Performance of Unreinforced Masonry Walls Retrofitted with Externally Anchored Steel Studs under Blast Loading

Six full-scale concrete masonry walls were tested under free-field blast loading using different charge sizes up to 250?kg of ammonium nitrate/fuel oil (ANFO) and at a constant stand-off distance of 15.0?m to cover a wide range of expected damage levels. Five walls were retrofitted with cold-formed steel studs anchored to the wall backs and were compared to the remaining as-built wall. Significant enhancement to the out-of-plane blast resistance of the retrofitted walls, compared to the as-built wall, was observed. This enhancement is attributed to the development of a tied-arch action in the retrofitted walls in which the masonry forms a compression strut while the steel studs serve as the tie. A simplified single-degree-of-freedom model was used to analyze the experimental results, and the model results agreed well with the observed damage levels and the resistances of the walls. In addition, the effectiveness of the proposed retrofit technique was evaluated in terms of strength enhancement and wall deflection reduction. The test results were also compared with those predicted by available blast damage assessment models for unreinforced masonry walls. However, it was found that available models, which do not account for the tied-arch mechanism, greatly underestimate the actual blast capacity of the retrofitted walls because of the assumption of a tensile flexural failure mode. Additionally, the proposed retrofit technique shifts the mode of failure from flexure to shear.     

Dynamic Response of Retrofitted Masonry Walls for Blast Loading

A full-scale blast test was conducted on eight masonry walls reinforced with two and four layers of carbon fibers and two types of polymer matrices. The walls were then subjected to a 0.45-kg pentolite booster suspended from the ceiling of a test structure. The pressure-time history caused by the blast and the resulting displacement response were measured during the test. This paper presents a summary of the test program and the corresponding results from a nonlinear single degree of freedom analysis. The results provide a basis for determining effective means of retrofitting existing masonry walls and designing new structures to withstand blast loads. The paper also outlines a fiber-reinforced polymer retrofit design procedure for walls subjected to blast loading.     

Murrah Building Bombing Revisited: A Qualitative Assessment of Blast Damage and Collapse Patterns

On April 19, 1995, a truck loaded with an ammonium nitrate and fuel oil bomb caused collapse of fully half of the total floor area of the nine-story, reinforced concrete Murrah Federal Building in Oklahoma City. The extent of the collapse, which extended well beyond the zone of direct structural blast damage, prompted studies of progressive/disproportionate collapse and development of new design guidelines for important buildings. While there is no question that the collapse was the result of the loss of only four columns, there is a common belief that direct blast effects destroyed three of those columns. Firsthand observation of debris, collapse patterns, damage patterns, and thousands of photographs taken during search and rescue activities at the building suggest the possibility that only one column was destroyed by direct blast effects, while the other three buckled due to loss of lateral support provided by beams and floor diaphragms that were destroyed by the blast. While the distinction may be subtle, it has significant implications for the design of tougher buildings. Specific lessons include ductile detailing, the necessity of maintaining the integrity of a three-dimensional frame, and explicit consideration of structural fuses to protect critical elements.     

Blast Resistance of FRP-Strengthened Masonry Walls. I: Approximate Analysis and Field Explosion Tests

An approximate analysis method is proposed to determine the blast resistance of fiber-reinforced polymer (FRP)-strengthened masonry walls. The method relates the static to dynamic response by incorporating the strain rate effect on the material strength and a dynamic load factor for the applied peak load. Based on the method, 18 full-scale masonry walls reinforced with three different FRP systems were designed and subjected to field explosions, using charges of 27-ton TNT in one test and 5-ton TNT in the other. For each test, the walls were placed at three different standoff distances and orientations to the blast source. The response of the strengthened walls under blast was monitored by high-speed data acquisition systems. Post-test observations indicated no visible damage, crack, or debonding in any of the walls, thus confirming the effectiveness of the FRP retrofit technique in blast protection. The data presented are valuable for validation of analytical or numerical models.     

Modeling Concrete Masonry Walls Subjected to Explosive Loads

Concrete masonry unit walls subjected to blast pressure were analyzed with the finite element method, with the goal of developing a computationally efficient and accurate model. Wall behavior can be grouped into three modes of failure, which correspond to three ranges of blast pressures. Computational results were compared to high-speed video images and debris velocities obtained from experimental data. A parametric analysis was conducted to determine the sensitivity of computed results to critical modeling values. It was found that the model has the ability to replicate experimental results with good agreement. However, it was also found that, without knowledge of actual material properties of the specific wall to be modeled, computational results are not reliable predictors of wall behavior.     

