Estimate the depth of wood rafters on the basis of the horizontal (not slope) distance from the outside wall of the building to the ridge board in a gable or hip roof and the horizontal distance between supports in a shed roof. A 2_ 4 rafter spans approximately 7 feet (2.1 m), a 2 _ 6 10 feet (3.0 m), a 2 _ 8 14 feet (4.3 m), and a 2 _ 10 17 feet-(5.2 m). The depth of wood light roof trusses is usually based on the desired roof pitch. A typical depth is one-quarter of the width of the building, which corresponds to a 6_12 pitch in a gable truss. Trusses are generally spaced 24 inches (600 mm) o.c. and can span up to approximately 65 feet (20 m). Estimate the depth of wood fl oor joists as follows: 2 _ 6 joists span up to 9 feet (2.7 m), 2 _ 8 joists 11 feet (3.4 m), 2 _ 10 joists 14 feet (4.3 m), and 2 _ 12 joists 17 feet (5.2 m). Estimate the depth of manufactured wood I-joists as follows: 9½-inch (240-mm) joists span 16 feet (4.9 m), 11 7_8 -inch (300-mm) joists 19 feet (5.8 m), 14-inch
(360-mm) joists span 23 feet (7.0 m), and 16-inch (400-mm) joists span 25 feet (7.6 m). Estimate the depth of wood floor trusses as 1_18 of their span. Typical depths of ß oor trusses range from 12 to 28 inches (305Ð710 mm) in 2-inch (51-mm) increments. 2 _ 4 wood studs 24 inches (600 mm) o.c. can support attic and roof loads only. Either 2 _ 4 studs 16 inches (400 mm) o.c. or 2 _ 6 studs 24 inches o.c. can support one ß oor plus attic and roof. 2 _ 6 studs 16 inches o.c. can support two ß oors plus attic and roof. Framing members in light frame buildings are usually spaced either 16 or 24 inches (400 or 600 mm) o.c. For actual sizes of dimension lumber in both conventional and metric units, see Figure 3.22. These approximations are valid only for purposes of preliminary building layout and must not be used to select Þ nal member sizes. They apply to the normal range of building occupancies such as residential, ofÞ ce, commercial, and institutional buildings. For manufacturing and storage buildings, use somewhat larger members. For more comprehensive information on preliminary selection and layout of a structural system and sizing of structural members, see Edward Allen and Joseph Iano, The Architect’s Studio Companion (4th ed.), New York, John Wiley & Sons, Inc., 2007.
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