A numerical and theoretical analysis of the structural performance for a new type of steel-concrete composite aqueduct

Highlights

• A new type of steel-concrete composite aqueduct is proposed.

• Advantages of composite aqueduct compared with other aqueducts are demonstrated.

• Finite element model is established to study the properties of composite aqueduct.

• Parametric analyses are carried out in the longitudinal and transverse directions.

• A fiber model with shear lag is presented to facilitate the stress estimation.

Abstract

A new type of steel-concrete composite aqueduct, made of steel trough and concrete cover connected by shear studs, is proposed. The composite aqueduct has better fluid tightness, lighter weight, higher capacity and faster construction speed compared with traditional prestressed concrete aqueducts; therefore, it has a broad potential to be applied in water transfer projects. To study the structural performance of the composite aqueduct, a fine finite element model is established by using ABAQUS, whose results are representative of the typical spatial deformations experienced by the aqueduct. Parametric analyses are carried out in the longitudinal and transverse directions. The results indicate that the longitudinal stress is higher than the transverse stress, and a significant shear lag is found on the steel bottom plate. Thus, design parameters are dictated by the longitudinal stress of the steel bottom plate. In order to facilitate the estimation of the longitudinal stress, a fiber model that considers the shear lag effect is developed. The formula of the effective width is fitted based on the results of a series of finite element-based parametric analyses. The fitted formula is more accurate and simpler compared to several national codes, and the error of the stress is usually less than 10%.

Introduction

In some countries, the distribution of water resources is extremely uneven, thereby stunting economic development areas that receive less water. Thus, the subject of optimal allocation of water is a subject of national importance. Particular to China, the Dongjiang-Shenzhen water Diversion Project, South-North water Diversion Project and other large-scale water diversion projects have been built to relieve the serious water shortage in affected regions. A key structure used in water diversion is the aqueduct.

The aqueduct is an overhead flume for transporting water across canals, valleys, depressions and roads. The scale and size of old aqueducts are small, and the design is generally borrowed from bridge structure theory. However, the rise of large-scale water diversion projects saw a great increase in the implementation of large-scale aqueducts in China. For example, the 2009 Shahe aqueduct in the South-North Water Diversion project was constructed with a single span length of 30 m, a section width of 8.6 m, a height of 8.3 m, and a span-depth ratio of 3.61, and acted as a deep flexural medium-long shell member. The simply-supported beam structure with an open trough section has been used widely in large aqueducts. Therefore, the work in this paper centers around this kind of beam structure. In order to reduce volatilization and avoid pollution, the aqueduct built in the United States is usually a closed pipeline structure, which is quite different from the aqueduct with an open trough section [1], [2]. The designs of large aqueduct structures are characterized by a large cross-sectional size accompanied by large dead weight and water body weight. Thus, prior design methods and experiences gained from engineering ordinary bridges and old aqueducts may not be applicable for new, large aqueducts.

The large aqueduct structure belongs to a family of wide channel beams. Reports of experimental studies, theoretical analyses and engineering cases about highway and railway channel beams [3], [4], [5] can be found in literature. For channel beams used for traffic engineering, only the bottom plate is under traffic load, and the depth-width ratio is about 1:2 [3]. Thus, the transverse stress and lateral deformation are not significant. However, due to the heavy hydraulic pressure on both the bottom plate and webs, together with a distinct depth-width ratio of about 1:1, the spatial structural behavior of large aqueducts can be quite different from that of channel beams used for traffic engineering, thus necessitating further study.

Most traditional large aqueducts use prestressed concrete structures (as shown in Fig. 1), in which prestressed tendons are installed to prevent concrete cracking, which can lead to leakage. In order to simplify computational tasks, the aqueduct body is divided into longitudinal and transverse plane structures, which can be analyzed through structural mechanics theory [6]. The simplification may cause inaccuracies in computing the structural behavior of large aqueduct, and thus three-dimensional finite element analysis is usually used to assist the design [7]. Different types of loads and actions can be simulated using finite element software. Wang et al. [8] adopted finite element software ANSYS in the design optimization of concrete aqueducts, and the thermal stress was considered during modeling. Xu et al. [9] used ABAQUS software to investigate the damage mechanism of concrete aqueducts under earthquake excitation. Aqueduct structures with large sections and high piers were found to be more vulnerable to earthquake damage.

