Mechanical Performance of Composite Truss Bridge with Double Decks

Mechanical Performance of Composite Truss Bridge with Double Decks Jun HE PH.D Tongji University Shanghai, China frankhejun@gmail.com Biao MA Civil Engineer Shanghai Municipal Engineering Design General Institute, China ma_b.js@smedi.com Haohui XIN PH.D Candidate Tongji University Shanghai, China 2011xinhaohui@ tongji.edu.cn Bin HAN MSc Tongji University Shanghai, China hb-147258369@163.com Yuqing LIU Professor Tongji University Shanghai, China yql@tongji.edu.cn Summary A new structure form of composite truss bridge which composed of upper and lower concrete decks and truss member is proposed to resolve the traffic jam in cities in this paper. Based on one engineering example of a composite truss bridge with main span of 96.0m,the mechanical behavior including deflection, stress distribution of concrete slab, concrete chord and steel web under different load conditions are investigated. In addition, the joint parts which connect steel truss and concrete slabs are the most important components in structure system. Different joints in terms of structure type and thickness of steel gusset plate are analyzed and determined by finite element analysis to prevent the local stress concentration of the steel gusset plate. All the results of static behavior for composite truss bridge with double decks in this study provide reference for the design and construction of such type bridges. Keywords: composite truss bridge, double decks, mechanical behavior, joint part 1. Introduction As the needs of more traffic space, multiline highways and new transportation infrastructures should be constructed to solve the traffic jam in cities. Therefore, based on viaducts system, to enlarge urban space in height direction, building urban viaduct with double decks is an effective measure to increase traffic volume of viaducts in central urban areas. Also, Composite truss bridge with double decks provides wide vision for drivers. Compared with concrete box girder, the composite truss bridge has smaller dead weight, can give fewer burdens on the substructure and be long-span. As for the mechanical performance, it transforms bending moment of the bridge into axial forces on upper and lower concrete decks, which fully use the good compression performance of concrete. At the same time, the shear force is changed into axial force on steel truss web members. In recent years,several composite truss bridges have been built. Sarutagawa bridge and Tomoegawa bridge, which adopt composite structures with steel-pipes, are the first multiple continuous rigid frame bridges[1]. The main spans for Sarutagawa and Tomoegawa Bridge are 110m and 119m respectively. With concrete upper and lower slabs and steel pipe webs, Shitsumi Ohashi Bridge is a 5-span continuous composite truss bridge which locates on the Kawamoto-Hata prefectural highway and passes through the Kando Gawa River[2]. The span arrangement is (64.0 + 75.0 + 60.0 + 45.0 + 34.0) m and the width of deck is 10.75m (effective width: roadway 7.25m + walkway 2.50m). Minpu Bridge in Shanghai is the first composite truss cable-stayed bridge in China, the main span is steel truss structure and the side spans are composite truss structures [3]. With a main span of 708 m and side spans of 63 m, the total span is 1212m. It has eight lanes on the upper deck and six lanes on the lower deck. In terms of studies on composite truss bridge, researchers mainly focus on joint structures. Xue[3]presented a model test and numerical finite element analysis on the mechanical behavior of a

composite joint in a truss cable-stayed bridge. Udomworarat [4] studied the fatigue performance and ultimate strength of welded steel tubular joints. An experimental study on the ultimate strength and fatigue strength of concrete filled and additionally the reinforced tubular K-joint of truss girders has been conducted by Sakai [5]. Furuichi [6] introduced various junction structures and explained their features. Two kinds of joint structures were provided by Sato[7], in order to clarify the stress transfer mechanism, a loading test in half scale specimens of the joint structure has been carried out. On the basis of one engineering example of a composite truss bridge with double decks, tmechanical behaviors including deflection, stress distribution of each component are investigated. In addition, different joints in terms of structure type and steel gusset plate thickness are analyzed to prevent the local stress concentration by finite element analysis. All the results of static behavior for composite truss bridge with double decks in this study could provide references for the design and construction of such type bridges. 2. Engineering example 2.1 General layout Deshenglu Bridge in Hangzhou (Fig.1) is the first simply supported composite truss bridge with double-decks in China. The length of main span is 96.0m, and the bridge has eight lanes on the upper deck and six lanes on the lower deck (Fig.2). Detail layout of upper deck is 2.9m maintenance path +33 m motorway +2.9m maintenance path, and the lower deck is 5.5m sidewalk +3.9m bicycle lane +28m motorway +3.9m bicycle lane +5.5m sidewalk. Fig.1: Deshenglu Bridge in Hangzhou, China (mm) Fig.2: Cross section (mm) 2.2 Bridge details The cross section of the bridge consists of the upper deck, lower deck and steel truss web members (Fig.3). The upper deck includes one interior prestressed concrete chord, two exterior prestressed concrete chords, prestressed concrete cross beam and reinforced concrete deck. The lower deck is made up of prestressed concrete deck, prestressed concrete cross beam and overhanging sidewalk slab, also includes one interior prestressed concrete chord and two exterior prestressed concrete chords. first second third forth fifth sixth seventh eighth ninth tenth (a) side view of steel truss web members (b) sectional view Fig.3: Steel truss web members (mm)

