Framing, in construction, is the fitting together of pieces to give a structure support and shape.[1] Framing materials are usually wood, engineered wood, or structural steel. The alternative to framed construction is generally called mass wall construction, where horizontal layers of stacked materials such as log building, masonry, rammed earth, adobe, etc. are used without framing.[citation needed]
Wood Frame Construction Manual Wood Frame Const...
Building framing is divided into two broad categories,[2] heavy-frame construction (heavy framing) if the vertical supports are few and heavy such as in timber framing, pole building framing, or steel framing; or light-frame construction (light-framing) if the supports are more numerous and smaller, such as balloon, platform, or light-steel framing. Light-frame construction using standardized dimensional lumber has become the dominant construction method in North America and Australia due to the economy of the method; use of minimal structural material allows builders to enclose a large area at minimal cost while achieving a wide variety of architectural styles.
Modern light-frame structures usually gain strength from rigid panels (plywood and other plywood-like composites such as oriented strand board (OSB) used to form all or part of wall sections), but until recently carpenters employed various forms of diagonal bracing to stabilize walls. Diagonal bracing remains a vital interior part of many roof systems, and in-wall wind braces are required by building codes in many municipalities or by individual state laws in the United States. Special framed shear walls are becoming more common to help buildings meet the requirements of earthquake engineering and wind engineering.
Historically, people fitted naturally shaped wooden poles together as framework and then began using joints to connect the timbers, a method today called traditional timber framing' or log framing. In the United States, timber framing was superseded by balloon framing beginning in the 1830s. Balloon framing makes use of many lightweight wall members called studs rather than fewer, heavier supports called posts; balloon framing components are nailed together rather than fitted using joinery. The studs in a balloon frame extend two stories from sill to plate. Platform framing superseded balloon framing and is the standard wooden framing method today. The name comes from each floor level being framed as a separate unit or platform.
Framed construction was rarely used in Scandinavia before the 20th century because of the abundant availability of wood, an abundance of cheap labour, and the superiority of the thermal insulation of logs; hence timber framing did not take off there first for unheated buildings such as farm buildings, outbuildings and summer villas, and for houses until the development of wall insulation.[3]
Balloon framing is a method of wood construction used primarily in areas rich in softwood forests such as Scandinavia, Canada, the United States up until the mid-1950s, and around Thetford Forest in Norfolk, England. The name comes from a French Missouri type of construction, maison en boulin,[17] boulin being a French term for a horizontal scaffolding support. It was also known as "Chicago construction" in the 19th century.[18]
Light-frame materials are most often wood or rectangular steel, tubes or C-channels. Wood pieces are typically connected with nail fasteners, nails, or screws; steel pieces are connected with pan-head framing screws, or nuts and bolts. Preferred species for linear structural members are softwoods such as spruce, pine and fir. Light frame material dimensions range from 38 by 89 mm (1.5 by 3.5 in); i.e., a Dimensional number two-by-four to 5 cm by 30 cm (two-by-twelve inches) at the cross-section, and lengths ranging from 2.5 metres (8.2 ft) for walls to 7 metres (23 ft) or more for joists and rafters. Recently,[when?] architects have begun experimenting with pre-cut modular aluminum framing to reduce on-site construction costs.
Wall sections usually include a bottom plate which is secured to the structure of a floor, and one, or more often two top plates that tie walls together and provide a bearing for structures above the wall. Wood or steel floor frames usually include a rim joist around the perimeter of a system of floor joists, and often include bridging material near the center of a span to prevent lateral buckling of the spanning members. In two-story construction, openings are left in the floor system for a stairwell, in which stair risers and treads are most often attached to squared faces cut into sloping stair stringers.[citation needed]
Light-frame methods allow easy construction of unique roof designs; hip roofs, for example, slope toward walls on all sides and are joined at hip rafters that span from corners to a ridge. Valleys are formed when two sloping roof sections drain toward each other. Dormers are small areas in which vertical walls interrupt a roof line, and which are topped off by slopes at usually right angles to a main roof section. Gables are formed when a length-wise section of sloping roof ends to form a triangular wall section. Clerestories are formed by an interruption along the slope of a roof where a short vertical wall connects it to another roof section. Flat roofs, which usually include at least a nominal slope to shed water, are often surrounded by parapet walls with openings (called scuppers) to allow water to drain out. Sloping crickets are built into roofs to direct water away from areas of poor drainage, such as behind a chimney at the bottom of a sloping section.
