Engineered Properties and Structures
FS-FPL-4714

Problem 3:
Serviceability of Solid and Composite Products

I. Background

The goal of this Problem is to extend the nation's forest resource by developing better methods for characterizing the end-use performance of structural lumber products as a function of environmental and industrial treatment processes.

II. Body

A. Durability & Serviceability

When kept dry, wood has historically performed well, and has become the premier structural material for residential and light-commercial use. However, moisture content and temperature, singularly and in combination, have a major impact on wood properties and upon the service life of wood structures. Extending the service life of wood structures reduces the need for new structures and helps conserve the forest resource.   In addition, chemical treatment processes designed to make wood more durable or fire resistant may have a major impact on structural performance and service life. The research work unit is recognized world-wide as a leading authority on the effect of environmental and chemical processes on the structural properties of solid sawn lumber.

One objective of the proposed research will be to extend our knowledge to structural composite lumber products made from either hardwood or softwood species and to compare their performance to that of solid sawn lumber. For example, previous research has established procedures for adjusting lumber properties for changes in moisture content.

Ongoing research is showing that allowable properties of solid sawn lumber may be greatly reduced at very low moisture contents. Yet little is known about the effect of moisture content on the properties of composite lumber products.

Likewise, little is known about the effect of preservative treatment on the properties of hardwood lumber or composites.  Knowledge gained on the effect of preservative treatment on softwood species can serve as a basis for evaluating potential effects on these newer products.

B. Predict Residual Service Life

A second objective will be to develop models to predict the service life of wood structural products. Previous research has focused more on characterizing changes in properties after environmental exposure.

A new approach will be to use this data to develop models to predict the properties at some future time and thus form the basis for evaluating when the member is no longer serviceable. For example, elevated roof temperatures caused by solar radiation in combination with fire-retardant (FR) chemicals and moisture can prematurely activate some FR-treated materials. This thermal degradation problem causes failure and requires costly roof replacement.

Over the past few years, extensive research has defined the mechanism of the problem, but now predictive methods are needed to evaluate the condition of FR-treated plywood and to estimate residual service-life. Such model development will allow code officials/regulators, contractors, and engineers to determine replacement time schedules for FR-treated roofing materials undergoing thermal degradation.

A separate but related example is the effect of exposure of untreated lumber to higher environmental temperatures over long periods of time. Such exposures may occur in the desert southwest or when wood structures are used in industrial situations. Several examples of roof truss failure having been reported when wood is exposed for decades at temperatures below the existing code-specified limit of 150 degrees F.

Ongoing research is helping to develop models to predict lumber strength at such conditions, and the research will be expanded to include structural composite products.

C. Research Goals

1. Develop more accurate models for predicting the effect of moisture content and ambient temperature on the performance of solid sawn and composite lumber products for both hardwood and softwood species.

i. Evaluate the effect of very low moisture contents on the tensile strength of structural lumber and wood-based products.

ii. Evaluate the effect of exposure to high ambient temperatures over long durations on the mechanical properties of structural lumber and wood-based products.

iii. Evaluate effect of cyclic humidities on the properties of structural lumber and wood-based products.

2. Develop improved methods for predicting the initial and long-term "in-service" effects of�chemicals, treatments, treatment processing, and decay on the performance of structural lumber and wood-based products for both hardwood and softwood species.

i. Define the effects of incising on the bending and shear strength of structural lumber and wood-based products.

ii. Evaluate the effects of waterborne preservatives on the mechanical properties of hardwoods, glulam, and structural-composite lumber.

3. Develop models for predicting the service life of structural lumber and wood-based products when exposed to cyclic humidity, temperatures, chemical treatments, or decay.

i. Develop serviceability model for fire-retardant treated plywood roof sheathing and fire-retardant treated structural lumber.

ii. Develop serviceability model to predict the effects of decay on the properties of structural lumber and wood-based products.

iii. Develop serviceability model to predict the effects of long-term "in-service" exposure to elevated temperatures on the properties of structural lumber, structural composite lumber, and wood-based panel products.

iv. Develop model to predict properties of structural lumber and wood-based products based on changing chemical composition of a number of wood species/product types.

III. Conclusion

These attainments help extend the Nation's forest resource by helping extend the useful life of wood members. Basic data generated in this Problem will characterize the effect of moisture content, temperature, and treating processes on the properties of solid sawn and composite lumber. Analytical models developed using this data will be used in engineering design standards to assure current structural safety. Analytical models of serviceability will estimate future performance of engineered structural members to prevent premature replacement of wooden structures.