Fig.4: Composite joint and cable tension adjustment. The composite joint is one of the most important parts for such bridge structure, and the mechanical behavior of the composite joint is very complex and difficult to analyze in practical design. As shown in Fig.4, the composite truss joint includes the steel diagonal web member, the shaped steel chord and the concrete chord. Headed studs are used to resist shear force in the interface between concrete chord and other steel members. The upper deck, which mainly bears compressive stress, just contains transverse internal prestressed tendons. The lower deck includes the transverse and longitudinal, internal and external prestressed tendons (Fig.5). The anchors for external prestressed tendons are adjustable,which can be used for cable replacement Fig.5: Layout of internal and external longitudinal tendons of lower deck 3. Finite element analyses 3.1 Finite element model The hybrid three-dimensional FEA model involving the whole bridge and the joint segment (Fig.6) were built by using ANSYS[8] to analyze the mechanical behavior of whole structure and joint part. The concrete structure components such as decks, cross beams and chords are modeled with SOLID45 elements; the steel truss web members are stimulated by beam188 elements; the prestressed tendons were modeled with LINK8 elements, the prestressed stress is applied by providing initial strain in LINK8 element. The Fig.6: Hybrid 3-D FEA model detailed model of joint segment was embedded in the whole bridge, the steel structure members are represented by SHELL63 elements, and spring elements COMBIN14 are chosen to represent the headed studs. In this study, the corresponding nodes of the concrete and steel are connected by spring elements in the tangential directions and coupled in the axial direction, the stiffness degradation of headed studs during the design life is not taken into account for simplification. 3.2 Material parameters The upper and lower concrete slab, transverse diaphragm and deviation adopt C50 concrete, while the substructure adopt C40 and C30 concrete. The modulus of elasticity of C50 concrete is 3.45 10 4 Mpa, the density is 2.6 10 3 kg/m 3. The steel truss web members are made use of the Q370qD steel, the modulus of elasticity of steel is 2.1 10 5 Mpa, Poisson's ratio is 0.3, and the density is 7.85 10 3 kg/m 3. High strength low relaxation steel strand are used for internal and external tendons with the ultimate tensile strength of 1860MPa, and the modulus of elasticity is 1.95 10 5 MPa. Considering prestressing loss, the practical tensile strength of the prestressed tendons is 1116MPa. The initial strain 1.2 10-5 is applied for LINK8 to consider the pretressing. The stud is made up of ML15, The diameter of outside and inside stud are (22 200)mm and (19 150)mm respectively. 3.3 Boundary condition and loading state Hyperboloid spherical steel bearing is used in this bridge. Vertical ultimate capacity of the bearing is 3000KN, while horizontal ultimate capacity is about 15% times of the vertical one. According to the actual support condition of bridge, there were two fixed bearings (all the degree of freedoms

were restricted in supporting 2 and 3), four one-way movable bearings (vertical-uz and horizontal displacement-uy were restricted in supporting 6 and 7; vertical-uz and longitudinal displacement-ux were restricted in supporting 1 and 4) and two two-way movable bearings (vertical displacement-uz was restricted in supporting 5 and 8), which are illustrated in Fig7. Y Boundary : X 1,4 UX UZ 2,3 UX UY UZ 5,8 UZ 6,7 UY UZ Fig.7: Boundary condition Three load cases are calculated in this article, i.e., balanced live load, and unbalanced live load. The involves structure own weight, the weight of bridge deck pavement and balustrade. The live load contains pedestrian load, vehicle load and motor vehicle load (Fig.8) as well as the. All parameters related to live load were used according to the design code [9]. (a) Balanced live load (b) Unbalanced live load Fig.8: Live load case 4. Analytical results and discussion 4.1 Deflection The maximum deflection under the, balanced live load and unbalanced live load are 47.37mm, 54.40mm and 58.91mm. All of them are less than L/600 (159.833mm) and fulfill the requirement of Code for design of highway RC and PC bridges and culverts [10]. 4.2 Stress of concrete deck The normal stress of concrete deck in mid-span by finite element analysis is shown in Fig.9, with the transverse distance of concrete deck as X-axis and normal stress as the Y-axis. It was found that the stress distribution is not uniform in the upper and lower slab along the transversal direction of the bridge, the stress concentration appear at the position of the connection between concrete and steel truss web, the connection between the concrete and tendons, so in these positions measures should be taken to reduce the stress concentration. -7 0 banlanced live load unbanlanced live load stress(mpa) -9-10 -11-12 banlanced live load unbanlanced live load stress(mpa) -2-13 0 8000 16000 24000 32000 40000 tranverse distance of deck(mm) 0 8000 16000 24000 32000 40000 tranverse distance of deck(mm) (a) Upper deck (b) Lower deck Fig.9: Normal stress of concrete deck

