Deck Attachment with Mechanical Fasteners

A diaphragm is defined by the American Iron & Steel Institute (AISI) as a roof, floor, or other membrane system that transfers in-plane forces to the lateral force resisting system, or structural framing. The diaphragm is comprised of metal decking sheets attached to structural framing supports, typically either joists or structural steel beams. The diaphragm is designed to hold dynamic loads of lateral perimeter, wind, and seismic forces as well as static loads like dead and snow loads.



A diaphragm may act as a structural element that transmits lateral load to the vertical resisting elements of a structure.

During the design stage, a structural engineer must consider a variety of  factors to properly design a metal deck diaphragm. Geometric configurations that influence the calculated capacity of the deck diaphragm include the deck profile and thickness, structural frame spacing, connection type, and connection spacing or pattern to address the loading conditions, among other design considerations.

Deck fastener selection during design can have a large impact on cost, productivity, and performance

Of these design parameters, connection type and quality can have tremendous impact on deck performance, total installation cost, and productivity on the jobsite. In the past, the construction industry relied on arc-spot welds, also known as puddle welds, as the conventional method of attaching a metal roof/floor deck to the structural framing. 

The puddle welds are formed when an intense and precise amount of heat is applied to the deck surface over the structural framing using an electric arc. During this process, a hole in the deck is formed in which filler metal can penetrate the structural framing material. Molten filler metal continues to “puddle” engaging the deck, thus forming a bond between the deck and structural support.

Although welds can provide high diaphragm shear capacity, there are many factors that affect the performance of these puddle welds. Among many factors, the quality, and thus the performance of a weld is subject to the amperage selected, type of electrode used, technique and skill of the welder, and time to complete the process. In the application of welds to attach metal deck, it is important to note that welds are not certified by the American Welding Society (AWS) for applications greater than two layers of deck to structural framing (American Welding Society (AWS) Standard D1.3 1.5.4). Missing the structural framing below, using amperage outside of the prescribed range, or even taking several seconds longer than necessary to complete the process can lead to burn-throughs, needing touch-up to the painted or galvanized deck or resulting in a connection that may not perform as intended.

Mechanical fastening with powder-actuated fasteners is becoming the standard

Because of the considerations and possible challenges that arise with puddle welding, Hilti developed innovative ways to fasten deck to steel framing supports. Mechanical fasteners are considered one of the most productive fastening methods due to the speed of installation, their high shear and tension pullout capacities, and superior reliability. The two types of mechanical fasteners we will discuss are powder-actuated fasteners (PAFs) and screw fastening systems.



Hilti frame fastener portfolio for attachment of metal deck to steel base materials.

Powder-actuated fasteners, commonly referred to as “pins” or “nails,” are driven through the deck into the structural framing steel, like joist or beam, using a low-velocity powder actuated tool (PAT) that uses powder cartridges to activate a captive-piston system which drives the pin into the base material.



Powder-actuated fasteners (PAFs) utilize friction, mechanical keying, soldering, and fusion to provide shear and uplift capacity.

PAF’s utilize four main holding mechanisms of friction, fusion, keying and micro-brazing to develop strength in steel framing base materials. The first holding mechanism in steel is friction. As the fastener is driven into the steel base material, the steel deforms around the fastener as it is displaced by the fastener and due to the steels elastic properties attempts to rebound to its original position, creating a friction hold around the fastener. Brazing occurs due to a thin coating of zinc on the fastener surface that is partially melted as heat is generated while driving the fastener. The tip of the fastener tends to achieve the highest temperature during the driving process, which results in fusion between the fastener, zinc coating, and steel base material. This fusion can be an important holding mechanism; however, this is only seen in fasteners that do not through-penetrate.


Hilti powder-actuated fasteners (PAFs) portfolio for metal deck attachment

Check out Hilti’s powder-actuated fasteners:
X-HSN 24 Metal Deck Fasteners
X-ENP 19 Metal Deck Fasteners

It is important to note that there are a variety of PAFs that are optimized based on geometry of the connection like thickness of base material or deck. In cyclical or dynamic testing, mechanical fasteners demonstrate ductile behavior, while welds are a brittle connection. PAF’s are installed with a tested system that allows flexibility through power regulation of the tool and driving energy, but still allows a higher level of certainty/consistency of the resulting connection.

