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Re: Granite Artwork - need anchorage advice

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First, welcome back to the list!  I knew you could not resist its "call"
for ever! <grin>

While I am looking at your project with the same lack of experience is
such matters, I do have some comments/suggestions.

If you proceed in the direction that you outlined (i.e. trying to have the
cap stone move relative to the three tall supporting stones), then I would
also think that you may want to try to modify the interface between the
supporting stones and the cap stone to make it more condusive to
potentially movement.  By providing some "slop" in the hole in the cap
stone for the attachment rod, you are providing the necessary slop for the
potential movement to occur, but you still are faced with a potential
significant force that can develop between the cap stone and the
supporting stone due to friction.  I am basing this on what I understand
your plan to be...the cap stone will still sit with a stone on stone
surface on the supporting stones...the elastomeric filler was to strictly
be around the slop in the hole where the connecting rod is located.  If
so, then you still have a stone on stone contact surface, which will still
generate significant force due to the static friction.  So, I would think
that looking at putting some sort of material between the two stones that
could reduce the friction might be helpful if preparing the system for
potential movement.

Regardless, you would still need to do some sort of projected movement of
the top stone relative to the supporting stones assuming that it can move
to determine if your expected movement will be greater than the amount of
"slop" that you have.  If so, then your "base isolation" system will hit
the limit of its permitted movement and STILL potentially impose some
serious lateral loads on the supporting stones.

If on the other hand, you don't hit the limits of the potential movement
and the top can move relative to the supporting stones, then it would
seem to me that you potentially have a good system, with the only other
area of concern being the moment imposed on the base of the supporting
stones due to their own inertia during a seismic event.  One way to
potentially solve this potential problem is to have some sort of stiff,
but still compressible fill layer below the supporting stones that would
still provide enough of a rigid base system to keep them standing, but
have enough give in the compressible layer so that not too much force is
developed at the edges of the supporting stone that would cause those
edges to crush or splinter.  I'll be honest in that I don't know if this
is even possible to do, but it is meant as "just a thought".

I think that you are likely on the best track in your approach, but you
could also still look at what might happen if you made a rigid structure
(i.e. no relative movement) in terms of stresses in the stone.  It is
possible that the stresses might still be small enough that the stone
could handle them.  You may have already done this.  Regardless, I am
assuming that the stresses would be too much...but you never know.

Another option that I can think of would be to provide for base isolation
at the base of the whole thing (i.e. at the base of the three supporting
stones).  In this case, you can "rigidly" attach the top stone, but
provide for your movement at the base of the supporting stones.  I think
this potentially presents more of a challege from an aesethic point of
view, but might be easier to achieve from an engineering point of view.
The tough part would be in providing the necessary bottom connection that
allows for the base isolation movement but then "hiding" that connection
from view as I doubt the artist would like it in plain view...kind of
would destroy that stone henge (i.e. old rocks sitting there) appearance.

If you want, go ahead and send me the JPGs privately and maybe it will
help stir some other potentially creative ideas.


Ypsilanti, MI

On Wed, 5 Feb 2003, Dennis Wish wrote:

> I have an unusual problem that I could use some advice. I have already
> laid out a design plan but could use some peer advice. If anyone is
> interested in offering an opinion, I can provide JPG photographs that
> will help explain further.
> The City where I live has paid for an art piece that consists of three
> granite monoliths roughly 24" x 30" and standing on a flat cut bottom
> 7'-0" tall. The monoliths, remind me of Stonehenge except that they are
> place roughly in the plan of an equal lateral triangle. Each stone
> weighs approximately 6,500 pounds and they taper to a smooth rounded
> top.
> A flat "cap" stone is hollowed out on the bottom to allow for it to fit
> on top of the vertical stones. The top cap stone weighs approximately
> 13,000 pounds.
> As the engineer, it is my responsibility to tie everything together and
> set the art piece on an adequate foundation. Here is my plan:
> The foundation will be one large concrete pad - larger than the
> equilateral plan of the vertical stones. I anticipate calculating the
> overturning of the stones and resisting the overturning in the following
> manner:
> 1) Each vertical stone has been cut so the base is flat. A 1-inch
> diameter SAE1018 threaded rod or #8 rebar (60ksi) will be epoxied
> (Wilbond Epoxy) at least 18-inches into the base of the rock. A 1/2"
> thick base plate will be welded to the rods and the base plate will be
> secured to the concrete foundation with four 3/4-inch diameter anchor
> bolts. The base of the stones and the base plates will be covered with
> an additional 3-inches of concrete which will be approximately 6-inches
> below grade.
> 2) The artist will drill through the cap stone and into each vertical
> granite stone and epoxy a 1-inch diameter threaded rod 18-inches into
> each vertical stone.
> 3) I wanted to oversize the holes in the cap stone and fill the area
> around the threaded rod with an elastomeric compound that will allow the
> stone to move, but dampen its movement similar to a base isolator. The
> rod will be recessed 3-inches into the stone so as to conceal it and an
> oversized plate washer will be installed.
> The rationale is to create as much fixity at the base of the column
> stones and allow the cap stone to move - albeit dampened by the
> elastomeric or rubber materials around the threaded rod. I believe
> (mostly intuition) that if I create a grouted or epoxy connection of the
> rod in the cap stone as well as the vertical stone columns, I would be
> creating a wedge action caused by relatively stiff materials that might
> blow out either the top stone or the narrow tapered top of each of the
> three column stones.
> The art piece is to be installed in a small park like setting outside of
> a new restaurant. The "Stonehenge" capped stone will be the focal piece
> while there will be smaller boulders for people to sit on that lie
> outside the capped stones. There will be a smaller round granite stone
> placed in the center of the triangle column plan to make it appear as
> though the round "ball" fell from the cap stone. It is possible for one
> or two people to move around below the cap stone or for one to sit on
> the round ball.
> As a piece of art, there is so much I can do to resist seismic motion as
> this was the concern of the City Council before they gave their
> approval. I can not prevent fractures of the stone or prevent the
> columns from fracturing above or around the anchors. In other words,
> there is only so much protection I can assure the public.
> Do any of you have a suggestion or can you confirm as to whether or not
> I am on the right track here. This is unusual for me and I am relying
> heavily on intuition of how the "system" will perform and even this
> assumes that it performs in the strong axis as there is not enough stone
> in the weak access to protect the cap stone from blowing everything
> apart.
> I'll be happy to forward the e-mail with photo's I sent to Williams Rock
> Anchors as I sought their opinion on the project as well.
> Dennis

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