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a frame shoring

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I have an existing structure whose roof is made of glu-lam a-frames spaced 15' o.c. The bottom of the glb members are supported on concrete footings designed to handle vertical and horizontal reactions.
 
The bottom of glb members are suffering from rot. I am designing the temporary shoring of the a-frames and a repair for the rotten ends.
 
My solution is to install a tie rod to take horizontal thrust when one glb end is cut for repair (only cutting and repairing one side at a time, not both at once). I am also installing vertical shoring to handle vertical load when glb end is cut.
 
Having evaluated my tie rod tension force and designing a connection to handle it, I am now thinking about the actual construction procedure. I want to be certain the tie rod is pretensioned such that there is minimal horizontal movement of cut glb when load is taken up by the tie rod.
 
I am stuck on how to measure the pre-tension and how much pre-tension to apply. I am thinking along the lines of calculating elongation of the rod for the amount of force it must carry and somehow tightening the turnbuckle an amount that equates to the rod elongation.
 
Any body interested in commenting on this?
 
TIA
 
Mark D. Baker