Case Study | 9.10.2015
Protection of Personnel, Plant and Equipment from Insurgent Attack
Our clients in the natural resource development, energy refining and production and energy transportation businesses often have consolidated manufacturing/processing and housing facilities located in remote and politically unstable regions and countries. PEC Sr. Principal Ed Conrath has supported those clients by designing, reviewing and recommending improvements to these facilities and compounds. Specifically, they have worked with customers to mitigate threats and protect staff from insurgents using indirect fire weapons targeted at their facilities. The structures we design and improve are typically conventionally designed and include vulnerabilities that could be exploited by insurgents. We regularly support our clients under compressed schedules.
An indirect fire threat consists of rockets and mortars, as illustrated below. These weapons can be defeated by effective design and implementation of overhead protection (OHP) consisting of a pre-detonation layer and shield layer and through sidewall protection. Our first step in the design/review process is to investigate and characterize these likely threats. Characterization includes defining the fragment size and velocity and weapon explosive weight. Sources of information for the design projects are regional threat assessments, insurgent capability descriptions and documented incidents provided by the client. We then consult weapon test reports and data for characteristics.
Once threats are established, we perform analyses to determine both local and global effects of weapon detonation on or near the structure. For the local effects, we determine whether fragments would perforate the shield layer or sidewall layer of the structure and whether the close-in applied impulse would breach or locally damage (spall) the shield layer or sidewall layer. The highly non-uniform impulse distribution tends to cause a plug of the shield layer to fail in shear beneath the area of high impulse.
For global response, we perform a single-degree-of-freedom (SDOF) analysis of critical members in the structure. These analyses characterize the flexural and shear response to the impulse applied by weapon detonation. We then compare member response to accepted response limits to ensure the critical members provide sufficient resistance to the threat. To finalize design, we perform appropriate checks or design any connections.
For some structures, we determine whether loss of a perimeter column due to weapon detonation would result in progressive collapse. We perform Alternate Path Analysis per DoD UFC 4-023-03 Design of Buildings to Resist Progressive Collapse. Based on the analyses performed, we develop recommendations for modifying the structure to provide required performance. Finally, we document the results of the weapon effects and progressive collapse analyses and recommendations in technical reports.
To provide more general design and site guidance for customers, we can develop design guidance and specifications for OHP to be used by third-party engineering companies. The guidance includes sections on threats describing the design weapons for OHP, numerical and simplified calculation of blast load for those weapons, and determination of design fragments. The structural design sections can include design for fragmentation using concrete, soil and steel; spall and breach of concrete and masonry; single-degree-of-freedom (SDOF) flexural and shear design; and progressive collapse design. The result of this effort is a self-supporting document on OHP design for reference and dissemination.