huangyhg

multi-ply wood constructed columns
i have constructed twenty eight 5-ply built up wood columns made with 2" x 8" treated lumber. each column is embedded in concrete(two foot diameter footing, four foot deep). these columns extend 13.5' above ground and support a cabin approximately 7 tons.
my question is what would an appropriate bolt pattern be using 1/2" diameter galvanized bolts. the cabin is located in a river flood plain and at present height is considered above the 100 year flood mark.
it has been suggested two bolts parallel to each other every 8". this seems excessive.
also, is there any other approved method of bonding the 2"x8" lumber together other than bolting that can be used in this application(lag screws ect...). no glue has been used in the construction of the columns only various lengths of screws.
thanks for your response.

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i assume that your finished column is 7.5" square with all of the 2x8s running the same direction. i also assume that their primary function is to hold vertical load. i further assume that the bolts you are referring to would be there just to hold the 5 2x8s together so that they all buckle together and act more like a solid 8x8. therefore there is really no load on the bolts, so i would suggest 3 bolts at the quarter points (about 3'-4" spacing) with a large plate washer (say 3"x3"x3/16").
i'll also assume that you have put diagonal or x bracing in to handle horizontal loads like wind or water borne debris.
that would be 7.25"x7.5" if i am not mistaken.
the nds will provide you with minimum fastenings you need for built-up columns and guidelines on whether to use nails, lags, or bolts.
two bolts at 8" o.c. does seem excessive but i am not experienced with the requirements so don't take my word for it. at the same time, three bolts seems too light. imho of course
lots of questions:
1. what type of pressure treated wood was used? is it the new stuff? if yes, you almost have to use stainless steel for fasteners.
2. what is the maximum vertical load on any column?
3. what is the maximum horizontal load on any column due to: wind on the cabin, water force on the columns, static and dynamic loadings on the columns due to stream debris and/or trees?
4. what is the maximum moment on the columns due to the cabin?
5. what are the columns founded upon? rock?
6. is all the lateral bracing provided by the columns embedded in the concrete footings? any diagonals?
7. any kind of wood in direct contact with concrete and some water will "rot" or deteriorate with time.
8. what is the stream velocity during flooding?
9. how high does the water get on the exposed columns during flooding?
10. is there the potential of stream scour around the column footings during the flood event?
you need to get the professional recommendations from a knowledgeable structural engineer.

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