The structural design of glass curtain walls and facades is a challenging issue, considering that building envelopes can be subjected extreme design loads. Among others, the soft body impact (SBI) test protocol represents a key design step to protect the occupants. While in Europe the standardized protocol based on the pneumatic twin-tire (TT) impactor can be nowadays supported by Finite Element (FE) numerical simulations, cost-time consuming experimental procedures with the spheroconical bag (SB) impactor are still required for facade producers and manufacturers by several technical committees, for the impact assessment of novel systems. At the same time, validated numerical calibrations for SB are still missing in support of designers and manufacturers. In this paper, an enhanced numerical approach is proposed for curtain walls under SB, based on a coupled methodology inclusive of a computationally efficient two Degree of Freedom (2-DOF) and a more geometrically accurate Finite Element (FE) model. As shown, the SB impactor is characterized by stiffness and dissipation properties that hardly match with ideal rigid elastic assumptions, nor with the TT features. Based on a reliable set of experimental investigations and records, the proposed methodology acts on the time history of the imposed load, which is implicitly calibrated to account for the SB impactor features, once the facade features (flexibility and damping parameters) are known. The resulting calibration of the 2-DOF modelling parameters for the derivation of time histories of impact force is achieved with the support of experimental measurements and FE model of the examined facade. The potential and accuracy of the method is emphasized by the collected experimental and numerical comparisons. Successively, the same numerical approach is used to derive a series of iso-damage curves that could support practical design calculations.
In current blast enhancement design strategies, to resist the effects of an accidental explosion, a facade system is commonly designed to behave in-elastically and undergo large deformations. The large deformation of the facade system leads to high blast energy dissipation, subsequently reducing the blast energy transferred to the main structure. In addition to the blast resistance of the facade system, human injuries due to glass fragmentation within the vicinity of the facade system should also be minimized in order to meet the required safety levels. Overall building safety can be optimized by balancing blast energy dissipation and glass fragmentation. Recently, Permasteelisa Group has developed an innovative design tool to optimize blast-enhanced facades using an equivalent MDOF approach. A novel fragmentation tool has been proposed to assist this design procedure. This paper presents various critical parameters considered in blast-enhanced facade analysis, the experimental validation of these parameters and their influence in the design optimization process.
Design requirements for bomb blast protection are a challenge for façade engineers because of complex interaction of various elements with different mass and nonlinear stiffness. There are different mitigation techniques available depending on the hazard rating. Rigid protecting structures are neither accepted by architectural design demands, nor by occupants who do not want to be affected by obvious protection in daily life. As a result, smart blast enhanced façade solutions that cannot be distinguished from conventional facades are required, being capable of providing the required safety level. With introduction of dissipative façade brackets the dynamic analysis of the MDOF system of the façade can be balanced due to beneficial inertia effects. The dissipative bracket attracts the blast wave energy in lieu of the glazing, so that a switch of the typical load chain becomes possible. The probability that the glazing remains in uncracked state increases while the dissipative bracket dissipates the energy. In addition, the reaction to the primary structure can be mitigated. Finally, the anchor channel that connects the bracket to the concrete slab can be designed to stay in the elastic range. This reduces the refurbishment costs after a blast event. Experimental test results of different crash absorbing materials, dissipative brackets, and anchor channels are used for the development of combined design charts for the selection of adequate brackets and anchor channels.
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