Scholarly article on topic 'Double-composite rectangular truss bridge and its joint analysis'

Double-composite rectangular truss bridge and its joint analysis Academic research paper on "Civil engineering"

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Abstract of research paper on Civil engineering, author of scientific article — Yongjian Liu, Zhihua Xiong, Yalin Luo, Gao Cheng, Ge Liu, et al.

Abstract This paper describes a novel composite tubular truss bridge with concrete slab and concrete-filled rectangular chords. With concrete slab plus truss system and joints reinforced with concrete and Perfobond Leiste rib, double composite truss bridge proved to be a fairly suitable solution in negative moment area. Perfobond Leiste shear connector (PBL) is widely implemented in the composite structure for its outstanding fatigue resistance. In this pilot bridge, Perfobond Leister ribs (PBR) were installed in the truss girder's joints, which played double roles as shear connector and stiffener. An erection method and overall bridge structural analysis were then presented. Typical joints in the pilot bridge were selected to analyze the effect of PBR. Investigation of the effect of PBR in concrete-filled tubular joints was elaborated. Comparison has revealed that concrete-filled tubular joints with PBR have much higher constraint capability than joints without PBR. For rectangular tubular truss, the punching shear force of the concrete filled joint with PBR is approximately 43% larger than that of the joint without PBR. Fatigue performance of the joint installed with PBR was improved, which was found through analysis of the stress concentration factor of joint. The PBR installed in the joints mitigated the stress concentration factor in the chord face. Therefore, the advantages of this new type of bridge are demonstrated, including the convenience of construction using rectangular truss, innovative concept of structural design and better global and local performances.

Academic research paper on topic "Double-composite rectangular truss bridge and its joint analysis"

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Original Research Paper

Double-composite rectangular truss bridge and its joint analysis

Yongjian Liu a'*, Zhihua Xiong a,b, Yalin Luo a, Gao Cheng a, Ge Liu a, Jian Yang c,d

a School of Highway, Chang'an University, Xi'an 710064, China b CCCC First Highway Consultants Co., Ltd., Xi'an 710075, China

c Department of Civil Engineering, The City College of New York, New York, NY 10031, USA d McLaren Engineering Group, West Nyack, NY 10994, USA

ABSTRACT

This paper describes a novel composite tubular truss bridge with concrete slab and concrete-filled rectangular chords. With concrete slab plus truss system and joints reinforced with concrete and Perfobond Leiste rib, double composite truss bridge proved to be a fairly suitable solution in negative moment area. Perfobond Leiste shear connector (PBL) is widely implemented in the composite structure for its outstanding fatigue resistance. In this pilot bridge, Perfobond Leister ribs (PBR) were installed in the truss girder's joints, which played double roles as shear connector and stiffener. An erection method and overall bridge structural analysis were then presented. Typical joints in the pilot bridge were selected to analyze the effect of PBR. Investigation of the effect of PBR in concrete-filled tubular joints was elaborated. Comparison has revealed that concrete-filled tubular joints with PBR have much higher constraint capability than joints without PBR. For rectangular tubular truss, the punching shear force of the concrete filled joint with PBR is approximately 43% larger than that of the joint without PBR. Fatigue performance of the joint installed with PBR was improved, which was found through analysis of the stress concentration factor of joint. The PBR installed in the joints mitigated the stress concentration factor in the chord face. Therefore, the advantages of this new type of bridge are demonstrated, including the convenience of construction using rectangular truss, innovative concept of structural design and better global and local performances. © 2015 Periodical Offices of Chang'an University. Production and hosting by Elsevier B.V. on behalf of Owner. This is an open access article under the CC BY-NC-ND license (http://

creativecommons.org/licenses/by-nc-nd/4.0/).

