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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).
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.
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 [3] 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 ...
The method for calculating the size of a flitch beam to be used in construction is straightforward, using the transformed-section method. The steel plate is treated as an equally stiff piece of wood, with its width modified by the ratio of their moduli of elasticity. This allows the deflection of the entire beam to be calculated as if it were ...
[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]
This is accomplished by using metal ties fastened to each rafter or truss. This is not normally a problem in areas not prone to high wind or extreme weather conditions. In the UK, a concrete tiled roof would normally have rafters at 600 mm (24 in) centers, roof battens at 300 mm (12 in) centers and ceiling joists at 400 mm (16 in) centers.
For longer span rafters, building materials manufacturers have created laminated veneer lumber (LVL) rafters that can be 2–5 times longer than typical wood rafter. In the US, most wood rafters have maximum length of 20 feet (6.1 m). [ 5 ]
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.