, 15%, and 11%, respectively; the Ke value is slightly reduced by -2%, -1%, and 4%, respectively; and the DI ratio is reduced by -3%, 6%, and 8%, respectively. The increasing value of Sv from 90 to 130, 170, and 240 mm actually increases faceplate slenderness ratio (λ=Sv/ts) from 30.0 to 42.9, 58.8, and 79.8, respectively, which results in the enlarged buckling length of the faceplates. Fig. 12 plots the buckling length of faceplates in specimens with Sv values of 90 to 130, 170, and 240 mm that equals to about 95, 129, 170, and 210 mm, respectively, which are close to their Sv values. Thus, these increased buckling length results in much lower buckling strain, which changes the plastic buckling to elastic buckling of faceplates as shown in Fig. 10. All these reduced the compression buckling resistances of faceplates, and thus the axial compression capacity of SUSSCB-ECs.

Effect of layout of ECs

Effect of faceplate thickness (ts)

Effect of vertical spacing of ECs (Sv)

Since the UHPC was used in the, the compression resistance of faceplates only contributes a small portion to total compression resistance of SUSSCW-ECs. This explains the limited reductions (less than 15%) through increasing Sv value from 90 to 210 mm. Since the Ke value of SUSSCW-ECs is determined at the loading level of 0.3Pu, the local buckling does not occur, which is the reason to limited influences of increasing Sv on Ke value of SUSSCW-ECs. Since the larger Sv value will produce earlier local buckling of the faceplates, this reduces the ductility of SUSSCW-ECs.

However, the local buckling of faceplates only partially influences the axial compression behaviour of SUSSCW-ECs, which is more governed by the compressive behaviour of UHPC. Thus, the limited influence on DI ratio (less than 8%) due to increasing Sv value from 90 to 210 mm can be explained. In addition, the influence of Sh value on ultimate compressive resistance of SUSSCWs needs to be considered, which is commonly ignored in most of the previous studies. This is because that the fc and Ec of UHPC is 2.2 and 2.0 times of those values of C50 NWC. However, as shown in Fig. 10(j), the DI ratio of SCSSCW-ECs is reduced by 12% due to the more brittle compressive behaviour of UHPC. Moreover, the local buckling of faceplate and crushing of NWC core occur almost at the same time as the specimen achieves its peak value as shown in Fig. 10(i). Thus, high strength steel with higher plastic buckling strains may be considered for UHPC as it is used for the sandwich composite walls.

Effect of horizontal spacing of ECs (Sh)

Effect of different type of core

Prediction equations on ultimate compressive resistance of SUSSCW-ECs

Theoretical models

Considering all the faceplate local buckling occurred prior to the crushing of UHPC, the confinement of the faceplates on the compression resistance of UHPC core is ignored. Thus, the ultimate compression resistance of SUSSCW-ECs (NT) consists of compressive resistance of UHPC core (Nc) and compressive resistance of two faceplates (Ns);

T c s N N N = + (1)
The compressive resistance of UHPC core (Nc) can be calculated as the following;

c c c N f A = (2)
where, fc is the UHPC compressive strength; Ac is the cross-sectional area of UHPC core.

As pointed out in section 3.5.4, the faceplates were divided into three different types of vertical strip (namely Strip A~C), which have varying buckling lengths as shown in

2 W are widths for strip A~C in Fig. 13, and Ws=2Lc for strip B; x, y, and z are quantities of strips A~C in a faceplate, respectively; K is the constant, and equals to 0.825; Sh and Sv are horizontal and vertical spacing of ECs, respectively; Es is the elastic modulus; fys denotes yield strength; i η is the coefficient for buckling length of faceplate strip.

y x z s s i s i s s j s j s s j s k s i j k N W t W t W t σ σ σ = = =   = + +     ∑ ∑ ∑ (3) ( ) 2 , 2 2 , min , 12 / s
s i ys i v i s E f K S t π σ η     =     (4) where,
Based on the experimental observations, the i η for different strips of faceplates can be determined as the following; 1.0, for strip, and strip in SUSSCW with side faceplate 1.0, 2 ,for strip, and strip in SUSSCW with free edge / 1.0, 2 , for strip, and strip in SUSSCW with free edge L is laterally unbraced length of the member.

i h v h v h v B A S S C A S S S S C A η   = ≤   - >  (5)

Code provisions

In Eurocode 4 [START_REF]Eurocode 4. Design of Composite Steel and Concrete Structures[END_REF], the ultimate compressive resistance of SCSSCW-ECs, NE, follows the below specifications;

, E pl Rd N N χ = (8)
, = 0.85

pl Rd y s c c N f A f A + (9) 2 2 1 1.0 λ χ Φ Φ - = ≤ + ( 10
) 2 0.5[1 ( 0.2) ] α λ λ Φ = + - + (11)
λ λ λ λ ϕ λ λ λ λ λ  - ≤   + + =    - + + - ≤  (15) 0 0 = / / y s s c c s s c c y f l E I I E E A A f f λ π + + (16)
where, 0 λ denotes generalized slenderness ratio. 12 and 0.12, respectively. However, the code equations in AISC, Eurocode, and CECS were mainly developed for composite wall with NWC but not for composite walls with UHPC, which may be the prediction discrepancies of these prediction equations.

Validations

Thus, the developed theoretical models are recommended to predict the ultimate compressive resistance of SUSSCW-ECs.

