|Engineered Wood Products
Creep and Creep-Rupture Effects
by Robert J. Tichy, Ph.D.
The development of standards to evaluate end-use performance of products is an obvious and critical step in assuring safe and efficient use by consumers. For almost 100 years, ASTM Committee D07 on Wood has developed standards for wood-based building construction products where model code recognition is required. Because most of these products are designed for use in structural applications often involving life-safety concerns, much attention is given to assure that the standards address building code requirements. This requires an in-depth understanding of code requirements.
Fortunately, a symbiotic relationship between D07 and our nations model building codes exists. Many members of D07 participate in writing these codes, thereby providing essential input into the standards development process. This paper discusses the intent and application of the new ASTM standard D 6815, Specification for Evaluation of Duration of Load and Creep Effects of Wood and Wood-Based Products.
During the United States first two centuries, population density was relatively low and forest resources plentiful conversion efficiency was not an overriding concern. However, the past few decades have witnessed an increasing emphasis on improved resource utilization worldwide. The forest products industry has responded by generating numerous engineered wood products that dramatically improve the utilization of our forest resources. These efficiencies are the result of innovative composite designs, advanced resin systems, improved strand and particle generation equipment, and nondestructive evaluation techniques.
As a consequence of these technological developments, innovative companies regularly work toward building code listings for their products. In an attempt to keep pace with product innovations, D07 committees have grown to address a wide variety of engineered wood products such as (but not limited to) metal plate-connected trusses, laminated veneer lumber, oriented strand board, I-joists and structural composite lumber products. In addition to enhancing home construction, these products have allowed dramatic growth in wood-based multi-story residential and commercial construction, dramatic arenas, domes, and bridges. This increased sophistication in both products and applications requires evaluation procedures of equal sophistication.
In support of these emerging engineered wood products, standards such as ASTM D 6815 have been promulgated. This particular specification was developed in response to a request made by both domestic and international model code organizations. Their request was simple and direct How do these newer engineered structural wood composites compare to the performance of those products for which we have adequate service history? In other words, Do these engineered wood products perform at least as well as solid wood? There was no answer to this question, so D07 created a task group to address the concern. The result was D 6815.
Creep and Creep-Rupture
Engineered wood products carry allowable design values used by engineers and architects to design buildings. When deriving allowable design values for any structural material (steel, concrete, wood, or composites), an extensive set of tests are conducted to verify product performance for the intended end use. One of the more important end-use criteria deals with the performance of these products under long-term loads. More specifically, this relates to the creep-rupture (or duration of load) and creep effects.
The question is, does the product lose strength and/or become more compliant as time progresses when subjected to design loads? A simple illustration of the importance of this concept might be the long-term performance of a beam over a door or window. If this beam were to creep over time, it may begin to deflect (sag) to an extent that interferes with the proper operation of the door or window clearly an unacceptable level of performance. Therefore, it is essential that design value assignments and subsequent adjustment factors consider creep and creep-rupture effects. With a few exceptions, duration of load and creep effects impact most building construction materials. This is particularly true for some of the newer polymer-rich composites emerging in the marketplace.
Specification D 6815
As previously stated, building code officials are generally comfortable with the duration of load (creep-rupture) and creep characteristics of traditional wood-based products. These products have exhibited a history of satisfactory performance. However, a new product with no historical database raises questions regarding its duration of load and creep performance. Specification D 6815 was developed to address this concern for engineered wood products. Its objective was to develop a relatively rapid testing procedure (a 90-day duration of load experiment) and method of evaluation to establish whether or not the product performed like those products currently listed by the model code organizations.
The standard simply addresses the performance of a new product within the context of materials for which satisfactory performance has been demonstrated. If the new product does perform similarly to other satisfactory products, the new product is allowed use of the duration of load and creep factors currently specified in the National Design Specification for Wood Construction. (1) However, if the product does not perform in like fashion, the manufacturer must adjust product properties or develop proprietary duration of load and creep performance adjustments. The latter approach is a costly and time-consuming activity.
Examples of Use
Manufacturers and inventors use this standard when developing new engineered wood products. The standard can be used as a screening tool to assess the creep and creep-rupture performance early in the product development process. Several wood product manufacturers, who understand the ramifications of duration of load testing, have chosen to assess their product performance early long before a code submission package is prepared.
The American Institute for Timber Construction, for example, is currently reviewing this standard to see if it would be applicable as a creep test method for new adhesives in glued-laminated beams (glulam). Currently, the acceptance of new adhesive types requires a seemingly excessive three-year creep test, making it very difficult to get approval of new adhesive types. The American Institute for Timber Construction is seeking a more reasonable test, so glulam manufacturers can take advantage of new adhesives much faster.
The American Forest and Paper Association, working on behalf of the wood products industry, has negotiated acceptance of D 6815 with Japans Ministry of Infrastructure, Land, and Transportation, thereby minimizing trade barriers to the Japanese import of U.S. manufactured engineered wood products. Not until the standard was well into the ASTM process would Japan allow certain engineered wood products to be sold in Japan.
Code accredited testing laboratories such as APA the Engineered Wood Association (Figure 1) and the Wood Materials and Engineering Laboratory at Washington State University (Figure 2) conduct the testing and analysis specified in D 6815 for companies requesting code listings for their products. For example, Washington State University regularly conducts proprietary testing programs for companies that have developed products for structural use and which need a code listing with model codes such as the International Building Code IBC 2000 (2) or the National Fire Protection Associations NFPA 5000. (3)
Summary and Conclusions
Improved forest resource utilization has stimulated the development of a growing number of sophisticated engineered wood products. To effectively integrate these products into construction applications, all pertinent code requirements must be addressed. D 6815 uses a 90-day duration of load experiment to assess the creep and creep-rupture performance of wood-based composites relative to traditional wood products with an excellent performance history.
This standard is used as an early screening tool for product development activities, a roadmap for code listing, and as technical support for removing international trade barriers. For nearly a century, Committee D07 has actively developed ASTM standards for wood-based building materials; the next one hundred years will be no exception. //
(1) American Forest and Paper Association, 2002. National Design Specification for Wood Construction. Washington D.C.
(2) International Code Council. 2000. International Building Code 2000. Falls Church, Va.
(3) National Fire Protection Association. 2003. NFPA 5000: Building Construction and Safety Code, 2003 edition. Quincy, Mass.
Copyright 2003, ASTM