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Wood Decks - Sample Calculations for Using the Span Table
Case I Solution: Refer to table for joist and beam sizes.
Example: A = 12', Post Spacing = 8'
Refer to the span table. Joist size may be either 2x8's 12" O.C. or 2x10's 16" O.C. Beam size may be either 3 - 2x8' or 2 – 2x10's.
Case II Solution: Use “a” for joist size and “a” + “b” to determine beam size (the length of “b” is restricted by both the length of “a” and the size of the joists).
Example: A = 8', B = 2', Post Spacing = 10'
Find the joist size required by looking under 8' on the table. Joist length is indicated as 2x6's 16" O.C. or 2x8's 24" O.C. For sizing the beam, use a joist length of 10' (8' + 2' = 10') and a post spacing of 10'. The table indicates that 4 - 2x8's or 3 – 2x10's are required for the beam.
Case III Solution: Use “a” or “b”, whichever is greater, to determine joist size. Use “a” + “b” to determine the size of Beam No. 1 and use joist length “b” to determine the size of the Beam No. 2.
Example: A = 6', B = 7', Post Spacing = 9'.
The joist length (7') is determined by the longest span joist “b”. The table indicates that 2x6's 16" O.C. or 2x8's 24" O.C. are required for a 7' span. For Beam No. 1, use joist length of 13' (6' + 7' = 13') and post spacing of 9'. The table indicates that 3 – 2x10's or 2 – 2x12's are required for Beam No. 1. For Beam No. 2 use joist length of 7' with a post spacing of 9'. The table indicates that 4 – 2x6's or 3 – 2x8's are required for Beam No. 2.
Case IV Solution: Use “A” or “B”, whichever is greater, to determine the size of Beam No. 1 and “B” + “C” to determine the size of Beam No. 2. (The length of “C” is restricted by both the length of “B” and the size of the joists).
Example: A = 7', B = 8', C = 2', Post Spacing = 12'
The longest joist span is 8'; therefore, the table indicates that 2x6's 16" O.C. or 2x8's 24" O.C. are required. For Beam No. 1, use joist length of 15' (7' + 8' = 15') and post spacing of 12'. The table indicates that 3 – 2x12's are required for Beam No. 1. For Beam No. 2, use joist length of 10' (8' + 2' = 10') and post spacing of 12'. The table indicates that 3 – 2x10's or 3 – 2x12's are required for Beam No. 2.
NOTES: Post size must be adequate to provide full beam bearing, ie., one-member and two-member beams must be placed on a 4x4 or 6x6 posts, and four-member beams must be placed on 8x8 posts.
Most of the boxes in this table contain two optional means of support. Wood members may be increased above those indicated in the table, but in no event may lesser members be used.
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