Conclusions

This paper firstly developed new type of sandwich composite walls using UHPC and enhanced C-channels. Followed, this paper performed compression tests on new SUSSCW-ECs to investigate their ultimate compression behaviours. Through the tests, the axial compression behaviours of SUSSCW-ECs and their key influencing parameters were investigated and revealed. Analytical models were also developed to predict the ultimate compression resistance of this new type of SUSSCW-ECs. From these studies, following conclusions can be drawn;

(

Figures 4 (

 4 Figures 4 (a)~(b) and (c)~(d) show the geometric details of SUSSCW-ECs with

Fig. 8 (

 8 Fig. 8(a) plots the failure mode of specimen UDW1-1. During the tests, minor local

Fig. 8 (

 8 Fig. 8(b) depicts the failure modes of specimen UDW2. As the testing proceeded,

Fig. 8 (

 8 Fig. 8(c) depicts the failure modes of specimen UDW4. As P increased to 2000 kN,

Fig. 8 (

 8 Fig. 8(e) depicts the failure modes of specimen UDW9. As P increased to 1130 kN,

Fig. 9 (

 9 Fig.9(a) plots the influences of layout of ECs on P-Δ behaviours of SUSSCW-ECs.

Fig. 9 (

 9 Fig. 9(b) plots the influences of ts on P-Δ behaviours of SUSSCW-ECs. Fig. 11(c)~(d)

Fig. 9 (

 9 Fig. 9(c) plots the influences of Sv on P-Δ behaviours of SUSSCW-ECs. Fig. 11(e)~(f)

Fig. 9 (

 9 Fig. 9(d) plots the influences of Sh on P-Δ behaviours of SUSSCW-ECs. Fig. 11(g)~(h)

Fig. 9 (

 9 Fig. 9(e) plots the effects of core type on P-Δ behaviours of SUSSCW-ECs. Fig.

Fig. 14 .

 14 Fig. 14. The Strip A is the edge strip that locates at two external sides of the

AISC360- 10 [

 10 [START_REF]AISC 360-16, Specification for Structural Steel Buildings[END_REF] specifies the ultimate compression resistance of SUSSCW (NuA) =Es Is + γEc Ic for cross section for SCSSCW-ECs; γ = 0.

  Ncr denotes the elastic critical buckling force that can be determined by the effective flexural stiffness ( ) s s e c c eff EI E I K E I = + ; Ke=0.6 is a coefficient.China code CECS[START_REF]Technical Standard for Structures with Concrete-Filled Multicellular Steel Tube Walls[END_REF] defines the ultimate compressive resistance of SUSSCW-ECs follows the specifications as the following;

Fig. 1 32 Fig. 4 Fig. 6 Fig. 7 Fig. 8 Fig. 9 Fig. 10

 1324678910 Fig. 1 SCSSCWs with different bonding connectors

Fig. 12 Fig. 13

 1213 Fig. 12 Influence of Sv on buckling length of faceplates

Table 2

 2 lists the experimental and predicted Pu values as well as test-to-prediction (TTP) ratios obtained by different prediction equations. Fig. 15 plots the scatters of TTP ratios for different prediction methods. These table and figure show that the prediction equations developed by the authors offer the most accurate predictions on Pu with an average TTP ratio and Cov of 1.02 and 0.10, respectively. The discrepancies of the predictions are probably due to scatter in strengths of both UHPC and steel material. Moreover, the ignorance on imperfection of the composite walls is also a probably reason. The equations in China code CECS offer the secondly most accurate predictions on Pu with an average TTP ratio and Cov of 1.03 and 0.11, respectively. AISC code equations offer the most conservative predictions on Pu with an average TTP ratio and Cov of 1.20 and 0.13, respectively. Eurocode 4 offers the secondary most conservative estimations on Pu with an average TTP ratio and Cov of 1.

ACKNOWLEDGMENT

The authors would like to acknowledge the Scientific Research Fund of Institute of Engineering Mechanics, China Earthquake Administration (Grant No. 2019 EEEVL0504) and Peiyang Scholar Foundation (grant no. 2019XRX-0026) under Reserved Academic Program from Tianjin University, China for the works reported herein. The authors gratefully express their gratitude for the financial supports.

specimen achieved its ultimate compression resistance.

(2) The P-Δ behaviours of SUSSCW-ECs under axial compression can be divided into three working stages, i.e., elastic, nonlinear, recession stage. However, the SUSSCW-ECs with UHPC exhibited more brittle P-Δ behaviours at the recession stage due to the brittleness of UHPC. The nonlinear behaviour of SUSSCW-ECs was mainly due to the local buckling of faceplates.

(3) Plastic local buckling occurred to the faceplates in most of the SUSSCWs, but elastic buckling occurred to specimens for specimens with large horizontal or vertical spacing of ECs.

(4) Compared with SUSSCW-ECs with V-web ECs, SUSSCW-ECs with H-web ECs exhibited 17% higher in Pu through alleviating stress concentration in the front toe of ECs under vertical compression. However, changing the layout of ECs did not have significant influence on Ke and DI ratio of SUSSCW-ECs.

