SYMPOSIA PAPER
Published:
27 August 2014
STP157720130137
Stress Relief in Terra Cotta Facades: Predicting the Unpredictable
SourceThe advent of the skeletal frame structural system and the use of terra cotta to clad skyscraper facades in the late 19th century introduced a new set of challenges, including how to accommodate the issues caused by the differential movement and volumetric changes between the steel or reinforced-concrete structural frame and the expansive clay cladding materials. The opposing forces such as frame shrinkage and the expansion of clay materials were further exacerbated by the growing heights of buildings. Although the distress caused by the differential movement in masonry-clad skyscrapers has been a long-known issue, preservation professionals have addressed this issue using different methods. Several key lessons have been learned from the various approaches over the last 40 years. One common technique for accommodation of the differential movement is known as strain relief. In the mid-1970s, this method was used at the Woolworth Building in New York City, which is an early and widely published example of the strain-relief technique described below. Stress relief in masonry-clad skyscrapers usually begins with a series of strain tests that are used to measure compressive stresses, which exist in the cladding portion of the facade. These tests involve adhering carbon-filament strain gauges to the cladding surface and then releasing the in situ stress by saw cutting around the gauged facade area. The strain value is measured before and after saw cutting, and the in situ residual facade stress is computed by multiplying the modulus of elasticity of the facade material by the measured strain change. This technique is particularly useful to determine the potential for cracking or compression failure of masonry facades. Compression failure is caused by the combination of axial load and bending associated with inelastic bucking of a brittle material such as clay masonry. Compression failure may result in outward lateral deformations, and subsurface failures, such as face shearing caused by inadequate support of the cladding. Once the stresses are computed from the strain testing, the design professional may suggest cutting horizontal relief joints into the facade at regular intervals to alleviate some of the locked-in stresses. This process has been used with varying degrees of success on terra cotta–clad facades with both steel and reinforced-concrete frames. The varied results are likely a reflection of the variations in the terra cotta material properties, restraints, unanticipated stress concentrations, and general behavior structure, which are unique to each building in general and specific to facade areas. This paper will generally explore the process of strain relief, what has been learned from the early strain-relief theories used at the Woolworth Building in 1976, and how these “lessons learned” have been implemented by the authors on various terra cotta building facades throughout the country to help preserve the urban fabric of our cities. This paper, by means of specific case studies highlighting terra cotta–clad skyscrapers, will also illustrate how one aspect of the preservation practice in facade repair and restoration, such as strain relief, while seemingly formulaic and scientific, often produces significantly varied results.
Author Information
Gerns, Edward, A.
Wiss, Janney, Elstner Associates, Inc., Chicago, IL, US
Will, Rachel, L.
Wiss, Janney, Elstner Associates, Inc., Chicago, IL, US
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Details
Developed by Committee: C07
Pages: 1–19
DOI: 10.1520/STP157720130137
ISBN-EB: 978-0-8031-7600-3
ISBN-13: 978-0-8031-7599-0