Assessing Wood from Hurricane-Downed Trees in Puerto Rico

After Hurricane Maria made landfall in September 2017, the storm left hundreds of thousands of downed trees in its wake. Many of the trees were species with commercially valuable wood, but which ones?

A huge pile of logs from hurricane-downed trees in Puerto Rico.
Wood from hurricane-downed trees in Puerto Rico. Species identification was needed to decide how to best use or dispose of the material.

To find out, an assessment of the post-hurricane wood, stored at 21 different locations around the island, was requested by Puerto Rico’s Department of Natural and Environmental Resources, or DNER. The Federal Emergency Management Agency supported this request through the Natural and Cultural Resources Recovery Support Function.  The Department of the Interior contacted the USDA Forest Service, and scientists Mike Wiemann of the Forest Products Laboratory and William Gould from the International Institute of Tropical Forestry developed an assessment of the species mix and log quality of the downed trees.

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What’s a Debris Launcher and How is it Used?

Last week we talked about safe rooms as a place to shelter from tornadoes and hurricanes. Standards for safe rooms are set forth by the Federal Emergency Management Agency (FEMA), and research on their construction is ongoing.

As part of their efforts to improve safe room design, researchers at FPL are conducting performance tests on a series of materials and wall designs that might be used in the construction of safe rooms. Using an air-pressurized debris launcher, FPL researchers have performed tests that follow FEMA standards.

These tests were performed using a specially designed debris launcher that meets International Code Council ICC-500 criteria of launching 2- by 4-inch lumber missiles at speeds of approximately 100 miles per hour. The launcher was donated to FPL by the nonprofit trade association APA – The Engineered Wood Association.

In the debris launcher tests, a 15-pound Southern Pine 2- by 4-inch missile is fired at 100 mph at a safe room wall to simulate tornado debris. A 9-pound Southern Pine 2- by 4-inch missile is fired at 34 mph to simulate hurricane debris. In the research done by James Bridwell and others, the wall was built using four-ply vertically laminated logs (nominal 6- by 8-inch) with tongue-in-groove joints. Various buffering and support systems have been tested to pass FEMA standards.

Debris Launcher Design Details

The main components of FPL’s debris launcher are pressure tanks, control valve, barrel, muzzle, and instrument panel.

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Debris launcher’s electric solenoid control valve system. The control valve is a modified sprinkler valve.

The pressure tanks are made of 52-inch long, 6-inch diameter PVC pipe, and they house the air needed to propel the missile to the target panel. To ensure safety during testing, the control valve allows the operator to remotely regulate air pressure in the tanks through an instrument panel.

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Instrument panel for the FPL debris launcher. Besides computing missile velocity, the instrument panel also serves as the control center for FPL’s debris launcher.

The barrel of the debris launcher is made from a 150-inch long piece of 4-inch diameter PVC pipe that when housed in the mobile firing platform, is supported every 24 inches by an aluminum block attached to the frame. Attached to the front end of the barrel is the muzzle, which houses two electro-optical sensors. As the missile passes through the muzzle, the light across the first and then the second sensor is interrupted, sending electrical signals back to a conditioner in the instrument panel. The signal conditioner then computes and outputs a velocity measurement to the operator based upon the calibrated distance between the sensors and the time between signals.

Besides computing missile velocity, the instrument panel also serves as the control center for FPL’s debris launcher. It enables the operator to route air to the pressure tanks, monitor tank air pressure, and activate the control valve to fire each missile all while maintaining a safe distance from any possible safety concern.

Missile Details

The typical missile used in the impact tests was a nominal 2-inch by 4-inch Southern Pine board that weighed between 15 and 15.5 pounds and was between 144 and 146 inches long. No specifications were made on the desired grade of the missile; however, FEMA-361, Design and Construction Guidance for Community Safe Rooms, stipulates that missiles should contain no knots within 12 inches of the leading edge. Before being loaded into the barrel, each missile had a sabot (a device used in a cannon to fire a projectile that must be held in a precise position) of approximately the same circumference as the inside of the cannon barrel attached to its end to minimize the amount of air pressure lost. 

Panel Design

FPL researchers have designed test panels to mimic the retrofitting of existing building structures; this provides increased impact resistance to the debris propelled by tornadoes and hurricanes. The tests done thus far at FPL have used a wide array of materials including oriented strand board (OSB), plywood, hardwood paneling, fiber-laminated hardboard, cross laminated timber (CLT), and bamboo composite panels. OSB, plywood, hardwood, and CLT are common materials to the U.S. building industry and were chosen for their widespread use and availability.

Along with a variety of materials tested, FPL researchers are trying different configurations of panels. At this time, new research is happening with panel design and materials that we will visit in future Lab Notes posts.

Shelter from the Storm? FPL Debris Launcher

Throughout the United States, hundreds of tornadoes and several hurricanes affect people’s livelihoods each year. Nearly every state east of the Rocky Mountains faces the possibility of devastating tornadoes, with the Midwest being particularly at risk. Living along the gulf and east coasts offers little respite, as powerful hurricanes are apt to cause similar, and potentially more widespread, damage. The following maps show this sobering possibility. Check out those wind speeds!

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Map from FEMA P-320 showing the number of tornadoes in the United States from 1950–2006. (Click to enlarge in Flickr.)

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Design wind zones from FEMA P-320 based on ASCE 7-05 criteria. The zones mirror the locations of high intensity storms, with the addition of higher design requirements along the coasts due to hurricanes. (Click to enlarge in Flickr.)

These natural disasters not only cause structural damage to property, they also cause numerous injuries, and regret­tably, far too many deaths of people caught in their path. When the envelope of a structure is punctured by de­bris, pressure changes from the wind can cause the structure to rapidly fail. In many areas, buildings were not built with resistance to debris in mind. However, with the extensive media coverage of recent disasters such as the devastating tornadoes in Joplin, Missouri, and Moore, Oklahoma, as well as Hurricanes Katrina and Sandy, greater numbers of home and small business owners are seeking to increase their probability of surviving a storm by voluntarily installing a “safe room” to shelter in.

A safe room is defined by the Federal Emergency Manage­ment Agency (FEMA) as “a space where you, your family, or friends and employees can survive a tornado or hurricane with little to no injury.” Guidelines for safe room design are detailed in FEMA P-361, Design and Construction Guid­ance of Community Shelters, and FEMA P-320, Taking Shel­ter from the Storm: Building a Safe Room Inside Your Home.

Safe room design must meet two main criteria. The first is that the room’s structural components must be able to withstand the basic wind load of the tornado or hurricane. The second design criterion is that the safe room must have resistance to flying debris, a particular danger of tornadoes and hurricanes.

As part of their ongoing efforts to improve safe room design, James Bridwell, Robert Ross, Zhiyong Cai, and David Kretschmann conducted performance tests on a series of materials and wall designs that might be used in the construction of safe rooms. Next week we will delve into this unique research described by Bridwell and others in the publication, USDA Forest Products Laboratory’s Debris Launcher.