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Article history:

Available online 6 June 2015

Keywords: Bridge engineering Composite truss bridge Joint analysis Perfobond rib Concrete-filled tube

1. Introduction

The composite tubular truss bridge has been built in western countries for 20 years since the closure of well-known pilot

project Nantenbach Railroad Bridge in 1993 (Brozzetti, 2000; Hanswille, 2008). The main span of Nantenbach Railroad Bridge is 208 m with two side spans of 83.2 m excluding approach bridge (Fig. 1(a)). The cross section of superstructure

* Corresponding author. Tel.: +86 29 82334577.

E-mail addresses: steellyj@126.com (Y. Liu), xiongzhihua_2013@126.com (Z. Xiong). Peer review under responsibility of Periodical Offices of Chang'an University. http://dx.doi.org/10.1016/j.jtte.2015.05.005

2095-7564/© 2015 Periodical Offices of Chang'an University. Production and hosting by Elsevier B.V. on behalf of Owner. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

(a) (b)

Fig. 1 - Typical composite bridges. (a) Nantenbach Railroad Bridge (Germany). (b) Arroyo Las Piedras Viaduct (Spain). (c) St. Kilian Viaduct (Germany). (d) Ganhaizi Bridge (China).

consists of two main rectangular truss frames spacing 6 m in transverse direction. The bottom chord is filled with concrete at piers to improve its stiffness. Top concrete slab and bottom composite sections form a double-composite structural type which has an outstanding performance in heavy and fatigue loading while train speeds over (Schwarz et al., 1992). This double-composite concept is adopted in Arroyo Las Piedras Viaduct, part of high speed railway line in Spain (Mato et al., 2007) (Fig. 1(b)).

At the mean time, circular hollow tube is an alternative choice in composite tubular truss bridge design. Typical applications such as St. Kilian Viaduct and Ganhaizi Bridge are shown in Fig. 1(c) and (d) respectively (Dorrer, 2007; Wang et al., 2011). St. Kilian Viaduct is a continuous bridge with maximum span of 61.5 m. The cross section is a three-chord system - a main circular truss tube (F610 mm) with a height of 5 m and two web truss tubes (F298.5 mm). The joints of bottom chord are cast iron. Ganhaizi Bridge is located in Ya'an, which is an area with a high risk of earthquake in China. The weight of composite truss bridge is significantly lighter than reinforced concrete bridge, which makes a best

solution for high seismic risk zone. However, since the curvature of the circular truss tube should be under restrict control during fabricating and welding (Liu et al., 2010), the manufacture quality of rectangular truss is more easily guarantied than the circular truss. The cost and simplicity of circular truss are less competitive than that of rectangular truss.

Substantial experiments (Liu et al., 2010; Packer, 1995; Sakai et al., 2004) and analysis of tubular structure indicate that the joints' strength and fatigue performance are governing factors in design (Hou et al., 2013; Liu et al., 2009; Machacek and Cudejko, 2009; Schumacher et al., 2009; Shi et al., 2012; Wang et al., 2013). In this paper, perfobond ribs (PBR) have been settled in chord joints for better performance than the hollow tubes. Based on the knowledge of double-composite action and special configuration of joints, the author proposed a new type of composite bridge fitting for the span range of 30-80 m, which is made of rectangular truss chord with concrete-filling and prestressed concrete slab. The bottom concrete-filled chord combining with the prestressed concrete slab forms a double-composite system.

2. Pilot composite rectangular tubular truss bridge

2.1. Structure overview

There is a pilot composite truss bridge which is under construction to validate its structural mechanical characteristic. The bridge is flying over a road with spans made up of 24 m + 40 m + 24 m (Fig. 2). The total width of cross section is 5.5 m, including 3.0 m wide carriageway, 1.5 m wide pedestrian pavement and 1.0 m of two barriers.

The bridge is situated on a straight alignment in plane and on a slope of 0.84% in vertical alignment. In terms of the durability, it is designed as an integral bridge through additional components welded on the truss girder at piers extending to the concrete piers. Movable bearings are set in the abutments. The substructure consists of 1.8 m pile foundation and short stem seat abutment.

