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Prestigious structures began using thin natural stone panels as wall cladding to give them historic character in the late 1960s. Easier fabrication methods increased stone production and enabled larger, thinner panels that were lightweight. Unfortunately, engineering and testing did not keep pace. Inadequate initial evaluation of material durability and panel strength resulted in varying degrees of distress developing in some claddings. Because early stone cladding was installed with little structural analysis, current evaluation should predict capability by checking characteristics initial engineering likely ignored. Comprehending all characteristics that affect the integrity of thin stone cladding must be the objective of a responsible maintenance program. The program must evolve with each evaluation's findings to adapt to the specifics of that building and its changes over time. Maintaining the safety of a building's skin is essential to extending the service life of the whole structure. Verifying safety requires more than a cursory visual check for exposed elements to find them before they fall. In the past, stone cladding was completely removed from too many structures that experienced distress. Some evaluations misunderstood or misdiagnosed affect of distress on integrity. They failed to investigate some characteristics, or did not relate others, yielding an incomplete assessment. Adding evaluation of characteristics common to traditional structural assessment to a cladding program adds rigor and objectivity, avoiding misdiagnosis and unjustified expense. In rare cases, remove-and-replace-all may still be required, but only if gross problems exist with support. However, in most cases, an appropriate program will not only identify the parts creating the highest risk, but the program can prescribe selective replacement instead of complete recladding as a long-term remedy.
Lewis, Michael D.
PresidentChairman, Facade Forensics, Inc.ASTM Subcommittee C18.06 on Dimension Stone Attachments & Systems, Cincinnati, Ohio