Anchor bolts are an important part of the foundation planning of transmission lines. Their function is to effectively transfer the vertical and horizontal loads acting on the tower to the foundation concrete.
[1] If there are problems in the planning, processing, and construction of the installation process, it may cause safety incidents in the tower or the line, such as the "Fengcheng '5·7' iron tower collapsed in Jiangxi on May 7, 2017. Casualties"
[2] The reliability of anchor bolts is related to the safety of the entire transmission line
Prone to quality problems
1. Planning stage
Nowadays, the electric power industry mainly selects anchor bolts according to the electric power industry standard "Technical Regulations for Overhead Transmission Line Fundamental Planning" DL/T 5219—2014, and adjusts the bolt specifications and quantity to meet the requirements by comparing the mechanical function value and demand value of the specification rules Strength, as shown in formula (1), the commonly used structural forms of anchor bolts have the following 5 types: L type, J type, pawl type, T type and I type. Compared with the tensile strength of the data, the planned tensile strength of anchor bolts is generally low. For example, the planned tensile strength of Q235 is 160 N/mm2, and the tensile value of Q235 data should be 375～460 N/ mm2, the planning safety factor is relatively high. It can be seen that the main problem in the planning process is whether the original data itself is qualified and able to meet the requirements.
2. Processing stage
In addition to the five types of anchor bolts mentioned above, a set of intact anchor bolts should also have matching nuts. Therefore, the processing of the external thread of the anchor bolt and the internal thread of the nut will be involved in the processing process. Processing. The processing of external threads simply presents quality problems such as random buckle, incorrect pitch and incorrect pitch diameter. Common quality problems of internal thread processing include rough appearance and corrugation, thread pitch diameter is too small, thread pitch diameter is too large, and thread profile is not equal. When the nut and bolt cooperate in the installation, the installation will not be smooth, the gap will be too large, the joint surface will be reduced, and the overall function of the bolt will be affected.
3. Construction installation stage
Under the situation that the State Grid Corporation of China vigorously promotes the general plan of transmission lines, different tower types in the same module are often used in the same project. If the level difference between the anchor bolts used in two adjacent tower types is small, In the case that the base-by-base packaging is not strictly implemented during the installation process, there may be a mismatch between the bolt and nut specifications, that is, the chaotic use of nuts and bolts with the same pitch and different specifications. Take the SJ2 and SJ3 tower types of the 2E5 module of the State Grid Corporation of China, the SZK and SJ1 tower types of the 2F4 module, and the ZM3-30 and ZM3-33 towers of the 1C3 module as examples. The relevant data are shown in the table 1 shown. According to the rule of Reference [5], the thread form of the anchor bolt is a coarse triangle. According to the rule of Reference [6], the basic form of the coarse triangle is shown in Figure 1. It can be seen from Figure 1 that when the thread pitch is the same, the thread of the bolt and the nut can be combined when the large diameter of the external thread is larger than the small diameter of the internal thread.