vectorsound
Structural
- Feb 23, 2025
- 1
In the NDS SDWPS the maximum unit shear diaphragm values for "Horizonal Lumber Sheathing" include nailing requirements for "Nailing at intermediate and Bearing Supports". However nowhere in the table or previous sections does it specify what the maximum or any spacing for that matter is required for these intermediate supports, i.e. what is the maximum rafter/joist spacing? 2x6 T&G decking can span up to 8'-0" for Roof Live Load, but does this mean the unit shear diaphragm value specified in SDWPS Table 4.2D of 100 plf was tested or applies to a rafter spacing of 8'-0" on center?
Thinking of a simple example, a simple square structure constructed with horizontal lumber diaphragm that has rafters at 2'-0" OC and therefore (2) 16d nails at each rafter (per Table 4.2D nailing req at intermediate supports), will have 4 times as many intermediate nails as the same structure constructed with rafters at 8'-0" OC. I understand diaphragm strength comes from the edges for plywood panels but for horizontal lumber it would seem the diaphragm with 2'-0" OC rafters with more nails would be stronger than the diaphragm with rafters at 8'-0" OC.
The NDS Diaphragm Construction Requirements sections alludes to a maximum rafter spacing for Blocked and Unblocked diaphragms by stating "maximum nail spacing shall be 6" on center when support spacing of 48" on center is specified and 12" on center for closer support spacing". I say "alludes" because the the NDS doesn't explicitly state support spacing cannot exceed 48" OC, altough one can assume rafters cannot be spaced greater than 48" OC. Even so this applies to blocked diaphragms with panel sheathing not lumber diaphragms. There is no mention of support spacing for lumber diaphragms. Appreciate any and all thoughts. Thank you, Alex.
Thinking of a simple example, a simple square structure constructed with horizontal lumber diaphragm that has rafters at 2'-0" OC and therefore (2) 16d nails at each rafter (per Table 4.2D nailing req at intermediate supports), will have 4 times as many intermediate nails as the same structure constructed with rafters at 8'-0" OC. I understand diaphragm strength comes from the edges for plywood panels but for horizontal lumber it would seem the diaphragm with 2'-0" OC rafters with more nails would be stronger than the diaphragm with rafters at 8'-0" OC.
The NDS Diaphragm Construction Requirements sections alludes to a maximum rafter spacing for Blocked and Unblocked diaphragms by stating "maximum nail spacing shall be 6" on center when support spacing of 48" on center is specified and 12" on center for closer support spacing". I say "alludes" because the the NDS doesn't explicitly state support spacing cannot exceed 48" OC, altough one can assume rafters cannot be spaced greater than 48" OC. Even so this applies to blocked diaphragms with panel sheathing not lumber diaphragms. There is no mention of support spacing for lumber diaphragms. Appreciate any and all thoughts. Thank you, Alex.
