真实试验的边界条件和约束的计算模型

The 12th International Symposium on Structural Engineering COMPUTATIONAL MODELING OF REALISTIC EXPERIMENT BOUNDARY CONDITIONS AND RESTRAINTS

Liping Kang1, Roberto T. Leon2, Xilin Lu1 1 College of Civil Engineering, Tongji University, Shanghai, 200092, P.R.China

2 The Via Department of Civil and Environmental Engineering, Virginia Institute of

Technology, VA 24061, USA

Abstract: Experimental simulation of an idealized structure in the laboratory typically requires a careful consideration to achieve the idealized boundary conditions and restraints; however, the real test conditions involve friction, rotational restraints, fabrication and construction errors that may have a significant effect on the structural response. Computational analyses with idealized boundary conditions and restraints cannot capture these realistic effects thus resulting in large errors in displacement, rotation, and capacity. This paper presents a computational modeling approach of realistic boundary conditions and restraints to simulate these effects in OpenSEES. Translational and/or rotational springs are used in the computational modeling to represent the friction and rotational restraints, and a methodology is presented to estimate the force vs. displacement and moment vs. rotation spring relations based on the global structural response. Additionally an approach to consider initial deformations due to assembly errors is also presented to account for locked-in stresses. The result shows that: 1). Friction and rotational springs increase the capacity of the analyzed connections; 2). Global unsymmetrical force vs. displacement behavior of the analyzed connection is obtained with unsymmetrical spring behavior or locked-in stresses from initial deformation; and 3). The proposed computational modeling approach captures the possible significance of each influencing factor on the global structural response. The results highlight the need to provide additional experimental data such as estimates of the frictional response and measured strain from rotational restraints and initial deformations during structural assembly so that the effect of these terms can be accounted for. With the additional experimental data and the significance of influencing factors, we can accurately predict the realistic structural response computationally. Keywords: Model Simulation, boundary condition, restraint, structural test, spring, OpenSEES

1 INTRODUCTION In an experimental setup of a structural component or assembly, in a push over analysis, vertical and lateral forces are usually applied on the top of columns by actuators. The actuator should apply a uni-axial loading to the structure, and transverse forces should not be resisted by the loading apparatus to allow the specimen to deform freely in the transverse direction according to GB 50152-92.(China, 1992). However, it is not practical to eliminate the friction between the specimen and the actuator completely. One clever solution designed to minimize friction in a push over analysis is a swivel with low-friction bearings as shown in Figure 1 (Perea, 2010). Unfortunately, this device has not been widely adopted because of

the high initial and maintenance expenses. In china, a sliding support with a line of rollers is a more common solution due to its cost effectiveness and ease of assembly (Figure 2) (Wang, Han et al., 2006; Han, Wang et al., 2008). Many researchers (Holmberg, 1984; Liu, Jin et al., 2005; Wu, Guo et al., 2006; Luo, Zhou et al., 2011) have studied the friction behavior of roller supports. Friction varies considerably due to the stiffness of the roller support assembly, contaminants on the contact surface, and the magnitude of the vertical force etc. (Wu, Guo et al., 2006). As expected, the real lateral capacity of the idealized structure will be overestimated because friction at the sliding support is assumed negligible. In addition, the loading beam prevents the top of the column from rotating freely. ·478·

For complex test setups, initial locked-in stresses will also be induced in the structural assembly when placing specimen into the testing rig. The level of such stresses is directly proportional to the initial deformations in the specimen, the tolerances at the specimen-loading frame interface, the setup sequence and the degree of control available to relieve those stresses through motions in the hydraulic actuators. All the effects above will affect the capacity of the specimens; however, very little research has been focused on the significance of these influencing factors. This paper will present an approach to simulate friction, rotational restraints and initial stresses with analytical models in OpenSEES. The simulation result will be compared to experimental test results available in the literature to quantitatively assess the significance of these influencing factors.