Out-of-Plane Strengthening of Unreinforced Masonry Walls with Openings

Collapse of unreinforced masonry (URM) walls is the cause of many casualties during extreme loading events. The objective of this current research was to investigate effective and practical approaches for strengthening URM block walls with openings to resist extreme out-of-plane loads. Five full-scale masonry block walls were constructed. The walls had different opening configurations such as a single center window, one window off center, two windows, a wide window and a door. The walls were tested when subjected to uniformly distributed lateral load up to failure. The walls were then strengthened using carbon fiber-reinforced polymer laminate strips and then retested. The walls were set up in a vertical test frame and were subjected to cyclic out-of-plane distributed pressure using an airbag. Failure of the unstrengthened URM block wall was along the mortar joints. In the strengthened walls, failure occurred in the mortar joints as well as in concrete blocks near the carbon strips. The lateral load carrying capacity of the strengthened walls was found to be significantly higher than that of the unstrengthened walls and had much more ductile performance.     

Collapse of Masonry Buttresses

This paper investigates the collapse of masonry buttresses under concentrated lateral loads. A fracture forms at the collapse state, significantly decreasing the resistance to overturning. Conventional analysis assumes that a masonry buttress acts monolithically to resist lateral loads. The current paper demonstrates that this approach is clearly unsafe, and the possibility of a fracture at the collapse state must be considered in the design and assessment of masonry buttresses. By treating the masonry as a continuum, infinitely strong in compression, with no resistance to tension and no possibility for sliding, the writers demonstrate the form of the fracture and determine the critical failure load for typical buttress forms. This approach follows in the tradition of limit analysis of masonry structures as developed by Heyman. General methods are proposed for the overturning analysis of masonry buttresses, and calculation examples are provided. Finally, methods for evaluating the safety of existing buttresses are presented and discussed.     

Resistance of Membrane Retrofit Concrete Masonry Walls to Lateral Pressure

This paper first describes the current state of analysis for the response of unreinforced concrete masonry walls subjected to lateral uniform pressure. The formulation is based on the initial elastic response, the subsequent initiation of cracks and the nonlinear rocking response, and the eventual large displacement and potential collapse. The necessary equations are developed for these phases in the form of a resistance function. The paper then incorporates membrane retrofit materials to strengthen the wall’s resistance to lateral pressure, and develops the necessary resistance function equations. In blast tests, membrane retrofit unreinforced masonry walls have experienced severe cracking and large displacements without collapse. This is of high interest to the Department of Defense, the protection of diplomatic facilities, and the construction industry impacted by hurricanes and other high wind events. The paper concludes with examples that demonstrate application of membrane retrofits indeed increase the resistance of the wall to lateral pressure.     

Experimental Response of Externally Retrofitted Masonry Walls Subjected to Shear Loading

Recent earthquakes have produced extensive damage in a large number of existing masonry buildings, demonstrating the need for retrofitting masonry structures. Externally bonded carbon fiber is a retrofitting technique that has been used to increase the strength of reinforced concrete elements. Sixteen full-scale shear dominant clay brick masonry walls, six with wire-steel shear reinforcement, were retrofitted with two configurations of externally bonded carbon fiber strips and subjected to shear loading. The results of the experimental program showed that the strength of the walls could be increased 13–84%, whereas, their displacement capacity increased 51–146%. This paper presents an analysis of the experimental results and simple equations to estimate the cracking load and the maximum shear strength of clay brick masonry walls, retrofitted with carbon fiber.     

Modeling the Nonlinear Behavior of Concrete Masonry Walls Retrofitted with Steel Studs under Blast Loading

This paper presents the results of an analytical investigation of one-way unreinforced masonry (URM) walls retrofitted with externally anchored steel studs and subjected to blast loads. Using the wall geometrical and material properties, deflected shape, and crack pattern as input, a nonlinear model is developed to predict the inward force-displacement relationship of the retrofitted walls. In addition, using a rigid body analysis, a simple bilinear force-displacement relationship is developed to model the outward force-displacement relationship of the walls. Utilizing these two force-displacement relationships (resistance functions), a generalized single-degree-of-freedom (SDOF) model is developed to capture the nonlinear out-of-plane dynamic response of the retrofitted walls under blast loads. The SDOF model captured the experimentally observed displacement responses of the tested walls with reasonable accuracy. The model was also used to investigate the influence of block thickness, wall slenderness ratio, blast load intensity, and blast pulse shape on the out-of-plane dynamic response of retrofitted walls. The results demonstrated that anchored steel-stud systems could significantly enhance the out-of-plane capacity of the retrofitted walls by increasing their out-of-plane capacity and reducing their displacement.     

Cellular Automata Model for Predicting the Failure Pattern of Laterally Loaded Masonry Wall Panels

This paper introduces a technique that directly predicts the failure patterns of laterally loaded masonry panels based on the results of existing typical panels tested in the laboratory. The technique is based on the use of the cellular automata (CA). In this technique, the CA modeling is established to propagate boundary effects to zones within a panel. The corresponding rules for the state values of zones are derived from the proposed CA model, using appropriate transition functions. These state values are then used by the CA to establish zone similarity between two panels. Finally, the zone similarity is applied to establish locations of cracks on the panel. The technique is used in a novel way that eliminates the use of any numerical tools such as finite-element analysis (FEA). This technique is purely based on comparing the failure pattern of the base panel (a panel whose failure pattern is known from the laboratory tests) and unseen panels (panels not tested in the laboratory by the writers or with unknown failure patterns) subjected to the same type of loading and with similar boundary conditions to predict the failure load of the unseen panels.     

