Function of Double Washers in HS Bolt Assemblies

High-strength (HS) bolts are widely used in structural steel connections, especially in bridges, buildings, and heavy equipment. These bolts are designed to handle high tensile forces and provide robust, secure connections under dynamic and static loads. To enhance their performance, a common practice is the use of double washers in HS bolt assemblies. These washers are not merely accessories but serve several critical functions that ensure the safety, integrity, and longevity of bolted joints.

1. Load Distribution

One of the most important functions of double washers is to distribute the clamping force generated when the HS bolt is tightened. In high-strength applications, a single washer may not provide sufficient surface area to prevent localized stress on the connected materials. Double washers increase the contact area, reducing the chance of surface deformation or crushing in softer or thinner plates, and ensuring that the load is evenly spread across a broader region.

2. Prevention of Embedment Loss

Embedment loss refers to the microscopic settling that occurs between the bolt head, nut, washer, and the connected plates after tightening. Over time, this can reduce bolt preload and compromise the joint. Double washers help mitigate embedment loss by providing a more stable and uniform bearing surface under both the nut and bolt head, especially in slip-critical connections. They reduce the potential for material flow and loss of tension due to surface irregularities.

3. Improved Tension Retention

In many HS bolt assemblies, especially those subjected to vibration or cyclic loading, maintaining consistent bolt tension is crucial. Double washers can play a role in tension retention by acting as a buffer that absorbs small movements and helps maintain the bolt’s grip. This helps prevent loosening over time and supports the long-term structural performance of the connection.

4. Protection of Coated or Treated Surfaces

Many structural components are coated for corrosion resistance or treated for durability. Double washers serve as a sacrificial layer between the bolt/nut and the coated surface, preventing direct contact that could damage protective finishes. This function is particularly important in outdoor or marine environments where corrosion resistance is a priority.

5. Assistance in Alignment and Assembly

During the installation process, double washers help in achieving proper alignment of bolt holes and ease the tightening process. They can compensate for minor misalignments in hole placement or surface irregularities, contributing to faster and more reliable assembly on construction sites.

Conclusion

The use of double washers in HS bolt assemblies is a practical engineering solution that enhances load distribution, reduces preload loss, maintains tension, protects surfaces, and simplifies installation. While the washers themselves may seem simple, their role is fundamental in ensuring the performance and reliability of high-strength bolted connections. For safety-critical structures, investing in proper washer configurations is not optional—it is a necessity.

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