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In order to study the progressive collapse resistance of plate-member composite reticulated shell structure and propose an applicable analysis method, the complete and member removal experimental models of composite reticulated shell structure were designed by Alternate Path (AP) analysis method, and the progressive collapse resistance test and numerical analysis were carried out. The results show that the honeycomb plate contributes greatly to the bearing capacity of the composite structure, the ultimate bearing capacity of member removal structure is 3.64% smaller than that of the complete structure, and the ultimate displacement is 30% smaller. The average error of load-displacement curves between the progressive collapse resistance test and numerical analysis is less than 10% in the elastic stage, and the overall failure mode of the numerical analysis model and the experimental model is consistent. The anti-progressive collapse analysis method based on modal sensitivity is suitable for the plate-member composite reticulated shell structure due to high computational efficiency and accuracy, which provides a reference for progressive collapse resistance analysis of plate-member composite reticulated shell structure.

This paper presents the behavior of six tests of planar prestressed concrete frames under the loss of a middle column. The six tests consist of two non-prestressed reinforced concrete (RC) specimens and four prestressed concrete (PC) specimens with bonded post-tensioning tendons (BPT). The structural response of the specimens with different flexural reinforcement ratio, span-depth ratio, and effective prestress level has been reported. In addition, the impact of parabolic BPT on the behavior of RC frames to resist progressive collapse is also evaluated. Experimental results indicated that the BPT cannot only increase the initial stiffness and yielding load of the RC counterparts, but also increase the ultimate load capacity in the catenary action stage. Moreover, it will impact the load-resisting mechanisms and the failure modes. Contrary to the commonly accepted sequential mobilization of compressive arch action and catenary action to resist progressive collapse of RC frames, no effective compressive arch action is developed in PC frames to mitigate progressive collapse risk. Based on experimental observations, it is found that higher effective prestress in BPT results in enhanced initial stiffness and yielding load but less deformation capacity and ultimate load capacity. It is also found that higher non-prestressed flexural tensile reinforcement ratio could improve the behavior of PC specimens to resist progressive collapse. Keywords: bonded; catenary action; compressive arch action; mechanism; post-tensioning tendon; prestressed concrete; progressive collapse.

This study investigates the dynamic response and failure mechanism of the polyline-shaped large-span double-layer grid space structures subjected to progressive collapse. A grid model was designed and fabricated to represent a typical area of a large-span double-layer grid space structure from a specific engineering project. Three representative locations were selected to simulate failure of the test model, and dynamic collapse tests were conducted. In the tests, four conditions were considered: D1 (120 kg, failure at A), D2 (120 kg, failure at B), D3 (120 kg, failure at C), and D4 (200 kg, failure at C). The dynamic response of the structure under various conditions was studied by comparing strain, displacement, and failure patterns derived from the test analysis. Furthermore, the collapse process and mechanism of the structure were analyzed. The results indicate that the upper chord rods are key components in collapse resistance design. Under test conditions D4, significant vertical displacement occurred, and out-of-plane deformation increased markedly after the lateral constraints were removed, causing the structure to tilt towards the side without a failure device. The strain and displacement changes were most significant under test conditions D3 and D4, especially near the failure locations. Under condition D3, the strain change is 1109 microstrain larger than that before the failure, with the maximum vertical displacement increase being 59.109 mm. Under condition D4, the strain change is -1126 microstrain larger than that before the failure, with the maximum vertical displacement increase being 74.795 mm. Through multi-condition testing, the collapse mechanisms at different failure locations in the structure were clarified. The failure of web members and lower chord rods led to a redistribution of internal forces, but the effect on the double-layer grid structure was minimal. After the failure of the upper chord rods, significant displacements occurred near the failure location, and buckling of surrounding members was observed.

