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Pergola Lateral Support

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I have a project that I've been scratching my head on and would like some suggestions.    I have an existing pergola structure in the garden area of a local hospital that needs to be rebuilt.    This thing has 12" diameter carved wood columns that are 7 ft long and sit on concrete piers/footings.   The 4 columns were tied together (doweled) at the top with 6x12 sawn beams with 3x6 purlins over that (no exposed hangers).    I think the base was set on nails cast into the concrete.   All the wood has decorative cuts, etc.     The thing was taken down over a year ago due to decay and they now want to put it back with all new wood and concrete except they want to reuse the 12" diameter wood posts.     This thing is 18 ft x 10 ft in plan and in a rhombus shape.    The concrete piers are 42" above grade where the wood posts will sit and span 7 ft up to the beams.   The new beams will be 5 1/8" x 12" pressure treated glu-lam beams (about 11 ft above grade).
 
The question is lateral support?    It never really had any and they really want it to look the same as before.     Currently no diaphragm and no fixed columns.   I'm thinking of requiring them to drill out the columns for steel posts that get cast into the new concrete.      Anybody else have lateral support ideas for an open pergola.   They don't want knee braces.    Seismic controls over wind using a limited wind exposure and I see this thing weighing only about 1500 pounds total.
 
 
Peder
Portland, Oregon