Metal plate connected wood trusses are sometimes used in applications or environments that require the trusses to be designed and constructed with chemically treated lumber. The two most common types of chemically treated wood used in trusses are preservative treated wood (PTW) and fire retardant treated wood (FRTW). Chemicals and treatment pro-cesses can require reductions in the strength and stiffness design properties of the wood or the lateral resistance strength (i.e., grip) of the connector plates. They can also require additional corrosion protection for the con-nector plates. The building designer is respon-sible for specifying any type of wood treatment to be used. Section 126.96.36.199(g)(6)(iii) of ANSI/ TPI 1-2014 requires construc-tion documents include infor-mation concerning moisture, corrosive chemicals and gases expected to result in corrosion potential from wood treatments or other sources that can be detrimental to trusses. Still, truss design technicians need a basic understanding of the potential effects the vari-ous treatments can have on the lumber and connector plate properties so appropriate adjustments can be made dur-ing the design process to ensure the trusses perform as intended by the building designer. Corrosive Environments THERE ARE SEVERAL SERVICE CONDITIONS that are potentially corrosive to truss connector plates. Corrosion is the term for the degradation of steel due to its interaction with the surrounding environment. Rusting, for example, is a com-mon type of corrosion that occurs when steel reacts with oxygen in the presence of water. The rate of rusting increases with increasing moisture content of the air or with the presence of salt or corrosive chemicals such as ammonia-and chlorine-containing compounds. The continuous formation of rust reduces the strength and cross-section of steel—a problem that can eventually lead to the failure of a metal plate connected wood truss. The metal connector plates used for wood trusses are typically fabricated from hot-dip galvanized steel meeting the requirements of ASTM A653/A653M, with a coating that meets or exceeds the designation G60. G60 indicates a minimum total weight of 0.6 ounces of zinc coating per square foot, taking into account both sides of the steel plate. The zinc coating protects the steel base metal by providing a sac-rificial layer that corrodes at a rate over 50 times slower than the uncoated steel. For trusses used in protected, well ventilated, dry-service conditions, the G60 coat-ing provides adequate protection against corrosion. However, some applications require connector plates with greater corrosion resistance. Factors that call for cor-rosion precautions can include: • Wet service conditions, where the equilib-rium moisture content of the wood members exceeds 19 percent. • The presence of chemically-treated wood, such as trusses constructed with preservative or fire retardant treated lumber. • Exposure to corrosive chemicals, as might be expected in the construction of indoor swimming pools or storage buildings for salt or bulk fertilizer. • Close proximity to salt water coastal areas, such as coastal high hazard and ocean haz-ard areas described in detail in FEMA’s Technical Bulletin 8, Corrosion Protection of Metal Connectors in Coastal Areas . Designing for PREVIOUS ARTICLES: A Designer’s Guide to PTW | A Designer’s Guide to FRTW 8 sbcmag.info • JUNE/JULY 2017 Stainless steel plates typically provide the highest level of corrosion protec-tion but they are expensive.