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Granite Artwork - need anchorage advice
- To: <Seaint(--nospam--at)seaint.org>
- Subject: Granite Artwork - need anchorage advice
- From: "Dennis Wish" <dennis.wish(--nospam--at)verizon.net>
- Date: Wed, 5 Feb 2003 19:54:31 -0800
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
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
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
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
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
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.