10th International Conference on Short and Medium Span Bridges

Quebec City, Quebec, Canada,

July 31 – August 3, 2018

ROAD BRIDGE CROSSING THE HUANGYAN HIGHWAY: A RECTANGULAR CONCRETE-FILLED STEEL TUBULAR TRUSS BRIDGE WITH PERFOBOND LEISTER RIBS

Jiang, Lei1, Liu, Yongjian2,6, Fam, Amir3, Liu, Bin4 and Xiong, Zhihua5

1 Chang’an University, China, and Queen’s University, Canada

2 Chang’an University, China

3 Queen’s University, Canada

4 Chang’an University, China

5 Tongji University, China

6 lyj.chd@gmail.com

Abstract: The road bridge crossing the Huangyan Highway, north of Huangling county, is the first rectangular concrete-filled steel tubular truss girder bridge in China. The total length of the bridge is 84 m including three spans with the longest span being 40 m and the truss depth is 2.5 m. The first structural innovation is using a rectangular steel tube in the lower chords with concrete filling at the hogging regions in compression, making use of the concrete confinement effect. Similarly, concrete-filled steel box sections were used for aesthetically pleasing Y-shaped piers. Another technical innovation is the use of longitudinally welded Perfobond Leister Ribs (PBLs) inside the inner faces of the chord and pier box sections. The static and fatigue designs of the joints of the welded steel truss are key considerations in design. Numerical analysis is performed for the innovative rectangular concrete-filled tubular joints stiffened with PBLs in the chords. This analysis demonstrated the much enhanced performance of this design compared to conventional rectangular hollow section joints. Construction of the road bridge has been completed in October 2015. The construction duration was only two months and an economic analysis comparing this design with conventional prestressed concrete girder system is also completed.

1. INTRODUCTION

Composite tubular truss bridges have been widely used in design because of their high efficiency and aesthetics. The steel tubular truss and concrete deck are combined in an efficient composite system, where the deck slab bears compression along with the compressive chord in the positive bending moment regions. On the other hand, for continuous girder bridges, the deck slab is under tension and no composite action can be considered in the negative bending moment regions, and as such some measurements have been proposed to optimize the structure at the internal supports of continuous girder bridges.

One of the major design optimizations is the tubular truss bridge with a concrete lower flange as adopted for the Nantenbach bridge in Germay, and the viaduct over Ulla River in Spain. Both bridges use a rectangular hollow steel tubular truss with a variable depth. The double composite action concept, the top deck slab and the concrete lower flange with their corresponding tubular chord could bear compression in the positive bending moment regions and negative bending moment regions, respectively, which is better suited for meeting strict clearance requirements. Another design optimization is to fill the lower chord with concrete, making full use of the confinement effect of concrete to improve its compressive strength. A typical application is Ganhaizi bridge in China. When circular hollow steel tubes are used, welding at the joints and connections between the steel truss and deck slab pauses some challenges and a cast-in-place concrete deck system is used. For this reason, the road bridge over Huangyan Highway in China using concrete filled steel tubes only in the negative bending regions was designed. The manufacture quality of the rectangular section truss is more easily guarantied and the precast concrete deck can be cast with arrangements to accommodate the prefabricated shear connectors on the top of truss, which make the cost more competitive compared to circular sections. Additionally, in order to compensate for the lower confinement efficiency of rectangular sections as opposed to circular sections, the longitudinally welded PBLs inside the chord faces were proposed to enhance the bonding behavior of the chords and the mechanical behavior of the joints.

1. LAYOUT OF THE BRDIGE

Huangyan Highway, connecting the cities of Huangling and Yanan, in Shaanxi province, China, is part of the Baomao Highway which runs from Baotou in Inner Mongolia to Maoming in Guangdong province. The pilot project, Road Bridge Crossing the Huangyan Highway, is the first rectangular concrete-filled steel tubular truss bridge in China. The total length of the bridge is 88 m that covers three spans (24+40+24 m) (Figure 1). The bridge has a straight alignment in plan with a 0.84% slope in elevation. To add an aesthetically pleasing effect and ensure that the bridge would blend into the surroundings, a Y-shaped rectangular concrete-filled steel tubular pier design was adopted.

