When should high-strength bolts be initially tightened?

Firstly, the specifications and technical requirements of high-strength bolt connections must meet the design requirements and relevant regulations. The factory certificate of conformity and the test (pre-tension reinspection) report should be checked. The friction coefficient of the joint surface must meet the design requirements; the surface must be free of iron oxide scale, burrs, weld scars, paint, oil, and other contaminants. The friction coefficient test report should be checked, and the appearance of the joint should be inspected: the direction in which the bolts are inserted should be consistent, and the length of the exposed threads should be at least two turns. All the tail collars of the torsional shear high-strength bolts were removed after final tightening. The clearance between the friction surfaces meets the specifications and design requirements.

Reply to 1# Chemical Engineer: The person above still hasn’t answered the original poster’s question, haha. High-strength bolts function based on the friction between their working surfaces; therefore, the installation surfaces need to be properly prepared. As for the initial tightening and final tightening, the original poster has a misunderstanding – there is no strict requirement for carrying out these steps at specific times. After the bolts are installed, the initial tightening should achieve a torque of 60% to 80%, and once all initial tightening is complete, the final tightening can be done. The purpose of this is to ensure even distribution of forces. When performing initial tightening, it’s important to follow a specific order of tightening according to the arrangement of the bolts; the exact sequence can be found in the equipment installation manual. Tightening should be done using a torque wrench

Since there are many high-strength bolted joints in this frame structure, I want to know whether the final tightening is done all at once at the end, or whether it is carried out after each joint is connected

Reply to 4# Chemical Engineer: The installation manual specifies that for this type of frame structure, ordinary bolts should be used as temporary bolts to join the components together. The number of ordinary bolts should be 1/3 of the total number of bolts required. These ordinary bolts are used to secure the frame structure, after which high-strength bolts are installed and tightened initially and finally. Once that is done, the temporary ordinary bolts are removed and replaced with high-strength bolts, which are then tightened again initially and finally. I hope my explanation is clear

Based on my experience in constructing large-scale steel structures for Shell, the joints used are generally high-strength bolt joints. After all the high-strength bolts have been inserted into a joint, a preliminary tightening is carried out; the order of this preliminary tightening is usually symmetrical, so as to ensure even stress distribution within that joint. For the initial tightening, ordinary wrenches are sufficient for most people to meet the requirements. The final tightening is carried out after the initial tightening. Final tightening is generally carried out using specialized power tools. For large structures, after a beam is installed, initial tightening is performed on both sides first; once the beam is level, final tightening is carried out.

The difference between high-strength bolts and ordinary bolts lies mainly in their materials and uses. For ordinary bolts, manual or mechanical tools can be used for initial tightening, as long as the installation accuracy requirements are met. High-strength bolts are prestressed bolts: friction-type bolts require a torque wrench to apply the specified prestress, while compression-type bolts involve removing the hex head after tightening.