EXPLORING CROSS-LAMINATED TIMBER USE FOR TEMPORARY MILITARY STRUCTURES: BALLISTIC CONSIDERATIONS
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ABSTRACT.
The design and construction of temporary military structures, built to house personnel in theater, have changed little since World War II. While lightweight, rapidly deployable, and quick to erect, these gage metal or wood frame structures provide minimal ballistic and blast protection for occupants. Cross-laminated timber (CLT) is a new building material gaining attention in the U.S. construction industry for its many unique characteristics. This prefabricated, engineered wood product, composed of three or more plies of 2x lumber with alternating ply directions, is strong, stiff, and has the potential to meet the requirements for temporary military structures. CLT structures can be assembled rapidly as they arrive to the site in large sections, are machined in the factory to fit together and interlock, and are typically fastened with self-tapping screws using cordless tools. Ballistic and blast resistance are key criteria for temporary military structures. Together, these requirements for military structures are known as force protection. In order to expand its use into temporary military structures, a better understanding of the force protection performance of CLT is needed. In the past, typical wood products such as dimension lumber have not been desirable as a protective material due to overall poor performance. However, the composite nature of CLT coupled with the energy absorbing capacity of the thicker wood panels have warranted further investigation into the viability of CLT for temporary military structures. The mechanical properties of wood influence the overall response of CLT to impact loads like those experienced in a blast or ballistic event. Mechanical and physical xxxvi properties such as hardness, density, and shear strength all aid in determining the expected response of the CLT panel and were evaluated for two types of CLT. Specimens in this research included commercially produced Spruce-Pine-Fir (South) CLT as well as Southern Pine CLT specimens fabricated as part of this research program. Use of CLT in temporary military structures would require a broad market availability of CLT – so the work focused on the two most common species available from Western U.S. forest (Spruce-Pine-Fir South) and Eastern U.S. forests (Southern Pine). Ballistic testing of both types of CLT indicate that the material’s inherent penetration resistance is significantly greater than that of dimension lumber and plywood used in current common temporary military structures. By conducting the first ballistic tests on CLT, the relationship between striking velocity and depth of penetration or residual velocity has been established. The study of penetration mechanics is complex with many influencing variables describing both the projectile and the target. A quantitative analysis of the data collected allowed for the calibration of classical penetration models and the parameter estimation to help predict the thickness of CLT required to prevent penetration. The research examined existing equations found in UFC 4-023-07, Design to Resist Direct Fire Weapons Effects and found that these equations dramatically over-predict the thickness of CLT needed for ballistic protection. Updated parameters are therefore proposed that more accurately capture the performance of CLT. To ensure CLT performance in a wide range of site environments, specimens subjected to high moisture environments were tested and exhibited no degradation in ballistic resistance relative to samples at typical kiln-dried moisture contents (8 percent to 12 percent). This initial investigation into the ballistic response of CLT will help shape future testing in this realm xxxvii with the aim of meeting performance ratings for the material such as those found in UL 752 The pre-fabricated, laminated nature of CLT allows for modification of the layup through the addition of one or more enhancing layers for improved ballistic resistance, creating enhanced CLT (or ECLT as it is referred to in this thesis). Fourteen panels representing eight different ECLT configurations were produced in the Digital Fabrication Laboratory (DFL) at Georgia Institute of Technology using various hardening materials including thin metal plates and gratings, polymer-based armors, and fiber-reinforced epoxy matrix panels. The enhancing layers were evaluated based on ease of production, ballistic resistance given a small initial test series, and a cost-benefit evaluation. Solid metal plate presented challenges in fabrication but performed well. Perforated metal offered a lighter option with good resistance as well. The fiber-based panels, composed of highperformance armoring materials, performed well, but are costly to procure and difficult to incorporate into the CLT stack. The E-glass-epoxy configurations were the least effective but showed improved resistance to baseline CLT at a relatively low cost. A qualitative analysis examined the damage to the CLT panels relative to the unique anisotropic and inhomogeneous properties of wood. This analysis focused on local failure modes and the local response of the wood under ballistic impact. The resistance mechanisms of wood were directly observed by dissection of the projectile paths through the CLT panels. The bi-directional layups of the multiple plies of CLT allowed a crosssectional cut to show the different failure modes based on grain orientation of the wood. Multiple failure modes of the material in a localized damage zone were observed with the ballistic penetration. Unexpected data, such as shallower penetration or faster residual xxxviii velocity, was critically examined for a variation in the wood structure to help better understand the variability observed in ballistic testing of CLT. In addition to ballistic response, blast resistance is a critical attribute of interest for materials to build temporary military structures. Based on data from live blast testing and use of the shear analogy method to account for the strength and stiffness of the CLT panel given the cross-laminations, a single-degree-of-freedom model was used to evaluate the response of a CLT panel under blast loads in the elastic regime. A CLT blast analysis tool was developed to help predict the blast response of different grades of CLT in different dimensional and geometric configurations. The initial version of the tool is limited to response in the elastic regime, due to a lack of available data on CLT panel response in the inelastic regime. Multiple requirements, constraints, and limitations to consider while meeting mission requirements create a complex decision for military leaders with regard to what material to use in the construction of temporary military structures. The construction industry uses multi-attribute tools to help assess risk and to determine design and construction alternatives. One potential tool to evaluate construction material system and assist in making decisions regarding temporary military structure construction is the analytical hierarchy process (AHP). In this process, materials are evaluated using pairwise comparisons and relative weights for multiple criteria of interest, with the goal of selecting the best material system for the temporary military structures within a given operational environment. A simplified tool based on a radar chart was developed to present a visual representation of the decision for military leaders. Ultimately, the decision is highly dependent on a thorough definition of the evaluation criteria. The research defines a xxxix tailored set of decision criteria for temporary military structures along with proposed methodologies for rating their performance. CLT presents significant potential for improved performance over traditional lightweight construction materials for use in temporary military structures. Improvements in ballistic resistance and blast response are critical force protection considerations for temporary military structures. While additional research is needed to definitively rate the performance of CLT for ballistic resistance, this research presents the first such tests ever conducted and provides a foundation for future work. The concept of enhanced CLT defined by this research holds much promise and has demonstrated that specialized variants of CLT can be designed to meet military specifications for ballistic resistance. The blast analysis tool shows good agreement with the response of CLT in the elastic regime and can be modified to include the inelastic regime with further blast simulation testing. Lastly, tailored evaluation criteria for comparative assessment of a construction material system, like CLT, for use in temporary military structures was developed and implemented in the context of decision making required of military leaders.
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