Manual offers guidance to engineers challenged
by historic load-bearing structures
By Charles Guidry
Long before the advent of
standardized building codes,
architects were tackling complicated
structural elements like arches,
domes and buttresses. While today's
architects and engineers marvel at the
endurance of these historic features,
awe sometimes turns to frustration
when faced with restoring them-especially
those that are load-bearing.
The struggle with load-bearing
buildings that have high ceilings and no
central support is amplified by the lack
of technical guidance on the subject. As
a result, these structures are in danger of
being restored improperly or replaced.
Using a PTT Grant, Thomas
Boothby and his research team from
Pennsylvania State University studied
structural modeling to assess load-bearing
masonry structures and developed a
manual to demystify these structures for
architects and engineers.
"A structural engineer is usually
confronted with a standing, apparently
competent, structure that seems to defy
most of the rules of structural behavior,
as incorporated in modern building
codes," Boothby said, "The engineer
must then choose between reinforcing
the structure according to a modern
understanding of material strength and
structural behavior or trying to make
sense of the behavior and anticipated
strength of the structure on a more
fundamental level."
The study used a testing procedure
called experimental modal analysis
(EMA). Using response transducers
and signal processing techniques, EMA
can describe a structure in terms of its
dynamic characteristics, such as damping
ratios, natural frequencies and mode
shapes.
The project team conducted detailed
assessment studies on four major
early twentieth-century buildings: the
National Cathedral in Washington,
D.C.; the Cathedral of St. John the
Divine in New York; the State Education
Building in Albany, N.Y.; and the
City-County Building in Pittsburgh.
In each of these studies experiments
were conducted on a key portion of the
building and used to verify the accuracy
of a diagnostic model.
The manual resulting from this
research describes the application of
different methods to the structural assessment
of two different types of structures:
two-dimensional arch structures
and three-dimensional vault structures.
It also gives guidance for the application
of frame analysis programs to linear
and non-linear assessment of masonry
arches and provides detailed instructions
for the development of solid models,
meshing, entering material properties,
boundary conditions, and loads for
models of complex three-dimensional
structures, such as domes and vaults.
"We sincerely hope we have provided
information that will assist consulting
engineers in making appropriate
decisions regarding these structures,"
Boothby said. "Since the engineering
profession is driven by safety, there is
a tendency to reinforce what we don't
understand. When the inherent strength
of load-bearing masonry can be understood,
it is easier to undertake appropriate
treatments for historic properties
made with this material."