Direct fastening technology is a system in which a hardened nail or stud is driven into steel, concrete, or masonry using a power-actuated tool in which the internal piston drives the fastener into the base material with a driving energy. Hammer-actuated tools were launched in the 1950’s. Since then, there have been innovations which have broadened and advanced the field of direct fastening, making these systems more productive, safe, and reliable.
There are three main components to a direct fastening system: a tool, driving energy, and fastener. While direct fastening originally used powder as an energy source, gas-actuated and battery-actuated driving energies have been added as more recent innovations in direct fastening. See the table below for more information on each direct fastening technology.
Table 1: Differences Between Various Direct Fastening Driving Energies
Depending on the driving force being used, there are three primary methods available when fastening to steel base materials: sharp tip, blunt tip, and blunt tip screw fasteners. See the table below for more information on each.
Table 2: Differences Between Various Fastener Types
In steel base material there are four main holding mechanisms: friction, keying, micro-brazing (or soldering), and fusion. For a sharp tip fastener, the resilience of the displaced base material as the fastener is being driven exerts a clamping force on the surface of the fastener. Thus, when a tensile force is applied externally to the fastener, the load is transferred to the base material by friction. If the sharp tip fastener has knurling, or grooves, along the shank, the coefficient of friction at the areas in contact are increased and mechanical interlock will form between the base material and the fastener to create a keying effect. Soldering occurs due to a thin coating of zinc on the fastener surface that is partially melted as high temperatures are generated at the surface of the fastener during the driving process. The tip of the fastener tends to achieve the highest temperature during the driving process, which may result in fusion between the fastener, zinc, and steel base material. This fusion can be an important holding mechanism; however, this is only seen in sharp-tip fasteners that do not through-penetrate.
Blunt-tip fasteners primarily experience fusion and friction (clamping), due to being driven into an undersized pilot hole, providing significant load capacities. Blunt-tip screw fasteners are driven into a pre-drilled pilot hole with the appropriate Hilti system and creates a keying effect by utilizing the self-tapping threads on the fastener.
Figure 1: Various Holding Mechanisms of Direct Fastener in Steel
If the base material is concrete, there are three main holding mechanisms: bonding/sintering, keying, and clamping. Similar to the soldering mechanism achieved with fasteners in steel, the heat generated during the driving process causes the concrete to sinter around the shank of the fastener. Sintering is the process in which the concrete around the fastener compacts and forms a solid mass around the pin due to the heat and pressure of installation. Finally, a clamping force is developed around the fastener shank. Additional pre-mounted washers on the fastener can further clamp the attached material to the concrete base material. In certain applications, an optional washer can be added to assist in transferring shear load and pullover resistance.
Once the basic holding mechanisms of direct fastening are understood, it is important to be familiar with other factors that can affect the capacity of the fastener. For steel, some of the factors are base material thickness and tensile strength, fastener hardness, fastener spacing and edge distance, and fastener shank diameter. When fastening on steel, penetration of the fastener into the base material also plays a role in capacity.
Analogously for fasteners in concrete, the factors are depth of penetration into concrete, compressive strength and condition of concrete, fastener hardness, fastener spacing and edge distance, and fastener shank diameter. Typically, as the concrete compressive strength increases, so does the fastener’s load capacity. However, there is an optimum performance range of fasteners installed in 2,000 psi to 6,000 psi concrete base material. Manufacturers may test fasteners in base material compressive strength up to 8,500 psi, but it is important to note that pre-drilling of the base material may be necessary for proper installation into higher strength concrete.
These influencing factors are evaluated and confirmed during testing in accordance with ASTM E1190 and either ICC-ES AC70 for general powder actuated applications into steel and concrete or ICC-ES AC43 for steel deck and floor systems. These testing standards provide consistency in evaluating fasteners. Manufacturer’s technical literature and code approvals provide guidance for design with consideration of these influencing factors. Some common failure methods that are assessed through testing include fastener pullout in steel, fastener pullout in concrete, sheet steel pullover, bearing, tearing, piling up, and fastener shear fracture.
Hilti has resources available for further questions on direct fasteners. Local field engineers can assist with training and guidance on design and specification. Product submittals can be generated using Hilti’s Submittal Generator. If specifically designing direct fasteners for metal roof and floor deck attachment, Hilti’s PROFIS DF design software can help structural engineers and designers with this process. Technical literature, design load tables, guideline specifications, CAD details, and approvals are all available in the Fastening Design Center. For specific information about using direct fastening when attaching to steel, refer to the Fastening on Steel Design Center. Hilti’s Technical Services team can be contacted at hnatechnicalservies@hilti.com or 877-749-6337 to assist as well.
Fastening to Steel Design Center
https://www.hilti.com/content/hilti/W1/US/en/engineering/design-centers/engineering/fastening-on-steel.html
Corrosion in Construction
https://www.hilti.com/content/hilti/W1/US/en/engineering/design-centers/engineering/fastening-on-steel/corrosion.html