Masonry Design | 29
Stronger is NOT Beter!
My wife and I both love historic architecture so
Italy naturally seemed like the perfect destination
for our honeymoon. After spending two weeks in
Cinque Terre, a string of historic seaside villages
along the Italian Riviera coastline, we ended our
stay in Milan. The city of Milan has always been
considered an industrial powerhouse and, as a
result, was heavily bombed during WWII. Of the
few remaining historic structures left standing,
the most impressive is undoubtedly the Duomo di
Milano.
Construction of Milan's Duomo officially began
in 1386 and it remains the fifth largest Christian
church in the world. Visiting Milan’s Duomo was
a truly jaw-dropping experience: it boasts 135
spires, 96 gargoyles and it is said to contain more
integral statues than any other building in the
world with an amazing 3,159 in total. As one
considers the magnificence of this structure it
is also worth considering a far less obvious but
equally important feature, its mortar.
It is easy to overlook mortar when appreciating a
building such as Milan's Duomo. Mortar
represents only a small and singular component
of a structure and our attention is naturally drawn
to a building's broader architectural features.
However, it is remarkable to consider that for
over 600 years mortar has not only acted as the
glue, which holds Milan’s Duomo together but it
has been responsible for protecting its intricately
carved masonry units from the effects of settling
and thermal expansion. What I find even more
remarkable is that historic lime mortars such as
those employed in Milan's Duomo are relatively
weak and rarely have compressive strengths in
excess of 900 pounds per square inch.
It has been my experience that contractors,
building owners, and specifiers place too much
emphasis on a mortar’s compressive strength
when selecting bedding or pointing mortar. While
it is true that mortars must be strong enough
to carry a wall’s compressive load, a mortar's
most critical mechanical role does not reside in
its compressive strength but rather its ability to
accommodate small structural movements by
means of deformation under stress.
UNI-AXIAL STRESS
Imagine you have a marshmallow on the table
in front of you and you are pressing firmly on it
with your palm. Are you visualizing the sides of
the marshmallow bulging out as it squishes into a
pancake? The uni-axial (single direction)
compressive stress exerted onto the
marshmallow has created tensile strain within its
mass as it stretches outward and squishes flat.
WORDS: MATT WOLF
PHOTOS: MATT WOLF, AND MIKE TATE OF GRAYMONT LIME