Tag Archives: machine stress rated lumber

How Grading Agencies Establish Lumber Design Values

How Grading Agencies Establish Lumber Design Values

Originally published in SBCA Magazine January 16, 2024

Nearly all roof trusses and floor trusses are created using cutting-edge software. Embedded in that software are engineering principles outlined in the National Design Specification, Design Values for Wood Construction (NDS), as well as published design values for the sizes and grades of softwood lumber used in North America. Consequently, it’s important to know where those published design values come from and how they are established.

The NDS Supplement provides a succinct answer in the introduction:

“Reference design values for most species and grades of visually graded dimension lumber are based on provisions of ASTM Standard D 1990-16 (Establishing Allowable Properties for Visually Graded Dimension Lumber from In-Grade Tests of Full-Size Specimens)…Reference design values for machine stress rated (MSR) lumber and machine evaluated lumber (MEL) are based on nondestructive testing of individual pieces…The stress rating systems used for MSR and MEL lumber is regularly checked by the responsible grading agency for conformance to established certification and quality control procedures.”

In other words, softwood lumber species groups are regularly subjected to established testing protocols to ensure they retain their published design values over time. In North America, there are seven grading agencies that have jurisdiction over establishing softwood lumber design values and are responsible for conducting ongoing testing to verify them. Those grading agencies are:

· National Lumber Grades Authority (NLGA)
· Northeastern Lumber Manufacturers Association (NELMA)
· Northern Lumber Manufacturers Association (NLMA)
· Redwood Inspection Services (RIS)
· Southern Pine Inspection Bureau (SPIB)
· West Coast Lumber Inspection Bureau (WCLIB)
· Western Wood Products Association (WWPA)

Each of these grading agencies follow grading rules approved by the American Lumber Standard Committee’s (ALSC) Board of Review (BOR) and are certified for conformance with the U.S. Department of Commerce Voluntary Product Standard (VPS) process. For softwood lumber used in the U.S., the VPS is maintained by the ALSC and is referred to as American Softwood Lumber Standard PS 20 (PS 20). The current edition is PS 20-20.

Once each grading agency completes its regular testing protocols, it’s required to report to the ALSC BOR the resulting data, along with any recommendations it has regarding to changes to current published design values. Fortunately, changes are rare. The most significant in recent years was with SPIB and their recommended changes to the design values of Southern Pine (SP) in 2012.

Again, these published design values are integrated into component design software. The parameters of each truss assumes that the lumber species, grade, and size used in the manufacturing process matches what was input into the design. This assumption emphasizes the importance of the lumber grade stamp placed on each stick of dimensional lumber that help users identify key structural properties. The next article in this series will look at how lumber manufacturers grade each stick of lumber and how to read its corresponding grade stamp.

Smartphone App to Test Lumber Strength

I have been a proponent of machine rated lumber since I bought my first truckload to be made into trusses at Coeur d’Alene Truss (http://www.cdabuilders.com/) back in 1978. Years later I spent five terms on the Board of Directors of the Machine Stress Rated Lumber Producers Council (http://www.msrlumber.org/).

I have opined previously on the merits on machine graded lumber: https://www.hansenpolebuildings.com/2012/12/machine-graded-lumber/.

Mississippi State University™ has taken lumber testing technologies and placed them directly into the hands of the public. Please read on in this MSU article:

The “Smart Thumper” app, available for download in the Apple Store, uses sound waves or vibrations to determine stiffness, a quality that relates to strength, for individual pieces of lumber. (Photo by David Ammon)

STARKVILLE, Miss.—Determining the stiffest piece of lumber is now easier with a new smartphone app created by scientists in Mississippi State University’s Forest and Wildlife Research Center. 

Called “Smart Thumper,” the app uses sound waves or vibrations to determine stiffness, a quality that relates to strength, for individual pieces of lumber.  

Developer Dan Seale, professor in MSU’s Department of Sustainable Bioproducts, said it will help carpenters, contractors, architects, engineers, lumber mill personnel and consumers. He pointed out that it can be particularly beneficial for the do-it-yourself market.

The Mississippi State team that has developed an app to determine stiffness of individual pieces of lumber include, from left, Frederico Franca, assistant research professor of sustainable bioproducts; Songyi “May” Han, an MSU sustainable bioproducts doctoral student; and professor Dan Seale. The team’s work is part of the university’s Forest and Wildlife Research Center. (Photo by David Ammon)

“All lumber is not the same, even though it may be graded the same. The grade is based on a range of values and characteristics,” Seale said. “Perhaps a consumer has a pack of lumber which meets the specification for No. 2 grade, but they need a couple of pieces for a header, something that might span the opening for a window or door. This app helps select the stiffest pieces that are least likely to sag over time,” Seale explained.

Frederico Franca, the app’s co-developer and an assistant research professor in sustainable bioproducts, first envisioned the app when he discovered that the equipment designed to test lumber costs around $84,000.

“The goal was to make something cheaper and more readily available to give consumers and stakeholders broader access to nondestructive testing equipment,” Franca said. “Now anyone with a smartphone can download the app to help pick out the stiffest pieces for whatever they are building.”

His love of physics, along with the desire to create something less expensive, fueled his idea for a smartphone app that would render lumber values through the use of sound and vibration. 

“With this app, I can show you which lumber pieces are stiffer and therefore stronger,” Franca explained. “This can’t always be done through visual inspection. You need vibration or you need sound.”

Lumber mills use both visual and mechanical means to grade all types of dimensional lumber. Pieces can be tested for strength and stiffness, and the numbers are crunched through an algorithm to determine grade.

“This app can help further evaluate lumber within established grades, potentially optimizing the longevity and cost efficiency of wood structures by selecting stiffer pieces for situations that demand higher performance,” Franca said.

Also a part of the code development team is Songyi “May” Han, an MSU sustainable bioproducts doctoral student whose 2017 master’s thesis relates to marketing the smartphone app.

The app is available for download in the Apple Store. Visit https://itunes.apple.com/us/app/smart-thumber/id1436858557?mt=8.

Learn more by visiting http://smartthumper.fwrc.msstate.edu/.