(5) Increasing ts value of faceplate from 3.0 to 4.8 and 5.8 mm increased the Pu (or Ke) value of SUSSCW-ECs by 8% (10%) and 12% (18%), respectively. (6) Increasing the Sv value from 90 to 130, 170, and 240 mm reduced the Pu value of SUSSCW-ECs by 12%, 15%, and 11%, respectively. However, this increased Sv value did not affect Ke and DI ratio of SUSSCW-ECs. (7) Increasing the Sh value from 140 to 290 mm reduced the Pu value of SUSSCW-ECs by 6% due to the early local buckling and larger local buckling length of vertical strips of faceplates between two columns of ECs. The buckling length far away from the ECs equals to about 2.0 times of that locating along the


MD PhD Michael Grelat
email: michael.grelat@gmail.com

MD PhD Benjamin Pommier

MD Sylvain Portet

MD PhD Aymeric Amelot

MD PhD Cédric Barrey

MD Henri-Arthur Leroy

MD Rachid Madkouri

Patients with Coronavirus 2019 (COVID-19) and Surgery: Guidelines and Checklist Proposal

Keywords: proposal Covid-19, Surgery, Operating Room, Health Care Workers, Checklist

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

In december 2019, pneumonia of unknown causes were described in Wuhan province [START_REF] Zhu | A Novel Coronavirus from Patients with Pneumonia in China[END_REF][START_REF] Wang | A novel coronavirus outbreak of global health concern[END_REF] . Patients were identified and a new coronavirus was described quickly (2019-nCoV) [START_REF] Zhu | A Novel Coronavirus from Patients with Pneumonia in China[END_REF] . Many cases have been described and the spread was quickly carried out in China but also internationally, with the virus reaching Europe [START_REF] Wang | A novel coronavirus outbreak of global health concern[END_REF][START_REF] Johnson | Potential scenarios for the progression of a COVID-19 epidemic in the European Union and the European Economic Area[END_REF][START_REF] Spiteri | First cases of coronavirus disease 2019 (COVID-19) in the WHO European Region, 24[END_REF][START_REF] Stoecklin | First cases of coronavirus disease 2019 (COVID-19) in France: surveillance, investigations and control measures[END_REF] . The disease seems to spread with humanto-human transmission [START_REF] Riou | Pattern of early human-to-human transmission of Wuhan 2019 novel coronavirus (2019-nCoV), December 2019 to[END_REF] implying professionals contaminated during the care of some patients [START_REF] Guan | Clinical Characteristics of Coronavirus Disease 2019 in China[END_REF][START_REF] Zhou | Protecting Chinese Healthcare Workers While Combating the 2019 Novel Coronavirus[END_REF] . As of February 11, 2020, 3019 Chinese health care workers were infected with Covid-19 and 6 died from it 9 . In recent decades, we have had examples of epidemic coronavirus. During the SARS and MERS epidemic, infected patients had to undergo emergency surgery [START_REF] Park | Infection Prevention Measures for Surgical Procedures during a Middle East Respiratory Syndrome Outbreak in a Tertiary Care Hospital in South Korea[END_REF][START_REF] Tien | Elective and emergency surgery in patients with severe acute respiratory syndrome (SARS)[END_REF][START_REF] Chee | Infection control measures for operative procedures in severe acute respiratory syndrome-related patients[END_REF][START_REF] Wong | Measures to prevent healtcare workers from contracting severe acute respiratory syndrome during high-risk surgical procedures[END_REF] . Cases of transmission to health professionals during operating theaters have also been described in previous epidemics and should be prevented [START_REF] Park | Infection Prevention Measures for Surgical Procedures during a Middle East Respiratory Syndrome Outbreak in a Tertiary Care Hospital in South Korea[END_REF][START_REF] Wan | When surgeons become SARS patients[END_REF] .

The situation is rapidly evolving and we can observe an alarming increase in cases in the world. The European situation is worrying: at the beginning of March 2020, 15 million people were placed in quarantine in Italy and thereafter the whole country, a significant increase in the number of cases is observed in other countries, especially in France now in quarantine (Figure 1). The situation subsequently spread worryingly to North America. Surgical emergencies may be multiple, and some infected patients will need surgical care. Facing this increase of cases, in the same way as China 8,15,16 , we will be confronted with patients needing emergency surgery. It is necessary to anticipate and prepare to take care of Covid-19 patients in the operating room.

The objective of this study is to establish recommendations for the management of a patient with covid-19 coronavirus in the operating room. It is essential to be prepared and to have received good information to deal with the management of these cases in order to reduce the risk of contamination.

Before Surgery

The degree of emergency must be assessed and it is preferable to postpone the surgical procedure if it is not urgently needed. It is necessary to use the protective material in the right situation and avoid shortages of protection. We recommend seeking the advice of the hospital's infectious disease department to consider surgery in a patient with the coronavirus Covid-19. In our centers, scheduled patients are hospitalized 48 hours before surgery. If the patient is asymptomatic, we perform a PCR screening test upon arrival. If the patient is symptomatic, we perform 2 screening tests 24 hours apart, combined with a chest CT-scan. The same procedure is applied for patients arriving in emergency adjusted to the required delay of surgery. Antibodies are not routine tests at this time. As soon as they will be available and reliable, they probably should be added to the screening strategy and coupled with the PCR technique to optimize preoperative screening.

Several situations can be encountered:

-If the patient is suspected or confirmed to have coronavirus Covid-19, it is necessary to take the utmost precaution for the management of this patient. A specific protocol should be followed in the operating room (OR). It is obvious that a PCR test must be performed to obtain a necessary diagnosis in these patients [START_REF] Corman | Detection of 2019 novel coronavirus (2019-nCoV) by real-time RT-PCR[END_REF] .

-If the patient is cured (resolution of clinical symptoms), we currently advise taking the same precautions in the operating room as for a confirmed patient. The precautionary principle should be followed given the absence of follow-up on this disease (risk of asymptomatic case, risk of second contamination of the patient, risk of carrying the virus longer).

-If a coronavirus diagnosis is made after surgery, it is necessary to identify all the surgical staff who have been in contact with the patient and consider quarantining them after advice from the infectious disease department.

