Journal of Performance of Constructed Facilities

Extensive research has been conducted to investigate the blast effects on building structures and the protective design methods using the fiber-reinforced polymer (FRP) strengthening concepts in resisting structural damage and preventing injuries against dynamic explosive impacts. Both numerical and experimental studies have proved the effectiveness of FRP in strengthening structures to resist blast loads. However, problems related to end anchorage, bond length, and premature peeling have been concerns when strengthening structures in flexure or shear using FRP. In this paper, numerical analyses of FRP-composite-strengthened RC walls with or without additional anchors are carried out to examine the structural response under blast loads. The results illustrated that an anchor system is often necessary when using external FRP laminates for strengthening RC walls to prevent premature peeling. This study presents three simulations of RC walls, namely, an unstrengthened RC wall, an FRP-composite-strengthened RC wall with end anchorage, and an FRP-composite-strengthened RC wall with both end anchorage and anchors applied at a minimum spacing across the width and height of the RC wall. Commercial software LS-DYNA is used to carry out the structural response analysis. Numerical results show that anchorage of the FRP sheet may prevent peeling damage and therefore enhances the capacity of the FRP-strengthened RC walls against blast loads. However, anchors result in stress concentration and may cause FRP rupture.

This paper presents the results of experimental and numerical studies of the comparative behavior of square hollow section (SHS) tubes filled with rigid polyurethane foam (RPF) and concrete undergoing transverse impact loading. A series of instrumented drop hammer tests were performed on mild steel and stainless steel SHSs for both filled and unfilled constructions. The concrete-filled tubes had the highest impact resistance and energy absorption capacity, followed by the steel tubes filled with RPF, and then the hollow tubes. The results also show that RPFs can be used as an effective infill material in structural steel hollow columns when expedient enhancement of the energy absorption capacity is required, e.g., to increase blast and impact resistance of hollow structural elements. Nonlinear dynamic finite-element analyses were carried out to simulate drop hammer test conditions. The predicted impact forces, deformation histories, and failure modes were found to be in good agreement with the experimental results.

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.

The strain growth phenomenon is an unsolved fundamental problem in the dynamic elastic response of containment vessels subjected to internal blast loading, which may seriously influence the design of multiple-use containment vessels. Because strain growth may cause increased local structural deformation in multiple-use containment vessels, it is important to consider this phenomenon when designing multiple-use containment vessels. In this paper, the mechanisms of strain growth, its influencing factors, and control methods are presented. Based on the understanding of the strain growth phenomenon, the guidelines and procedures for the design of multiple-use containment vessels are proposed. Employing the design guidelines, application examples are presented to demonstrate how to control strain growth in containment vessels. Because the strain growth phenomenon has not been considered in the design method or design code for multiple-use explosion containment vessels, understanding of the strain growth mechanisms and the proposed design guidelines may contribute to the safe design of containment vessels.

The mode approximation method (MAM) is an effective approach when analyzing dynamic structural responses and damages to blast loading. The pressure-impulse (P-I) diagram generated from the MAM provides an efficient and accurate estimation of structural damage of simple or combined failure modes. In the previous works for damage assessment of underground reinforced concrete structures, the soil-structure interaction (SSI) was simplified as a constant damping or stiffness effect, and the pulse shape effect of the blast load on the P-I diagrams was not discussed. In the present study, a generalized iteration procedure is adopted in derivation of the P-I equations on the basis of the MAM; therefore, the nonconstancy of SSI and the pulse shape effect on underground RC structure damage to blast load can be analyzed. Result of the present study shows that the nonconstancy of SSI and the pulse shape effect has certain effects, and they cannot be simply ignored in damage assessment of buried structures. The generalized iteration procedure is also proved a helpful tool in carrying out such analysis.

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.

Annealed glass is by far the most commonly used glazing material in building construction. The resistant properties of glazing panels to wind loading has been studied for a long time based on quasistatic modeling. More recently, the vulnerability of infrastructure and its protection against acts of terrorism has been receiving a great deal of attention, as pioneered by the work of Wu and Hao from within Australia, whereas vulnerability studies by Stewart were specific to glass façades in terrorism scenarios. Emphasis has been on modeling the blast induced pressure and impulses, as they are applied to the panels while the resistant properties of the glazing materials have been based on assumptions used currently in design. Research undertaken by the writers over the past two to three years have revealed scopes of possible improvements to the current models that have been used in defining the resistant properties of glazing materials. Models based on the well known Weibull distribution, although widely assumed in practice, have been found to be not fully consistent with evidence produced experimentally or by analytical simulations. The interaction of the principal stresses surrounding the crack tip and higher mode effects have also been found to have significant effects on the ultimate behavior of the panel. The accurate modeling of these phenomena is particularly important for large panels. This paper provides an update of the redevelopment of a realistic behavioral model for annealed glazing panels subject to impact action. A new simulation procedure that takes various described phenomena into account is presented in this paper.

In some terrorist attacks, it is possible that RC structures might be subjected to more than a single explosion. RC structures designed without the consideration of blast effects tend to lose their capacity after the first explosion. The use of a fiber reinforced polymer (FRP) sheet has been proven to enhance the performance and resistance of an RC member under a single explosion test. However, there appears to have been no experimental programs conducted to assess the performance of FRP-strengthened RC members subjected to multiple explosions reported in the literature. This paper, therefore, presents experimental results for the behavior of RC slabs strengthened by an FRP sheet after undergoing single, double, and triple independent explosion testing. Results from these blast tests indicate that the FRP sandwich RC slab tested was able to sustain the subsequent second explosion of greater impact. A brittle shear failure with FRP debonding was observed following the third explosion on this FRP-strengthened RC slab.

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.

This research study examines the use of a precast concrete panel system for blast protection of facilities with exterior light gauge metal stud walls. The structural retrofit is designed for the specific case where internal operation of the facility cannot be interrupted. To meet this design requirement, a series of precast concrete panels are installed exterior to the building envelope with connections to the foundation at ground level and to the steel building frame at upper floor levels. To validate the retrofit concept, two explosive detonations representing relatively low and high blast threat levels are examined. An exterior insulation and finishing system (EIFS) clad stud wall and a precast concrete protected stud wall are examined under each demand level. The measured responses of both systems are compared with each other and with basic dynamic predictive models. In addition, a finite element study of the connection is conducted to estimate support demands for the blast retrofit. The research results show that the precast wall system provides effective protection of the exterior wall. The research also shows that EIFS clad metal stud wall systems retain significant resilience under blast demands. The dynamic responses of the systems are predictable using standard elastic-plastic dynamic modeling assumptions.

Aluminum foam is a newly developed mobile and lightweight material with excellent energy absorption capacities. Applying aluminum foam as a sacrificial protection layer on the bearing faces of protected structures can mitigate blast effects on the resistance capacities of structures against impact or blast loading. The aluminum foam undergoes great plastic deformation under transient dynamic loads before becoming fully densified, making it excellent for mitigating blast effects on these structures. In this paper, we conducted quasi-static testing on two types of aluminum foam specimens and obtained the primary parameters for the mechanical properties of aluminum foam specimens. We then used these two types of aluminum foams to protect the reinforced concrete (RC) slabs, and we conducted a series of tests to investigate the performance of the aluminum foam–protected RC slabs against blast loads. We tested a total of five foam-protected slabs and one control RC slab in the blast test program. The test results, including displacement and acceleration histories, performance of specimens, and maximum and permanent deflections, were fully reported. We then discussed the efficiency of aluminum foam to mitigate blast loads on protected RC slabs.