# Investigating the Effect of Screw Size on the Stress Level in MERO Joint for Space Frame Structures
## 1 Introduction
The most important features of the space frame structures with MERO ball joints include its capability to receive various covers (paint and plating) due to the limited dimensions of structural members, and remove or displace the supports of the structure with minimal time and variations in the structure, cost, and interference with other on-site construction activities. In addition, no heavy machinery or specific equipment for installation and implementation of the structure are required and the regular execution tools of the project can be considered during the design and production phases [1]. Generally, joints considerably influence the dynamic behavior and integrity of the structure [10; 11; 12; 13], hence their proper modeling in civil engineering structures is an important step in analysis and design of these structures. Behavior of joints is not either  
Figure 1: Main components of the space frame structure. completely rigid or pinned. In reality, joints behave semi-rigidly as pinned joints exhibit some rotational stiffness and joints that are considered rigid always possess some flexibility [14; 15; 16]. In recent years, extensive research has been conducted regarding the behavior of the joints used in space frame structures [17; 18; 19; 20; 21; 22], as well as other kinds of civil engineering structures in general. Some studies indicated that the joint rigidity is regarded as the most significant factor affecting the behavior of the single-layer dome structures [23; 24; 25; 26; 27]. Structural health monitoring of space frame structures is another significant achievement for monitoring any damage caused to these structural systems. [PERSON] et al. [28] studied damage detection in both joints and structural members for space structures with semi-rigidity behavior of the connection joints. Therefore, the effect of the joint should be taken into account in structural analysis in order to achieve more accurate results. [PERSON] and [PERSON] [29] evaluated the load-displacement relationship from a double-layer grid by applying a centralized load in real scale, and concluded that the analytical responses of the structures, regardless of the effect of the joint, significantly differ from their real responses. Considering the actual behavior of the space frame structures, the joints are influenced by a combination of axial, shear and bending moments. [PERSON] and [PERSON] [30] and [PERSON] et al. [31] indicated that in a ball joint, the force is transmitted differently under tension and compression. The existence of different components with different mechanical and geometrical properties which are connected by screws, discontinuities between the joint components, and the pre-stressed condition resulting from tightening of the screw and the spiral nature of the thread, exacerbates complexity of the joint behavior. As a result, in the present study, the surfaces under tensile or compressive loading were selected more precisely. To the best knowledge of the authors, no study has been conducted on the optimal screw size for double-layer ball joints. Therefore, the present study aimed at evaluating the effect of the screw size on the maximum stress level in MERO-based joints in space frame structures. Further, the optimal screw size with the highest factor of safety (FOS) is suggested for load-bearing purposes considering the strength of the materials used for the ball joint. For the purpose of numerical simulations, the model is validated through a single-axis mode and based on accurate calculations via FEM analysis using ANSYS.