The surgical staff that will take care of this type of patient must have been trained in this risk management and aware of the protocol established in the institution. In this protocol, the patient's journey must have been planned in advance by the OR referents.

It is necessary to delimit a precise route, to avoid as much as possible the patient's contact with other unprotected personnel as well as other patients: dedicated elevator and corridors (Figure 2). In this course, the patient's route should avoid buffer zones such as the recovery room or the induction room. OR must be dedicated for these patients.

The patient must wear a surgical mask when transferring to the OR. Personnel must use a dedicated stretcher with disposable protections that will be well disinfected after use. Moreover, caregivers should take their temperature before surgery.

During Surgery

Operating rooms are generally under positive pressure. In these cases an OR must be dedicated and should be switched to negative pressure [START_REF] Park | Infection Prevention Measures for Surgical Procedures during a Middle East Respiratory Syndrome Outbreak in a Tertiary Care Hospital in South Korea[END_REF][START_REF] Tien | Elective and emergency surgery in patients with severe acute respiratory syndrome (SARS)[END_REF][START_REF] Chee | Infection control measures for operative procedures in severe acute respiratory syndrome-related patients[END_REF][START_REF] Wong | Measures to prevent healtcare workers from contracting severe acute respiratory syndrome during high-risk surgical procedures[END_REF][START_REF] Tao | Recommendations for general surgery clinical practice in novel coronavirus pneumonia situation[END_REF] . We must therefore anticipate this situation and plan to modify the environment in these rooms. In fact, the passage of air from the room to the outside must be limited as much as possible [START_REF] Chow | Conversion of operating theatre from positive to negative pressure environment[END_REF] . This procedure should be planned upstream with the hospital engineers, to obtain a negative room pressure (-5 Pa) [START_REF] Park | Infection Prevention Measures for Surgical Procedures during a Middle East Respiratory Syndrome Outbreak in a Tertiary Care Hospital in South Korea[END_REF] . We recommend for each surgical team to approach their engineering team in order to study the feasibility and prepare their structure to anticipate such a situation.

A clear display must be set up outside the room to inform other health care worker. The number of people in the Operating room must be reduced to the strict minimum. It is necessary to establish traceability of the health care worker present during the surgery. All the equipment necessary for the surgery procedure must be prepared and present in the operating room. It is necessary to avoid entering and leaving the room as much as possible during surgery. Disposable equipment is preferred.

All health staff in the OR area should use the following protection 11 : head cover, shoes cover, surgical gowns, surgical goggles, two pairs of surgical gloves, respiratory masks FFP2. If the material is available, it is possible to use a system powered air-purifying respirator [START_REF] Park | Infection Prevention Measures for Surgical Procedures during a Middle East Respiratory Syndrome Outbreak in a Tertiary Care Hospital in South Korea[END_REF] . The staff must be trained to wear the equipment in good conditions, in particular the FFP2 respiratory masks.

The anesthesiologist must wear the same protective equipment when faced with a very high risk of contamination during intubation and extubation procedures. [START_REF] Tien | Elective and emergency surgery in patients with severe acute respiratory syndrome (SARS)[END_REF] . The patient must benefit from an adequate sedation, preferably associated with a curarization in order to avoid agitation and cough episodes. After the intubation, a wet compress can be added to the patient's nose to avoid spreading the virus in the air in the event of a cough [START_REF] Tao | Recommendations for general surgery clinical practice in novel coronavirus pneumonia situation[END_REF] . It is also important limiting disconnection of the circuit when positioning the patient.

The surgeon must take the utmost precaution during the surgical procedure in order to reduce the risk of accidents involving exposure to body fluids and tissues. In the event of an exposure accident, the establishment's standard protocols should be followed to deal with a blood exposure accident. The infectious disease department should immediately be contacted and a test could be considered [START_REF] Si | Countermeasures and treatment for aortic acute syndrome with novel coronavirus pneumonia[END_REF] .

The patient should be awakened in the OR. The patient should wear a surgical mask as soon as he is awake. Once again the recovery room should be avoided and the patient should be brought back to his room directly if possible [START_REF] Park | Infection Prevention Measures for Surgical Procedures during a Middle East Respiratory Syndrome Outbreak in a Tertiary Care Hospital in South Korea[END_REF] . 30 minutes ventilation of the room at the end of surgery should be achieved [START_REF] Chee | Infection control measures for operative procedures in severe acute respiratory syndrome-related patients[END_REF][START_REF] Peng | Infection control and anesthesia: lessons learned from the Toronto SARS outbreak[END_REF] .

After Surgery

At each transfer, the patient should wear a surgical mask when leaving the operating room. A strict disinfection of the OR should be achieved. The OR should be closed for 2 hours and then switched to the laminar flow again. We recommend the elimination and destruction of body fluids and tissues using bags that allow double closure, and waste and liquids to be destroyed with infectious medical waste. For bacteriological or anatomopathological samples, also use a hermetically sealed bag with a double closure and call the center to recover and inform about the situation.

The health care staff must undress at the end of the procedure, helped by another caregiver because of the risk of contamination during this procedure. Strict decontamination of the OR is required. The infectious disease department of the hospital or hygiene department may be requested to perform tests in the OR to check the presence of viruses. The reusable material used must be carefully cleaned with the usual decontaminating detergent products.

