In-Situ Evaluation of Compressive Stresses

Abstract

The use of terra cotta as a cladding material emerged in the 1880s and began to decline in the late 1930s. During this period, high-rise buildings employing terra cotta cladding included no provisions for differential movement between the cladding and structural framing. Numerous terra cotta clad buildings are showing signs of distress due to weathering and induced stresses from frame shortening under load. Determination of residual compressive stresses within the terra cotta cladding is often necessary for evaluation and repair of the distressed material. The level of stresses induced in the cladding of a high-rise building in San Francisco was measured by performing strain relief tests. Uniaxial strain gauges were adhered vertically to the face of the terra cotta blocks and initial balance readings were obtained. The mortar bed joints were sawn along the terra cotta units and the strains monitored during and after the cutting. Samples of the terra cotta block were cut, instrumented, and tested in compression to determine elastic properties and ultimate compressive strength. Having obtained the physical properties of the terra cotta, compressive stresses due to frame shortening in the structure were determined. Strain data obtained indicated stress levels which were low enough to eliminate the need for stress relief. Optimum saw cutting locations were determined had stress levels been high enough to require stress relief.