Designed by Todd Brady and Stephen H. Miller, the CDTC cold formed (CFSF) (also known as “light gauge”) frame was originally an alternative to wood, but after decades of aggressive work, it finally played its part. Like carpenter-finished wood, steel posts and tracks can be cut and combined to create more complex shapes. However, until recently there has been no real standardization of components or compounds. Each rough hole or other special structural element must be individually detailed by an Engineer of Record (EOR). Contractors do not always follow these project-specific details, and may “do things differently” for a long time. Despite this, there are significant differences in the quality of field assembly.
Ultimately, familiarity breeds dissatisfaction, and dissatisfaction inspires innovation. New framing members (beyond the standard C-Studs and U-Tracks) are not only available using advanced shaping techniques, but can also be pre-engineered/pre-approved for specific needs to improve the CFSF stage in terms of design and construction. .
Standardized, purpose-built components that conform to specifications can perform many tasks in a consistent manner, providing better and more reliable performance. They simplify detailing and provide a solution that is easier for contractors to install correctly. They also speed up construction and make inspections easier, saving time and hassle. These standardized components also improve workplace safety by reducing cutting, assembly, screwdriving and welding costs.
Standard practice without CFSF standards has become such an accepted part of the landscape that it’s hard to imagine commercial or high-rise residential construction without it. This widespread acceptance was achieved in a relatively short period of time and was not widely used until the end of World War II.
The first CFSF design standard was published in 1946 by the American Iron and Steel Institute (AISI). The latest version, AISI S 200-07 (North American Standard for Cold Formed Steel Framing – General), is now the standard in Canada, USA and Mexico.
Basic standardization made a big difference and CFSF became a popular construction method, whether they were load-bearing or non-load-bearing. Its benefits include:
As innovative as the AISI standard is, it does not codify everything. Designers and contractors still have a lot to decide.
The CFSF system is based on studs and rails. Steel posts, like wooden posts, are vertical elements. They usually form a C-shaped cross-section, with the “top” and “bottom” of the C forming the narrow dimension of the stud (its flange). Guides are horizontal frame elements (thresholds and lintels), having a U-shape to accommodate racks. Rack sizes are usually similar to nominal “2×” lumber: 41 x 89 mm (1 5/8 x 3 ½ inches) is “2 x 4″ and 41 x 140 mm (1 5/8 x 5). ½ inch) equals “2×6″. In these examples, the 41 mm dimension is referred to as “shelf” and the 89 mm or 140 mm dimension is referred to as “web”, borrowing concepts familiar from hot rolled steel and similar I-beam type members. The size of the track corresponds to the overall width of the stud.
Until recently, the stronger elements required by the project had to be detailed by EOR and assembled on site using a combination of combo studs and rails, as well as C- and U-shaped elements. The exact configuration is usually provided to the contractor and even within the same project it can vary greatly. However, CFSF’s decades of experience have led to the recognition of the limitations of these basic forms and the problems associated with them.
For example, water can accumulate in the bottom rail of a stud wall when the stud is opened during construction. The presence of sawdust, paper, or other organic materials can cause mold or other moisture-related problems, including deterioration of drywall or attracting pests behind fences. A similar problem can occur if water seeps into finished walls and collects from condensation, leaks, or spills.
One solution is a special walkway with holes drilled for drainage. Improved stud designs are also in development. They feature innovative features such as strategically placed ribs that flex in cross section for added rigidity. The textured surface of the stud prevents the screw from “moving”, resulting in a cleaner connection and a more uniform finish. These tiny improvements, multiplied by tens of thousands of spikes, can have a huge impact on a project.
Going beyond studs and rails Traditional studs and rails are often sufficient for simple walls without rough holes. Loads may include the weight of the wall itself, the finishes and equipment on it, the weight of the wind, and for some walls also include permanent and temporary loads from the roof or floor above. These loads are transmitted from the top rail to the columns, to the bottom rail, and from there to the foundation or other parts of the superstructure (eg concrete deck or structural steel columns and beams).
If there is a rough opening (RO) in the wall (such as a door, window, or large HVAC duct), the load from above the opening must be transferred around it. The lintel must be strong enough to support the load from one or more so-called studs (and the attached drywall) above the lintel and transfer it to the jamb studs (RO vertical members).
Likewise, door jamb posts must be designed to carry a greater load than regular posts. For example, in interior spaces, the opening must be strong enough to support the weight of the drywall over the opening (i.e., 29 kg/m2 [6 lbs per square foot] [one layer of 16 mm (5/8 inch) per hour of wall.) per side of plaster] or 54 kg/m2 [11 pounds per square foot] for a two-hour structural wall [two coats of 16 mm plaster per side]), plus seismic load and typically the weight of the door and its inertial operation. In external locations, openings must be able to withstand wind, earthquake and similar loads.
