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Measuring movement
To determine how much Low's dome is moving, Silman Associates
had Vibra-Tech, a Mount Holly, N.J.–based consulting
firm, install a remote vibration monitoring system. Eighteen
devices, called vibrating wire crack meters, were installed
at Low. These can accurately monitor changes in the width
of a crack of only one hundredth of an inch. The ongoing readings
from each are recorded by a computer called a data logger
every 15 minutes throughout the year. The data logger is connected
to Silman's office by modem for study. Temperature and moisture
meters were also installed around the building so that movement
can be corroborated against changes in the weather.
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It was especially important that crack
meters be installed over open joints in the faces of the arches,
at their apexes, which are presumed to have existed from the
time when construction of Low was just complete. The erection
of a scaffold would have taken months, cost hundreds of thousands
of dollars, and prevented the use of one of the university's
most important gathering spaces. It was impossible to bring
a cherry-picker into the rotunda—there simply wasn't
any way to maneuver it into the space. So the team called
upon Kent Diebolt, whose New York City–based company,
Vertical Access, specializes in inspection and nondestructive
testing in "extreme locations," to install these
sensors. Diebolt also used his high-wire act to support himself
in midair while "sounding" the plaster dome—that
is, striking it with a hammer to determine that the plaster
was still securely attached—and to conduct a video survey
of areas that would have been impossible for other team members
to see.
Helpern Architects and Robert Silman
Associates have concluded that Low's dome is quite safe. Cracks
are present where computer analysis indicates that they should
be expected, and according to data recorded from the remote
monitoring system, they are stable. Has the testing therefore
been a colossal waste of money? Hardly, considering that the
building is irreplaceable, and safety is a concern—the
rotunda is often filled with hundreds of people. Prior to
the initiation of this project, no one could really say if
the dome was safe. It is a bonus that it has probably been
studied more thoroughly than any unreinforced masonry dome
besides that of the Duomo in Florence. What has been learned
will contribute much to structural engineers' understanding
of masonry dome behavior. Irwin Lefkowitz adds, "Low
has served Columbia University well for more than 100 years.
We want to ensure that it will serve us for 100 more."
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