In the case of steel framing, the cross-sectional shape of the frame member, size and the thickness and grade of steel must be considered. In the case of wood framing these include the species, size and grade of lumber used. The choice and installation of framing depends on a number of f actors.
This upper structure lends itself well to individually framed walls and floor systems.CEMCOs new patent pending TAB TRACK provides a non mechanical (screwless) stud-to-track attachment for metal stud wall framing. The base structure is generally used for parking or retail space, with an upper structure that is of another use, such as apartments, dorms, senior living, hotels or other private spaces. The most common scenario is a base structure, commonly referred to as the podium. Specification for the Design of Cold-Formed Steel Structural Members has designBuilding designers are often challenged with an increasing demand for high-density, combined-use buildings in urban locations. Track with the wall studs nested into the track (see Figure 1) with no attachment of the wall sheathing.
Combining non-combustible materials such as cold-formed steel with concrete and hot-rolled steel results in higher building heights for these structures at an economical price. With this increased density comes the need to provide a safe, non-combustible framing system that is also strong to achieve the desired overall building height. Once the studs are nested within the TAB TRACK the tabs may be pushed inward on either side.
This permits a separate and distinct determination of each of the areas based on the allowable area limitation, continuity of firewalls, type of construction, and number of stories. There are three sections in IBC 2015 Chapter Five pertaining to podium designs that describe permissible building heights and story limits and are critical to understand when designing cold-formed steel upper structures over a concrete podium to maximize the benefits of non-combustible steel construction.The first provision, Section 510.2, allows an upper structure of any construction type to be built over a lower podium where the two structures are treated as separate and distinct structures. Shaft assemblies are.The International Building Code (IBC) requires the upper and lower structures to be separated for fire and other safety reasons. For the lateral force resisting system, shaft assemblies constructed from CFS could be the best option. How High Can Cold-Formed Steel Go?Nested jamb with studs and track. Cold-formed steel provides an ideal solution for all of these design challenges.
For example, by using the residential occupancy and cold-formed steel framing over the top of retail space, one could get the maximum stories of floors and still be under the overall building height limitations when coupled with a multi-story podium. These heights are associated primarily with Type IIA and IB, which require one-hour and two-hour fire-rated assemblies, respectively. For cold-formed steel, there are various UL assemblies as outlined in A Guide to Fire and Acoustic Data for Cold-Formed Steel Floor, Wall and Roof Assemblies ( The most common range for mid-rise non-combustible building materials is five to twelve stories and 85 feet to 180 feet. This is achieved by providing a fire protection system for the bearing walls that meets an hourly rating between one and three hours.
This design would likely be most cost-effective when the building has no need for a parking deck and ample space in the lot on which it is being built. Below-ground parking would still require a three-hour fire separation. This provision does not require a podium level separation but does require a 1½-hour fire-rated first elevated floor system. It presents a rare opportunity for a nine-story, 100-foot-tall, Type II-A building when there is at least a 50-foot lot line separation. CFS over parking structure podium.The third provision, Section 510.6, governs Group R-1 and R-2 buildings of Type II-A construction.
This helps to clarify the use of the framing element as both the main frame of the building and a component. Separately, there is a bending check for components and cladding loading without the effect of axial loading. In this case, AISI S240, North American Standard for Cold-Formed Steel Structural Framing, allows for the design of these elements as MWFRS ( Section B1.1 allows for live load reduction of the framing along with a check for combined MWFRS loading and axial loading. The lightweight nature of cold-formed steel provides a direct benefit for seismic design (less mass) but also offers a significant benefit since the inherent stiffness of cold-formed steel makes it easy to comply with the ASCE 7-10 requirement that the fundamental period of the entire structure not be greater than 1.1 times the period of the upper structure alone.For wind loading, the cold-formed steel framing is the building’s main frame and therefore the primary wind force resisting system (MWFRS). This analysis procedure treats the upper and lower portions of the structure as two distinct structures, with the base shear of the superstructure applied to the base in a second analysis. The two-stage analysis procedure recognizes the unique performance characteristics of a lightweight and flexible superstructure over a stiff base which is ten times stiffer than the superstructure.
This is where cold-formed steel is uniquely qualified. If the shear walls are non-bearing walls, the seismic forces may be significantly higher than gravity loads. When this is the case, cold-formed steel shear walls can be used to supplement or even provide the entire lateral force resistance. However, they alone may not be capable of providing the torsional resistance due to their dimensions and location within the building’s footprint. Shaft assemblies are typically placed in the central core area of a building and are inherently strong due to the construction and fire-rating requirements. Nested jamb with studs and track.For the lateral force resisting system, shaft assemblies constructed from CFS could be the best option.
Nested LDM with steel plate. More discussion on this topic is found in AISI D110-16, Cold-Formed Steel Framing Design Guide, 2016 Edition ( Figure 4. As seen in Figure 4, connections to the podium can range from post-installed anchors to welded embed plates. This is achievable with cold-formed steel. In this situation, the design called for more lateral resisting elements than what was available in open wall locations, so an overlapping method was employed. Figure 3 depicts an example of creative overlapping of thin cold-formed steel lateral strap bracing.
The loading from a single floor can be handled by the header over the opening. Overlapping shear straps for lateral resistance.Another challenge is the design of gravity framing around stacked openings like doors and windows. As discussed in CFSEI Tech Note G200-15, Chase the Loads: Load Path Considerations for Cold-Formed Steel Light-Frame Construction, the goal of every framing system is to provide a concise and direct load path ( Figure 5. Unlike some alternates, cold-formed steel can also resolve concentrated loads into singular or closely grouped resisting elements when appropriately detailed. It is easy to achieve uniform loading that is evenly spaced, and many materials can handle this load scenario. Cold-formed steel framing is dimensionally and geometrically stable, which can be a major consideration when evaluating the longevity and durability of a mid-rise building.Often, it is the detailing and concentration of load into a single isolated connection that creates the greatest challenge for the designer.
Figure 5 shows an example of nested studs and tracks to form a compact jamb condition. This allows more room for electrical, plumbing, and other necessary items that occupy the wall cavity. With cold-formed steel framing, there are many ways to keep the framing concentrated into an area that is relatively narrow in the wall without dimensional build-up that occurs with other materials. Jambs can accumulate quite large forces in taller buildings when openings align.
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