2.2. Truss system design

The general layout of truss is shown in Fig. 3. Two main truss girders are 2.78 m apart with a depth of 2.5 m. The general

_9796_ _

¡;490^, 2400 t|1 4000 (|< 2400

J Unit: cm

Fig. 2 - Pilot composite tubular bridge elevation.

Fig. 3 - Typical section of the superstructure.

Fig. 4 - Tendon scheme.

shape of up and bottom chords is a 300 mm x 400 mm x 14 mm rectangular tube. The truss section height increases to 500 mm linearly near support. The top and bottom chords are filled with concrete. The diagonal truss is made of 250 mm x 250 mm x 14 mm hollow rectangular steel tube except the tubes in hogging regions which are concreted and increased to 400 mm x 250 mm x 14 mm. Horizontal link beam is placed in 4 m spacing using H-shaped section.

2.3. Deck design

The top slab is designed as precast concrete component and stored in factory for at least half a year to reduce the shrinkage and creep action. Based on the provision of Eurocode 4-2, the concrete slab is prestressed with longitudinal tendons to limit the crack width of the slab near support.

Fig. 4 demonstrates a plan view of the tendon lay-out. The type of slab tendon is 15-12 (2176.4 mm2). The slabs are classified into different types due to anchors and length variation. Each segment of slab has been left with holes to

cast concrete after the tendons prestressing, which is called post-connected slab system. Every slab segment forms a whole slab through construction joint. The deck distributes the live and dead loads through the shear studs welded at top chords to the truss girder.

2.4. Joint with perfobond rib design

The perfobond rib is installed in each joint in the bridge to enhance the static and fatigue performance. The standard truss segment is shown in Fig. 5, and the configuration of PBR is shown in Fig. 6. It is made of 14 mm thick steel sheet aligned with opening of diameter 40 mm in a row. The angle between two diagonal braces is 78.2°.

3. Erection of the bridge

To eliminate the tensile stress in concrete slab near support produced by the bottom chord and filled concrete in sagging

Fig. 5 - Standard truss segment.

Fig. 6 - PBR configuration.

region, concrete in the bottom chord is filled when the whole major bridge structure is done before the wearing of the deck. The major erection sequence is described as follows.

Phase 1: Constructing the substructure, establishing the scaffold;

Phase 2: Assembling the truss girder, casting the concrete in the top chord and the joints near support; Phase 3: Placing the precast slabs in sagging regions in the midspan and sidespan, casting the construction joints of the slab;

Phase 4: Placing the precast slabs in the hogging regions

near support, casting the construction joints of the slab;

Phase 5: After hardening of the concrete in the hogging

regions, prestressing the tendons of the slabs;

Phase 6: Concreting the shear stud pockets left;

Phase 7: Removing the scaffolds supporting the

superstructure;

Phase 8: Casting the concrete in the bottom chord, ending up with the deck wearing and accessory component.

4. Structural analysis of overall bridge

With the structural analysis software Midas Civil, overall bridge was discreted as beam elements in finite element analysis to investigate the stress status during construction and service. The truss system is constraint in the main pier and with movable link in the abutment. During the stage of placing concrete slab, the upper chord showed up the maximum compressive stress 100.3 MPa in midspan (Fig. 7). The maximum tensile stress 155.2 MPa was found in hogging moment in the upper chord. Under the combination of serviceability limit state, the maximum of compressive stress occurs in the same place as construction stage, while the maximum tensile stress 154.2 MPa took place in web near bearing (Fig. 8). Since the effect of prestressed tendon, the concrete slab in the hogging moment area remained in compression in serviceability limit state. The maximum tensile stress 3.5 MPa was found in the anchorage spot in the slab (Fig. 9).

Fig. 7 - Normal stress of truss in the stage of placing concrete slab (unit: MPa).

Fig. 8 - Normal stress of truss in the load combination of serviceability limit state (unit: MPa).

Fig. 9 - Normal stress of slab in the load combination of serviceability limit state (unit: MPa).

