## 1 Introduction  
Porcelain electrical equipment is an important part of power systems and plays an essential role in their stability and security. Earthquake disaster investigations have shown that porcelain electrical equipment is easily damaged, making it the part of the power system with the weakest seismic resistance [1; 2; 3; 4]. There is a large volume of published studies assessing the susceptibility of porcelain electrical equipment but without a deeper investigation of its mechanical mechanisms [5; 6; 7; 8]. In porcelain electrical equipment, a joint is formed at the connection of the flange and porcelain bushing. This joint is the point that is the most vulnerable to earthquake damage [9; 10; 11]. The joint is made of two different materials: a cup-shaped iron flange and a cylindrical porcelain bushing, which are bonded by cement or clamped with a spring. Owing to the complexity of the joint structure, it is difficult to determine the bending stiffness accurately, which is an important constitutive parameter of the joint. Therefore, determining the bending stiffness of the joint connecting the porcelain bushing and flange has become an essential problem in the seismic design of electrical structures. The porcelain bushing and flange of ultra-high voltage (UHV) electrical equipment are usually connected using one of two forms: cement bonding or spring clamping. Cement-bonded joints are widely used in engineering, but their constitutive behavior is very complex. Therefore, this study focuses on the bending stiffness of this type of joint. When numerical methods are used to simulate the dynamic behavior of electrical equipment, the standard procedure is to regard the porcelain bushing and flange joint as a rigid connection. However, recent studies have shown that this assumption overestimatesthe bending stiffness, and the resulting numerical simulation results are very different from the results measured in practical engineering and experiments [12; 13; 14]. The flange and porcelainushing are filled with cement material. The resulting joint has good stiffness in the translational direction, which is similar to a rigid connection. However, the bending stiffness of the cement-bonded joint is between the bending stiffness of a hinge joint and a rigid joint, and it should thus be regarded as an elastic connection. The current code for the seismic design of electrical installations in China [15], which is based on the corresponding Japanese code [16] and experimental studies, provides a formula for calculating the bending stiffness of cement-bonded joints connecting flanges and porcelain bushings. For general electrical equipment of the porcelain column type, this formula can be applied to calculate the bending stiffness of the cement-bonded joint connecting the porcelain bushing and flange. However, there is an empirical parameter in this equation obtained by statistically fitting the experimental data. Moreover, there are no variables in this equation to express the physical and mechanical properties of the joint material. Therefore, this equation cannot reveal the mechanical mechanism of bending stiffness [17]. In addition, the outer diameter and height of the porcelain bushing of UHV electrical equipment and the thickness of the cement bonding between the flange and porcelain bushing are very different from those in ordinary high-voltage electrical equipment, which may lead to a large difference between the actual bending stiffness of the joints of UHV electrical equipment and the bending stiffness calculated according to the code [18]. Whether this equation is suitable for UHV electrical equipment is questionable.