Blind Bolts for Structural Steel: A Buyer’s Guide
Quick Answer: Blind bolts are structural fasteners that install entirely from one side of a connection. They are the standard method for bolting to Hollow Structural Sections (HSS), where the inside of the tube cannot be reached. Blind bolts install with basic hand tools, require no welding or on-site drilling of the supported structure, and are available with third-party structural approvals including Lloyd’s Register Type Approval, DIBt, and ICC seismic approval (ESR-3217). BoxBolt® by LNA Solutions is available in six diameters from 1/4″ to 3/4″ and ships from Buffalo, NY with free PE-stamped engineering support within 24 hours.
Already know what you need? View BoxBolt® blind bolts or send your connection details to an engineer for a free sizing recommendation and quote.
What Are Blind Bolts?
A blind bolt is a mechanical fastener designed for connections where only one side of the joint is accessible. A standard nut-and-bolt connection needs access to both sides: one wrench on the head, one on the nut. A blind bolt replaces the nut with an expanding sleeve mechanism that anchors itself inside the steel section as the core bolt is tightened.
That single difference solves one of the most common problems in steel construction: connecting to Hollow Structural Sections. HSS tubes are closed by definition. You cannot put a hand, a wrench, or a nut inside them. Before blind bolts, the options were welding, through-bolting the entire section, or shop-drilling and tapping. Each adds cost, time, or hot work permits to the job.
The BoxBolt® from LNA Solutions is a high-strength blind bolt for HSS and blind connection applications. It works with rectangular, square, and circular hollow sections and requires only a clearance hole drilled to suit its outside diameter, which can be drilled off-site for new projects or on-site for retrofits.
When Should You Specify Blind Bolts?
Blind bolts are the right specification when one or more of these conditions apply:
- The connection is to an HSS member. Square, rectangular, or round tube where the far side is sealed.
- Only one side of the joint is accessible. Connections against walls, inside box girders, or in congested retrofit conditions.
- Hot work is restricted or banned. Occupied buildings, chemical plants, food processing facilities, and confined spaces where welding permits are costly or impossible.
- The existing structure must not be modified. Blind bolting preserves the supporting steel’s surface integrity and protective coatings.
- Speed matters. Installation requires only basic hand tools, so crews without welding certifications can make structural connections.
Typical applications include curtain wall brackets on hollow structural steel, guard railing, steel flooring, arena seating connections, and solar panel frame attachments.
Blind Bolts vs. Welding vs. Through-Bolting
Each connection method makes a trade. Here is how they compare for HSS and restricted-access connections:
| Factor | Blind Bolts | Welding | Through-Bolting |
|---|---|---|---|
| Access required | One side only | One side, plus clearance for welder | Both sides |
| Hot work permit | Not required | Required | Not required |
| Skilled labor | Basic hand tools, any skilled labor | Certified welder | Drilling access and alignment |
| Coating damage | None to supporting steel surfaces | Burns coating, requires repair | Penetrates both walls |
| Removable / adjustable | Yes | No | Yes |
| Verified load data | Published, third-party tested | Depends on weld quality and inspection | Standard bolt values |
For a deeper look at the bolting vs. welding decision, see our guide on bolting vs. welding steel connections. If your specific problem is reaching inside a tube, start with how to connect hollow structural sections from one side.
What Sizes and Finishes Do Blind Bolts Come In?
BoxBolt® is available in six bolt diameters: 1/4″, 5/16″, 3/8″, 1/2″, 5/8″, and 3/4″. Each diameter comes in three body sizes (Size 1, 2, and 3) covering different clamping ranges, so the fastener matches the combined thickness of the materials being joined. At the top of the range, the 3/4″ Size 3 covers a clamping range of 2-9/16″ to 4″.
Three finishes cover the full spectrum of service environments:
- Zinc plated for less aggressive indoor environments
- Hot-dip galvanized for more aggressive and exterior environments
- Stainless steel for the most demanding applications
Not sure which size fits your connection? Our BoxBolt sizing guide walks through the selection process step by step.