As shown in the Fig.10, the full width of upper deck and lower deck is 7.75m. According to the maximum and average stress, effective width of upper and lower decks can be obtained, which were listed in Table 1. It can be found that the average effective width ratio is 0.761 and 0.691 for upper and lower deck respectively. Moreover, the effective ratio remained virtually unchanged under different loading cases. outside of the exterior concrete chord center of the interior concrete chord outside of the exterior concrete chord center of the interior concrete chord Fig.10: Effective width Table.1: Effective width details The upper deck The lower deck Dead load Balanced live load Unbalanced Live load Dead load Balanced live load Unbalanced Live load Max. stress/mpa 11.027 12.290 12.560 5.962 5.747 5.751 Effective width ratio 0.768 0.776 0.738 0.715 0.673 0.685 Full width(b)/m 7.75 7.75 7.75 7.75 7.75 7.75 Effective width(bf)/m 5.95 6.02 5.72 5.54 5.22 5.31 The normal stresses of PC chords in the upper and lower deck are shown in Fig.11 and Fig.12 respectively. The stresses on the outside of the exterior concrete chord and the center of the interior concrete chord are figured out by finite element analyses. From the results, it is considered that the stress is up and down along with the distance because of the contribution of cross beam. In addition, all the concrete chords in upper deck are in compression under different load cases, the maximum stress is about 9MPa, which much less than the design compressive stress for C50 concrete. The stress on the lower deck also fluctuates at the positions of cross beams. Most of the stress on the lower deck is in compression, the maximum stress is about 16MPa. But the stress on outside of the exterior concrete chord in mid-span is in tension, and the maximum value is about 1.8MPa, less than the design tensile stress for C50 concrete. stress (MPa) 0-2 -6 balanced load unbalanced load stress (MPa) 0-2 -6 balanced load unbalanced load -10 0 20000 40000 60000 80000 100000 longitudinal distance of deck ( mm) 0 20000 40000 60000 80000 100000 longitudinal distance of deck ( mm) (a) Outside of the exterior concrete chord (b) Center of the interior concrete chord Fig.11: The stress of the concrete chord in upper deck

0 0-2 stress (MPa) -12 balanced load unbalanced load stress (MPa) -6-10 -12 balanced load unbalanced load -16-14 0 20000 40000 60000 80000 100000 longitudinal distance of deck ( mm) -16 0 20000 40000 60000 80000 100000 longitudinal distance of deck ( mm) (a) Outside of the exterior concrete chord (b) Center of the interior concrete chord Fig.12: The stress of the concrete chord in lower deck 4.3 Force of steel truss web members The axial force and bending moments of steel truss web members under different load cases are shown in Table3. The maximum axial force appears in the first (Fig.3) diagonal steel truss web member near support points. While the maximum longitudinal bending moment appears in the third diagonal steel truss web member, about 1/4 of the span; and the maximum transverse bending moment happens in the fifth diagonal steel truss web member, about the middle span. Combined with the axial force and bending moments, the Mises stress on the steel truss member was obtained. The maximum value was calculated as 172MPa under unbalanced live load, which was much lower than the yielding stress of Q370qD steel. 4.4 Joint stress The joints which connect steel truss and concrete slabs are the most important components in structure system. While the local stress concentration on the steel gusset plate of the joint should be paid attention. So, different steel joint in terms of steel gusset plate types (Fig. 13) and different thicknesses (involves 55mm and 60mm) is analyzed by finite element analysis. There are two stiffeners on the gusset plate in Structure 1, while Structure 2 has one stiffening plate on the center. Compared with Structure 2, the stiffening plate on center in Structure 3 extended to the end of the gusset plate. Differences are shown in red circles. Table3. The moment and axial force of steel truss web Axial force(kn) Longitudinal bending Transverse bending Load case moment (KN m) moment (KN m) Maximum Minimum Maximum Minimum Maximum Minimum Dead load 17000-19800 1.24 10 6 1.74 10 6 1.03 10 6-9.9 10 5 Balanced live load 18600-21400 1.4 10 6 1.93 10 6 9.47 10 5-9.47 10 5 Unbalanced live load 19400-22100 1.49 10 6-2.02 10 6 9.44 10 5-9.71 10 5 Structure 1 Structure 2 Structure 3 Fig.13: Front view of the steel joint (mm)