Concerning patients, during postoperative time, he presents a high risk of pneumonia, fevers, infections, with an increase in the level of white blood cells, which can make diagnosis of postoperative infection difficult. In addition to regular temperature monitoring for patients, the surgical staff in contact must also monitor their temperature twice a day [START_REF] Tao | Recommendations for general surgery clinical practice in novel coronavirus pneumonia situation[END_REF] . Postoperative temperature monitoring is necessary for patients not at risk being treated for another pathology. Indeed, we can cite the situation in a Wuhan hospital with the post-operative appearance of several cases of Covid-19 coronavirus, either in asymptomatic patients [START_REF] Gou | Treatment of pancreatic diseases and prevention of infection during outbreak of 2019 coronavirus disease[END_REF] .

We recommend the daily washing of hands for staff, patients and families of patients. Family visits should be limited to the strict minimum. However, in the current situation in our centers, family visits are prohibited. In cases of visits the patient's family should also wear a surgical mask when visiting the patient, and staff should pay attention to contact with families. Indeed, they can also carry Covid-19 so it is necessary to apply the protection procedures for suspicious cases.

Conclusion

The management of patients with Covid-19 represents a new challenge for medical and surgical teams not accustomed to a large flow of patients. It is important, as of now, that each establishment and structure became aware of the development and prepare for an adequate crisis management.

It is obvious that these recommendations are the result of thinking and research between health care professionals working in the operating room (Figure 3). These recommendations will be confirmed by additional studies with more perspectives on the situation concerning this new virus.

Patient supervision in the operating room must be considered and organized previously to guarantee the safety of the surgical staff and to limit the spread of the disease. Each operating room in the world must prepare with thinking by developing a protocol and patient route. The surgical staff must be aware of the procedure for the care of patients with Covid-19, well trained in the establishment and prepared to deal with it, including during emergency.

Conflict of interest statements: None

Figure 1 :Figure 2 :

 12 Abbreviations: SARS: Severe acute respiratory syndrome MERS: Middle East respiratory syndrome PCR: Polymerase chain reaction OR: Operating Room FFP2: Filtering Facepiece Particles 2 Pa: Pascal

Figure 3 :

 3 Figure 3: Management of Suspicion of Covid-19 in Operating Room (OR): Safety Checklist of mandatory points to limit the surgical staff exposure and the spread of the disease.


domain: sdu shs sde

text:
Marco Filippi
email: marco.filippi@unimi.it

Davide Zanoni
email: davide.zanoni@unimi.it

Jean-Marc Lardeaux
email: lardeaux@unice.fr

Maria Iole Spalla

Guido Gosso
email: guido.gosso@unimi.it

Borgo San

M Tenibres

St Étienne De Tinée

Isola Bagni

Vinadio Beuil

Valdeblore St

Martin-Vésubie

Vinadio Mollières

M Demonte

M Matto

C Malinvern

P Ma Ghiliè

Ta Gèlas

Frisson

Evidence of Tethyan continental break-up and Alpine collision in the Argentera-Mercantour Massif, Western Alps

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Morbid obesity has recently reached epidemic proportions worldwide [START_REF] Angrisani | Bariatric Surgery Worldwide 2013[END_REF] and bariatric surgery (BS) has been shown to be the only therapeutic means leading to long-term sustained weight loss [START_REF] Lazzati | Bariatric Surgery and Liver Transplantation: a Systematic Review a New Frontier for Bariatric Surgery[END_REF] . Furthermore, obesity is associated with non-alcoholic fatty liver disease (NAFLD) that may proceed to non-alcoholic steatohepatitis (NASH), cirrhosis with end-stage liver disease and/or hepatocellular carcinoma (HCC) [START_REF] Charlton | Frequency and Outcomes of Liver Transplantation for Nonalcoholic Steatohepatitis in the United States[END_REF] . In parallel to the exponential increase of BS, liver transplant surgeons may be confronted with patients with such background [START_REF] Iannelli | Bariatric surgery and liver transplant[END_REF] . While biliopancreatic diversion with duodenal switch (BPD-DS) is an uncommon bariatric procedure, for its technical difficulties and increased risk of postoperative complications, it may still be possible to come across for transplant surgeons. In this paper, we report the case of a patient with a previous history of BPD-DS who became a candidate to liver transplantation (LT) because of HCC complicating hepatitis C related liver cirrhosis.

Case Report

A 51-year-old Caucasian man was referred to our department for a chronic entero-cutaneous fistula complicating a BPD-DS for morbid obesity done 6 months earlier in another center. His past medical history included blood hypertension, dyslipidemia, type 2 diabetes, hypothyroidism, hepatitis C and a NAFLD related liver cirrhosis. Hepatitis C (genotype 1b) was first detected in the '90s and treated by Interferon, while cirrhosis was diagnosed in 2000. In 2006, the patient underwent a transjugular intrahepatic portosystemic shunt (TIPS) for portal hypertension. In 2010 he underwent a laparoscopic sleeve gastrectomy (SG) for morbid obesity (weight: 160 kg; height: 183 cm; BMI: 47.7 kg/m²). A systematic second step DS was done at one year (weight: 120 kg; height: 183 cm; BMI: 35.8 kg/m²) by the same surgeon. The postoperative course was complicated by a leak at the duodeno-ileostomy, which required surgical treatment with laparotomy. In 2012, the patient was admitted in our department for chronic leak with a BMI of 24.8 kg/m² and underwent surgical revision of the duodeno-ileostomy that was not effective as the chronic leak recurred (Fig. 1). In 2013, two HCCs, in segment III and VII were found on follow-up imaging (Fig. 2). No extra-hepatic metastases were present. The patient met the Milan criteria and was enlisted for LT. Radio-frequency ablation (RFA) was performed while awaiting for LT. When added on the transplant list, the patient had no nutrient deficiencies and had an albumin level of 38.5 g/L (normal range 35.0-52.0), a vitamin B9 level of 8.3 ng/mL (normal range 3.0-18.0) and a vitamin B12 level of 323 pg/mL (normal range 200-1000). The measurement of psoas area at L3 showed slight sarcopenia with the right area of 1392 mm² and the left area of 1282 mm² (Fig. 3) [START_REF] Golse | A new definition of sarcopenia in patients with cirrhosis undergoing liver transplantation: GOLSE ET AL[END_REF] . He finally underwent LT in September 2014 (MELD = 12) with a duct-to-duct biliary reconstruction. At the time of LT, the enterocutaneous fistula tract was resected and the anastomosis fashioned again on fresh tissues. The postoperative period was uneventful and the patient was discharged on day 21. Pathology of the native liver revealed liver cirrhosis, 50% microscopic steatosis, two entirely necrotic HCCs of both 15 mm and a third one of 16 mm. The patient was given calcium, iron, and vitamin supplementation along with immunosuppressive treatment (tacrolimus (8-12 ng/mL up to month 4, then 6-10 ng/mL during months 4-12, then 4-8 ng/mL after 12 months), mycophenolate mofetil and prednisone). To date, the patient is currently alive and disease-free at 9 years from his first BS and 5 years from LT (weight: 75.5 kg; height: 183 cm; BMI: 22.5 kg/m²). His most recent nutritional state presents an albumin level of 32.6 g/L and a sarcopenic state with a right psoas area of 986.6 mm² and a left area of 829.1 mm² (Fig. 3).