The casualties and economic loss in historic events have revealed that progressive collapse performance of buildings has to be evaluated in structural design to prevent such disastrous events. Integrity and resilience are important characteristics for buildings to prevent total collapse or disproportionate collapse once an unpredictable terrorism event unfortunately occurs. Compared to the extensive studies on behavior of cast-in-place reinforced concrete (RC) buildings for progressive collapse resistance, there is less research on precast concrete (PC) buildings to mitigate progressive collapse. Thus, in this study, three one-story, two-bay large-scale frame-floor subassemblies (one RC and two PC) are tested under pushdown loading regime to investigate the effect of PC floor units and transverse beams on progressive collapse resilience of PC moment-resisting frames. It is found that the PC beams and slab systems could provide substantial compressive arch action and compressive membrane action, similar to the cast-in-place RC buildings. However, as PC slabs are discontinuous, insignificant tensile membrane action is able to develop in PC slab systems and the ultimate load capacity in enormous deformation stage is mainly attributed to the catenary action developed in PC beams. Keywords: load-resisting mechanism; monolithic connection; precast concrete; progressive collapse; resilience.

An approach to improve the progressive collapse resistance of conventional reinforced concrete (RC) frame structures was put forth by using unbonded post-tensioning strand (UPS). Two UPSs with straight profiles were mounted at the bottom of the beam section. A static loading test was conducted on an unbonded prestressed RC (UPRC) beam-column subassemblage under a column removal scenario. The structural behaviors of the test specimen such as load-carrying capacity, failure mode, post-tensioning force of the UPSs, and reinforcing bar strain were captured. By analyzing the results of the tested substructure, it was found that the compressive arch action (CAA) and catenary action (CTA) were sequentially mobilized in the UPRC subassemblage to avert its progressive collapse. The presence of UPSs could significantly improve the load-carrying capacity of conventional RC structures to defend against progressive collapse. Moreover, a high-fidelity finite element (FE) model of the test specimen was built using the software ABAQUS. The FE model was validated by experimental results in terms of the variation of vertical load, horizontal reaction force, and post-tensioning force of the UPSs against middle joint displacement (MJD). Finally, a theoretical model was proposed to evaluate the anti-progressive collapse capacities of UPRC subassemblages. It was validated by the test results as well as the FE models of the UPRC subassemblages, which were calibrated using the available experimental data. Keywords: finite element (FE) model; progressive collapse performance; reinforced concrete (RC) frame structure; theoretical model; unbonded post-tensioning strand (UPS).

This paper presents an experimental program on structural behavior of four reinforced concrete frames under column removal scenarios, simulating progressive collapse. The specimens were designed with conventional non-seismic and seismic detailing in terms of stirrup arrangement and different boundary conditions. Each specimen, consisting of a two-bay beam, a middle joint, and two side columns, was quasi-statically tested by increasing the beam deflection until the complete failure. The load-deflection relationships show the sequential mobilization of compressive arch action and catenary action in the beams. Test results indicate that beam-column connections are the most critical components in developing catenary action, and confirmed the concern in current engineering practice that the longitudinal reinforcement in beams may fail to function as effective ties due to fracture of bars under large rotations. The bar fracture was ascribed to local rotations at the connections heavily dependent on the development of fixed-end rotation.

Background The widespread use of large-space structures has led to a subsequent increase in the demand for the inspection and monitoring of engineering structures. Digital construction of engineering structures poses a challenge to conventional measurement methods. The development of noncontact measurement methods based on computer vision and photogrammetry technologies has made these measurements possible. Objective In this study, cable tension and progressive collapse processes of suspen-dome structure were investigated using unmanned aerial vehicle (UAV)-assisted close-range photogrammetry and multi-camera stereo-digital image correlation (stereo-DIC). Methods Based on the principles of close-range photogrammetry, three-dimensional (3D) points reconstructed by a digital single-lens reflex (DSLR) camera and camera mounted on UAV are registered, and the global coordinate system is established. In the cable tension process, a stereo-DIC system enhanced by parallel computing was used to monitor the displacement of the local structure in real time, and the tension process was accurately controlled using real-time monitoring data. The UAV was then used to measure the whole-field static displacement of the upper control points after tension. Finally, the progressive collapse displacement monitoring of the structure is realized by a multi-camera stereo-DIC system comprising 12 high-speed cameras, and the coordinate system of the 12 subsystems is unified to the established global coordinate system. Results The results indicate that the displacement sensitivity of a multi-camera stereo-DIC system is higher than 0.05 mm. The measurement results show that the structure meets the design index after tension. The static displacement of the nodes before and after tension can be measured accurately based on the UAV and close-range photogrammetry, which directly reflects the overall deformation trend of the structure after tension. The collapse test results indicate that the structure collapsed quickly after slow deformation for approximately 2.5 s. Conclusions The UAV-assisted close-range photogrammetry and multi-camera stereo-DIC system accurately captured the 3D displacement data of both cable tension and progressive collapse processes. The accurate measurement of these data has a great value for engineering applications, model tests, and numerical analysis.

