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LH-Series Joists have been standardized in depths from 18 inches (460 mm) through 48 inches (1,200 mm), for spans through 96 feet (29,000 mm). DLH-Series Joists have been standardized in depths from 52 inches (1,300 mm) through 120 inches (3,000 mm), for spans up through 240 feet (73,000 mm).
[5] [3] As a rule of thumb, the depth should be 1 ⁄ 24 of the span. [5] The width of the ribs is typically 130 mm (5 in) to 150 mm (6 in), and ribs usually have steel rod reinforcements. [5] The distance between ribs is typically 915 mm (3 ft). [3] The height of the ribs and beams should be 1 ⁄ 25 of the span between columns. [3]
There are approved formulas for calculating the depth required and reducing the depth as needed; however, a rule of thumb for calculating the depth of a wooden floor joist for a residential property is to take half the span in feet, add two, and use the resulting number as the depth in inches; for example, the joist depth required for a 14-foot ...
Sizes vary according to the I-joist's intended load and span. Depths can range from 9 + 1 ⁄ 4 to 24 inches (230–610 mm) and reach up to 80 feet (24 m) in length, although 40 to 42 feet (12–13 m) is more common.
Because of these very strong and rigid joints, some of the bending moment in the rafters is transferred to the columns. This means that the size of the rafters can be reduced or the span can be increased for the same size rafters. This makes portal frames a very efficient construction technique to use for wide span buildings.
Greek: the ridge height is 1 ⁄ 9 to 1 ⁄ 7 the span (an angle of 12.5° to 16°); Roman: the ridge height is 2 ⁄ 9 to 1 ⁄ 3 the span (an angle of 24° to 34°); Common: the rafter length is 3 ⁄ 4 the span (about 48°); Gothic: the rafters equal the span (60°); and; Elizabethan: the rafters are longer than the span (more than 60°). [7]
I-joists, I-beams engineered from wood with fiberboard or laminated veneer lumber, or both, are also becoming increasingly popular in construction, especially residential, as they are both lighter and less prone to warping than solid wooden joists. However, there has been some concern as to their rapid loss of strength in a fire if unprotected.
Key: 1: queen posts, 2: tie beam, 3: straining beam, 4: principal rafters. A queen post truss has two principal rafters and two vertical queen posts. [5] The queen post truss extends the span, and combined with spliced joints in the longer members extends the useful span for trusses of these types.