A.Frank Woeste, P.E., professor of wood construction and engineering at Virginia Tech, responds: The built-up post you describe may be as strong as the 6x6, depending upon the grade and species of the lumber, and how the 2x6s are nailed together. Assuming the post and 2x6s are No. 2 Southern Pine, the nailing used to laminate the three 2x6s is the main issue in determining if three pieces of 2x6 will replace the 6x6 post.
When the deck column is loaded, the nails or screws slip or give slightly, allowing the column to start bowing. If the nailing is inadequate, the bowing will continue and the capacity of the deck column is reduced. Even when such a column is assembled in accordance with rigorous National Design Specification for Wood Construction (NDS)...
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