Bomb blast simulator looks to 'harden' structures

A University of California, San Diego, team thinks it can extract precise computer-aided design parameters that characterize a "bomb-proof" material's strength with the $4.2 million simulator.

"These controlled and repeatable tests we will do with the blast simulator will allow us to create and validate computer tools that can then be used to tailor the design and assessment of important facilities," said Frieder Seible, principal investigator on the bomb blast simulator at UCSD.

Today, validating whether bomb-proof materials are strong enough means blowing up a sample out in the desert somewhere. Hardening important facilities to be bomb-resistant is more an art than a science, Seible said.

In the UCSD technique, the material to be characterized is placed inside the simulator, where hydraulic actuators can deliver the power of increasingly strong bomb blasts. The precise response parameters can then be fed to the CAD model by measuring the damage the material has sustained.

Besides new materials, theoretical hardening techniques, such as applying composite overlays to existing structures, can also be tested, the UCSD team said. Composite overlays were first invented to harden facilities against earthquakes, but they have already proven effective in real bomb blast tests. The bomb blast simulator will be able to characterize the materials precisely, without using real explosions, so that CAD models can apply hardening "virtually" to any building and measure the degree of protection afforded, the team believes.

The UCSD bomb blast simulator laboratory is funded by the Technical Support Working Group, an anti-terrorism organization within the Department of Homeland Security. TSWG calls the $4.2 million effort its Explosive Loading Laboratory Testing Program, a federal interagency effort involving other contractors besides UCSD.

The overall blast mitigation effort is to result in a final "design manual" that will prescribe proven methods and metrics for hardening high-risk buildings against terrorist bombs.

A bomb blast doesn't physically "blow over" the walls and columns supporting a building but instead creates a shock wave that "vibrates" walls and columns with such force that they literally pull themselves apart. If enough of the supporting structures are thus pulverized, local collapsing ceilings can create a domino effect, leading to the progressive collapse of the entire structure.

As a result, one key to hardening a facility is to encase its load-bearing columns, beams, girders, walls, ceilings and floors inside a tough composite shell. In real blast tests, composite encapsulated concrete-and-steel structures were typically able to withstand the bending and stretching of shock waves, while untreated concrete crumbled to dust.

The UCSD simulator delivers its blast effects by virtue of servo-controlled hydraulic actuators. The critical load-bearing elements are put inside the simulator, and the velocity-generating hydraulic actuator delivers a precisely calculated blast. The actuators, which resemble those used in automotive crash tests, are being engineered in cooperation with MTS Systems Corp., a company that produces crash test systems for military vehicles and weaponry.

The new blast simulator laboratory will be located at UCSD's existing secure location, the Powell Structural Research Laboratory at Camp Elliott, eight miles east of the UCSD campus. Powell, where ground was broken in December 2002, will also house the world's first outdoor "shake table," designed to simulate the effect of an earthquake on the structures assembled on top of it. Both the shake table and the bomb blast simulator are slated to be completed during 2005.

The UCSD effort to retrofit earthquake simulators for bomb blast simulation dates back to 1998, when full-scale real bomb explosion tests were performed. Those tests validated the earthquake prevention devices, such as com- posite-encased structures. As a result, several foreign embassies and military installations have already been retrofitted with UCSD's composite technology, as well as a similar method that encases columns in steel jackets.

"By confining and containing concrete in load-bearing columns, we're actually strengthening columns so that they can take large structural deformations such as bending or swaying without collapsing," said Seible. "Concrete is brittle and can break apart in an explosion, but when we wrap it with these materials we can contain the concrete for the short duration of the shock wave."

To protect walls and floors, the laboratory will design a specific configuration of the simulator designed to test configurations of surfaces, including both load-bearing and non-load-bearing structures.