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2305.2 Design of Wood Diaphragms
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Wood diaphragms are permitted to be used to resist horizontal forces provided the deflection in the plane of the diaphragm, as determined by calculations, tests or analogies drawn therefrom, does not exceed the permissible deflection of attached distributing or resisting elements. Connections shall extend into the diaphragm a sufficient distance to develop the force transferred into the diaphragm.
Permissible deflection shall be that deflection up to which the diaphragm and any attached distributing or resisting element will maintain its structural integrity under design load conditions, such that the resisting element will continue to support design loads without danger to occupants of the structure. Calculations for diaphragm deflection shall account for the usual bending and shear components as well as any other factors, such as nail deformation, which will contribute to deflection. The deflection (D) of a blocked wood structural panel diaphragm uniformly nailed throughout is permitted to be calculated by using the following formula. If not uniformly nailed, the constant 0.188 (For SI: 1/1627) in the third term must be modified accordingly.
|A||=||Area of chord cross section, in square inches (mm2).|
|b||=||Diaphragm width, in feet (mm).|
|E||=||Elastic modulus of chords, in pounds per square inch (N/mm2).|
|en||=||Nail deformation, in inches (mm).|
||Modulus of rigidity of wood structural panel, in pounds per square inch (N/mm2).|
|L||=||Diaphragm length, in feet (mm).|
|t||=||Effective thickness of wood structural panel for shear, in inches (mm).|
|ν||=||Maximum shear due to design loads in the direction under consideration, in pounds per linear foot (plf) (N/mm).|
|Δ||=||The calculated deflection, in inches (mm).|
|Σ(ΔcX)||=||Sum of individual chord-splice slip values on both sides of the diaphragm, each multiplied by its distance to the nearest support.|
Size and shape of diaphragms shall be limited as set forth in Table 2305.2.3.
|MAXIMUM LENGTH -
|Wood structural panel, nailed all edges||4:1|
|Wood structural panel, blocking omitted at intermediate joints||3:1|
|Diagonal sheathing, single||3:1|
|Diagonal sheathing, double||4:1|
Shear panels shall be constructed of wood structural panels, manufactured with exterior glue, not less than 4 feet by 8 feet (1219 mm by 2438 mm), except at boundaries and changes in framing. Boundary elements shall be connected at corners. Wood structural panel thickness for horizontal diaphragms shall not be less than set forth in Tables 2304.7(3) and 2304.7(5) for corresponding joist spacing and loads, except that 1/4 inch (6.4 mm) is permitted to be used where perpendicular loads permit. Sheet-type sheathing shall be arranged so that the width of a sheet in a shear wall shall not be less than 2 feet (610 mm).
Design of structures with rigid diaphragms shall conform to the structure configuration requirements of Section 220.127.116.11 of ASCE 7 and the horizontal shear distribution requirements of Section 18.104.22.168 of ASCE 7.
Open front structures with rigid wood diaphragms resulting in torsional force distribution are permitted provided the length, l, of the diaphragm normal to the open side does not exceed 25 feet (7620 mm), the diaphragm sheathing conforms to Section 2305.2.4, and the l/w ratio [as shown in Figure 2305.2.5(1)] is less than 1.0 for one-story structures or 0.67 for structures over one story in height.
Exception: Where calculations show that diaphragm deflections can be tolerated, the length, l, normal to the open end is permitted to be increased to a l/w ratio not greater than 1.5 where sheathed in compliance with Section 2305.2.4 or to 1.0 where sheathed in compliance with Section 2306.3.4 or 2306.3.5.
Rigid wood diaphragms are permitted to cantilever past the outermost supporting shear wall (or other vertical resisting element) a length, l, of not more than 25 feet (7620 mm) or two-thirds of the diaphragm width, w, whichever is the smaller. Figure 2305.2.5(2) illustrates the dimensions of l and w for a cantilevered diaphragm.
Structures with rigid wood diaphragms having a torsional irregularity in accordance with Table 1622.214.171.124, Item 1, shall meet the following requirements: The l/w ratio shall not exceed 1.0 for one-story structures or 0.67 for structures over one story in height, where l is the dimension parallel to the load direction for which the irregularity exists.
Exception: Where calculations demonstrate that the diaphragm deflections can be tolerated, the width is permitted to be increased and the l/w ratio is permitted to be increased to 1.5 where sheathed in compliance with Section 2306.3.4 or 2306.3.5.
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