The wall thickness of prestressed concrete aqueducts is relatively large and contributes greatly to structural weight. Coupled with the heavy water weight the large aqueduct may be especially vulnerable to seismic excitations. Thus, there have been several research efforts targeted towards the better understanding of dynamic properties of prestressed concrete aqueducts. Wu et al. [10] and Zhang et al. [11], [12] studied the dynamic responses of a 3D aqueduct–water coupling system through the Arbitrary Lagrangian Eulerian (ALE) method. Results showed that the distribution and the intensities of hydrodynamic pressures and stresses in the aqueduct can change greatly due to large water sloshing effects induced by earthquakes or wind loads. The effectiveness of the prestressed steel bar arrangement can be greatly reduced by the change of distribution and value of stresses in the aqueduct structure. An advanced design for implementing adequate three-dimensional prestress may be needed as indicated by these findings. Furthermore, the transverse displacements of aqueducts with great water depths can be excessive in a seismic or wind event and thus necessitate damping equipment or bumper blocks [11].

As for the construction of prestressed concrete aqueducts, difficulties are focused on appropriately grading the three-dimensional prestress and grouting. In order to ensure crack resistance and overall stiffness of the aqueduct, a detailed construction process and quality control scheme must be implemented [13]. However, loss of prestress is an inevitable challenge in long-term operation. Loss of prestress can lead to a host of problems, such as cracking and leakage, resulting in a waste of water resources. At the same time, infiltration of water into cracks can cause the corrosion of rebar and generate hidden dangers. These problems have been exposed gradually for concrete aqueducts, and routine inspection and maintenance are needed to prevent sudden failure [14], [15], [16], [17]. Hadavandsiri et al. [14] developed a novel approach for automatic, preliminary detection of damage in concrete structures using ground-based terrestrial laser scanners, which was tested on a concrete aqueduct in Canada. Cheng et al. [15] presented a safety grade evaluation method of aqueduct structures based on fuzzy cloud theory analysis. They carried out a comprehensive evaluation of a U-type aqueduct in China using this method. Their results showed that it is necessary to pay attention to the maintenance and repair of the components to maintain adequate overall safety levels. For damaged aqueducts, engineering solutions were introduced by Wang et al. [16] and Liu et al. [17] to repair and reinforce cracked joints, concrete defects and erosions. However, the effectiveness of these solutions needs to be tested over time.

Therefore, the main problems facing the design and implementation of large aqueducts are as follows:

• The cracking control is the major problem for design. The technology and methods used to construct prestressed members are complex [13]. The cracking of the aqueduct body cannot be effectively avoided, and can lead to harmful leakage [14], [15], [16], [17].

• The heavy structural weight of the aqueduct, coupled with the immense water weight, causes the aqueduct structure to perform poorly against dynamic loads such as seismic excitation [10] and winds [11], [12].

• Research into the structural properties of large aqueducts is limited. The spatial structural behavior of large aqueducts under water load is not clear, and the design is usually based on a simplified method [6] that may be inappropriate for large open sections.

Steel-concrete composite structures have been widely used in large engineering projects. The forms of composite structures are varied, aiming at exploiting the respective advantages of steel and concrete to meet engineering requirements. For example, the two-way composite floor [18], [19] was applied to many long-span infrastructures to reap significant technical and economic benefits. Example of applications include the Wuchang Railway Station, and the Shenzhen Wanxuan Complex Building. Guo et al. [20] introduced the use of steel-concrete-steel composite structures in large-scale immersed tunnels, which improved the structural properties as well as construction efficiency. Steel-concrete composite girders composed of a steel box girder and a concrete slab were adopted in long-span suspension bridges, which were studied through overhanging tests by Li et al [21]. Wu et al. [3] conducted an experimental study on steel-concrete composite channel girder that was specifically applicable to railway bridges. The interior of the U-shaped steel beam is cast with concrete, and studs are used to ensure the cooperation of the steel and concrete. This new type of composite channel girder possesses many advantages, including good structural properties, convenient construction and low maintenance cost. To sum up, steel-concrete composite structures can provide effective solutions to various problems in large engineering projects and await implementation in large aqueducts.

In order to solve the problems experienced by traditional prestressed concrete aqueducts, innovation of the structural form is necessary. In this paper, a new type of steel-concrete composite aqueduct is proposed to comprehensively improve structural performance and provide an alternative choice for large-scale aqueduct projects. The section structure and construction method of steel-concrete composite aqueduct are described in Section 2. The advantages of composite aqueducts are demonstrated by comparison with prestressed concrete aqueducts. In Section 3, a three-dimensional elaborate model is established to investigate the spatial structural behavior of the composite aqueduct. Using this numerical model, parametric analyses are carried out along the longitudinal and transverse directions to study the influence of design parameters in Section 4. Numerical results suggest that the design of the composite aqueduct is usually controlled by the longitudinal normal stress of the steel bottom plate at the mid-span. Therefore, a fiber model with shear lag effect for computing the stress of the normal section is introduced in Section 5. The model provides an accurate and convenient method for estimating the longitudinal normal stress during the design stage.