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Coastal Construction in Hurricane Prone Regions

August 20th 2013 in Wood Products - #tech talk

The effects of Super Storm Sandy—the second costliest hurricane in United States history resulting in $68 billion in damages—are still being felt today, nearly one year after Sandy made landfall along the Jersey Shore. New building height requirements for residential structures along the New Jersey coastline will serve as a permanent reminder of the devastation caused by Sandy.

Both the International Residential Code (IRC) and the Federal Emergency Management Agency (FEMA) have guidelines and requirements for building in coastal flood zones (known as raised floor coastal construction). These requirements primarily address the height of the lowest structural members relative to a Design Flood Elevation and the use of Flood Resistant Materials below the Base Flood Elevation (BFE).

Determining the type of construction and building height requirements relative to the BFE is important in determining what materials are appropriate, and how they should be designed. More information and your project specific Zone classification can be found on FEMA’s web site. See also: Flood Hazard Data for New Jersey and New York.

There are typically several factors to be considered by the designer in sizing and selecting appropriate treatments for wood products including:

• Appropriate treatment for the application.
• Use Category as defined by the American Wood Preservers Association (AWPA).
• Expected in service moisture conditions where wood products will be used.

Treatment Types

Treatments come in two types of systems; Waterborne and Oil-borne.

Waterborne treatments use water as the carrier for the preservative chemicals and can result in higher moisture contents after treatment. Kiln drying after treatment (KDAT) works to reduce the excessive moisture. Without KDAT, shrinkage of the treated material may occur after construction. Design properties may require adjustments based on the in-service moisture content. Waterborne treated products, if KDAT, can be stained or painted immediately after installation.

Oil-borne treatments use mineral spirits or heavy oil as a carrier for the preservative chemicals. An advantage to the oil-borne treatment process is the treatment does not cause dimensional change, so kiln drying is not needed. However, greater time is required for the product to dry prior to painting or staining and the wood may still change in dimension due to weathering in the field. A disadvantage to oil-borne treted products is they should not be use in interior or enclosed spaces.

Use Category

In coastal residential construction the location of building materials relative to the structure directly impacts how wet the materials will get. Use category UC3A is commonly applied to joists while UC3B is applied to floor beams. Both are above ground. However, the side and/or ends of a beam are more likely to be exposed. Framing members that actually contact the ground are commonly viewed as UC4A or UC4B. Refer to Table 3: AWPA Service Conditions for Use Category Definitions for further explanation.

More information on wood treatments types and use categories can be found at the American Wood Protection Association site at http://www.awpa.com/.

Service Conditions

A Wet Service Factor (or CM) must be applied to design properties for wood products exposed to moisture. The National Design Specification (NDS) defines wet use as conditions in which the moisture content exceed 19% for dimension lumber and 16% for Structural Composite Lumber or Glulam members. The NDS commentary more clearly defines wet use conditions as applications in which the structural members are used in exterior exposures that are:

• Regularly exposed to rain
• Not protected from the weather by a roof, overhang, or eave
• Subject to water exposure for a sustained period of time.

Using this definition it is possible for the designer to influence or design an environment that can result in a dry use application. Should it not be possible to ensure a dry use condition, appropriate wet use strength reductions should be applied.

Product Options

Products located above the Base Flood Elevation, with proper detailing to ensure a dry use environment can be constructed with non-treated materials including I-joists. Note that detailing to ensure dry use is critical. Parallam® PSL, TJI® joists and other untreated Weyerhaeuser products are acceptable in these applications.

Products that are built above the BFE but exposed to the elements such as wind driven rain should be treated and designed for wet use service conditions. Joists and beams located below the Base Flood Elevation must be made of flood resistant materials, meaning wood members need to be treated.

Parallam Plus PSL is a preservative treated version of our Parallam PSL product. Standard beam and header sizes are suitable for use category UC4A (above ground) conditions, column sizes are suitable for use category UC4B (in ground) conditions.

For more information on Parallam Plus PSL see our July blog, What’s so Special About Parallam Plus PSL or visit our Parallam Plus PSL product page.

Additional Considerations

Some other factors to consider when selecting a product include the need to treat field cuts and holes, the visual appeal of the material, the sizing of the product, the overall cost, ease of installation, and product availability.

The International Residential Building Code (IRC) requires the field treatment according to AWPA M4 standards for dimension lumber, and glulam beams. Parallam Plus PSL members do not require field treatment of holes and notches. Is the product to be painted or stained? Kiln dried after treatment products including Parallam Plus PSL can be painted or stained immediately after installation, whereas other products my require time after installation to acclimate prior to painting or staining. Can columns be eliminated in favor of deep depth beams to enhance the visual appeal of the project? Deeper beams like Parallam Plus PSL allow for longer spans which can create more open sight lines.

As the Jersey Shore and surrounding areas continue to recover and rebuild following Super Storm Sandy, building designers in these areas must be mindful of and balance design considerations including:

• Elevation of materials relative to the Base Flood Elevation
• Use category
• Protection of materials from the elements
• Strength and span characteristics of framing material
• Material finish (painted or stained)
• Cost and availability of materials

Careful consideration of these factors, coupled with an understanding of building materials like Parallam Plus PSL that can be used in these environments, provide an excellent foundation towards preparing for the next Super Storm.