Analytical and Experimental Evaluation of Precast Sandwich Wall Panels Subjected to Blast, Breach, and Ballistic Demands

Abstract Due to heightened security concerns federal as well as many public facilities require some level of blast design, whether it be intentional or accidental. In addition, with the increasing cost in utilities and continuous rise in global warming, a movement has begun to streamline the construction process and limit the environmental footprint of every building. In response, the federal government now requires that all government buildings not only be designed for blast loads, but also sustainability.Insulated wall panels are capable of meeting both the blast and sustainable requirements due to the inherit strength of a reinforced concrete slab and the thermal resistance provided from the insulating layer; however, limited experimental testing is available to prove that insulated wall panels are an ideal system for both blast and sustainability. The objective of this research is to develop the tools to design a blast and ballistic resistant insulated wall panel system. As part of this research, experimental tests were conducted on insulated panels to validate models developed to predict panel behavior observed. Using the results of the research an approach was developed to create a 1) Thermally efficient, 2) Blast Resistant, 3) Spall/Breach Resistant and 4) Ballistic Resistant panel.Insulated wall panels are inherently thermally resistive due to the insulating foam located between the two layers of concrete. Parametric studies were performed via analytical calculations to determine the efficiency of the wall system. The calculations indicated that the insulating layer is fundamental to the resistance of the panel; an 8in. solid concrete panel had a thermal resistance of less than 10% of a panel 2in. of insulation sandwiched between two 3in. concrete wythes. Additionally, the parametric study indicated that the shear connectors located between the interior and exterior wythes can have a significant effect on the overall panel thermal resistance due to the thermal bridging phenomenon. Three panels were modeled with identical layout and wythe connectors with identical dimensions but different material: concrete, steel, and low-conductive material. The panel with concrete and steel wythe connectors saw a reduction in thermal resistance compared to the low-conductive material of nearly 78% and 62%respectively. Thus, to decrease the panel resistance while maintaining strength, a strong thermally resistive material must be used as a shear connector.To improve the response to far-field detonations, experimental tests were performed on small solid panels as well as larger insulated panels. Locally unbonding the small solid panels allowed the panel to reach support rotations past the 10° specified by the United States Army Corps of Engineers as the highest threat level while the bonded panels reached less than 5° before softening. Additionally, testing of insulated wall panels revealed that the panel behavior is highly dependent on the shear tie constitutive property and location along the span. A numerical model was created to predict the behavior of an insulated and as a result, a new shear tie was developed to improve the flexural response of the panel while at the same time, decreasing the production cost.To assess the response of insulated wall panels to close-in detonations, experimental tests and numerical models were conducted. The tests revealed that the insulation results in a detriment to panel performance as a panel with 2in. of insulation sandwiched between two 3in. thick concrete wythes breaches the exterior wythe while a 6in. thick solid concrete panel does not breach under the same demand. As the insulating layer thickness is increased, the panel does not breach due to the increased standoff created by the additional thickness. Additionally, the empirical formulas developed by the Unified Facilities Criteria for solid panels were shown to be inaccurate when used for insulated wall panels, while numerical simulations were able to bound the response of an insulated wall panel.To investigate the performance of insulated wall panels to ballistic and fragment demands, a probabilistic method was developed. The method results in the creation of fragility curves allowing a designer to assess the probability of perforation and residual velocity for a given threat at any wall thickness. Additionally, the likelihood of injury occurring to personnel behind the wall panel was assessed by using organ threshold tolerances provided in literature. Using the method developed, engineers can design the thickness of an insulated wall panel to achieve an acceptable probability of occurrence for injury.Finally, all of the material learned through the first four stages were combined to create a comprehensivedesign example. An 8in. thick panel with 2in. of insulation was designed using the newly designed shear tie as well as a ductile shear tie with the same strength, and then subjected to the demands reviewed throughout the research project. The tie system allowed the wall to reach a support rotation of 10° while behaving in a moderate to heavy damage level when subjected to the far-field detonation demand. From the conclusions of the close-in detonation study, the panel is known to breach under the load prescribed. Ballistic fragility curves were developed showing that the panel stops a low threat ballistic with 100% certainty, but under a high ballistic threat the projectile has an 86.5% chance of perforating the wall system. For the fragmenting munition considered in the study, the wall system has a 15.4% chance of causing injury to personnel behind the wall. Finally, by using the new shear tie system developed, the wall system results in a reduction of less than 3% in the total R-value when compared to an insulated panel without thermal bridges due to the low thermal conductivity of the shear tie material.