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 22.214.171.124(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. A Designer’s Guide to FRTW BUILDING CODES ALLOW fire retardant treated wood (FRTW) to be used in certain applications where noncombustible materials are otherwise required. Truss technicians encountering this situation should know to check with the treatment supplier, the truss design engineer and the connector plate supplier to determine the appropriate adjustments to make to the wood and plate strength values as well as the type of corrosion protection to use on the connector plates. Fire retardant treatment does not make wood “noncombustible,” as defined by the building codes, but it does reduce the wood’s ability to contribute to flame spread and the subsequent growth of a fire. A key distinction here is that the focus is on reduced flame spread. Non-combustion is not the same as fire endurance, a perfor-mance capacity determined by an ASTM E119 fire endurance assembly test. FRTW does not improve the performance or the fire endurance rating of lumber. Similarly, the ASTM E84 (UL 723) test method that is used to classify the fire performance and smoke development of interior wall and ceiling finishes should not be confused with FRTW. 2015 IBC 2303.2 Fire-retardant-treated wood. Fire-retardant-treated wood is any wood product which, when impregnated with chemicals by a pressure process or other means during manufacture, shall have, when tested in accordance with ASTM E84 or UL 723, a listed flame spread index of 25 or less and show no evidence of significant progressive combustion when the test is continued for an additional 20-minute period. Additionally, the flame front shall not progress more than 10½ feet (3200 mm) beyond the centerline of the burners at any time during the test. ANSI/TPI 1-2014 Section 126.96.36.199 Fire Retardant Treated (FRT) Lumber. Designing for Corrosive Environments 28 sbcmag.info • APRIL 2017 NEXT IN THE SERIES: All FRT lumber used in Trusses shall be re-dried after treatment to 19 percent maximum moisture content at temperatures not to exceed 160°F (71°C). FRT lumber design values shall be developed from approved test methods and procedures that consider potential strength-reduction characteristics, including effects of elevated temperature and moisture. Design values shall be approved by the authorities hav-ing Jurisdiction.