New Barrier Technology Development
To avoid deep excavations, some new concepts have been recently proposed, including shallow foundation mounted bollards, which typically employ large strong steel frames cast into a base concrete slab. If a vehicle crashes into it, the huge rigid impact forces will spread out over large areas, thereby limiting foundation damages. However the vast foundation work may create many other construction and cost issues.
Another innovative idea is the exertion of a relatively constant “calibrated force” by cushioning energy-dissipaters, such as hydraulic systems or crushable aluminum Hexcel® blocks. Such energy dissipaters allow the barrier to stop an explosive-laden truck differently than traditional methods.
Figure 5 Vehicle Momentum versus Barrier Resistance
Stopping the momentum of a terrorist vehicle requires changing its impact speed to zero forward velocity, without permitting significant penetration towards the target structure. Based on Newton’s second law of motion, the vehicle’s momentum (the vehicle’s mass times velocity) can be successfully brought to rest at a zero velocity if a relatively low and constant deceleration force can be exerted for sufficient duration of time without failure, as shown in Figure 5.
Precast reinforced concrete barriers containing energy absorbers become ideal because of the flexibility of both structural and geometric design, large stiffness and strength, compatibility to connections, and its secure nature against normal destructions. The barrier’s decelerating energy dissipaters can be accurately set at any force and stroke required by analytic studies and crash validations. Such “calibrated” deceleration forces assure the deceleration of a vehicle to zero velocity outside the secured perimeter, while at the same time controlling the forces imposed on the foundation. The controlled shear force is transmitted to the foundation, either a sidewalk or deck, using mechanical interlocking at its underside as well as optional soil anchors that lock the existing diaphragm to the earth below. The proposed barrier technology eliminates any deep foundation needs by employing an effective load transfer mechanism with calibrated decelerating forces.
Barrier Evaluation and Comparisons
It should be noted that both DoS and ASTM perimeter barrier crash test standards only consider right-angle impact scenarios, excluding any uncertainties inherent in the barrier and vehicle characteristics as well as potential impacts in the real world. Such facts need to be taken into account by the clients or professionals. Simply picking off-the-shelf products with universal solution imaginations may not work for all site-specific requirements. Comprehensive comparisons and evaluations of barriers have to be made to achieve the final decisions. The authors have made side-by-side comparisons for stationary barriers. Table 2 (attached below) summarizes the comparisons on multiple aspects covering performance, constructability, cost, and appearance.
Numerical Crash Simulations and Prototype Crash Validations
Both high-tech computer-based numerical simulations and full-scale prototype crash validations can be used to analyze and evaluate the proposed perimeter security barrier technologies.
Numerical simulations of a truck crashing into a physical barrier can be executed using advanced computer programs such as LS-DYNA3D, one state-of-the-art software in analyzing real-time dynamic behaviors considering actual geometry and material characteristics for both truck and barriers. It is always meaningful for the barrier developers to perform numerical simulations before directly jumping into field tests. High-tech expertise and powerful computer software as well as hardware make it possible to cover different barrier design options and many vehicle crash scenarios. The developers can then fine-tune the technical design based on results in the virtual world. The clients can also gain a good understanding of the barrier’s performance under real attack predicted from the simulations.
For full-scale prototype crash validations, these are the primary objectives: 1) to verify the barrier’s actual anti-crash performance, following ASTM F 2656-07 Standard and 2) to further optimize the barrier design through observed crash results, recorded data, pictures, and videos. One crash validation only illustrates one impact scenario; however in the real world, potential terrorist attacks could happen in situations different from the crash validation. There are many possibilities in combining vehicle impact speed and angle or location, which can not be covered in one crash validation. Therefore, the worst crash scenario shall be investigated beforehand and followed in both design and necessary validations to assure the robust functionalities of proposed barriers in any future applications.
Figure 6 Field Crash Validation Compared with Computer-Based Numerical Simulations
Figure 6 shows the representative results from both numerical simulations and prototype validations during a recent barrier development, following the ASTM F 2656-07 Standard M50 Designation. More than 20 simulations were analyzed in LS-DYNA3D, and two crash tests were performed considering two stringent impact scenarios. During the crash, the new barrier successfully stopped the 15,000 lbs Ford F-800 truck crashing into barriers at 50 mph. The simulations have accurately predicted the behaviors of both barrier and truck. The side-by-side comparison in Figure 6 showed that numerical engineering simulation is an effective tool that supplements physical crash testing and can be used to optimize barrier design before an expensive test is performed. The fact that actual crash observations and analytic results match one another so closely enables potential owners to modify security and decoration requirements without additional prototype crashes.
Because the science behind choosing the right protective barrier system directly relates to protecting human life and property, it must be a thorough process. To provide the most reliable and effective security solutions for clients, security professionals should utilize multi-disciplinary expertise to perform a systematic and comprehensive risk assessment, scientific structural analyses and barrier evaluations, and then recommend the right security barrier type and arrangement based off their client’s particular needs.
As terrorism concerns underline the necessity of more and more anti-crash and anti-blast barriers throughout city landscapes, architects and designers have found many innovative ways to blend them into the urban environment based off their stakeholders’ demands. Nevertheless, security professionals will need to work ever more closely with architects and city planners in the design and implementation of vehicle control barriers to address more pressing functional requirements.
♦ Marc Caspe, PE, SE, is Manager of Engineering at KKCS, Oakland, CA 94612. He can be reached at 415-299-9914 or firstname.lastname@example.org.
♦ Jun Ji, PhD, PE, is Senior Engineer at KKCS. He can be reached at 217-721-2501 or email@example.com.
♦ Lin Shen, PhD, PE, is Engineer at KKCS. He can be reached at 217-493-3418 or firstname.lastname@example.org.
♦ Qian Wang, PhD, PE, LEED AP, is Engineer at KKCS. He can be reached at 319-331-5734 or email@example.com.
Source Materials for this article included:
- U.S. Department of State – DS 9. SD-STD-02.01 Specification for Vehicle Crash Test of Perimeter Barriers and Gates, 1985.
- U.S. Department of State – DS 9. SD-STD-02.01, Revision A Test Method for Vehicle Crash Testing of Perimeter Barriers and Gates, 2003
- Unified Facilities Criteria (UFC), DoD Minimum Antiterrorism Standards for Buildings, 2003
- ASTM Standard F 2656-07, Standard Test method for Vehicle Crash Testing of Perimeter Barriers, 2007