Response of Curved Sandwich Panels Subjected to Blast Loading

Curved sandwich panels with two aluminium face sheets and an aluminium foam core under air blast loadings were investigated experimentally and numerically. Specimens with two values of radius of curvature and different core/face sheet configurations with the same projected area were tested for three blast intensities. All four edges of the panels were fully clamped. The experiments were carried out by a four-cable ballistic pendulum with corresponding sensors. The impulse acting on the front face of the assembly, the deflection history at the center of the back face sheet, and the strain history at some characteristic points on the back face were obtained. Then the deformation/failure modes of specimens were classified and analyzed systematically. The commercial software LS-DYNA was employed to simulate those physical processes. The finite-element (FE) model was validated by the data from experiments. Detailed deformation and energy dissipation mechanisms were further revealed by the FE models. The valuable experimental data and results from FE models show that the initial curvature of a curved sandwich panel changes the deformation/collapse mode with an extended range for bending-dominated deformation mode, which suggests that the performance of the sandwich shell structures slightly exceeds that of both their equivalent solid counterpart and a flat sandwich plate in certain blast intensity ranges.     

Behavior and Design of Commercial Multistory Buildings Subjected to Blast

The behavior of nonmilitary buildings subjected to blast is considered. Case studies from World War II are described, as well as more recent events from the detonation of large vehicle borne devices in the Middle East, North America, and Europe. Conventional methods for nonseismic design are shown to lead to frames with overstrong beams connected together by relatively weak connections. This may explain much of the evidence from bomb damaged buildings in which building connections have been observed to fracture in a brittle manner when subjected to blast. The risk of progressive collapse may be minimized by strengthening beam to column connections located at close proximity to potential vehicle borne devices and a capacity design method for such strengthening is advocated.     

Analysis of Dynamic Response of Architectural Glazing Subject to Blast Loading

The effect of blast loading on civilian structures has received much attention over the past several years. The behavior of architectural glazing is of particular interest owing to the disproportionate amount of damage often associated with the failure of this component in a blast situation. This paper presents the development of a simple yet accurate finite element-based tool for the analysis of architectural glazing subjected to blast loading. This has been achieved through the creation of a user-friendly computer program employing the explicit finite-element method to solve for the displacements and stresses in a pane of glass. Both monolithic and laminated panes have been considered, in single and insulated unit configurations, and employing several types of glass. In all cases, the pane of glass has been modeled as a plate supported by an array of boundary conditions that include spring supports, and two failure criteria are employed. Furthermore, the program is designed to predict the hazard level, given a particular glazing configuration and blast load.     

Vulnerability Screening and Capacity Assessment of Reinforced Concrete Columns Subjected to Blast

In this study, a nonlinear model is developed to study the response of blast-loaded reinforced concrete (RC) columns. The strain rate dependency and the axial load and P?Δ effects on the flexural rigidity variation along the column heights were implemented in the model. Strain rate and axial load effects on a typical RC column cross section were investigated by developing strain-rate-dependent moment-curvature relationships and force-moment interaction diagrams. Analysis results showed that the column cross section strength and deformation capacity are highly dependent on the level of strain rates. Pressure-impulse diagrams were developed for two different column heights with two different end connection details (ductile and nonductile) and the effects of the axial load on the column midheight deflection and end rotation at failure were evaluated for both connection types. Based on the results of this study, a pressure-impulse band (PIB) technique is proposed. The PIB technique presents a useful tool that covers practical uncertainties associated with RC column reinforcement details as well as possible increase of column axial loads resulting from different blast-induced progressive collapse scenarios. Finally, the uses of the PIB technique for vulnerability screening of critical infrastructure or postblast capacity assessment of RC columns of target structures are presented.     

Use of High-Efficiency Energy Absorbing Device to Arrest Progressive Collapse of Tall Building

The World Trade Center collapse has brought attention to progressive collapse of tall buildings and the study of possible countermeasures. From the viewpoint of energy transfer, this analysis explains why the collapse could not stop by itself once began. By introducing a design parameter called collapse stability index that controls design against progressive collapse, it is found that conventional design of a tall building usually leads to an inherently unstable structure in the event of a progressive collapse. In a subsequent feasibility study in this paper, a heavy-duty metal-based honeycomb energy absorbing structure is proposed. Using a finite element analysis, it is demonstrated that the structure is capable of absorbing potential energy released in a tall building collapse. The added energy absorbing devices will only occupy a small percentage of the total floor space. By properly designing and installing such devices, a progressive collapse, should it happen in a tall building, may be arrested within a few floors, and hence, the building is inherently stable to the progressive collapse. The theory is also elaborated with the example of the World Trade Center collapse.