In the traditional CFSF design, the headers and sill posts are made on site by combining standard slats and rails into a stronger unit. A typical reverse osmosis manifold, known as a cassette manifold, is made by screwing and/or welding five pieces together. Two posts are flanked by two rails, and a third rail is attached at the top with the hole facing up to place the post above the hole (Figure 1). Another type of box joint consists of only four parts: two studs and two guides. The other consists of three parts – two tracks and a hairpin. The exact production methods for these components are not standardized, but vary between contractors and even workers.
Although combinatorial production can cause a number of problems, it has proven itself well in industry. The cost of the engineering phase was high because there were no standards, so rough openings had to be designed and finalized individually. Cutting and assembling these labour-intensive components on site also adds to costs, wastes materials, increases site waste, and increases site safety risks. In addition, it creates quality and consistency issues that professional designers should be especially concerned about. This tends to reduce the consistency, quality, and reliability of the frame, and can also affect the quality of the drywall finish. (See “Bad Connection” for examples of these problems.)
Connection systems Attaching modular connections to racks can also cause aesthetic problems. Metal to metal overlap caused by tabs on the modular manifold can affect the wall finish. No interior drywall or exterior cladding should lie flat on the metal sheet from which the screw heads protrude. Raised wall surfaces can cause noticeable uneven finishes and require additional corrective work to hide them.
One solution to the connection problem is to use ready-made clamps, fasten them to the posts of the jamb and coordinate the joints. This approach standardizes connections and eliminates inconsistencies caused by on-site fabrication. The clamp eliminates metal overlap and protruding screw heads on the wall, improving the wall finish. It can also cut installation labor costs in half. Previously, one worker had to hold the header level while another screwed it into place. In a clip system, a worker installs the clips and then snaps the connectors onto the clips. This clamp is usually manufactured as part of a prefabricated fitting system.
The reason for making manifolds from multiple pieces of bent metal is to provide something stronger than a single piece of track to support the wall above the opening. Since bending stiffens the metal to prevent warping, effectively forming microbeams in the larger plane of the element, the same result can be achieved using a single piece of metal with many bends.
This principle is easy to understand by holding a sheet of paper in slightly outstretched hands. First, the paper folds in the middle and slips. However, if it is folded once along its length and then unrolled (so that the paper forms a V-shaped channel), it is less likely to bend and fall. The more folds you make, the stiffer it will be (within certain limits).
The multiple bending technique exploits this effect by adding stacked grooves, channels, and loops to the overall shape. “Direct Strength Calculation” – a new practical computer-assisted analysis method – replaced the traditional “Effective Width Calculation” and allowed simple shapes to be converted into appropriate, more efficient configurations to get better results from steel. This trend can be seen in many CFSF systems. These shapes, especially when using stronger steel (390 MPa (57 psi) instead of the previous industry standard of 250 MPa (36 psi)), can improve the overall performance of the element without any compromise in size, weight, or thickness. become. there have been changes.
In the case of cold-formed steel, another factor comes into play. Cold working of steel, such as bending, changes the properties of the steel itself. The yield strength and tensile strength of the processed part of the steel increase, but the ductility decreases. The parts that work the most get the most. Advances in roll forming have resulted in tighter bends, meaning that the steel closest to the curved edge requires more work than the old roll forming process. The larger and tighter the bends, the more steel in the element will be strengthened by cold working, increasing the overall strength of the element.
Regular U-shaped tracks have two bends, C-studs have four bends. The pre-engineered modified W manifold has 14 bends arranged to maximize the amount of metal actively resisting stress. The single piece in this configuration may be the entire door frame in the rough opening of the door frame.
For very wide openings (i.e. over 2 m [7 ft]) or high loads, the polygon can be further reinforced with appropriate W-shaped inserts. It adds more metal and 14 bends, bringing the total number of bends in the overall shape to 28. The insert is placed inside the polygon with inverted Ws so that the two Ws together form a rough X-shape. W’s legs act as crossbars. They installed the missing studs over the RO, which were held in place with screws. This applies whether or not a reinforcing insert is installed.
The main benefits of this preformed head/clip system are speed, consistency and improved finish. By choosing a certified prefabricated lintel system, such as one approved by the International Code of Practice Committee Evaluation Service (ICC-ES), designers can specify components based on load and wall type fire protection requirements, and avoid having to design and detail each job, saving time and resources. (ICC-ES, International Codes Committee Evaluation Service, accredited by the Standards Council of Canada [SCC]). This prefabrication also ensures that blind openings are built as designed, with consistent structural soundness and quality, without deviations due to on-site cutting and assembly.
Installation consistency is also improved as the clamps have pre-drilled threaded holes, making it easier to number and place joints with jamb studs. Eliminates metal overlaps on walls, improves drywall surface flatness and prevents unevenness.
In addition, such systems have environmental benefits. Compared to composite components, the steel consumption of one-piece manifolds can be reduced by up to 40%. Since this does not require welding, the accompanying emissions of toxic gases are eliminated.
