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Re: Granite Artwork - need anchorage advice

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Dennis,
 
Stone, being a natural material, has properties that we are not used to dealing with in an engineered construction.  Important in your application is that it is not reliable in tension.  This is because stone was formed by growth of grains toward boundaries defined by adjacent grains.  Each grain boundary is a potential micro-crack across which no structural tension can be developed.  The locations and orientations of micro-cracks are random and unknown, and can result in unexpected weakness in tension.  Anchor tests in granite stone masonry on one of my projects resulted in pulling half a granite stone out of the wall, broken in tension at the end of the anchor. 
 
I recommend that the capstone be tested under its support condition, for capacity to support its weight plus an overload to represent a conservative seismic vertical load plus seismically induced tension due to restraint by the base-anchored vertical stones.
 
In lieu of anchoring the vertical stones to the foundation, and interconnecting them at the top, consider analyzing them by Housner's old paper, "The Behavior of Inverted Pendulum Structures During Earthquakes", published 1956 by EERI.  I'll fax a copy to you if you'd like.  It analyzes stability of a structure rocking on its base due to seismic shaking.  Judging from the proportions you describe, my guess is that you will determine the assembly stable -- if not, perhaps the artist would be open to some suggestions on proportion from the structural engineer.  [Don't laugh, he might be.]  Anchored bases may result in flexural cracking of the vertical stones in an earthquake.  In addition, restraint of the vertical stones could induce additional tension n the capstone in an earthquake.
 
If the cap stone is amply recessed to receive the shapes of the vertical stones, and if the vertical stones have flat bases, there seems a good potential for stability without additional connections using modern materials.
 
Dennis, it was good to hear from you on the list.
 
Nels
 
Nels Roselund
Structural Engineer
South San Gabriel, CA
njineer(--nospam--at)att.net