## 2 Retrofitting Test on Steel Angles
### Specimen Design  
The proposed schematic diagram of the steel angle reinforcement method is shown in Figure 1. The steel angle reinforcement composite component consists of a steel angle, reinforcement plate, and fixture. The reinforcement plate is placed on the inner side of the weak axis of the steel angle, and the reinforcement plate is chamfered to ensure full contact with the inner side of the steel angle. There are two rows of bolt holes on the fixture and reinforcement plate, which are connected to the fixture through high-strength bolts of grade 8.8. The tightening torque should strictly meet the requirements to ensure that the reinforcement plate and steel angle can work together. The width of the reinforcement plate can be adjusted within a certain range to meet different bearing capacities and construction requirements. In the experiment of built-up steel angle members, the length of the reinforcement plate is slightly smaller than the length of the steel angle, which avoids direct compression of the reinforcement plate, facilitates the installation of the reinforcement plate, and is consistent with the actual engineering reinforcement method. In order to study the actual effect of a new prefabricated reinforcement form and analyze its reinforcement mechanism, five commonly used steel angle specifications (L80 \(\times\) 7, L100 \(\times\) 8, L125 \(\times\) 10, L160 \(\times\) 12, L180 \(\times\) 14) in steel towers were selected for component testing conduct comparative experiments between reinforced and unreinforced specimens, taking into account factors such as fixture spacing, slenderness ratio, and fixture size in the design of each set of test specifications to study the applicability of this reinforcement  
Figure 1: Retrofitting method. form and provide a basis for optimizing the reinforcement form. The test components are divided into two types: reinforced components and unreinforced components. The reinforced component is numbered LR, the unreinforced component is numbered LS, and there are five different steel angle specifications, L80 \(\times\) 7, L100 \(\times\) 8, L125 \(\times\) 10, L160 \(\times\) 12, L180 \(\times\) 14 are numbered 1-5 in sequence, and the specimen design is shown in Figure 2. For ease of loading, end plates are welded at both ends of the steel angle, and a 5 mm gap is left between the reinforcement plate and the end plate, as shown in Figure 2a. This test designs two types of fixtures, namely single-row bolt form and double-row bolt form, and verifies the influence of different fixture sizes on the bearing capacity of reinforced specimens through comparative experiments. The partially reinforced plate between the end plate and the last fixture cannot work together with the steel angle due to insufficient constraints. Therefore, this is the weak section of the reinforced specimen. If the weak area is too long, the reinforced specimen will first experience local buckling here. To avoid local buckling, this article adds an additional pair of fixtures, such as LR2-2-2, to the ends of some specimens with longer weak areas to constrain the two ends of the reinforced plate and ensure that it and the steel angle can synergistically bear the force. The thickness of the reinforcement plate in the reinforcement component is equal to the thickness of the steel angle limb, and the width of the reinforcement plate is the steel angle limb width\(/\sqrt{2}+80\) mm (80 mm is about 3d, which is the minimum bolt edge distance \(\times\) 2). The bolt adopts grade 8.8 M24, and the pre-tightening torque of the bolt is based on the requirements of the \"Technical Regulations for High Strength Bolt Connection of Steel Structures\". The pre-tightening force value is 175 kN, and the summary of specimen size is shown in Table 1.  
Figure 2: _Cont._