Discussion

In 2016, procedures performed for morbid obesity reached the 200.000 level in the US and this rate will probably continue to increase as obesity is on the rise [START_REF] Hales | Prevalence of Obesity Among Adults and Youth: United States[END_REF][START_REF] English | American Society for Metabolic and Bariatric Surgery estimation of metabolic and bariatric procedures performed in the United States in 2016[END_REF] . BS has been proven to be the most effective and sustainable treatment for obesity and its comorbidities such as hypertension, diabetes, obstructive sleep apnea and dyslipidaemia 2 . On the same slope, NAFLD concerns more than 30 % of Americans. Indeed, NASH has become the second indication for LT in the US [START_REF] Cholankeril | Liver Transplantation for Nonalcoholic Steatohepatitis in the US: Temporal Trends and Outcomes[END_REF] . Although there is some evidence that BS may improve NASH lesions [START_REF] Lazzati | Bariatric Surgery and Liver Transplantation: a Systematic Review a New Frontier for Bariatric Surgery[END_REF][START_REF] Hafeez | Bariatric Surgery as Potential Treatment for Nonalcoholic Fatty Liver Disease: A Future Treatment by Choice or by Chance?[END_REF] , end-stage liver disease and/or HCC may still occur. Given that, it should be considered that more and more LT will be done in patients with history of BS because of the wide diffusion of the latter. Whether obesity is a relative contraindication for LT, due to the risk of postoperative morbidity and the reduction of long-term survival, is still a matter of debate [START_REF] Conzen | Morbid obesity in liver transplant recipients adversely affects longterm graft and patient survival in a single-institution analysis[END_REF][START_REF] Schiavo | Nutritional issues in patients with obesity and cirrhosis[END_REF] . However, as NAFLD is becoming the main indication for LT, the management of obesity treatment and LT is critical and the impact of prior BS on LT needs more clarity.

Idriss et al studied the impact of prior BS on candidacy for LT and found an increased risk of death on the waiting list and a higher rate of delisting in patients with Roux-en-Y Gastric Bypass (RYGB) [START_REF] Idriss | Impact of Prior Bariatric Surgery on Perioperative Liver Transplant Outcomes: Liver Transplantation[END_REF] .

Malabsorptive mechanisms involved in RYGB might induce more important malnutrition and/or a sarcopenic state, which can contribute to worse outcomes while being on the waiting list and after LT patients. Indeed, the patient herein presented showed before LT psoas muscle area slightly below the range accepted to define the sarcopenic state [START_REF] Golse | A new definition of sarcopenia in patients with cirrhosis undergoing liver transplantation: GOLSE ET AL[END_REF] . Two years after LT sarcopenia worsened as the psoas muscle area decreased of approximately 30% on CT scan. Indeed, it is well known that proteic malnutrition is a complication of biliopancreatic diversions techniques including the DS.

The unavoidable worsening of sarcopenia may be explained by the malabsorption due to BPD-DS in addition to the surgical stress linked to LT as well as the following immunosuppressive therapy 13 .

Probably, the patient could have taken advantage of a rehabilitation programme before LT. As a matter of fact, evidence for the benefits of nutrition intervention associated with resistance exercise in improving skeletal muscle strength and mass is growing. Healthier dietary patterns, such as adequate intake of protein, vitamin D, antioxidants and long-chain polyunsaturated fatty acids seem to benefit on muscle mass and function [START_REF] Dent | International Clinical Practice Guidelines for Sarcopenia (ICFSR): Screening, Diagnosis and Management. The journal of nutrition[END_REF] . Anyway, this shortcoming could serve as a future suggestion to consider the possible benefits of nutritional supplementation in patients with sarcopenia before a demanding surgical intervention such as LT.

