The anchor bolts, as the key large-sized “screws” connecting the transmission tower to its foundation, play a crucial role in ensuring the stability of the tower when it bears external loads. Once these large-sized “screws” have quality issues, it is highly likely to cause tower collapse accidents.
This article sorts out and summarizes some relevant knowledge about the anchor bolts of transmission towers, hoping to be helpful to everyone.
01 Foundation Bolt Categories
Classification by type
The foundation bolts are composed of a bolt, a nut, a washer and an anchor component at the end. According to the type of anchor at the end, the foundation bolts of transmission towers can be classified into L-type, J-type, T-type and double-head type.
L-shaped anchor bolt
J-bolt foundation anchor bolt
T-bolt foundation anchor
Double-headed anchor bolt
Classified by tensile strength grades
The anchor bolts of transmission towers are classified into grades 4.8, 6.8, 8.8 and 10.9 based on their tensile strength. The grade is determined by the tensile strength of the bolt (rounded down after dividing by 100) and the ratio of yield strength to tensile strength.
02 Design Principles
The anchor bolts, serving as the link between the transmission line tower and the foundation, must meet the design value of the tower’s uplift force during the design process. When the anchor bolts are symmetrically arranged with respect to the design uplift force, the effective area of a single cross-section should be calculated according to the following formula:
03 Manufacturing Process Requirements
Material Preparation
High-quality steel materials that have passed mechanical property tests, such as carbon steel and alloy steel, are selected and cut to the specified lengths as required.
Thread processing
According to the standards GB/T 196-2003 “Basic Dimensions of Common Threads” and GB/T 15756-2008 “Limit Dimensions of Common Threads”, the threads of anchor bolts should be processed by rolling.
Heat treatment
For grade 8.8 and 10.9 anchor bolts and nuts, the heat treatment process should be quenching followed by tempering. For grade 4.6 and 5.6 anchor bolts and nuts, to ensure all performance aspects, heat treatment of the steel and products should be carried out as appropriate.
Bend
Anchor bolts can be bent by hot bending or cold bending. The temperature for hot bending should be controlled between 800℃ and 950℃. Hot bending process should not be adopted for 8.8 grade anchor bolts after heat treatment.
Welding
When selecting materials for T-bolts, the weldability of the materials should be taken into account, and verification process tests should be conducted before welding. 8.8 grade, 10.9 grade and 5.6 grade bolts that have undergone quenching and tempering treatment should not be welded during processing and construction.
Hot-dip galvanizing
According to the provisions of the standard DL/T 284-2021 “Hot-dip Galvanized Bolts and Nuts for Transmission Line Towers and Power Hardware”, for anchor bolts in complex service environments, local or full hot-dip galvanizing of the threaded part can be adopted for anti-corrosion. The length of local hot-dip galvanizing should not be less than twice the length of the thread.
Assembly or configuration
The anchor bolts should be assembled with nuts of corresponding performance grades in accordance with the standard DL/T 1236-2021 “Anchor Bolts and Nuts for Transmission Tower Foundations”. The outer end face of the outermost nut should be flush with the outer end face of the anchor bolt thread.
04 Installation Precautions
The installation of anchor bolts begins with drilling holes in the foundation, followed by pouring concrete, and finally embedding the anchor bolts into the foundation and securing them with nuts. During the installation process, the following points should be noted:
Installation diagram of anchor bolts
Selection of anchor bolts
According to the State Grid Infrastructure [2018] No. 387 document, to prevent the mismatch of anchor bolts and nuts, anchor bolts should be selected from the specifications of M24, M30, M36, M42, M48, M56, M64, M72, M80, M90, and M100.
Verticality of anchor bolts
When installing anchor bolts, they should be vertical without any inclination. If they are not installed vertically, it will cause errors in the installation coordinates of the bolts and make the installation difficult. In addition, non-vertical bolts will reduce the external force they can withstand.
Vertical Installation Diagram
The laying of anchor bolts
When laying the anchor bolts, it is necessary to prevent foreign objects from entering the holes and ensure that the end of the anchor bolts is not in contact with the bottom.
If the anchor bolts are directly fixed to the foundation reinforcement, a special mold should be used for tightening to maintain verticality. To avoid deformation of the mold during pouring, which may cause residual stress in the anchor bolts.
Tightening of anchor bolts
The anchor bolts can be tightened only when the concrete in the reserved holes reaches more than 75% of its designed strength. The tightening force of each bolt should be uniform. After tightening, the bolts should protrude from the nuts, and the protruding length is preferably 1/3 to 2/3 of the bolt diameter.
Keep the corresponding construction records.
During the installation process, keep detailed construction records, accurately reflecting the model and specification of the anchor bolts, to provide effective technical data for future maintenance and replacement.
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