تحقیق اجزاء محدود در رفتار سازه از اتصالات فولادی پیچ شده سرد

A finite element model with three-dimensional solid elements was established to investigate the bearing failure of cold-formed steel bolted connections under shear. It was demonstrated (Chung and Ip. Engineering Structures 2000;22:1271–1284) that the predicted load–extension curves of bolted connections in lap shear tests followed closely to the measured load–extension curves provided that measured steel strengths and geometrical dimensions were used in the analysis. Furthermore, it was shown (Chung and Ip. Proceedings of the Second European Conference on Steel Structures, Praha, May 1999, p. 503–506) that stress–strain curves, contact stiffnesses and frictional coefficients between element interfaces, and clamping forces developed in bolt shanks were important parameters for accurate prediction of the deformation characteristics of bolted connections. This paper presents an extension of the finite element investigation onto the structural behaviour of cold-formed steel bolted connections, and three distinctive failure modes (Ip and Chung. Proceeding of the Second International Conference on Advances in Steel Structures, Hong Kong, December 1999) as observed in lap shear tests are successfully modelled: bearing failure; shear-out failure; and net-section failure. Furthermore, a parametric study on bolted connections with different configurations is performed to provide bearing resistances for practical design, and the results of the finite element modelling are also compared with four codified design rules. It is found that the design rules are not applicable for bolted connections with high strength steels due to reduced ductility. Consequently, a semi-empirical design formula for bearing resistance of bolted connections is proposed after calibrating against finite element results. The proposed design rule relates the bearing resistances with the design yield and tensile strengths of steel strips through a strength coefficient. It is demonstrated that the design rule is applicable for bolted connections of both low strength and high strength steels with different ductility limits.

Galvanized cold-formed steel sections are found in various building applications, ranging from purlins and steel framings, to roof and wall cladding, and floor decking. The advantages of using cold-formed steel are derived from their long-term durability together with high yield strength and high buildability. In building construction, hot-rolled steel sections are commonly used as primary structural frames while cold-formed steel sections are used as secondary structural members to support claddings in forming external building envelopes. The most common cold-formed steel sections are C and Z sections, and the thickness of these sections typically ranges from 1.2 to 3.0 mm. Both steels with yield strength of 280 and 350 N/mm2 are commonly used. Connections between hot-rolled steel and cold-formed steel members are commonly achieved with bolts and hot rolled angles as web-cleats. At present, many design recommendations for cold-formed steel connections may be found in the literature which give design rules for the load carrying capacities of fasteners such as bolts, screws and rivets against bearing failure. However, they are empirical expressions developed from test data of specific ranges of material properties such as steel strength and ductility, and of geometrical dimensions such as steel thickness and bolt diameter. Most of the test data are derived from lap shear tests where deformations in the form of axial extensions are large, typically in the range of 3–10 mm, depending on deformation limits adopted during data analysis. These design rules are primarily developed for simple connections under lateral loads, and tension connections under axial forces where connection deformation is not critical to the structural performance of connected members. However, for bolted connections under moment, the bearing resistances of the connected parts in cold-formed steel sections may only be fully mobilized at large extensions together with large rotation, leading to moment connections of low stiffness and strength. In general, moment connections between cold-formed steel members are not commonly used in practice due to the lack of information on their structural behaviour and appropriate design guidance. In recent years, due to advances in steel technology, cold-formed steel strips with high yield strength up to 550 N/mm2 become available for building products. However, the ductility of high strength steels is found to be reduced significantly with an elongation limit typically <10%; the elongation limits in low strength cold-formed steels and hot-rolled steels are typically 15 and 25%, respectively. With reduced ductility, there is concern about the structural adequacy of high strength steels in term of deformation capacity, especially at connections where highly localized deformations are expected. Moreover, it is also important to recognize that codified design rules in most design recommendations may not be adequate for high strength low ductility steels as they are originally developed from test data with low strength high ductility steels at large deformations. Those design rules are unlikely to provide sufficient safety margin in assessing the connection resistances of high strength low ductility cold-formed steels. Furthermore, they may be inappropriate for moment connections where joint rotations between connected members are limited, and thus incapable of mobilizing the full bearing resistances as in simple connections.

A finite element model is established to investigate the structural behaviour of bolted connections between cold-formed steel strips and hot-rolled steel plates under shear. By incorporating both solid and contact elements, the model is able to capture non-linearities associated with geometry, materials and boundary conditions. It is demonstrated that the finite element model may predict successfully the three typical modes of failure in cold-formed steel bolted connections, namely: 1. bearing failure; 2. shear-out failure; and 3. net section failure. A calibration exercise on the finite element model against bearing failure is carried out and a number of lap shear tests with both low strength steels and high strength steels of different thicknesses were conducted. It is demonstrated that the finite element model may predict the deformation characteristics of bearing failure in bolted connections accurately. Moreover, any reduction in the ductility of cold-formed steel strips will have significant adverse effect on the bearing resistances of bolted connections. Consequently, the use of the proposed finite element model enables a systemic investigation on the bearing resistances of bolted connections. By varying the steel grades and thicknesses, and the clamping forces in bolt shanks, the bearing resistances of bolted connections at specific extensions with various configurations may be assessed readily. A parametric study with the finite element models over a range of practical connection configurations is also carried out and the finite element resistances are compared with the results obtained from codified design rules. It is found that the design rules in AISI, BS5950: Part 5, CSA: S136 and Eurocode 3: Part 1.3 are not applicable to bolted connections of high strength cold-formed steel strips due to reduced ductility, in particular, with thick steels. Moreover, there are significant discrepancies in the bearing resistances obtained from the four codified design rules; three major reasons for the discrepancies are also presented. Based on the results of the parametric study, a semi-empirical design rule is proposed which relates the bearing resistance of bolted connections at 3 mm extension with a number of parameters, namely, the design yield and the design tensile strengths of steels, the steel thickness, and also the bolt diameters. The bolt is assumed to be installed by a torque which induces a nominal clamping force of 12 kN developed in the bolt shank. The proposed design rule is shown to always give safe bearing resistances for cold-formed steel bolted connections with consistent safety margin over a practical range of connection configurations. The proposed design rule will be particularly useful in predicting moment resistances of bolted connections with limited joint rotations.