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(a)

Figure 1: Elevation drawing of the road bridge crossing the Huangyan Highway, China: (a) Schematic of finished bridge (Unit: m); (b) Photograph of the finished bridge

1. OPTIMIZATION OF TRUSS CHORDS AND PIER COLUMNS

Rectangular tube sections are more convenient to fabricate and weld than circular steel sections. This made the rectangular sections ideal for welding shear connectors to the inner surfaces. Because of its ease of constructability, Liu et al. 2014 proposed an innovative rectangular concrete-filled steel tube (RCFST) system stiffened with PBL (Figure 2), and applied it to both the lower chord of the truss girder and the pier. Extensive research related to the compressive behavior, flexural behavior, bond behavior, and buckling performance of this system have been conducted (Liu et al. 2015, Liu et al. 2017, Gao et al. 2017). The results indicate that welding PBLs inside the steel tube walls offers the following functions:

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(b)

Figure 2: RCFST stiffened with PBL: (a) Diagram; (b) Photograph

a) PBL is used as shear connector to enable the shear transfer between the steel profile and concrete core. The interaction between the concrete and PBL connectors with holes in them is effective to prevent slip and debonding failure. b) PBL has the function of a stiffener, which improves the local stability of the steel tube. Figure 3 shows the buckling mode of a steel plate stiffened with PBL and other without it. Under uniform axial load, the behavior of the plate changes from a single wave buckling to a double wave buckling. This enables a larger width-to-thickness ratio for the steel plate in design. c) Comparing with circular-concrete filled steel tubes, the confinement effect in RCFST without PBL is lower. However, providing PBL can directly bear additional axial load as part of the steel tube. Additionally, it restrains the out of plain movement of the flat wall by having it anchored to the concrete core. This enables the wall to be better engaged in confinement. Figure 4 illustrates this point by showing that the confined region of RCFST stiffened with PBL is larger than conventional RCFST.

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Figure 3: Buckling modes under uniform load: (a) RCFST wall; (b) RCFST wall stiffened with PBL

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Figure 4: Effective confinement area of: (a) RCFST; (b) RCFST stiffened with PBL

1. DESCRIPTION OF THE SUPERSTRUCTURE

3. Main Truss Girder

The elevation arrangement of the main truss girder is Warren type. The height of the truss from the upper edge of the deck slab to the lower edge of the lower chord tube is 2.5 m, and is constant along the span. Figure 5 shows a cross-section of the bridge, including the road and the two truss girders. The total width of the cross-section is 5.5 m and the width between the outer edges of the two truss girders is 3.08 m. The slenderness (L/H) ratio of this lightweight superstructure is 16, where L is the longest span and H is the height. Compared to the triangular arrangement of the bridge cross-section, the stresses in the lower chords are smaller and the lateral stability is better in the rectangular arrangement (Figure 5).

Figure 5: Cross section (Unit: mm)

The truss consists of 300 mm × 300 mm rectangular tube sections for the lower chords and 300 mm × 200 mm for the diagonals. The upper chord consists of three PBL steel plates to form a 300 mm × 360 mm closed section embedded into the concrete slab (Figure 6(a)). Lower lateral bracings are placed at 4 m spacing using H shape sections (Figure 6(b)). The steel plates are up to 16 mm thick in the upper chords.

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Figure 6: Detail of upper chord: (a) Simulation of upper chord; (b) Photograph of upper chord

Another new innovation in this bridge is filling the steel tubes with concrete at the hogging sections. Also, the deck slab was prestressed for crack control. The concrete fill in the lower chord was provided only within a 2.5 m length at end supports and a 13 m length at pier supports. Figure 7 shows the calculated compressive stress in the steel tube of the lower chord for the cases of hollow tube and concrete-filling at hogging regions. It can be seen that the maximum compressive stress in the RCFST system is 188 MPa, which is 26% lower than that of the hollow section (253 MPa).