Wide Flange Studs Traditional studs are made by joining (screwing and/or welding) two or more studs. Although they are powerful, they can also create their own problems. They are much easier to assemble before installation, especially when it comes to soldering. However, this blocks access to the stud section attached to the Hollow Metal Frame (HMF) doorway.
One solution is to cut a hole in one of the uprights to attach to the frame from inside the upright assembly. However, this can make inspection difficult and require additional work. Inspectors have been known to insist on attaching the HMF to one half of the doorjamb stud and inspecting it, then welding the second half of the double stud assembly into place. This stops all work around the doorway, may delay other work, and requires increased fire protection due to on-site welding.
Prefabricated wide-shoulder studs (specially designed as jamb studs) can be used in place of stackable studs, saving significant time and material. The access issues associated with the HMF doorway are also solved as the open C side allows for uninterrupted access and easy inspection. The open C-shape also provides full insulation where the combined lintels and jamb posts typically create a gap of 102 to 152 mm (4 to 6 inches) in insulation around the doorway.
Connections at the top of the wall Another area of design that has benefited from innovation is the connection at the top of the wall to the upper deck. The distance from one floor to another may vary slightly over time due to variation in deck deflection under different loading conditions. For non-load-bearing walls, there should be a gap between the top of the studs and the panel, this allows the deck to move down without crushing the studs. The platform must also be able to move up without breaking the studs. The clearance is at least 12.5 mm (½ in.), which is half the total travel tolerance of ±12.5 mm.
Two traditional solutions dominate. One is to attach a long track (50 or 60 mm (2 or 2.5 in)) to the deck, with the stud tips simply inserted into the track, not secured. To prevent the studs from twisting and losing their structural value, a piece of cold rolled channel is inserted through a hole in the stud at a distance of 150 mm (6 inches) from the top of the wall. consuming process The process is not popular with contractors. In an effort to cut corners, some contractors may even forego cold rolled channel by putting studs on rails with no means of holding them in place or leveling them. This violates ASTM C 754 Standard Practice for Installing Steel Framing Members to Produce Threaded Drywall Products, which states that the studs must be attached to the rails with screws. If this deviation from the design is not detected, it will affect the quality of the finished wall.
Another widely used solution is the double track design. The standard track is placed on top of the studs and each stud is bolted to it. A second, custom-made, wider track is placed above the first and connected to the top deck. Standard tracks can slide up and down inside custom tracks.
Several solutions have been developed for this task, all of which include specialized components that provide slotted connections. Variations include the type of slotted track or the type of slotted clip used to attach the track to the deck. For example, secure a slotted rail to the underside of the deck using a fastening method appropriate for the particular deck material. The slotted screws are attached to the tops of the studs (according to ASTM C 754) allowing the connection to move up and down within approximately 25 mm (1 inch).
In a firewall, such floating connections must be protected from fire. Below a grooved steel deck filled with concrete, the fire retardant material must be able to fill the uneven space below the groove and maintain its fire-fighting function as the distance between the top of the wall and the deck changes. The components used for this joint have been tested in accordance with the new ASTM E 2837-11 (Standard Test Method for Determining the Fire Resistance of Solid Wall Head Joint Systems Installed Between Rated Wall Components and Non-Rated Horizontal Components). The standard is based on Underwriters Laboratories (UL) 2079, “Fire Testing for Building Connecting Systems”.
The advantage of using a dedicated connection at the top of the wall is that it can include standardized, code-approved, fire-resistant assemblies. A typical build is to place the refractory on deck and hang a few inches above the top of the walls on either side. Just as a wall can slide up and down freely in a mortise fixture, it can slide up and down in a fire joint as well. Materials for this component may include mineral wool, cemented structural steel refractory, or drywall, used alone or in combination. Such systems must be tested, approved and listed in catalogs such as Underwriters Laboratories of Canada (ULC).
Conclusion Standardization is the foundation of all modern architecture. Ironically, there is little standardization of “standard practice” when it comes to cold formed steel framing, and innovations that break those traditions are also standards makers.
The use of these standardized systems can protect designers and owners, save significant time and money, and improve site safety. They bring consistency to construction and are more likely to work as intended than built systems. With a combination of lightness, sustainability and affordability, CFSF is likely to increase its share of the construction market, no doubt spurring further innovation.
Todd Brady is President of Brady Construction Innovations and inventor of the ProX manifold roughing system and the Slp-Trk wall cap solution. He is a metal beam specialist with 30 years of experience in the field and contract work. Brady can be contacted by email: bradyinnovations@gmail.com.
Stephen H. Miller, CDT is an award-winning writer and photographer specializing in the construction industry. He is the creative director of Chusid Associates, a consulting firm providing marketing and technical services to building product manufacturers. Miller can be contacted at www.chusid.com.
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Post time: Jul-07-2023