However, most patients with a history of BS listed for LT have currently a SG 2 . Indeed, SG is considered by most the procedure of choice in patients with liver cirrhosis that are or may become candidates to LT [START_REF] Zamora-Valdes | Long-term outcomes of patients undergoing simultaneous liver transplantation and sleeve gastrectomy[END_REF] . Restrictive procedures have less protein malnutrition and micronutrient deficiencies than malabsorptive procedures avoiding any limitations linked to the intestinal bypass, and that is why it might be a better choice before LT. Moreover, Safwan et al studied 11 patients that had previous BS and stressed that, in case of RYGB and BPD-DS, endoscopic access to the biliary tree in case of biliary complications, that were as high as 27% in his series, is often impossible [START_REF] Safwan | Outcome of liver transplantation in patients with prior bariatric surgery: Safwan et al[END_REF] . In our patient, a duct-to-duct reconstruction was done but no postoperative complication occurred.

After a bariatric procedure, a rare but authentic complication to BS remains the possibility of acute or chronic liver failure, even more after BPD-DS, mainly due to bacterial overgrowth and protein deficiency [START_REF] Addeo | Liver transplantation for bariatric surgery-related liver failure: a systematic review of a rare condition[END_REF][START_REF] Geerts | The Multicenter Belgian Survey on Liver Transplantation for Hepatocellular Failure after Bariatric Surgery[END_REF] . Nonetheless, this complication stays rare with much fewer liver-damages after BPD-DS than after jejunoileal bypass (JIB). In a recent review of the literature, we found that BPD with or without DS may lead to acute liver failure although only 17 cases were found over the last 18 years [START_REF] Addeo | Liver transplantation for bariatric surgery-related liver failure: a systematic review of a rare condition[END_REF] .

After a BPD-DS, Geertz et al recommend following these patients strictly with routine monitoring of liver tests and adequate supplementation [START_REF] Geerts | The Multicenter Belgian Survey on Liver Transplantation for Hepatocellular Failure after Bariatric Surgery[END_REF] . The possibility to lengthen the common channel should be considered at the time of LT or later if malabsorption impairs postoperative course. In the case herein reported, we chose not to alter the bariatric anatomy as the patient underwent LT for HCC and not because of BS related LF. Interestingly, the short common channel did not affect the immunosuppressive treatment. Indeed, Tacrolimus is also absorbed in the colon [START_REF] Tsunashima | Assessment of Tacrolimus Absorption From the Human Intestinal Tract: Open-Label, Randomized, 4-Way Crossover Study[END_REF] .

This case report indicates that LT can be safely performed in the setting of a BPD-DS. Indeed, while the patient fulfilled the Milan criteria, known to be associated with optimal survival results after LT, one might have been tempted to decline LT favouring alternative therapeutic options because of the theoretical risk of complications linked to the BPD with DS. Excellent results were obtained in our patient because he was on a regular nutritional follow-up after LT, had compensated liver cirrhosis, and was managed by a team experienced in both LT and BS [START_REF] Mosko | Increased Perioperative Mortality Following Bariatric Surgery Among Patients With Cirrhosis[END_REF] . Close monitoring of vitamins, minerals and nutritional status is mandatory in the setting of patients with a history of BS undergoing LT in parallel to the level of immunosuppressive drugs.

Conclusion

BS is expanding its range of activity over the years and takes on more and more role in the scene of transplantology. To our knowledge, this is the first case of BPD-DS followed by LT for HCC. While the malabsorptive component of BS adds risk, it should not be a reason for liver surgeons to retreat, particularly if associated with a well-planned physical and nutritional rehabilitation. AmpI1 core, appinite AmpI1 core, comb layered appinite AmpI1 rim, comb layered appinite AmpI1 core, melano-spessartite AmpI1 rim, melano-spessartite AmpI1 core, spessartite AmpI1 rim, spessartite AmpI2b, leucocratic vein AmpI1, appinite AmpI2a, appinite AmpI1, leuco-appinite AmpI2a, leuco-appinite AmpI1, spessartite AmpI2a, spessartite Act Hbl AmpM1, spessartite AmpM1, appinite AmpM1, comb layered appinite AmpM1, melano-spessartite AmpM2, spessartite AmpM2, appinite AmpM2 comb layered appinite AmpM1, spessartite AmpM1, appinite AmpM1, leuco-appinite

Haut Boréon Valscura

Haut Boréon Valscura

AmpM2a, appinite AmpM2b, spessartite AmpM2b, appinite AmpM2a, spessartite Al / (Al + Si) (apfu / apfu) Al / (Al + Si) (apfu / apfu) Na / (Na + Ca) (apfu / apfu) Na / (Na + Ca) (apfu / apfu)

  GDEM is a product of METI and NASA

.87 -6.18 1.82 -2.13 0.33 -0.54 0.20 -0.35 0.48 -0.68 1.81 -1.94 0.78 -0.92 0.11 -0.14

