Shear performance of high-strength bolt connector considering different pad and bolt hole

Abstract

Better than traditional stud connectors, the high-strength bolts enable rapid assembly of steel–concrete composite beams and replacement of decks. However, the researches on the shear performance of high-strength bolt connectors in prefabricated steel–concrete composite beams are relatively limited. Existing studies have not considered the effect of bolt holes in concrete slabs on structural performance. In this paper, the shear performance of high-strength bolt connector of prefabricated steel–concrete composite beams was investigated. The shear bearing capacity, shear stiffness and failure mode were obtained by push-out test. The impact of different bolt hole diameters and pad sizes were considered. Bolt hole diameter of 24 mm, 28 mm and 32 mm, pad size of 50 mm × 50 mm, 80 mm × 80 mm, 200 mm × 100 mm were used in the push-out tests. The results showed that all the specimens were damaged due to crushing of concrete along with shear deformation of bolts. Increasing the pad size can effectively reduce the cracking of concrete slab and improve the slip load and post-slip shear stiffness of specimens. The slip load and peak load of specimens are mainly dependent on the diameter of bolt hole. Specimens with small bolt hole achieve significantly higher pre-slip and post-slip shear stiffness compared to specimens with large bolt hole diameter.

Introduction

Steel-concrete composite beam has been widely used in bridge construction as a structure with good mechanical performance [1]. However, shear connection is an important feature in composite structure. Shear connector is the key component of the combination of steel beam and concrete slab. Its main function is to resist the relative sliding between steel and concrete slab and transfer the longitudinal shear force between steel beam and concrete interface. Welded stud connector is one of the most common connection methods. The mechanical properties of traditional welded stud connectors have been widely investigated, and the methods that can guide corresponding engineering design and construction have been proposed [2], [3], [4], [5]. However, traditional steel–concrete composite beams with welded stud connectors are mostly cast-in-place, which are not suitable for prefabrication and assembly, and cannot be quickly erected. In order to speed up the construction of steel–concrete composite beam bridges and realize the assembled construction of steel–concrete composite beams, a steel–precast concrete composite beam with high-strength bolts working as shear connector was proposed.

Dallam [6] conducted a feasibility study on the use of high-strength bolts as shear connectors for composite beams. In his research, the high strength bolts exhibited a greater useful capacity and ultimate strength than comparable studs. Pavlović et al. [7] performed push-out tests and finite element analyses. The research showed that bolt shear connectors with single embedded nut (grade 8.8) achieved approximately 95% of shear resistance for static loads of traditional welded headed studs shear connectors and has showed brittle behavior. Kwon et al. [8], [9] presented a test on three types of post-installed shear connectors including double nut bolt, high-tension friction-grip bolt and adhesive anchor under static and fatigue loading, which showed those post-installed bolt shear connectors had a significantly higher fatigue strength than conventional welded shear studs. Moynihan and Allwood [10] carried out three composite beams, of 2 m, 10 m and 5 m length, using M20 bolts as demountable shear connectors. The results showed that all specimens had higher strengths than predicted using Eurocode 4 [11]. In addition, the longer specimens had performance similar to comparable welded-connector composite beams. Chen et al. [12] performed static push-out tests on bolt shear connectors. Parameters such as bolt diameter, bolt pretension, and steel–concrete contact surface properties were considered. Their research showed that the bolt connectors exhibited a low initial slip load, however, achieved similar ductility and bearing capacity as conventional shear stud. Pathirana et al. [13] conducted experiments on full-scale beam specimens to study the feasibility of utilizing blind bolts as shear connectors to develop demountable steel–concrete beams. Finite element simulation on beam tests was also carried out. The experimental and numerical results showed that the ability of blind bolts in steel-concrete beam to realize and maintain the composite action is comparable to that of welded stud connectors under flexural load. Liu et al. [14], [15], Ataei et al. [16] conducted a series of experimental and theoretical studies on composite beams with geopolymer precast concrete slabs. The experimental program comprises of two series of tests. In the first series, push-out tests were conducted to investigate the ultimate bearing capacity and the load-slip characteristics of Post-installed Friction-grip Bolted Shear Connectors (PFBSCs). The results showed that load-slip response of the HSFGB shear connectors exhibited three distinct regimes compared with conventional headed stud shear connectors. The second series of tests were conducted on four full-scale composite beams. In all specimens, composite action between the steel girders and concrete slabs was provided by PFBSCs. The research showed that the bolts had good ductility compared with traditional shear connectors and this type of composite beams could be deconstructed easily at the end of the service life. Liu et al. [17] established a finite element model to study the mechanical properties of composite beams with bolts as shear connectors. The parametric study showed that the ultimate strength of composite beams increased with the increase of the degree of partial shear connection. In the composite beams with the same shear connection, the ductility and economy of the composite beams with large diameter bolts were better than those with small diameter bolts.

Zhang et al. [18] combined the numerical model and test results of push-out test with high-strength bolt connectors to investigate the effects of bolt pretension, the bolt diameter, and the compressive strength of concrete on the failure mode and the load-slip characteristics of high strength bolt connectors. The results showed that the shear bearing capacity of the bolt connector was mainly depended on the bolt diameter, the bolt tensile strength and the concrete strength. Moreover, design formulas to predict the shear bearing capacity of bolt connector under different failure modes were proposed. Abdolreza ataei et al. [19]applied low cycle and high amplitude loads to nine steel–concrete composite connections to investigate their cyclic behavior. In addition, three monotonic tests of steel–concrete composite connections were also carried out to evaluate yield slip, yield load, ultimate slip and ultimate shear strength.

The above reviews show that the existing studies mostly focused on the load-slip characteristics and ultimate strength of bolted shear connectors and ignored research on local structural measures and ease of installation. Considering the local crushing problem of concrete caused by excessive stress near the bolt nut, the influence of bolt pad size on the performance of the test piece should be investigated. In addition, deviations in bolt hole positioning often occurred during the fabrication of prefabricated slabs and steel beam. In order to ensure the quality of on-site installation, the constructor often wants the diameter of the bolt holes in the precast concrete slab to be as large as possible. Therefore, based on the parameters of the pad size and the bolt hole diameter, five groups of push-out specimens were tested under static loading in this paper. Cracking of the concrete slab, load-slip characteristics, as well as influence of bolt hole diameter and pad size on the shear bearing capacity and shear stiffness of the specimens were analyzed. According to the influence of bolt hole diameter and pad size on shear bearing capacity, the design calculation formula of shear bearing capacity of bolt connector was put forward, which has reference value for the application of high-strength bolt connector in assembled composite beam bridges.