Mechanical Screw Fastening System; an alternative to powder-actuated fastening

Screws have been most commonly used for base materials less than 1/8” thick, for example in cases where metal deck is being attached to light gage steel. A base material this thin is outside the range of powder-actuated fasteners. Recent innovations to screw fasteners for metal deck attachment have been developed for use in steel base materials up to 3/8” thick.


Hilti frame fastener screw portfolio for metal deck attachment

Hilti screw systems bring key benefits to deck attachment including:
•     A hex-head screw with large integrated washer providing greater clamping and pullover resistance  against uplift loads
•     A larger shank diameter generally providing more robust shear capacity and increased bearing area
•     Testing for fastening in applications of greater than two sheets of deck
•     Reduced potential damage to base steel when compared to puddle welds.

Check out Hilti’s screw frame fasteners:
S-RT5+ Screw
S-RT5 Screw

The benefits of mechanical fastening

Hilti’s X-HSN 24, X-ENP 19, RT5+ and RT5 mechanical fastenings can be installed more reliably and consistently using calibrated, smart tools to help ensure proper power regulation to drive fastener to a consistent penetration of base materials each time, regardless of user experience. Mechanical fastening tools are designed so that the installer remains upright in a comfortable standing position, making proper installation easier and safer for the installer and helping to reduce errors associated with worker fatigue.

Depending on weld diameter, a proper weld needs 8 – 12 seconds to meet specification, whereas Hilti mechanical fasteners can be properly installed quickly (1-2 seconds) increasing jobsite speed and productivity. Mechanical fasteners also have little to no limitations in wet or moist conditions on the deck due to weather, no burn throughs, and little to no touch-up required with painted or galvanized deck.

Hilti’s mechanical fasteners have been tested and evaluated in accordance with Acceptance Criteria AC43 (ICC-ES). With this additional third-party testing and evaluation, Hilti has been able to achieve high-capacity diaphragm shear and uplift performance. Hilti mechanical fasteners show more ductile connections versus welded connections.

Design with mechanical fasteners using PROFIS Engineering Diaphragm Design

 
Hilti has your projects covered from design to installation with the recently released PROFIS Engineering Diaphragm Design module. Hilti’s newly enhanced Diaphragm Design module helps optimize your deck design while also reducing design time. The module supports the design of multiple fastening solutions and allows for comparison of multiple fastening methods side-by-side helping you to select the best fastening method for your steel deck project. With multiple deck types from multiple manufacturers, the Diaphragm Design module can help support most steel deck projects and the optimize feature can help you find the best pattern and fastener for your loading conditions.
 
Log in PROFIS Engineering Software Suite to try out the new Diaphragm Design Module.
 

Resources

Hilti has resources available for further questions on mechanical deck fasteners. Local field engineers can assist with training and guidance on design and specification. Product submittals can be generated using Hilti’s Submittal Generator. Technical literature, design load tables, guideline specifications, CAD details, and approvals are all available in the Decking Design Center. Hilti’s Technical Services Corporate Engineering team can be contacted at hnatechnicalservies@hilti.com or at 877-749-6337.

For further study into decking diaphragms, please consider the following technical articles:
 
Essa, H. S., Tremblay, R., & Rogers, C. A. Inelastic Seismic Response of Metal Roof Deck Diaphragms for Steel Building Structures. 12th European Conference on Earthquake Engineering. Paper reference 482.

Geise, C. (2012, July). Lessons Learned from the Joplin Tornado. Structure, 30-32. Retrieved from https://www.structuremag.org/wp-content/uploads/F-Joplin-Geise-July121.pdf.

Snow, G. L. (2008). Strength of Arc Spot Welds Made in Single and Multiple Steel Sheets. Retrieved from https://vtechworks.lib.vt.edu/handle/10919/33200.