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.
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.
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.
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.
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.