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Fastening cold-formed steel framed members together as well as attaching other materials to steel framing (such as plywood sheathing) is most commonly accomplished using screws. Screws, although relatively inexpensive, are time consuming to install. To speed up the connection process, several new technologies and connection methods have emerged that have been gaining wider usage particularly among panel manufacturers. Some of these fastening methods are listed below:
- Clinches
- Self piercing rivets
- Pins and pneumatic fasteners
Clinches
Clinching is a method of joining two pieces of sheet metal by pressing them together into a die that forms a connection similar to a rivet. Fasteners like rivets or self-tapping screws are not required to complete a clinched connection. This technology has only recently seen use for attaching residential steel framing members.
There are several different types of clinching equipment. The majority of clinched connections are made with pneumatic or hydraulic tools, although manual clinchers are available. A clincher makes a connection by driving a punch into a die through overlapping material. When the material is forced to the bottom of the die, the die begins to mushroom. The die expands to allow full development of the connection. When the punch reaches its final position, it is withdrawn and the die returns to its original shape. The result is a connection very similar to a riveted connection. Typical point sizes of the punches are 3/16- and 5/16-inches. The strength of a clinched connection is approximately the same as that of a self-drilling screw. Clinching tools are available that will connect up to 68 mil (14 gauge) material. The clinching process does not harm the galvanized coating on framing members.
Self-Piercing Rivets (SPR)
This technology has been widely used in the automotive and trucking industry for decades. SPR connections consist of joining two or more sheets of cold-formed steel with a pin-like rivet that is installed with proprietary tools, without making a hole in the steel. The connection is formed by driving the rivet into steel sheets and piercing the top layer(s) with a rivet that is under the influence of a die under the sheet(s). The rivet flares at the bottom layer without penetrating it.
Self-pierce riveting systems are available to suit a wide variety of applications and production styles. Equipment types include the simple, hand held tools, useful for repair work, on-site riveting and low volume production. For higher volume production, hydraulic powered systems are available, offering fast cycle times and automatic rivet feed. For use in heavily automated, high volume production, robot mounted self piercing tools with separate control cabinets are highly effective.
Pneumatic Pins
Pneumatic pins and nails are specifically designed with spiral grooves or knurls on the nail shaft to penetrate the steel. Like automatic nail delivery in wood framing, drive pins and nails are applied with air guns. Pin guns have an advantage over screw guns in that they are much faster. Unlike rivets or clinches, pneumatic pins are not limited to steel connections; they can also be used to fasten other building products to steel.
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Rivets, pin nails, and clinches have a long history of sturdy performance in manufactured product connections.
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There are several manufacturers of clinching, riveting and pin tools. Many clinching and riveting tools are limited to use in a warehouse/factory environment while pneumatic pins can be used in either environment. Clinched and riveted connections can be difficult to loosen when necessary. Both clinching and riveting technology/equipment for cold-formed steel framing are not widely available. Unlike pneumatic pin tools, the use of clinching and riveting tools at the field jobsite for steel framing is not as practical as in the plant because of several limitations:
- The equipment is too bulky and heavy to move around on a site
- Access to both sides of an assembly is required and this is often hard to achieve on site. The tools are too large for many applications, or the use of double or triple members is so frequent that access is restricted.
- Screw guns, pneumatic guns and clinching and riveting tools would need to be used, which would usually require switching back and forth between the two on the same assemblies.
Pneumatic pin tools are easy to use and are very similar to nail guns. These tools come with different pins that can be used to attach any building product to cold-formed steel (such as plywood to steel, Densglass to steel, baseboard to steel, or gypsum board to steel).
Initial investment in the clinching tools would be the highest cost among the three fastening options, followed by the riveting tool. The pneumatic pin tools require the smallest initial investment. However, non-load bearing members can be clinched together using economical hand-held clinching tools. The most economical use of the clinching and riveting tools is in a production setting where repetitive tasks are performed.
Pin nailer costs compare similarly to that of the pneumatic nail guns that are used for wood framing. So, an initial investment would require guns, hoses, and air compressor.
Clinching tools require several dies for different material thicknesses. The dies do wear out and will require replacement. Clinched connections have no fastener cost.
Operational costs for riveting tools are not as high as those for clinching tools because the same die will accommodate different steel gauges and dies do not wear out as frequently.
Operational costs for pneumatic pin guns are similar to those of nail guns.
Fastener costs for riveted and pinned steel connections are higher than the cost of screws, but the added material cost can be offset by labor savings. Rivets and pins can be installed at rates 5 to 10 times faster than screws. The cost of a pin is twice that of a comparable screw and the cost of a rivet is now three to four times that of a screw. However, unlike economic activity in mature markets, these costs are expected to decrease as these products become more widely used.
Some fastener manufacturer (ET&F for example. www. etf-fastening.com) offer worksheets which allow you to calculate the cost savings of pins based on specific labor rates.
Clinched Connections
The majority of manufacturers of clinching technology have not yet obtained national code approval. Many manufacturers do have testing information available on the strength of their connections. One clinching manufacturer ATTEXOR, Inc. of Springfield, MA currently has an ICBO ES report (ER 5439) which can be used as a basis for acceptance. An acceptance Criteria (AC 137) has also been developed by ATTEXOR, Inc. for clinched connections. This Acceptance Criteria is available for clinching manufacturers to develop other evaluation reports.
Riveted Connections
The current manufacturers of self-piercing rivets have not yet obtained national code approval. Some manufacturers do have testing information available on the strength of their SPR connections (such as Henrob Corp.) An acceptance Criteria (AC 344) has been developed by Henrob Corp. for riveted connections. This Acceptance Criteria is available for riveting manufacturers to develop evaluation reports.
Pneumatic Pin Connections
Almost all manufacturers of pneumatic pins used for cold formed steel framing have obtained evaluation reports for their pins. The evaluation reports contain strength values for different thicknesses of steel. Racking shear values are also available for steel walls with pins. On a one-to-one basis, screws usually offer greater holding strength than pins when withdrawal strength is measured. This may not be true when pull over strength is compared, due to similar head diameters of the two fasteners. However, for most applications, the installation of a few more pins than screws, allows the assembly made with pins to carry the same loads as an assembly built with screws.
Attexor Corporation: Oahu, Hawaii
The installation of riveted, clinched and pinned connections is very simple. For riveted and clinched connections, the material to be joined is placed between the jaws of the tool and then the trigger is pulled. Typically, these tools are used in an industrialized panel plant where walls are assembled in a manufacturing environment. Air guns and equipment used for pneumatic pins perform in a similar manner to the pneumatic nail guns used for wood framing.
Warranties of tools vary by manufacturer.
Riveting and clinching have the following benefits/costs:
- They reduce the amount of screws or welds needed.
- Connections can be performed in a second or less.
- Hand injuries from sharp screws and metal burrs are reduced.
- There are no toxic fumes or loud noises.
- Connections are less susceptible to loosening with building movement.
- The fastening devices are better suited to a factory environment.
- Costs per connection (labor and material) have been reported to be less than those of screw connection.
Pneumatic pins have the following benefits/costs:
- Connections are fast and easy to make.
- Connections take about the same time as framing pneumatically-driven nails (many per minute).
- Connections can be more susceptible to loosening over time than other types of steel connections.
- Pneumatic pins facilitate the panelization process.
- Costs per connection (labor and material) have been reported to be about the same as connections installed with screws.
- Care should be taken to follow manufacturer recommendations carefully to avoid problems such as squeaky floors.
Clinched or riveted connections, once they are made, are very difficult to take apart. The clinching and riveting mechanism does not adversely impact the corrosion resistance of the steel.
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