One of the most common methods of construction is the use of concrete masonry units (CMU) in the walls of buildings. However, they are vulnerable to blast, and result in collapse, fragmentation, and severe injury to occupants. An understanding of the behavior of CMU walls during blast is key to developing mitigation techniques. Research has been conducted using the finite element method to simulate structural failure due to blast. A common problem faced by model developers is the selection of constitutive relationships that appropriately simulate the behavior of materials subjected to shock loading. This project examined the effect of blast impulse loading on CMU blocks. Finite element models were used to perform direct transient analysis using various material cards available in LS-DYNA, and the results were compared to the results of full-scale blast tests conducted by AFRL. The material card that best agreed with the test results was recommended for use in the models of polymer reinforced masonry walls.
Mitigation techniques are currently sought to ensure public safety in the event of intentional or accidental explosions. The use of concrete masonry walls in civilian and military buildings is one of the most common methods of construction. These walls, however, are vulnerable to impulse loads, and can result in collapse, fragmentation and severe injury to occupants. Over the past several years, the Airbase Technologies Division of the Air Force Research Laboratory has investigated methods of retrofitting concrete masonry walls to better resist blast loads from external explosions. One method that was demonstrated to be very effective is the application of thin membranes of high elongation materials to the inside surface of the walls. Due to the nonlinear behavior of concrete masonry walls, the use of advanced simulation techniques provides certain advantages over experiments for full understanding of their structural responses under explosive loads. In the present study, several finite element models were developed according to blast test conditions, and analyzed using LS-DYNA explicit code. Input sensitivity studies were conducted to investigate the variations of a wide range of parameters on wall deformation, damping coefficients, boundary conditions and arching action. The effort has led to cost effective analysis techniques for use by structural engineers in designing membrane retrofit concrete masonry walls subjected to blast loads. This report summarizes the simulation methodologies, challenges, techniques and comparison to full-scale dynamic tests for membrane retrofit concrete masonry walls.
Computational models were developed and used to simulate polymer reinforced masonry walls subjected to blast loading and the models were used to understand the response of the structure. LS-DYNA, a nonlinear finite element solver, was used. Model development challenges were considered, and appropriate input parameters were determined. With these pedestal values, a baseline model of one unit width of concrete masonry block was developed, and the response under two load conditions was studied. Dimensional and mechanical variants involved in the system were varied to study their effect on wall behavior. The effects of door and window openings on the performance of the polymer reinforcement were evaluated. This report also presents an analysis of strain rate that occurs in the polymer coating and results were compared to theory-based closed form solutions. Finally, the static nonlinear capabilities of LS-DYNA were used to describe the static resistance of the system, and a theoretical description of a simply supported membrane subjected to pressure load is provided and compared with nonlinear finite element results.
This volume contains papers presented at the Ninth International Conference on Structural Studies, Repairs and Maintenance of Heritage Architecture. The conference provides an ideal forum for professionals in the area to discuss problems and solutions, and exchange opinions and experiences.
The increased threat against government and public facilities in the United States and abroad has highlighted the need to provide an economic and efficient method to retrofit existing conventional structures. Hollow, unreinforced, concrete masonry unit (CMU) infill walls, commonly used in reinforced concrete or steel framed structures, are particularly vulnerable to blast loads. Facilities that incorporate CMU walls must either be hardened or retrofitted for explosive events. Conventional retrofit techniques that focus on increasing the overall strength of the structure by adding steel or concrete are difficult to implement, time consuming, expensive, and in some cases, increase the debris hazard. The current research presents an alternative retrofit system for CMU walls that involves the application of an elastomeric material applied to the interior surface of the wall to prevent secondary debris in the form of CMU fragments from entering the structure when it is exposed to blast loads. The experimental program used to evaluate the alternative retrofit systems was divided into three phases. In Phase one, resistance functions for seven different retrofit systems were developed in 24 subscale static experiments. In Phase two, the structural response of the retrofit systems subjected to blast loads was evaluated in 25 subscale experiments. The final phase of the experimental program consisted of 18 full-scale high-explosive (HE) experiments used to validate the structural response observed in the subscale dynamic experiments. Data generated from the experimental program were used to develop a single-degree-of-freedom (SDOF) model to predict the mid-span deflection of the retrofitted CMU walls subjected to blast loads. The subscale resistance functions from Phase one were scaled and used in the SDOF model. The full-scale experimental results and the predicted results from the model were compared and the retrofit systems were ranked according to the qualitative and quantitative results obtained from the experimental and analytical research. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/151905
Concrete masonry units (CMU), commonly referred to as concrete blocks, are the most common construction material utilized throughout the United States and the world for exterior walls of conventional structures. While masonry provides adequate strength for conventional design loads, it does not meet the minimum design standards mandated for blast protection of new and renovated government facilities. One of the most dangerous aspects of blast response is debris hazard, defined as high-velocity fragments originating from walls, windows, light fixtures, equipment, and furniture. Retrofits for conventional structures have evolved over the years from blast hardening through the addition of mass using concrete or steel, to the application of lighter, more resilient and ductile materials. Research at ERDC has focused on the use of elastomeric materials to mitigate debris hazards resulting from blast events. A series of sub-scale and full-scale experiments was conducted by ERDC to investigate the potential benefit of elastomeric retrofit systems when applied to hollow, unreinforced, CMU walls subjected to an explosive event. This study discusses both the 1/4-scale static and dynamic experiments and the full-scale dynamic CMU wall experiments conducted over the past few years. The CMU wall response to static loading was characterized by resistance functions, and normalized pressure and impulse diagrams were used to characterize the dynamic loading.
