The HSS Connection Challenge

Hollow structural sections offer excellent strength-to-weight ratios, but their closed shape makes traditional bolting difficult. When access to the inside of a tube or box section is limited or impossible, installers cannot place a nut on the far side of the steel. Welding, though common, introduces heat, distortion, and safety issues that slow down installation and increase costs.

Traditional Fixes and Their Limitations

For years, fabricators relied on temporary fixes such as plug welding or inserting loose backing plates, both of which require specialized labor and inspection. Through bolting is another option, but it demands precision drilling and access to both sides of the steel. These methods create additional work, raise safety risks, and often fail to meet modern efficiency or environmental standards. If welding access is available, see our guide on bolting vs. welding structural connections to compare safety and cost.

How Blind Bolts Work

Blind bolts solve the problem by anchoring from inside the hollow section. The BoxBolt® blind bolt consists of a cone, a sleeve, and a nut. When tightened, the cone draws the sleeve upward, causing it to expand and lock against the interior walls of the HSS. This internal expansion creates a structural connection that distributes load evenly without damaging the steel surface.

The result is a repeatable, tested connection that can be installed quickly with common hand tools, ideal for new builds or retrofits where backside access is blocked.

Explore BoxBolt® Blind Bolts for HSS Connections →

Installation Process

1. 1. Drill a hole through one wall of the hollow section to the required diameter.
   2. Insert the BoxBolt® assembly into the hole until the head rests flush against the surface.
   3. Use the BoxSok™ installation toolattached to a standard impact wrench to tighten the bolt.
   4. As torque is applied, the internal sleeve expands and secures the connection automatically.
   5. Visually inspect for correct engagement, ensuring the sleeve expands evenly against the inner wall.

Testing and Certification

Standard BoxBolt® Testing & Certification
Standard BoxBolt® blind bolts are physically tested in hollow structural sections and carry Lloyd’s Register Type Approval, confirming consistent structural performance. They are also approved by DIBt, meeting DIN 18800 and Eurocode 3 design methodologies for bolted steel connections

BoxBolt® Type C (ICC-Approved) Testing & Certification
For applications requiring seismic or slip-critical performance, the BoxBolt® Type C is specifically engineered and tested for HSS-to-steel connections. This version is ICC-approved under ESR-3217, meeting the requirements of AC437 and approved for use in all Seismic Design Categories A–F

Technical Data Availability
All load capacities, shear and tensile values, dimensional requirements, and torque specifications for every BoxBolt® type are published in the official LNA Solutions catalog.

Applications

• Curtain walling and façade systems where clean, visible surfaces are required.
• Modular and prefabricated buildings needing rapid assembly and minimal field welding.
• Solar mounting frames where site conditions limit access for welding crews.
• Retrofit and maintenance projects requiring secure, removable connections.

Wherever you need a reliable connection inside hollow steel, BoxBolt® provides the speed, safety, and structural confidence demanded by modern construction.

LNA Solutions engineers are available to provide connection design guidance and verify the right BoxBolt® selection for your HSS application. Contact us today to discuss your project requirements.