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Figure 7: Comparison of compressive stresses in the steel tube of lower chords (Unit: MPa) when: (a) concrete fill was provided at hogging regions; (b) no concrete fill was provided

3. Truss Joints

The joints design is important for truss bridges due to the complex state of stress in these regions where members meet. The spatial fully welded RCFST joints stiffened with PBL are adopted at the hogging cross-section. Both the chords and braces have the same width of the members. Welded rectangular section joints are favored by engineers because of their simple configuration and regular shape compared to the circular hollow section joints and bolted joints. The RCFST joints stiffened with PBL offer several advantages as will be discussed next.

1. Static Behavior

For traditional hollow section joints, the typical failure modes are chord face (top flange) plastification for width ratio (β) less than 1 (Figure 8 (a)) and chord side wall failure for β=1, where β is brace width-to-chord width ratio. Filling concrete in chord is expected, as a rigid base, to limit the inward deflection on the chord face. The failure mode is governed by the tension brace transforming from the chord face plastification to the punching shear failure of the chord. In addition, the shear action between concrete and PBL limits the outward deflection on the chord face. The design concept of joints having β=1 enables the transfer of the load to the chord web. Therefore, the innovative joints are more likely to exhibit the shear yielding of chord at the gap (Figure 8 (b)).

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Figure 8: Failure mode comparison of different joints: (a) Chord face plastification for rectangular hollow section joints (β<1); (b) Shear yielding of chord in the gap of RCFST joints stiffened with PBL (β=1)

1. Fatigue Behavior

The fatigue behavior of welded joints is a well-recognised problem in the design of tubular truss bridge. The hot spot stress method is a common practice in design codes. In this method, stress concentration factor (SCF) is a dimensionless factor that is used to quantify how concentrated the stress is in a material. The high stress concentrations at the weld toes in the chord are mostly caused by local bending, a result of chord face deformation for rectangular hollow section joints. A review of current literature shows that concrete-filling of the chord is an effective way to reduce the stress concentrations. Furthermore, PBL welded inside the chord also can reduce the local bending of chord. Two finite element models are established to compare the SCF between rectangular hollow section joints and RCFST joints stiffened with PBL (Figure 9). A 1 MPa tensile stress is applied on the brace and a quadratic extrapolation method is used for the rectangular section (Figure 10).

Figure 9: Finite element model of the joint

Figure 10: Boundary condition and quadratic extrapolation

As shown in Figure 11, the maximum SCFs in the chord for the RCFST joint stiffened with PBL and the hollow section joint are 1.95 and 2.71, respectively. The maximum SCFs in the brace for the RCFST joint stiffened with PBL and the hollow section joint are 2.65 and 3.52, respectively. The results reveal that PBL limit the outward deformation of top plate, reducing the bending stress and making the stress distribution more uniform at the weld toe. Therefore, the maximum SCFs on the chord and brace for stiffened RCFST joints reduce by 25% compared with the maximum SCFs in hollow section joint. Overall, the fatigue behavior of RCFST joint stiffened with PBL is improved.

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Figure 11: Maximum SCFs comparison of different joints with and without a concrete fill: (a) in the chord; (b) in the brace

1. SUBSTRUCTURE

The substructure was essentially a RCFST Y-shaped pier stiffened with PBL. This design has been inspired by the branches of tress (Figure 12). Each pier consists of two Y-shaped columns connected by three horizontal braces. The cross-section of each column is 400 mm × 300 mm × 16 mm. The tubes in the pier were manufactured using structural steel of grade Q345 that conforms to the Chinese Standard GB/T 700, in which grade Q345 steel has a minimum nominal yield stress of 345 MPa. According to the finite element analysis of the pier, the maximum compressive stress in the tube at service is 155 MPa which is equivalent to only 56% of the allowable stress in Q345 steel. The concrete core and inner PBL stiffener also help resisting compression. At the top of the pier, the tube member is directly welded to the bottom of the lower chord (Figure 13). As for the connection between pier and the pile cap, several PBL plates are embedded in the pile cap to achieve a rigid connection (Figure 14).