(a) Structure 1 with 55mm plate (b) Structure 2 with 55mm plate (c) Structure 3 with 55mm plate (d) Structure 1 with 60mm plate (e) Structure 2 with 60mm plate (f) Structure 3 with 60mm plate Fig.14: The Mises stress of different steel gusset plate The Mises stresses on different steel gusset plates are shown in Fig14. It is found that, the local stress concentration of Structure 1 with 55m plate is obvious and the maximum stress is much greater than 300MPa. When the thickness is changed to 60mm, the local stress concentration still exists but the maximum stress reduces to 300MPa. When the force is transferred from oblique web members to gusset plate, the center plate has not been strengthened and the local stress concentration appears. To solve this problem, Structure 2 and Structure 3 have been designed. In terms of Structure 2, the local stress concentration deduced and maximum stress change from 270MPa to 240MPa when turning the thickness from 55mm to 60mm, as shown in Fig.14(b) and Fig.14(e). As to Structure 3, the local stress concentration disappears and maximum stress obviously decrease. In compassion of all three structure types with different plate thickness, Structure 3 with 60mm plate was applied as the optimal choice. 5. Conclusion A study on the Composite truss bridge with double decks has been conducted and the following conclusions can be made based on the design and finite element analysis. A new structure form of composite truss bridge which composed of upper and lower concrete decks and truss member is proposed to resolve the traffic jam in cities in this paper. Based on one engineering example of a composite truss bridge,the mechanical behavior including deflection, stress distribution of concrete slab, concrete chord and steel web under different load conditions are investigated by finite element analysis. The calculated maximum deflection of the bridge is less than 1/600 of the main span and fulfills the requirement of stiffness. Also the calculated maximum compressive and tensile stress on the concrete decks meets the requirement of the code. In addition, the joint parts which connect steel truss and concrete slabs are the most important components in structure system. Different joints in terms of structure type and thickness of steel gusset plate are analyzed and optimized to prevent the local stress concentration of the steel gusset plate. All the results of static behavior for composite truss bridge with double decks in this study provide reference for the design and construction of such type bridges. References [1] YAMAGUCHI T., UEHIRA Y., AOKI K. Advanced structure of composite bridges. In: Materials Forum Materials and Testing Conference 2005, pp.90-95 [2] MIYAWAKi, T., FUJIHARA, H., USHIROSHOJI, S., SHOJI, A. Design and Construction of the Shitsumi Ohashi Bridge, Bridge and foundation Engineering, 2005(11), pp 5-11 [3] XUE D., LIU Y., He J., MA B. Experimental study and numerical analysis of a composite truss joint, Journal of Constructional Steel Research, No. 67, 2011, pp. 957-964.

[4] Udomworarat P., Miki C., Ichikawa A. Fatigue and ultimate strengths of concrete filled tubular k-joints on truss girder, Journal of Structural Engineering, ASCE, 2000(46).pp.1627-35. [5] Sakai Y., Hosaka T., Isoe A. Experiments on concrete filled and reinforced tubular K-joints of truss girder, Journal of Constructional Steel Research 2004(60).pp. 683-99. [6] Furuichi K., Hishiki Y., Yoshida K. The proposal of a design method of the joint structure for the steel/concrete truss bridge, Proceedings of the Japan Society of Civil Engineers 2006(62), pp.349-66 (In Japanese). [7] Sato Y., Hino S., Sonoda Y. Study on load transfer mechanism of the joint in hybrid truss bridge, Journal of Structural Engineering JSCE 2008(547) p. 785 (In Japanese). [8] ANSYS Release11.0. ANSYS University Advanced. ANSYS Inc., 2005. [9] Ministry of communications. General Code for Design of highway bridges and culverts, Beijing: China Communication Press, 2004 (In Chinese). [10] Ministry of communications. Code for design of highway reinforced concrete and prestressed concrete bridges and culverts. Beijing: China Communication Press, 2004 (In Chinese).