If a Tree Falls in the Middle of the City…

If a tree falls in a forest and no one is around to hear it, does it make a sound?”

This question has served as an icebreaker for generations of introductory philosophy courses — highlighting the sometimes murky nature of reality and the role observation plays in determining it.

But what happens if a tree falls in the middle of a city? Whether or not it makes a sound is the least of concerns to the unfortunate city dwellers below.

Trees within an urban community provide aesthetic, social, ecological and economic benefits. Recent research even points to measurable cognitive and psychological benefits by introducing more nature to the daily regimes of urban workers. Despite these benefits, urban trees remain large physical structures in close proximity to people and property, and their failure can cause damage to individuals and infrastructure. Recognizing this, researchers at the Forest Products Laboratory (FPL) devised several methods to evaluate a tree’s condition and detect decay and defects that could jeopardize public safety.

These nondestructive evaluation techniques (NDE) allow researchers to investigate and verify the condition of a tree without causing it any damage. This way, experts avoid damaging healthy trees while they weed out the bad ones. NDE techniques vary, but can range from visual inspection regimes to cutting-edge electronic scanning technologies.


Decay detection in red oak trees using a combination of visual inspection, acoustic testing, and resistance microdrilling.

In FPL’s home, Madison, Wisconsin, a tree stability survey was conducted on 153 trees surrounding the Capitol using NDE methods. Researchers from the laboratory first conducted visual inspections of all directly and indirectly observable defects, and then followed up with acoustic testing. Acoustic testing uses sound waves to determine the structural soundness of a material — in the case of trees, even beneath the bark.

Thanks to lab benchmarks, researchers know how sound should travel through healthy wood. If the sound waves move slower through a tree undergoing acoustic testing, there might be decay beneath the surface. In the case of the Capitol’s trees, if the sound waves had more than a 25% reduction in speed, they failed the test.

Finally, some trees received resistograph testing. The resistograph is a fine-tipped drill that detects decay by measuring the resistance where the drill bit meets the wood (or more specifically, the electrical power consumption in the motor driving the bit). Since low density, decayed wood requires less power and torque to drill through, if the resistograph drill is consuming less power, there’s a good chance that it’s chewing through a decayed part of the tree.

A 300-page report was submitted to park managers detailing the cases of internal decay and defects revealed using these combined nondestructive testing methods. Select tree removal and pruning were followed by arborists based on the report’s conclusions. Sadly, even a 100-year old red oak was given a poor bill of health and had to be removed.

Just weeks before the last two trees were removed, a violent storm with unusually high velocity winds passed through the park. Of the remaining 143 trees in the park, the only ones to fail were two that had been identified as high risk using the NDE techniques. Luckily, they caused no harm to people or buildings, but their loss did verify the effectiveness of the researcher’s work.

Whether or not they made a sound however, is still up for debate.

For more information, please see FPL’s publication Nondestructive Evaluation of Wood: Second Edition.


Chronicling Covered Construction : NDE Helps To Record Historic Bridges

Covered bridges are part of the fabric of American history, and several hundred historic covered bridges still exist to this day. Although much effort is expended to preserve these structures, the high cost of restoration, neglect, vandalism, and arson often take their toll. Many are lost forever.

One of the more famous bridges from The Bridges of Madison County movie was burned in 2003, and Hurricane Irene destroyed a number of New England bridges in 2011.

Because we cannot completely prevent these types of incidents from occurring, the National Park Service’s Historic American Engineering Record (HAER), in cooperation with the Forest Products Laboratory (FPL), has efforts under way to document these historic structures.

A single laser scan of the Roseman Bridge, Madison County, Iowa. (NRRI).

With the help of laser imaging technology, these two organizations are ensuring that the construction triumphs of yesterday are recorded and preserved for future generations.

Three-dimensional (3D) laser scanners are nondestructive evaluation (NDE) instruments that record precise and accurate surface data of objects in a nondestructive manner. These instruments use an infrared beam of light to calculate and record the distance to an object, typically as data points with spatial coordinates.

These data are then analyzed using various types of computer software to generate a detailed image of coordinates and dimensions.

Three-dimensional laser scanners have successfully been used to digitize objects of various sizes ranging from small diagnostic artifacts to large, complex sites of monumental architecture — and now, nestled between these two extremes, are some of our country’s historic bridges.

For more information, please see Chapter 13 of the FPL’s publication Nondestructive Evaluation of Wood: Second Edition.

‘Landmark Book’ Covers Nondestructive Testing of Wood

ross cover CROPPEDThe USDA Forest Service Forest Products Laboratory (FPL) has assembled the most comprehensive publication ever regarding the nondestructive testing and evaluation (NDE) of wood materials.

Geared toward industry professionals, Nondestructive Evaluation of Wood, offers guidance, analysis, and practical application of NDE techniques, including the use of lasers, x-rays, and ultrasound, to assess and report on the condition and integrity of wood.  These techniques, which do not damage the objects being evaluated, can be used on structures, bridges, standing trees, and even historic artifacts.