Loads generated in explosions that result from terrorist attacks and industrial accidents create devastating hazards for buildings and their occupants. The objective of this dissertation is to develop design guidelines and methodologies for protective/hardening strategies used to mitigate blast hazards in reinforced concrete and concrete masonry walls. Commonly, guidelines and methodologies are developed from experimental data. Field testing with live explosive is a reliable experimental method for demonstrating the performance of blast resistant concepts, but it is expensive, time consuming, and often produces low quality data. Static testing is another experimental method that allows researchers to clearly observe behavior and failure modes of structural components; however this too is limited because it cannot account for the rate effects associated with blast loads. The UCSD Blast Simulator was developed to offers an alternative method for testing structures to loads generated in an explosion without the difficulties and limitations associated with field and static testing. For this dissertation, tests were conducted with the blast simulator to study reinforced concrete walls protected with frangible panels, concrete masonry walls strengthened with carbon fiber reinforced polymer composite, and unreinforced masonry walls retrofitted with polyurea catcher systems. The objective of the dissertation was achieved through a succession of tasks that included; the development of a test protocol, validation and implementation of numerical models to predict loads delivered to specimens during blast simulator tests, development of method to correlate blast simulator loads to air blast loads, generation of high quality data on specimens with mitigation strategies for validation of numerical models to predict response of hardened/protected reinforced concrete and concrete masonry walls, and investigation of design variables with parametric studies. The investigation of concrete masonry walls demonstrated that the addition of carbon fiber reinforced polymers can increase the resistance to blast loads, but may result in a brittle failure mode. The study of reinforced concrete walls showed that frangible panels can improve the response by adding mass to the system. Finally, the research performed on unreinforced masonry walls with polyurea catcher emphasized the need for proper connection detailing.
Unreinforced masonry (URM) walls are commonly found in existing and heritage buildings in Canada, either as infill or load-bearing walls. Such walls are vulnerable to sudden and brittle failure under blast loads due to their insufficient out-of-plane strength. The failure of such walls under blast pressures can also result in fragmentation and wall debris which can injure building occupants. Over the years, researchers have conducted experimental tests to evaluate the structural behaviour of unreinforced masonry walls under out-of-plane loading. Various strengthening methods have been proposed, including the use of concrete overlays, polyurea coatings and advanced fiber-reinforced polymer (FRP) composites. Fabric-reinforced cementitious matrix (FRCM) is an emerging material which can also be used to strengthen and remove the deficiencies in unreinforced masonry walls. This composite material consists of a sequence of one or multiple layers of cement-based mortar reinforced with an open mesh of dry fibers (fabric). This thesis presents an experimental and analytical study which investigates the effectiveness of using FRCM composites to improve the out-of-plane resistance of URM walls when subjected to blast loading. As part of the experimental program, two large-scale URM masonry walls were constructed and strengthened with the 3-plies of unidirectional carbon FRCM retrofit. The specimens included one infill concrete masonry (CMU) wall, and one load-bearing stone wall. The University of Ottawa Shock Tube was used to test the walls under gradually increasing blast pressures until failure, and the results were compared to those of control (un-retrofitted) walls tested in previous research. Overall, the FRCM strengthening method was found to be a promising retrofit technique to increase the blast resistance of unreinforced masonry walls. In particular, the retrofit was effective in increasing the out-of-plane strength, stiffness and ultimate blast capacity of the walls, while delaying brittle failure and reducing fragmentation. As part of the analytical research, Single Degree of Freedom (SDOF) analysis was performed to predict the blast behaviour of the stone load-bearing retrofit wall. This was done by computing wall flexural strength using Plane Section Analysis, and developing an idealized resistance curve for use in the SDOF analysis. Overall, the dynamic analysis results were found to be in reasonable agreement with the experimental maximum displacements.
