# The Bearing Performance and Sectional Design Method of Yielding U-Shaped Steel Support
## 1 Introduction  
With the depletion of shallow coal resources in China, coal mining has been developing towards deeper strata [1]. Deep coal mining is faced with complex geological conditions such as high stress and extremely soft rock. The surrounding rock of roadways deforms significantly and the deformation lasts for a long time. Traditional high-strength rigid support is difficult to effectively adapt to the large deformation of the surrounding rock [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]. Yieldable support not only provides support resistance to maintain the stability of the surrounding rock but also allows the support structure to generate an appropriate amount of deformation to relieve the pressure on the surrounding rock. It has been widely used in deep soft rock engineering projects. Common yieldable support structures includehigh-toughness bolts, flexible steel meshes, and retractable support [14; 15]. Among them, the yielding U-shaped steel support has the advantages of high resistance, retractability, and convenient installation, and it is a commonly used form of yieldable support. In the actual application process, due to the complexity of the stress environment, the yielding U-shaped steel support is prone to yield failure first at the stress concentration positions, resulting in difficulty in fully exerting its yieldable and bearing performances [16]. To improve the bearing and yieldable performance of the yielding U-shaped steel support and enhance the surrounding rock control effect, scholars at home and abroad have carried out a large number of related studies. [PERSON] et al. [17; 18] studied the sliding mechanism of the lapped section of the U-shaped steel, tested the friction performance between different materials at the nodes of the U-shaped steel support, and proposed an optimization method for improving the sliding performance of the lapped section, which improved the retractable performance at the lapped part of the U-shaped steel support. [PERSON] et al. [19] analyzed the main reasons for the structural instability of the U-shaped steel support, put forward a support stable-resistance technology, the compensation principle and compensation technology of the support structure, and improved the overall bearing capacity and structural stability of the U-shaped steel support. [PERSON] et al. [20] analyzed the influence of factors such as backfill behind the wall, torque of cable clamp nuts, and longitudinal tie rods on the overall bearing capacity of the U-shaped steel support, and proposed improvement measures for the overall bearing capacity of the support. [PERSON] et al. [21] analyzed the instability characteristics of the U-shaped steel support under different loading conditions, and proposed the combined support technology of \"U-shaped steel support + backfill behind the wall + anchor cable\", which improved the stability of the surrounding rock of the roadway under the influence of intense mining. [PERSON] et al. [22] studied the load distribution law of the support and the deformation characteristics of the roadway before and after the implementation of pressure relief boreholes, and proposed the control measures of \"shed support + pressure relief\" for the roadway in extremely soft coal seams, which improved the stress state of the U-shaped steel support. [PERSON] et al. [23] improved the U-shaped steel connecting sleeve and proposed the support system of \"metal mesh + backfilled chemical grouting material + U-shaped steel group\", which ensured the stability of the surrounding rock of deep roadways. [PERSON] et al. [24] found that the bearing capacity of U-shaped steel supports and the ability to withstand deformation of surrounding rock are largely dependent on the action of the connection, and the preload generated by the bolts under different torques was studied by 2010 ANSYS software. [PERSON], [PERSON] et al.