	Site	STAGE	GENESIS Rock type Generation		Si	MINERAL ASSEMBLAGE Al IV	Ti	Al VI	Mg#	Ca	Na	K
	I0 Valscura	Appinite	igneous	AmpI1			6.00 -6.92	Amp, Cpx 1.08 -2.00	0.11 -0.48	0.14 -0.38	0.56 -0.71	1.57 -1.88	0.51 -0.91	0.07 -0.11
	I1 (core, rim)		igneous	(37 data)		Amp, Pl, Qz, Kfs, Mgt, Ilm, Ap, Bt 6.36 ± 0.21 1.64 ± 0.21 0.23 ± 0.09	0.26 ± 0.05	0.64 ± 0.04	1.71 ± 0.08	0.79 ± 0.08	0.08 ± 0.01
		I2 (a,b)	late-igneous AmpI2a			7.13 -7.73		Ab, Amp 0.28 -0.87	0.01 -0.14	0.06 -0.17	0.42 -0.64	1.37 -1.86	0.15 -0.48	0.02 -0.07
		M1	subsolidus, hydrothermal (10 data)	Ab, Act, Bt, Chl, Ep, Hem, Kfs, Py, Qz, Ttn 7.45 ± 0.20 0.55 ± 0.20 0.08 ± 0.04	0.11 ± 0.03	0.55 ± 0.07	1.58 ± 0.16	0.31 ± 0.10	0.05 ± 0.02
		M2 (a,b)	subsolidus, syn-D4 Leuco-appinite AmpI1		Act, Bt, Chl, Ep, Kfs, Pl, Qz, Ttn, Wm 6.01 -6.05 1.95 -2.00 0.34 -0.39	0.25 -0.36	0.67 -0.71	1.81 -1.87	0.78 -0.87	0.08
					(5 data)			6.03 ± 0.02	1.97 ± 0.02	0.36 ± 0.02	0.30 ± 0.04	0.69 ± 0.02	1.84 ± 0.02	0.82 ± 0.04	-
					AmpI2a			7.40		0.60	0.12	0.12	0.75	1.81	0.33	0.05
					(1 data)			-			-	-	-	-	-	-	-
			Spessartite	AmpI1			5.74 -6.46		1.60 -2.26	0.26 -0.51	0.20 -0.34	0.57 -0.78	1.72 -1.96	0.59 -0.87	0.06 -0.12
	(B)				Regorda et al. (2017) (19 data) 6.04 ± 0.16 AmpI2a 6.65 -7.60	1.96 ± 0.16 0.40 -1.36	0.36 ± 0.07 0.04 -0.21	0.27 ± 0.04 0.05 -0.31	0.70 ± 0.05 0.53 -0.70	1.86 ± 0.06 1.23 -1.94	0.79 ± 0.07 0.16 -0.58	0.09 ± 0.02 0.02 -0.05
					(9 data)	lower plate	7.22 ± 0.37	0.78 ± 0.37	0.11 ± 0.06	0.15 ± 0.09	0.62 ± 0.07	1.73 ± 0.21	0.29 ± 0.13	0.04 ± 0.01
	Haut Boréon	Appinite		upper plate AmpI1 core	5.92 -6.12		1.88 -2.08	0.31 -0.45	0.10 -0.33	0.54 -0.64	1.69 -1.91	0.81 -0.94	0.09 -0.13
					(34 data)		6.03 ± 0.05	1.97 ± 0.05	0.38 ± 0.03	0.26 ± 0.04	0.67 ± 0.05	1.87 ± 0.04	0.84 ± 0.03	0.11 ± 0.01
			Melanocratic	AmpI1 core		5.74 -6.06		1.94 -2.26	0.41 -0.67	0.19 -0.37	0.55 -0.68	1.84 -1.94	0.79 -1.05	0.10 -0.14
			spessartite		(17 data)		5.94 ± 0.09	2.06 ± 0.09	0.54 ± 0.07	0.25 ± 0.05	0.62 ± 0.03	1.89 ± 0.03	0.89 ± 0.06	0.12 ± 0.01
					AmpI1 rim		6.03 -6.41		1.59 -1.97	0.23 -0.48	0.15 -0.35	0.41 -0.62	1.73 -1.88	0.85 -1.10	0.10 -0.18
					(15 data)	0.5	6.16 ± 0.12	1.84 ± 0.12	0.37 ± 0.06	0.28 ± 0.05	0.50 ± 0.05	1.81 ± 0.04	0.98 ± 0.08	0.14 ± 0.02
					AmpI2b			6.19 -6.32		1.68 -1.81	0.26 -0.36	0.11 -0.15	0.25 -0.34	1.71 -1.79	0.86 -1.01	0.28 -0.31
					(4 data)			6.27 ± 0.05	1.73 ± 0.05	0.30 ± 0.05	0.13 ± 0.01	0.28 ± 0.04	1.75 ± 0.03	0.93 ± 0.05	0.29 ± 0.01
			Comb-layered 5appinite AmpI1 core (27 data) 6.00 ± 0.09	2.00 ± 0.09	0.45 ± 0.06	0.28 ± 0.04	0.60 ± 0.06	1.89 ± 0.04	0.86 ± 0.04	0.12 ± 0.01
					AmpI1 rim		6

.26 -6.28 1.72 -1.74 0.29 -0.30 0.21 -0.33 0.44 -0.47 1.76 -1.81 0.85 -0.91 0.12 -0.14

	Site	Rock type	Generation	Si	Al
			(3 data)	6.27 ± 0.01	1.73 ± 0.01	-	0.27 ± 0.05	0.45 ± 0.02	1.79 ± 0.02	0.89 ± 0.02	0.13 ± 0.01
		Spessartite	AmpI1 core	5.78 -6.21	1.79 -2.22	0.32 -0.52	0.13 -0.38	0.51 -0.75	1.79 -1.96	0.66 -1.11	0.10 -0.17
			(48 data)	6.00 ± 0.10	2.00 ± 0.10	0.41 ± 0.05	0.27 ± 0.05	0.68 ± 0.05	1.87 ± 0.04	0.80 ± 0.06	0.13 ± 0.01
			AmpI1 rim	6.10 -6.96	1.04 -1.90	0.21 -0.35	0.21 -0.32	0.37 -0.71	1.70 -1.90	0.31 -1.16	0.06 -0.18
			(7 data)	6.47 ± 0.31	1.53 ± 0.31	0.28 ± 0.05	0.28 ± 0.04	0.58 ± 0.10	1.82 ± 0.06	0.72 ± 0.27	0.12 ± 0.04

Acknowledgements

No funding was provided by any source.