Designing for Corrosive Environments

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 processes 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 connector plates.

The building designer is responsible for specifying any type of wood treatment to be used. Section 2.3.2.4(g)(6)(iii) of ANSI/TPI 1-2014 requires construction documents include information 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 various treatments can have on the lumber and connector plate properties so appropriate adjustments can be made during the design process to ensure the trusses perform as intended by the building designer.

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 common 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 sacrificial 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 coating provides adequate protection against corrosion. However, some applications require connector plates with greater corrosion resistance. Factors that call for corrosion precautions can include:

• Wet service conditions, where the equilibrium 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 hazard areas described in detail in FEMA’s Technical Bulletin 8, Corrosion Protection of Metal Connectors in Coastal Areas.

If metal plate connected wood trusses are to be used in any of these service conditions, additional corrosion protection should be considered for the connector plates. This typically means using connector plates with a thicker zinc coating (G185 coating or plates that are hot dip galvanized per ASTM A153 after plate manufacturing),
connector plates made of stainless steel, or connector plates protected by an approved coating applied in the field over the embedded plate.

Field-applied coatings might seem like an easy option, but they do require care. For instance, two epoxy-based coatings are specified in ANSI/TPI 1-2014 as corrosion protection options. When either is used, the surface of the connector plates must be cleaned in accordance with the SSPC-SP 1, Solvent Cleaning standard, and the paint must be applied in accordance with the SSPC-PA 1, Shop, Field and Maintenance Painting of Steel standard. Using a brush to apply the coating is critical to ensuring the paint is worked into the slot openings and the exposed underlying wood, but it can be difficult to coat those portions of the plates that extend beyond the wood. When properly applied, both epoxy coatings are an acceptable alternative to double dip galvanized or stainless steel plates.

Stainless steel plates typically provide the highest level of corrosion protection but are very expensive and not available from all truss plate manufacturers. In addition, ANSI/TPI 1-2014 warns that some types of stainless steel can’t be used near swimming pools as they are susceptible to a phenomenon known as stress corrosion cracking (SCC). SCC results from exposure to chlorine-containing compounds that are the by-products of disinfectants used to clean pool water. Acceptable options are to use types of stainless steel that are not susceptible to SCC, such as 3171 MN and 904L, or to isolate the trusses from the pool environment with a vapor barrier and a separate ventilation system.

The responsibility for specifying which option to use for truss plate corrosion resistance in any specific application always belongs to the building designer. The decision to use one of the coatings specified in ANSI/TPI 1-2014 or another alternative solution must be made by the building designer with respect to the particular conditions of the building. ANSI/TPI 1-2014 requires the construction documents include information concerning moisture, corrosive chemicals and gases expected to result in corrosion potential from wood treatments or other sources that can be detrimental to trusses.

While it is not the responsibility of a truss design technician to specify corrosion protection for truss plates, truss designers can benefit from a basic understanding of the service conditions that can require corrosion protection. When truss designers keep an eye out for corrosive environments, adjustments can be made as needed during the design process to ensure the trusses perform as the building designer intends.

Checklist: Designing for Corrosive Environments

• Contact the truss design engineer and metal connector plate supplier to discuss the truss application before bidding the job. The engineer and supplier can determine the appropriate design criteria and connector plate type to use for the trusses.
• If the project specifications require the use of a specific corrosion protection measure, consult with the customer and the building designer for review and approval of any plate treatment or substitution.
• Apply the appropriate adjustments to the design properties of the lumber and connector plates to account for the type of treatment and design conditions. Typically, this means selecting the appropriate plate in your design software, which already includes the strength properties. The truss design engineer can determine the best procedure to follow.
• If applicable, obtain a copy of the warranty from the paint manufacturer to verify there are no additional limitations on the product for the application in which it will be used.
• Provide the material safety data sheets (MSDS) for the treated lumber with the truss shipment.