The Construction Fight Against Terror
However, in recent years, the threat of terrorism has forced clients and the A/E/C Community alike to consider building more robust and durable buildings. Fortunately, there are a host of technologies available that provide designers and clients with affordable solutions to increase the durability and decrease maintenance costs while protecting the lives of people occupying the building in the event of a terrorist attack.
Background to AT/FPJune 26, 1996, was like any other miserable day in the desert for U.S. servicepersons stationed at Al-Khobar, Saudi Arabia. It was hot, lonely and a world away from home. However, as ordinary as the day began, few knew that by day's end it would be one that would force the hand of the United States to seriously change the way it addresses the issue of Anti-Terrorism/Force Protection.
Shortly after 10 p.m. that date, a truck packed with an estimated 5,000 pounds of high explosives was parked next to a fenced area adjacent to Khobar Tower #131, the retrofitted building the U.S. Air Force used as barracks. After parking the truck, the culprits, members of the Lebanese terrorist group Hezabollah, quickly detonated the explosives and within a blink of an eye, the force of the blast had claimed 19 U.S. serviceperson's lives, heavily damaged and/or destroyed six high-rise apartment buildings and shattered windows in virtually every other building in the compound. An enormous crater, 85 feet wide by 35 feet deep, was left where the truck had been. If the bombing of the Marine Corps Barracks in Beirut 13 years earlier was a precursor of things to come, the bombing at Khobar Towers solidified the real threat U.S. servicemen and citizens face.
As a result of the devastation, the U.S. Joint Staff, the commanding generals of each of the U.S. military services, moved quickly to address the issue of Anti-Terrorism and Force Protection design. After much discussion and research, the AT/FP Standard was developed and passed by Congress for all the country's government buildings with standoff distances less than 82 feet.
At the heart of providing for blast mitigation and containment is standoff distance. In most cases, 82 feet is the minimum distance standoff before a building begins experiencing the devastating effects of an explosive event. In the instance of Khobar Towers, though, a standoff distance of 82 feet would have helped little since the truck bomb was located approximately 105 feet from the building and still created a massive explosion.
For situations where standoff distance is at a premium, such as heavily traveled corridors in downtown city areas, the solution is the use of blast resistant materials and design configurations that help mitigate the effects of a blast. One such design feature that can be maximized with concrete is redundancy. Redundancy in the structural system permits redistribution of forces in the event of a failure of key elements. Without capacity for redistribution, a localized failure in a supporting column, beam or connections can cause a global failure or a partial/total collapse of the building.
Force Protection Equipment DemonstrationOver the years, the United States has seen first-hand the risks associated with terrorism. From the Alfred P. Murrah Building in Oklahoma City to New York's World Trade Center, the effects of terrorism are not only costly but more importantly take the lives of innocent victims. In response to this growing problem, the U.S. Joint Staff has created the Force Protection Equipment Demonstration, which is held every two years at Quantico Marine Corps base in northern Virginia to allow government procurement officials to view how commercial off-the-shelf products perform during real-time events such as ballistics and blast demonstrations.
Knowing the resilient behaviors foam displays in impact testing and the sheer mass protection concrete provides during explosive events, the Insulating Concrete Form Association agreed to participate in FPED IV & V in 2003 and 2005 respectively, to showcase the mitigating properties it possesses.
Forty-five days before FPED, representatives from ICFA member companies, as well as ready mix concrete producers from the Northern Virginia Concrete Promotion Council, met in Virginia to construct ICF reaction boxes. Each box consisted of three 8-foot-by-8-foot walls, arranged in a U-shape with a 6-inch thick concrete slab and a 6-inch thick concrete roof. The fourth side or back was left open to allow for inspection of the interior after the trial. The 3/8-inch-aggregate, 4,000-psi concrete mix was reinforced with #4 steel bars at 16 inches on center, typical of how ICFs would be built in most residential applications.
The standard design used in the ICF reaction boxes was important for the purpose of proving that ICFs can reduce the damage and risk associated with explosive events using materials normally used in residential construction. It was lightly reinforced and did not require the special reinforcing detail typically recommended for use in blast resistant walls.
However, there was another consideration that needed to be addressed during the design phase. With the bomb generating forces in excess of 10 times the weight of the boxes, they needed to be constructed heavy enough to withstand the pressure of the blast without tipping over or being picked up and thrown downrange. In the end, a design was chosen to satisfy both concerns, and fully constructed each box weighed over 26,000 pounds apiece.
