November 03, 2014
New Proposed Perimeter Barriers for the Department of State
by Hinman Team
Click here to watch the test video: TTI Crash Test | September 2014
A very interesting video of a crash test performed by Texas A&M Transportation Institute (TTI) has been circulating the internet recently. The test evaluated a new perimeter barrier designed for the State Department to use at U.S. Embassies. As frequent consultants on U.S. embassy projects, Hinman Consulting Engineers knows how difficult it can be to balance stringent security requirements and design aesthetics. In recent years, the State Department has put an emphasis on vehicle barriers that (1) are visually appealing enough to not make the embassy look like a fortress, (2) are simple enough that they can be constructed reliably by contractors in a foreign country, and (3) have shallow enough foundations to not affect underground utilities in urban environments. The barrier, which is made up of a curved, 18-inch diameter round tube that spans 15 feet horizontally, appears to accomplish all three of these design goals while still stopping the impact vehicle. The test used a 15,000 pound flatbed truck at 50 miles per hour, signifying that the barrier successfully gained an impact designation of M50 in accordance with ASTM F2656-07: Standard Test Method for Vehicle Crash Testing of Perimeter Barriers.
The most interesting thing about this barrier is what it doesn’t try to stop: the front wheels, engine block, and rails that run along the body of the truck. While most barriers, like standard vertical bollards, are designed to stop those stiff and heavy components head-on, this system allows the components at the front of the vehicle to pass under the horizontal tube. Because the truck bed is where the potential explosive payload would be located on a truck bomb, ASTM F2656-07 specifies that the all important vehicle penetration measurement, which determines whether a crash test in a success, be taken from the barrier device to the leading edge of the truck’s bed. In addition to focusing on stopping the part of the truck that matters for the test, the design allows the energy of the impact to be dissipated over a longer duration of time. While the stiffer elements at the front of the truck pass under the horizontal tube, impact energy is burned as the truck’s cab crumples and tears.
While the curved horizontal tube design is both simple and effective, it also boasts an 18-inch deep foundation, which makes it much easier to use in urban environments that have utilities close to the surface. Barriers with shallow foundations are difficult to design in large part because there isn’t much mass preventing the whole system from prying and rotating out of the ground (if you watch from the side view, you can see the front edge of the foundation lift during impact). TTI’s design prevents the uplift of the foundation by making sure that the brunt of the impact energy is being transferred once the truck is already on top of the foundation. The truck’s weight helps to prevent the front of the foundation from uplifting, and the front wheels and rails of the truck seem to wedge the foundation to the ground to prevent further rotation of the system.
The barrier’s design appears to be efficient, simple and robust; however, some caution is in order. The large horizontal tube is at the perfect height to allow the rails and engine block of the truck through, but to still catch the bed of the truck. While this was most likely a conscious design decision, it runs the risk of making the barrier’s performance dependent on the impact vehicle’s geometry. If a taller vehicle’s engine block was firmly caught by the horizontal tube, would it still perform as well?
Additionally, ASTM F2656-07 specifies that the vehicle’s line of impact be centered on the most vulnerable section of the test article. A vehicle impact at the location where the horizontal tube turns down into the foundation would cause the barrier to act more like a traditional vertical bollard and engage the stiff elements at the front of the vehicle. This could have the potential to alter the failure mode of the barrier, and could even allow the impact vehicle to ramp up and over the device where the tube slopes down into the ground. It is possible that the State Department’s intent is to place many of these barriers end to end, so that a vehicle would never be able to strike just one turned down end of a single barrier.
While it seems like further testing could be warranted, the curved horizontal tube barrier designed by TTI appears to be simple, elegant, and effective. The design thinks outside the box by choosing which of the vehicle components to impact, dissipating the vehicle’s impact energy over a longer time duration in a more ductile manner.