Light-frame buildings in areas with shallow or nonexistent frost depths are often erected on monolithic concrete-slab foundations that serve both as a floor and as a support for the structure. Other light-frame buildings are built over a crawlspace or a basement, with wood or steel joists used to span between foundation walls, usually constructed of poured concrete or concrete blocks.
Nationwide, there has been an increase in the demand for multi-story mixed-use and multi-residential structures. Common configurations include up to five stories of residential use over retail, commercial, office, and parking occupancies, similar in configuration to the building shown in Figure 1. Podium designs are one way to maximize the number of stories, increase unit density, and lower construction costs. This article covers important design considerations and traditional approaches related to the design of a five-story wood-framed structure over a two-story concrete or masonry podium.
Example floor plan configurations typically encountered in mid-rise multifamily construction are shown in Figure 2. These plans are frequently rectangular in shape with or without exterior shear walls, or they can have multiple horizontal offsets and wings. The lateral force resisting system for the flexible upper portion is typically built with wood-framed shear walls sheathed with wood structural panels (WSP). Many, if not all, of the walls separating the dwelling units are used as interior shear walls in the transverse direction. Lateral forces in the longitudinal direction are typically resisted by the exterior walls and corridor walls. If a rigid diaphragm analysis is warranted, the transverse walls would also act to resist torsional forces.
Framing systems for gravity loads in the upper portion commonly consist of load-bearing wood-framed wall configurations as shown in Figure 3. Semi-balloon framing can be used to reduce vertical shrinkage. This system utilizes top flange joist hangers to support the floor framing off the bearing walls. Considerations with this system include the eccentric gravity load effects on the wall studs caused by top flange joist hangers. This must be accounted for in the design and can in some cases increase the size of the studs. Another consideration is the detailing and added framing challenge of placing the interior wall sheathing between or behind the joist hangers. Several connector manufacturers now have joist hangers that are unique for this type of installation, which simplifies the process. The other option is to use platform framing, which is easier to install, reduces stud heights, takes less time to install, and can eliminate the joist hanger and costs associated with semi-balloon framing. However, this method of framing has an increased potential for vertical shrinkage. Proper detailing for either framing system can address this issue.
Current practice for light-frame construction commonly assumes that wood diaphragms are flexible for the purpose of distributing horizontal forces to shear walls. ASCE 7-10 Section 12.3.1.1 (c) allows diaphragms in light-frame structures to be idealized as flexible when 1Â inches or less of non-structural topping, such as concrete or a similar material, is placed over WSP diaphragms, and each line of vertical elements of the SFRS complies with the allowable story drift of ASCE 7-10 Table 12.12-1. Using the flexible diaphragm assumption would allow distribution of diaphragm forces to shear walls to be based on tributary area. In 1999, the Structural Engineers Association of California Code and Seismology Committees recommended that relative flexibility requirements outlined in ASCE 7 Section 12.3.1 be considered for wood framed diaphragms.
Even though diaphragms may be idealized as flexible, it is sometimes good engineering judgment to consider other flexibility conditions. Currently, some designers only perform a flexible diaphragm analysis and some a rigid diaphragm analysis, but a few use semi-rigid modeling (enveloping). On that basis, some confusion and lack of consistency exist regarding which type of diaphragm analysis should be employed for a given project. Verifying the diaphragm flexibility is becoming increasingly more important given trends toward larger openings in exterior shear walls, shorter wall lengths, and a greater number of wood frame stories over the podium.
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