“Nondestructive testing of wood is an exciting area of research and has the potential to greatly enhance the wise use of wood,” said Bob Ross, the book’s editor, an author or co-author of several chapters, and a supervisory research general engineer at FPL. “Wood, in any form—trees through timber bridges—is highly variable because of how it grows, where it comes from, and what it is exposed to. Nondestructive evaluation technologies are the scientific foundation for all assessment and grading of wood-based materials,” Ross added

The book’s 13 chapters contain information from many of the industry’s foremost experts in the world, on topics such as static bending, transverse vibration, resistance drilling, piezoelectricity, acoustic assessments, and laser methodology. The book also provides information concerning more traditional evaluation techniques, such as machine grading, and advice for practical application in urban environments.

“To make the best, highest use of our forest resources,” Ross explained, “we need to have technologies that help us assess what the quality of a particular tree, log, or piece of lumber is. We can then utilize it appropriately. One of the fastest growing sectors of the wood products industry—engineered wood products—relies heavily on the use of nondestructive evaluation technologies.” NDE techniques described in the book have been employed around the world in many projects:

  • Use of sound waves to evaluate the quality of timber in National Forests
  • Use of ultrasound technology to locate decay in urban trees
  • Evaluation of structural performance potential of logs, veneer, lumber, and timbers before installation
  • Inspection of historic covered bridges
  • Inspection of historic artifacts,including the USS Constitution and a 2,500-year-old mummy coffin from Egypt

“This landmark book continues the proud legacy we have established at the Forest Products Laboratory as a cutting-edge scientific institution,” said FPL Director Michael T. Rains. “It represents years of research across the full spectrum of scientific endeavor, from technical journals and research reports, to the proceedings from various symposia. The book will serve as a guide to the public and a touchstone for future generations of scientists and land managers, as we continue to find better ways to utilize one of our planet’s most cherished and renewable resources—wood.”

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Wood vs. Water: Who Will Win - the Cabin or the Creek?

One thing we know for sure at the Forest Products Laboratory (FPL) is that wood and water don’t mix, and keeping wood dry is essential to long service life. So really, 80 years seems like a good, long life for a log cabin built with untreated logs, situated just 10 meters from a river in an area with high snowfall, and subjected to the subsequent spring melting that follows each winter. For this cabin, maintained by Michigan Technological University’s School of Forest Resources and Environmental Science, staying dry has been nearly impossible.

Otter River cabin, 2013.

Otter River cabin, 2013.

Situated on the north branch of the Otter River in the Portage Township of Upper Michigan, the cabin was built in 1934-1935 by the Civilian Conservation Corps and housed workers who used the site as a trout hatchery. In 1955, it was transferred to Michigan Tech with promise that public rights of hunting, fishing, and trapping would be maintained. Over the years, continuous maintenance was needed due to periodic flooding and the resulting log deterioration.

Since 1998, Michigan Tech and FPL have cooperated to conduct periodic inspections of the cabin. The latest inspection, completed in 2013, utilized various techniques to analyze the condition of the log cabin.

Some decay was obvious with visual inspection alone.

Some decay was obvious with visual inspection alone.

Visual inspection is the simplest method for locating deterioration. Here, researchers look specifically for evidence of water intrusion and damage, especially near the foundation, and evidence of structural failure of the timbers. Many of the logs showed signs of severe deterioration, with the ends of several having been entirely destroyed.

Sound transmission measures the time it takes for stress waves to travel between sensors placed on opposite sides of a timber. Significantly longer transmission times, relative to the base times known for several species, indicate the presence of deteriorated wood. Nearly half of the tested logs produced results in the ‘deteriorated’ or ‘severely deteriorated’ range.

Micro-drilling resistance test of wall timber.

Micro-drilling resistance test of wall timber.

Micro-drilling resistance is a commercially developed technique based on the underlying premise that degraded wood is relatively soft and will have low resistance to drill penetration. Researchers conducted micro-drilling resistance tests in areas of the timbers they believed contained decayed wood based on results from the visual assessments and stress-wave testing. Test results showed that many logs were deteriorated, with most having an outer shell of solid material but a severely degraded core.

In the end, these test results showed that water had finally won the war over wood. Researchers recommended the cabin be dismantled, salvaging any possible historic materials (hardware and any sound timber), and suggested a smaller structure be built on the site using the salvaged materials where appropriate.

For more information on the project, see this FPL Research Note.


18th International Nondestructive Testing and Evaluation of Wood Symposium September 24-27, 2013, Madison, Wis.

NDEcoverRegister now for the 18th International Nondestructive Testing and Evaluation of Wood Symposium. If you are involved or interested in nondestructive testing and evaluation of wood, wood-based materials and products, this event is for you.

The forum will bring together the international nondestructive testing and evaluation research community, including users of various nondestructive testing technologies, equipment development and manufacturing professionals, representatives from various government agencies, and other groups to share research findings and new nondestructive testing products and technologies.

The 18th Symposium also marks the 50th anniversary of the symposium series.  The first symposium was held at FPL in the fall of 1963. At that meeting, nearly 100 scientists, engineers, and industry leaders discussed the possibilities of a wide range of scientific means for testing wood nondestructively. Seventeen symposia have been held to-date at sites in China, Germany, Hungary, Switzerland and the United States.

Attendees will receive a complimentary electronic copy of a special publication in commemoration of the 50th anniversary of the first meeting. The publication includes a brief history of the symposium series, summaries of each symposium, and searchable electronic copies of the proceedings from each symposium.