Trusses In Corrosive Environments
Question:
I am a building designer on a salt storage building project. I would like to use metal plate connected wood trusses in the design. What should I specify for the metal connector plates?
Answer:
Metal plate connected wood trusses will perform adequately in most construction environments provided that after permanent installation they are protected with the traditional materials and roofing practices. However, special protection measures may be required when metal plate connected wood trusses are used in adverse conditions. These conditions include exposure to high humidity (for example, when trusses are installed over swimming pools) and to highly caustic chemical atmospheres (for example, when trusses are used in fertilizer storage sheds, brooder chicken houses, near salt-water environments or in salt storage buildings). Metal connector plates are galvanized for standard use conditions where moisture content of the lumber is 19 percent or less and there is no exposure to corrosive substances. If the salt storage facility you are designing has free-flowing air throughout the building, it should be fine with no additional steps. If this is not the case, additional steps should be taken in order to resist plate corrosion.
For several years, truss plate manufacturers supplied stainless steel truss plates when special protection was required. Stainless steel connector plates were supplied as a substitute when specifications for projects near the ocean called for metal plates that were “double dip” galvanized. Contact your component manufacturers or their plate supplier for more guidance on this option for your project. Another option, and potentially a better long-term solution when special protection is necessary, is a protective coating applied to the connector plates after the trusses are installed. The coating, such as Epoxy Polyamide, Coal Tar Epoxy or Asphaltic Mastic, seals the plate to the wood, fills in between the slots, coats the edges of the exposed steel, and provides a tight membrane protection for the plate. Its use has been tested on steel structures subjected to direct salt contact at oceanfront locations on rolled steel members, or exposed metal connector plate joints.
You can find more information about plate coatings in section 6.5 of the Truss Plate Institute's ANSI/TPI 1.