Hollo-Bolt: The perfect architectural connection for structural steel

Hollo-Bolt, the original expansion bolt for structural steel was invented by Lindapter as a fast cost-effective connection for structural hollow section (SHS). The blind fixing connection requires installation access to only one side of the steel section for exceptional convenience.

In comparison to alternative methods such as welding, a Hollo-Bolt connection can be quickly installed by simply inserting the fastener into pre-drilled holes and tightening with a torque wrench.

Lindapter pioneered the first blind fixing in Bradford in 1934, and since then, the product range has been continuously developed to include new head types, lengths and finishes. The case studies below highlight the different architectural styles that can be achieved.

The Hollo-Bolt Flush Fit, creates immense opportunity for architectural design as the unique fixing is concealed within the steelwork with no protruding bolt head or nut.

HafenCity, Hamburg is currently Europe’s largest inner city development, a purpose-built district spanning an area of 1.5 million m² and scheduled for completion by 2025. The prominent glazed feature of the Germanischer Lloyd building guarantees expansive views over the old warehouse district and creates a striking entrance to the company’s new head office.

The M12 Hollo-Bolt Flush Fit was used to construct the glazing support frame, securing the wide spanning glass roof and multi-storey windowed front. The connection design included splice joints inside the adjoining pre-drilled SHS sections, allowing the fixings to be simply inserted and tightened with a torque wrench. The cost effective solution required no specialist equipment or labour and dramatically less work at height in comparison to traditional welding or through-bolting methods. The Hollo-Bolt heads were then painted the same colour as the steelwork, making the connection virtually invisible. Resulting in that the steelwork is not only structurally sound, but aesthetically pleasing, presenting a clean symmetrical appearance.

After Martifer designed the stunning façade, Network Rail stipulated that self-drilling screws were not suitable because the fixing must exceed the 40 year life expectancy of the stainless steel façade panels. Therefore, Network Rail's Civil Engineer for Birmingham New Street Gateway Project explored various cladding fixings from the UK, European and USA markets. He even considered using British Aerospace, BAE System, jet fighter's body riveted connection. The Civil Engineer decided that Lindapter’s Hollo-Bolt was the best connection solution for this iconic structure as it met Network Rail’s safety and maintenance requirements and the Council Planners needs.

The Engineer took advantage of Lindapter’s free connection design service and the research and development department to receive an engineered solution. The Lindapter R & D team developed a bespoke M5 countersunk Hollo-Bolt in stainless steel, which exceeded the maintenance and safety requirements, complemented the design of the façade and provided a quick and easy installation process.

The ‘Flows Two Ways’ sculpture spans 8 storys and was designed to complement the main entrance to neighbouring skyscraper VIA 57 West, dubbed The Great Pyramid of Manhattan. The monumental cladding was made from a stainless steel mounting matrix to which 35 interlocking panels and 400 18m aluminium tubes were installed.

The engineers evaluated various connection methods for their ability to accommodate forces generated by wind and earthquakes. They concluded that the Hollo-Bolt by Lindapter was the best choice for the application because it is approved by ICC-ES for use in all seismic design categories (A-F) for resisting wind loads and seismic loads. After contacting the Lindapter technical department, American Signcrafters used 250 stainless steel Hollo-Bolts to secure the components to the stainless steel grid.

Using the Hollo-Bolts resulted in a discreet architectural finish, which complemented the design of the sculpture. In addition, the high strength long lasting materials ensured that the connection would out-live the other parts of the facade, avoiding unnecessary maintenance. Eliminating the need for onsite drilling and welding and allowed a convenient, quick and easy installation, without damaging the metal components.

The Lindapter Hollo-Bolt was specified due to its range of independent technical approvals, including COLA (City of Los Angeles) approval and full seismic approval from ICC-ES for all seismic design categories (A-F).

Contractors used over 3,000 M20 Hollo-Bolts to connect circular steel sections which form the curved atrium structure. Hollo-Bolts were installed from just one side, rapidly achieving discreet splice connections without the need for drilling or welding.

The extremely cost effective installation did not require specialist equipment or specialist labor. It dramatically reduced the amount of work at height in comparison to traditional welding or through-bolting methods. The design also eradicated the possibility of tube deformation that can be associated with through-bolting if the bolts are over-tightened. The end result was aesthetically pleasing, clean and discreet splice connections which complement the design of the exposed steel.

As a British pioneer in the design and manufacture of steelwork fixings, Lindapter has a wide range of product approvals. The CE marked Hollo-Bolt is independently approved as a structural connection for primary steel and features in the SCI/BCSA joint publication ‘Joints in Steel Construction – Simple Connections’ (Or Green Book as it is commonly referred to).

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