5. Investigation of PBR installed in joints

5.1. Static strength analysis

The joints of the bridge could be classified into two types. One is the standard joints, the other is the joints next to piers. In standard joint, four PBR are welded inside the chord faces, diagonal webs are hollow rectangular tubes. While in joints next to piers, diagonal webs are filled with concrete to increase its stiffness, and the size of section is also larger than standard joints. In order to find out the effect of PBR installed in standard joints, ultimate load tests are carried out analyzing four types of joints as shown in Table 1. In the table, and "x" denote with and without specifically. Dimensions of the four types of joints are originated from standard joints in the bridge. Detail of the section information is listed in Table 2, where b is width, h is height, t is thickness, b is width ratio of web to chord, 6 is inclined angle. Numerical simulation set-up is similar with the experiment of Sakai et al. (2004) using software ABAQUS. In the load process, one diagonal web behaves as tension member, the other behaves as compression member. At the bottom and end of the chord, two plates are set as contact blocks to limit its transverse translation (Fig. 10(a)). In the finite element model, the chord and web are simulated with shell elements (S4R), while the concrete is simulated as solid elements (C3D8R) with concrete plasticity damage model. The material of steel truss is Chinese standard Q345 with yield strength of 345 MPa, and the Mises plasticity model of steel is incorporated in FEM. The interface behavior between concrete and chord is modeled as friction contact. The friction coefficient is 0.3. The PBR is modeled as a solid element which is embedded in the concrete element.

Table 1 - Classification of joints.

Joint Classification

J1 Concrete-filled (V)

PBR (V)

J2 Concrete-filled (V)

PBR (x)

J3 Concrete-filled (x)

PBR (V)

J4 Concrete-filled (x )

PBR (x )

Table 2 - Dimension of joints.

Component b (mm) h (mm) t (mm) b flf)

Web 250 250 14 0.625 67.6

Chord 300 400 14 0.625 67.6

PBR - - 14 - -

The result of joint components is shown in a sequence of concrete plastic strain, chord plastic strain and PBR strain (Fig. 10(b)-(d)). It should be pointed out that, concrete and steel plastic strains are defined as 0.0006 and 0.0015 respectively. PBR installed could significantly improve the concrete assistance while loading, which is indicated by comparison between J1 and J2 (Fig. 10(b)). Reaching the peak load, the concrete axial plastic strain is attained in the thickness path as shown in Fig. 11. Concrete in J1 is almost in a crush state while concrete in J2 is basically in elastic phase. Since the load mode, the failure mode is punching shear (Fig. 10(c)). J1's ultimate strength is 4271 kN which ranks the first in three specimens indicated in Fig. 12. According to the equation proposed by Packer (1995).

fyt ( _2^ + b + b Pssin(q) Wn(q) + b + bep

where Fk is design punching shear force, bep is effective punching shear width, fy is steel yield strength.

The design punching shear force in J2's tensile web Fk-J2 equals to 2649 kN. With the assumption of the same safety factor consideration, the design punching shear force in J1's tensile web Fjj could be determined using the data in Fig. 12, which equals to 3799 kN. This method applies to the derivation of J3's Fk-J3 = 1236 kN based on the empirical formula result of J4's Fk-J4 = 1015 kN. In view of connection effect, PBR is an effective component which utilizes the maximum strength of concrete and chord. In typical hollow rectangular truss joint, PBR works as a stiffener plate to improve its stiffness.

Stress concentration factor

Fatigue verification is an essential point which needs to be carefully handled in tubular truss bridge design. The hot spot stress method is a common practice in provisions drafted by International Institute of Welding (IIW) and Comite International pour le Developpement et l'Etude de la Construction Tubulaire (CIDECT). The core of the hot spot stress method is to determine the stress concentration factor (SCF) of the joints. As the SCF is obtained, either the fatigue life or the acceptable nominal force range could be calculated from the S-N curve. Many experiments and researches have been done to find out the SCF of the hollow circular and rectangular tubular joint. The SCF of the circular concrete-filled tubular joint has been proved to be different from the non-filled joint (Wang et al., 2013). A finite element model is established to obtain the SCF of the concrete-filled joint (J2) and joint with PBR (J1), which could supply important data for the fatigue evaluation of the pilot composite bridge proposed by the author. Hollow section (J4) is also analyzed as a verification of the FEM because the SCF formula is already known. For