Twelve specimens representing reinforced concrete frame beams were tested to investigate their gravity load-carrying capacity against progressive collapse. In these tests, the beams within the frame subassemblies were restrained longitudinally against axial deformation. The tests indicated that the compressive arch action due to longitudinal restraint can significantly enhance the flexural strength of a beam subjected to vertical loads. The compressive arch action was observed to be a function of flexural reinforcement ratio and ratio of beam span to depth. The test results validated an analytical model that has considered the axial restraining effects on beam loading capacity. The application of compressive arch effect to the prevention of progressive collapse is discussed. [PUBLICATION ABSTRACT]

The catastrophic events of September 11, 2001, stand out as a major motivation for research on improving the understanding of structural behaviour in fire. These events included the first complete collapse of a tall steel framed structure solely due to fire. World Trade Center 7 (WTC7) was a 47-storey office building within the WTC complex that collapsed due to a fire initiated by debris from the collapse of WTC1. In the following years, detailed investigations were carried out by expert teams to pinpoint the cause of the progressive failure of WTC7. Each of the expert teams analysed the fire and structure and made varying conclusions with regards to the mechanisms responsible for initiating and propagating the collapse of the building. This paper revisits the collapse of WTC7 and its investigation, and then explores the hypothesis that a potential hydrocarbon fire may have compromised the large transfer structure within the mechanical space of the building. This is done via two OpenSees finite element models. The first model explores the thermomechanical response of the mechanical floors to a potential diesel fire, and the second investigates the response of the structure to a failure caused by that fire. The outcome of the analyses shows that it is feasible that a mechanical room fire could lead to a failure in the transfer structure, which would then result in the loss of support to at least two columns within the building core. The failure of these columns may unbrace the eastern-most core columns and precipitate in the failure of the structure as observed on 9/11.

Previous studies on reinforced concrete (RC) beam-column subassemblies under a column removal scenario are helpful to understand the load-resisting mechanisms of RC structures against progressive collapse, but most of these studies failed to simulate actual boundary conditions, which were simplified as fixed boundaries to allow sufficient development of the load-resisting mechanisms. These studies were unable to reflect the response of joints and side columns under progressive collapse. To fill this gap, an experimental program on six half-scale beam-column subassemblies with joints and side columns was designed and tested to fully understand the effects of boundary conditions on the structural behavior of RC planar frames against progressive collapse. Three subassemblies were specially designed, while the other three were ordinarily designed to quantify the benefits of special detailing. The test results show that the effects of boundary conditions on the development of load-resisting mechanisms are marginal, whereas the effects of special detailing are significant. Specifically, specimens under a middle-column removal scenario and a penultimate-column removal scenario develop similar compressive arch action (CAA) capacities and catenary action (CA) capacities. The CAA capacity dominates the load resistance of specimens with ordinary detailing. In contrast, the CA capacity governs the load resistance of specimens with special detailing mainly due to the larger areas of longitudinal reinforcing bars and the greater rotation capacities of beam ends. However, boundary conditions can greatly affect the failure mode of specimens with ordinary detailing. Finally, an analytical study was performed to demonstrate the contributions of axial force and shear force to load resistance. According to test results and analytical analyses, RC frames with special detailing have sufficient rotational capacity to develop adequate tie forces to resist progressive collapse.