Figure 12: Design concept of pier

Figure 13: Connection between main truss and pier

Figure 14: Connection between pier and pile cap

1. MANUFACTURING AND CONSTRUCTION OF BRIDGE COMPONENTS

Steel structures enable accelerated construction. This bridge took only two months to construct. As for the manufacturing, the main truss is divided into several prefabricated portions with lengths of about 10 to 12 meters. The pier is divided into the irregular Y-shape segment and the tube straight segment. After manufacturing, the steel segments are then transported to the bridge site. The construction sequences are described as follows: a) erecting the tube segment of pier to pile cap, establishing the scaffolding system, erecting and assembling the Y-shape segment (Figure 15), b) assembling the truss girder, hositing the span on the left end support and then the span on the left pier support, c) hositing the span on the right end support and then the span on the right pier support, d) hositing the closing span, casting the concrete in the lower chords (Figure 16 shows the construction progress of truss girder), and e) the completed truss girder serve as a scaffolding system to cast the deck slab, the framework is established and reinforcement is assembled, concrete deck slab is then cast-in-place (Figure 17 shows the construction progress of deck slab).

Figure 15: Construction progress of pier

Figure 16: Construction progress of truss girder

Figure 17: Construction progress of deck slab

1. ECONOMIC ANALYSIS

In order to understand the economic advantages of this innovative bridge, the frequently used bridge design in China, namely prestressed concrete box girder bridge, was selected to make comparison for the same span arrangement (Table 1) in terms of quantities. In terms of superstructure, the steel quantity ratio between scheme 1 (proposed design) and scheme 2 (conventional design) is (1.3:1). Counting the substructure and foundation quantities, the economic advantage of scheme 1 is obvious because of the transparent and lightweight pier. The total steel and total concrete quantity ratios between scheme 1 and scheme 2 are 0.85:1 and 0.70:1, respectively.

Table 1: Cost analysis of quantities comparing proposed new design with prestressed box girder bridge

Scheme

Spans

(m)

Superstructure

Substructure and foundation

Total

Concrete

(m3/ m2)

Steel

(kg/ m2)

Concrete

(m3/ m2)

Steel

(kg/ m2)

Concrete

(m3/ m2)

Steel

(kg/ m2)

Scheme1: RCFST truss girder

24+40+24

0.528

308.9

1.135

78.5

1.663

387.5

Scheme2: PC box girder

24+40+24

0.929

238.2

1.433

219.7

2.361

457.9

1. CONCLUSIONS

The application of RCFST with PBL is a new innovation in road bridge engineering in China. It has been demonstrated that this design improves the interfacial composite action and bond properties, buckling behavior and confinement effect. Numerical analysis of RCFST joint stiffened with PBL has shown that, failure mode of the joint is changed and stress concentration factor is approximately 25% lower compared with conventional rectangular hollow section joint. Accelerated bridge construction is easier to achieve using RCFST truss bridges and the economic advantage compared to conventional prestressed concrete box girder bridges has been demonstrated.

1. ACKNOWLEDGEMENTS

This project was funded by National Key R&D Program of China (Grant 2016YFC0701202), the National Natural Science Foundation of China (Grant 51378068) and the Fundamental Research Funds for the Central Universities (Grant 310821175015). The authors are grateful to Dustin Brennan at Queen’s University, for his editing of the manuscript.

1. REFERENCES

Gao, Y.M., Liu, Y.J., Jiang, L., Liu, J.P., Liu, X.H. 2017. Experimental Research on Flexural Behaviour of Rectangular Concrete Filled Steel Tubular Truss Stiffened with PBL. Journal of Architecture and Civil Engineering, 34(5): 171-180. (in Chinese)

Liu, Y.J., Cheng, G., Zhang, N., Zhang, J.G. 2014. Experimental Research on Concrete-filled Square Steel Tubular Columns Stiffened with PBL. Journal of Building Structures, 35(10): 39-46. (in Chinese)

Liu, Y.J., Xiong, Z.H., Luo, Y.L., Cheng, G., Liu, G., Yang, J. 2015. Double-composite Rectangular Truss Bridge and Its Joint Analysis. Journal of Traffic and Transportation Engineering (English Edition), 2(4): 249-257.

Liu, Y.J., Xiong, Z.H., Feng, Y.C., Jiang, L. 2017. Concrete-filled Rectangular Hollow Section X Joint with Perfobond Leister Rib Structural Performance Study: Ultimate and Fatigue Experimental Investigation. Steel and Composite Structures, 24(4): 455-465.

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