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Re: Openings in "uncoursed" stone walls

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Keith,

I've never designed a cut for an opening into a stone wall, but here are
some thoughts on how I might approach it.

I'm not familiar with the stone that you are dealing with.  If it is
reasonable to drill into it, I would begin before cutting the opening by
installing "shoring pins", which are steel rods set in acrylic adhesive
[Epcon A7 is like epoxy adhesive, but is thixotropic and will not flow from
an overhead hole] in holes drilled diagonally upward and inward through the
inner and the outer wythes from just above the soffit of the new opening.
The function of the pins would be to anchor the stones over the proposed
opening to stones that will be supported by "corbel action" from the
proposed jambs.  Two or three in each direction from each side [a total of
8] may be sufficient.  Stones may then be removed from the wall to produce
the opening.  Reset in fresh mortar any stones to remain that have been
loosened by the removal process.

I would look for jambs and lintels of other openings of the building, see
how they were done, and copy them with matching new stones built into the
new opening, building the new lintel on centering that must remain in place
until the mortar has cured.  Fill the space between the top of the lintel
and the remaining existing stone above last.  Additional shoring pins
installed through the new lintel into the masonry above.

If the stone is granite or similar, I would use portland cement mortar.  If
it is more friable than granite, say more like sandstone, I'd make the
mortar using sand and hydraulic lime as available from Transmineral USA
www.transmineralusa.com .  The Transmineral folks are very helpful with
guidance in matching available grades of hydraulic lime to the stone you are
dealing with.

Give installation instructions for the shoring pins to be sure that they are
fully embedded in adhesive -- any holidays will subject steel pins to
corrosion that will become a future headache for someone.  For more
insurance, use stainless steel [316] or fiberglass pins.

Nels

Nels Roselund
Structural Engineer
South San Gabriel, CA
njineer(--nospam--at)att.net



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