Events of the past two decades have vastly increased the need and demand for higher perimeter security in places where it was not previously thought necessary. Terrorist attacks in Europe in the 1990s, the Oklahoma City bombing, 9/11 and the upsurge in terrorist attacks worldwide since then have made perimeter security a top-of-mind issue.

This environment has changed the face of security fencing. The technology of anticlimb and anti-cut fence materials has advanced. More than ever, fencing is only one element of a perimeter security system that may also involve hardening measures to prevent vehicle crash-throughs and electronics to detect and locate potential and actual intrusions.

The reigning theory of perimeter security is referred to as the Four D’s: Deter, Detect, Delay and Defend. The concept is that the right fence can keep out all but the most determined intruders, and those can be slowed down to give time for security personnel to get to the breach and defend against the intrusion. But what is the right fence?

CHAIN LINK

High security used to mean a chain-link fence woven of eight-gauge wire in two-inch squares, eight feet tall, with four stands of barbed wire at the top.

“For years, that was the security standard,” said Bill Schenke, vice president of sales and marketing for Ameristar Perimeter Security. “It’s a low-entry-cost product, and it keeps the majority of honest people out. But since 9/11, we’ve seen an elevation in security solutions for the perimeter.” In the current environment, he considers chain link a limited deterrent. “It does a pretty job at deterring honest people,” he said.

In response to increased threats, some new chain-link fences are harder to climb, sometimes 10 or 12 feet tall with more of an aggressive topping such as concertina wire. But if someone really wants to break in, he probably can. The two-inch openings provide a relatively easy toehold for climbing, and a blanket thrown over the top can render the barbed wire harmless. Moreover, it can be cut through fairly quickly.

Jim Knott, chief executive officer of Riverdale Mills, recalled a test of security fences conducted by the U.S. Army in which Riverdale participated. It took a trained Special Forces “intruder” just six seconds to get through the chain link.
“I do chain link as a safety product more than security product,” Knott said.

BEYOND TRADITIONAL CHAIN LINK

“Chain link has evolved over the years,” said Evan Winston, president of Hercules Fence of Maryland. “For example, there’s vinyl-coated wire and mini-mesh instead of two-inch diamond. You can get chain link as small as a five-eighths opening. That makes it an anti-climb fence. You can also increase the gauge of wire so it’s anti-climb and anti-cut.”

Even so, the change in status of chain link has led to the deployment of other solutions. The selection of material depends on the type of threats the system is being designed to defend against and the aesthetic requirements of the site.

“If it’s a nuclear plant in North Dakota, nobody cares how the fence looks,” said Winston. “On the other hand, we just did Walter Reed Hospital, where soldiers are coming back wounded from service. It’s in an urban setting where there’s a lot of traffic. Aesthetic concerns are a big factor.”

For more aesthetically pleasing security fencing, ornamental metal pickets are usually the choice. Ornamental pickets are typically square in cross-section, three-quarters or one inch across. They may be aluminum, iron, or steel and could be tubular (hollow) or solid metal.

“You can’t take a pair of fence pliers and just clip wires and walk through,” said Schenke. “You’ve got to do something more robust to breach the perimeter. We can also put spikes on top and curved pickets that make it more difficult to climb. It increases deterrence and increases delay time. But because of picket spacing, it’s easy to get hand tools in there to cut it.”

“Even if you have a one-inch solid picket,” Winston said, “if someone wants to take a car jack to it, you could pry the pickets out side to side and have a diamond-shaped opening.”

“Next, we went to a palisade-style fencing,” Schenke said. “A palisade picket is roll-formed, almost like a W in cross-section, and it would usually be a minimum of 14-gauge steel. It’s almost impossible to bend apart. It’s punched to have a nasty-looking spear top called a splay. It’s more structural, and the pickets are usually spaced tighter, which makes it more difficult to breach the fence from outside.”

Other security-fencing materials utilize the strategy of close spacing too. Expanded metal mesh is one type. A bollard wall (a row of closely spaced round pipes set into the ground) is another option, although it is extremely expensive.

THE LATEST

“The next level that started to emerge over the last three or four years,” said Schenke, “are fences constructed with a light wire mesh in a non-climb configuration.” This type of welded wire is often called 358 mesh. It is a closely spaced rectangular grid. The term “358” derives from the dimensions of the most common type: three inches between the verticals, half an inch between horizontals, eight-gauge wire. (358-type mesh is also made with even heavier wire such as six gauge.)

Those tight dimensions give it good anticlimb and anti-cut properties. The horizontals are so closely spaced that it is almost impossible to get a toehold or finger hold or to get in a cutting tool. This was proved convincingly in the aforementioned Army test. Riverdale Mills’s WireWall product was in the test too. It’s a 358-type fence supported on a grid of heavy steel U-channel posts and rails. (Some 358-type fences use round posts rather than square channel.) Unlike the six second chain link, WireWall kept out the Special Forces for 45 minutes.

Knott said welding rather than weaving adds a huge delay factor. An intruder can’t just clip one or two wires and unweave it. The mesh has to be cut every half-inch.

WireWall was originally developed as an underwater fence for ocean applications called Aquamesh. It is galvanized after welding and coated in PVC, giving it high corrosion resistance. (For land applications, it can be used either coated or uncoated.)

The rails in this type of fence provide a protected channel for running wires for electronics such as intrusion-detection systems. To prevent tunneling under the fence, the ground can be trenched out about six feet down during installation, the mesh extended below grade, and the trench then backfilled.

WireWall is in use along part of the U.S.- Mexico border and is in the running for the extended border “wall” currently under development by the U.S. government.

THE IMPORTANCE OF TRANSPARENCY

One of the key aspects of using fencing, as opposed to a solid wall, for security is transparency in terms of both light and wind. Pickets, wire mesh and expanded metal allow people on one side to see what’s happening on the other, enabling security personnel to monitor outside activity; 358-type mesh offers some visual transparency but not complete, and that has value for some security clients. Ameristar has been supplying this type of fencing to the power-utility industry, where the visual screening property is in demand.

“It obscures your vision slightly,” said Schenke. “They’re building these fences 16, 20 and 24 feet tall to deter line of sight from positions outside the utility. They want to deter someone taking potshots from up on a hill with a high-powered rifle.”

Wind transparency is environmentally desirable. It also affects a significant design consideration: wind load on the fence. Each material has a different level of wind transparency and requires posts and post foundations of appropriate strength. For example, two-inch chain link has about 85 percent open space, whereas 358 mesh has about 65 percent. A tall fence experiences greater wind loading than a short one too. This is one of several reasons why a security fence needs to be engineered for wind loading as well as for the designated threat.

OTHER CONSIDERATIONS

Materials that are well galvanized or otherwise coated against corrosion are considered very low maintenance. Of course, physical damage, by human action or natural forces, requires repair to keep the fence’s integrity.

None of these fence types in their ordinary configuration provides protection against vehicle-crash intrusion. They can be hardened for that purpose by adding heavy steel cables stretched horizontally across the interior side of the fence. The cables are tensioned and anchored to special posts with strong foundations.

Hardening systems are tested for a 15,000-pound truck traveling at a specific speed, the highest level being M50, 50 mph. There is an ASTM test that rates how far into the perimeter the truck penetrates (>1 meter, 2-3 meters, etc.) The cable system is so effective that the driver in such a crash-in usually does not survive.