Skip the guesswork: send your flange thickness and section size to our engineering team and get a sizing recommendation with PE sign-off within 24 hours, free.
What Approvals Should a Structural Blind Bolt Have?
This is where blind bolt products separate. Any fastener can publish a load table. The question is who verified it.
BoxBolt® published loads come from physical testing in hollow structural sections, verified by Lloyd’s Register Type Approval, and the product is also tested and approved by DIBt (Deutsches Institut für Bautechnik). Published loads carry a 0.75 reduction factor applied to ultimate tensile and shear strength; the designer applies further factors relevant to the application and governing code.
ICC Seismic Approval: The Type C BoxBolt
For projects under IBC jurisdiction with seismic demands, the ICC Approved BoxBolt® (Type C) is engineered for HSS-to-structural-steel connections with static dominant lateral loads in slip-critical conditions, with a modified design to resist seismic loads. It is evaluated under ICC ESR-3217 to Acceptance Criteria AC437 and is supplied hot-dip galvanized. BoxBolt was the first blind bolt to receive an Evaluation Service Report from the ICC Evaluation Service.
If your specification requires code-recognized seismic performance for a blind HSS connection, ESR-3217 is the document to reference. Our article on bolt certifications explained breaks down what ICC, DIBt, and Lloyd’s approvals each mean for structural connections.
How Are Blind Bolts Installed?
Installation happens entirely from the accessible side: drill a clearance hole, insert the blind bolt, hold the body, and tighten the core bolt to the specified torque. The BoxSok™ installation tool holds the hexagon shoulder on the body to prevent rotation while tightening, which speeds up repetitive installations.
For the full procedure and torque guidance, see the step-by-step BoxBolt installation guide. Before your crew starts, it is worth reviewing the top 5 installation mistakes to avoid.
Frequently Asked Questions About Blind Bolts
Are blind bolts as strong as welding?
Blind bolts are engineered structural fasteners with published, third-party verified tensile and shear capacities. Whether a bolted or welded connection is appropriate depends on the load case and the design code, not on one method being universally stronger. The practical advantage of blind bolts is delivering verified structural capacity without hot work, certified welders, or coating repair.
Can blind bolts be used on round tube?
Yes. BoxBolt® is suitable for rectangular, square, and circular hollow sections. Only a clearance hole matching the bolt’s outside diameter is required.
Are blind bolts removable?
Yes. Because the connection is mechanical rather than welded, blind bolts can be removed, which suits temporary works, staging, and structures that may be modified later.
Do blind bolts work for seismic applications?
Yes, with the correct approval. The standard BoxBolt® carries Lloyd’s Register and DIBt approvals. For code-recognized seismic performance, specify the ICC Approved BoxBolt® Type C, evaluated under ICC ESR-3217 to Acceptance Criteria AC437.
How do blind bolts compare to Hollo-Bolt?
Both are expansion-type blind fasteners for HSS. The differences come down to approvals, sizing, head styles, and availability. We cover the full side-by-side in BoxBolt vs. Hollo-Bolt.
Where can I buy blind bolts in the United States?
LNA Solutions manufactures and ships BoxBolt® blind bolts directly from Buffalo, NY across North America, in zinc-plated, hot-dip galvanized, and stainless steel finishes, with free engineering support on every order.
Get a Sizing Recommendation and Quote
Send us your connection details: section size, flange or wall thickness, load direction, and environment. Our engineers will confirm the right blind bolt size, finish, and approval for your project, with free PE sign-off within 24 hours.
Request a Quote View BoxBolt® Products
BoxBolt is a registered trademark of LNA Solutions. Hollo-Bolt and Lindapter are registered trademarks of Lindapter International Ltd. LNA Solutions is not affiliated with, sponsored by, or endorsed by Lindapter International Ltd. Trademarks are used here for identification and comparison purposes only.
Last reviewed: September 2, 2026