The BlastEach day, the ICF reaction boxes were placed on the range and faced a 50-pound charge of TNT located anywhere between 31⁄2 feet to 10 feet away.
During the blasts, the TNT created a huge fireball as the explosion sucked the oxygen out of the surrounding area to fuel its destruction.
"The blast was awe inspiring," says Greg Moody of SI Concrete. "Even though we were more than 1,300 feet away, we could see the enormous size of the fireball and could feel the wave of the blast hitting us."
Despite being more than a quarter of a mile from the blast, the air pressure created by the bomb shook the metal buildings housing munitions and made more than a few people watching the blast demonstrations jump.
After each blast, the boxes were inspected and not surprisingly, the EPS was compressed against the face of the concrete.
Patrick Murphy of American Polysteel attended the demonstrations and said, "It was amazing to see the amount of compression the EPS on the face of the wall displayed, showing a measurable amount of absorption from of the force of the blast. Web ties that were manufactured half-a-foot below the surface of the EPS on the systems were completely visible after the explosion."
At 6 feet, only small cracks were visible-none more than 2 millimeters across-with no deflection or structural damage occurring.
"FPED offered the opportunity to test a minimally reinforced ICF wall under blast loading," says Baldridge, who also served as the ICFA's blast designer at FPED. "At the 2001 FPED, a wall with 2x4 studs was tested with a 50-pound TNT charge at 35 feet-the wall was destroyed. The ICF wall was tested with a 50-pound charge at 6 feet and there was only minor cracking in the wall."
It is important to note that although the EPS played an important role in the success of the demonstration, a large portion of the credit must be given to the sheer mass of the concrete and both the steel and fiber reinforcement in the wall system. Without the hardened structure as a backing for the EPS to compress against, the wall system would fail.
The Federal MarketThe heightened Force Protection Standard provides the ICF community with a great opportunity to expand into a market that is well suited to the ICF building system.
"With the emerging need for inclusion of AT/FP requirements in the latest design requirements for government facilities, the opportunities for concrete applications, including ICFs, have increased," says John Sullivan, director of infrastructure markets with the Portland Cement Association. "The ability to combine a degree of force protection (durability) with energy efficiency and sustainability (environmental) provides an enhanced opportunity for ICF use in government construction projects."
Since the AT/FP Standard's inception in 1999, ICFs have been used in several federal projects, none more impressive than the Florida Armed Forces Training Center, located in Pinellas Park. Built by Hunt Construction Group and completed in June 2004, the 235,000-square-foot, multi-purpose facility covered with an exterior acrylic finishing system was designed not only to meet the AT/FP Standard but also several value-added considerations of the client, the U.S. Department of Defense.
The lead designer of the Florida Armed Forces Reserve Training Center, Bill Murray, AIA, of URS Corp. says, "We selected the ICFs because it presented a more effective method of achieving the reinforcing requirements for exterior walls, as well as advantages of superior energy efficiency and sound control."
In addition to the structural requirements needed, the expanded polystyrene insulation on the exterior of the ICF allowed the designers to detail raised shapes into the exterior walls to fit the aesthetics and design details of other buildings in the area.
"Taking into account the need for a more durable and solid building system, ICFs provide designers and contractors the opportunity to add visual interest with EPS shapes at an economy that cannot be achieved by any other building system," says Kent Stumpe, director of marketing for Degussa Wall Systems Inc.
Perhaps Hunt Construction Group's biggest payoff for using ICFs, though, was the labor savings it achieved. By using ICFs, Hunt estimated it shaved an estimated five weeks off the construction schedule, increasing its profit on the project and pleasing the owner by allowing them to move into the building ahead of schedule. As budget issues become more prevalent and as subcontractor scheduling grows tighter, the need for speedier building systems will be among the chief considerations of construction firms.
Matthew McCoy, of South River Construction, an ICF installer and subcontractor of several trades, believes the values of ICFs are built-in.
"How we add value to general contractors is to accelerate their schedules by performing multiple tasks at once," says McCoy. "ICFs achieve that by being an insulation system, a structural system, a vapor/moisture barrier and a backing for interior and exterior systems all in one shot."