Fig. 10 - FEM model and results of joints. (a) FEM of joint. (b) Concrete plastic strain comparison between J1 and J2. (c) Typical punching shear failure mode of specimens. (d) PBR plastic strain at failure of J1.

the special purpose of the FEM, it is strictly built to meet the requirement of fatigue strength analysis of offshore steel structures drafted by Det Norske Veritas (DNV). The hot spot's area element length is about 6-8 mm (0.5t). The element type is 20-noded solid element (C3D20). The area far from the hot spot is modeled as an 8-noded element with a coarse mesh (Fig. 13). PBR in J4 is simulated as a solid element embedded in concrete as the same in Section 3.

Fig. 11 - Concrete axial plastic strain comparison through depth.

Symmetry is used to reduce the computation scale. The load mode is self-balanced with the tension of 10 MPa in web. Quadratic extrapolation curve is used for the fitting of SCF. Extrapolation distance is 0.4t-1.4t (Wingerde, 1992) (Fig.14).

As shown in Figs. 15 and 16, SCF for J1 and J2 are totally different. Concrete in joint has mitigated the SCF in web, but it has little effect on chord as shown in J2. PBR installed in joint has significantly improved the SCF in chord, while it increases the SCF in web. The FEM result of SCF of J4 for verification

5000 4000

/ fi-i: —n — J2 --J3 ...... J4

fl /■

10 15 20 25 30

acemen

ension)(mm

Fig. 12 - Load-displacement of four joints.

Fig. 13 - 1/2 FEM of J2.

Weld toe Distance

Fig. 14 - Load condition and hot spot stress.

° tr-

-o- Chord

-O Web

4 8 12 16 20 24 Distance from weld toe(mm)

Fig. 15 - SCF of J1.

turns out to be 6.03 in web and 11.75 in chord respectively (Fig. 17), which discloses a good accordance to the SCF value in web given by CIDECT (Table 3). Because the weld shape is not involved and the bond effect between concrete and steel is excluded in the FEM, it causes the SCF in chord to be

12 10 8

u 6 cn

-O- Chord

__ -O-Web

----- ....."T>—O—©—O

OAt I . l-4r I I

4 8 12 16 20 24 Distance from weld toe(mm)

Fig. 16 - SCF of J2.

Fig. 17 - SCF of J4.

Table 3 - SCF of joints.

Joint Chord Web

J1 2.36 9.13

J2 11.20 3.20

J4 (with accordance to CIDECT) 9.63 5.10

relatively higher than the value given by CIDECT. In a whole, the result is acceptable.

6. Conclusions

(1) A new type of composite tubular truss bridge is designed. The bridge's superstructure is a double composite section. It is composed of rectangular tubular chords with filled-concrete and prestressed concrete slab, which is expected to improve local strength and fatigue performance. To prevent the hogging region crack, the post-connected slab system and prestressed tendon are combined as erection methods which are validated in several construction practices.

(2) Through a numerical analysis of the four joints specimens originated from the design, for rectangular tubular truss, the punching shear force of the concrete-filled joint with PBR is approximately 43% larger than that of the joint without PBR. For hollow tubular truss, the joint with PBR's design punching shear force is approximately 22% larger than that of the joint without PBR. PBR is proved to enhance the concrete assistance performance in the truss chord while loading.

(3) The stress concentration factor of the pilot bridge joint is analyzed. It is found that PBR installed in joint of the pilot bridge could reduce the SCF of chord up to 30% than that of the hollow one.

(4) Segment static and dynamic experiments are needed to carry out to further the study of this innovative structure.

Acknowledgments

This research was supported by the National Natural Science Foundation of China (No. 51378068) and Foundation of

Transportation Construction Research for West China (No. 2013318812410).

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