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When a steel frame is subjected to an abnormal load, a sudden local failure due to the loss of a load carrying member may result in progressive collapse of the entire structure. In this study, experiments and numerical simulations were performed to investigate the anti-collapse performance of double half-span assemblies (a column connected with two half-span beams) with welded unreinforced flange–bolted web (WUF-B) connections in a column removal scenario. Three types of bolted webs—two bolts in one row, three bolts in one row, and four bolts in two rows—were considered. For the post-peak response, the connections exhibited a significant resistance recovery owing to the catenary action. The bolt-bearing areas on the beam web were prone to fracture owing to excessive compression, and the failure modes of the web varied significantly with the bolt arrangement. The test results demonstrate that increasing the number of bolt rows is a fairly effective approach for enhancing the anti-collapse performance of the WUF-B connections. Numerical models were used to further investigate the effects of the bolt arrangement on the anti-collapse performance of the WUF-B connections. The results indicate that the bolt arrangement can directly affect the response of the connections after the first peak load, especially in the catenary phase. [ABSTRACT FROM AUTHOR]

Although the reduced beam section (RBS) connection is an excellent seismic design for transferring plastic hinge, it is prone to fracture under the condition of large deformation. In addition to good seismic response, the performance of RBS connections needs to be improved to ensure good anti-progressive collapse response. In this study, a novel principle is proposed, in which the RBS connection is used to meet the plastic hinge transmission, and kinked reinforced bars are installed in the beam–column connection to realize a second path. Experimental and numerical investigations were conducted on a frame substructure with kinked reinforced bars, considering their diameter d , kinked height a and length l of the reinforced bars. The deformation capacity, vertical force vs. deformation response, failure mode, and contributions of the flexural and catenary actions on the reinforced bars were evaluated. The test results demonstrate that the kinked reinforced bars started to work when the reduced-beam-section connection with reinforced bars (RRBS) is in a plastic state. When the reinforced bars with a > 1.5 d were installed at the beam end, the bearing capacity of the RRBS specimens increased with an increase in the diameter d and decreased with an increase in the length l of the reinforced bars. In addition, When the cracking first occurred at the reduced section, the bearing capacity decreased with an increase in the kinked height a. It is suggested to select the kinked height of the reinforced bar according to the deformation capacity of the beam. • Tests on a novel principle with kinked reinforced bars are reported. • Specimens include bars with various diameters, kinked heights, and lengths. • Various fracture and failure modes occurred in the connection types. • The effect of the reinforced bars on catenary is proved to be considerable. • The kinked height should match the deformation capacity during beam cracking. [ABSTRACT FROM AUTHOR]

• Tests on a novel principle with kinked reinforced bars are reported. • Specimens include bars with various diameters, kinked heights, and lengths. • Various fracture and failure modes occurred in the connection types. • The kinked height should match the deformation capacity during beam cracking. • The plastic zone range and installation space should be considered for the length. Various disasters may trigger a progressive collapse of frame structures, resulting in a high demand for new preventive structural strategies. Among the strategies adopted, increasing the joint rotation capacity to improve catenary action and forming a second path in the joint area are the most effective. In this study, a novel principle is proposed, in which the web opening is used to meet the rotation capacity, and kinked reinforced bars are installed in the beam–column joint to realize a second path. Experimental and numerical investigations were conducted on a frame substructure with kinked reinforced bars, considering their diameters, kinked heights, and lengths. The deformation capacity, vertical force vs. deformation response, failure mode, and contributions of the flexural and catenary actions on the reinforced bars were evaluated. The test results demonstrate that the kinked reinforced bars in the reduced-web-section connection (RRWS) specimens started to work after the plastic state. Although they had little influence on the flexural action, they were very important to the catenary action. When the kinked height a was less than 2 d , the bearing capacity and ductility of the specimen increased with a , but further increasing the kinked height a beyond 2 d would reduce both the bearing capacity and the ductility. In addition, the failure modes of the specimens were related to whether the plastic hinge was formed first or the reinforced bar had a major role first. Therefore, it is suggested to select the kinked height of the reinforced bar according to the deformation capacity of the beam, and the diameter of the bar as half of the kinked height. [ABSTRACT FROM AUTHOR]

• The paper reports tests on specimens with castellated steel beams. • Specimens include various opening diameter, spacing, and opening position. • Various modes of fracture and failure took place in the connection types. • Larger spacing of openings has minor influences on mechanical properties. • The recommended position of the first opening is equal to the beam height. In addition to strengthening or weakening the connections, castellated steel beams can realize plastic hinge rotation, which is a common strategy for improving the resistance of steel structures against progressive collapse. This paper presents experimental and numerical investigations of a substructure with castellated steel beams under a column removal scenario considering the diameter D of the openings, first opening position A , and spacing B between openings. The deformation capacity, vertical force–deformation response, failure mode, and contributions of the flexural and catenary action were studied as having different parameters for the openings. The test results demonstrate that the first web opening underwent the greatest deformation, changing from a circular to shuttle shape at the cracking moment, whereas the second and subsequent openings exhibited only small deformations in all specimens. The vertical force vs. deformation response can be divided into four states: elastic, plastic, catenary transition, and catenary states. The final resistance against progressive collapse was dependent on the catenary action caused by excessive axial tension. Moreover, the bearing capacity and deformations in the specimens having a small diameter of openings, D < 0.6 h , decreased with an increase in the opening diameter, whereas the respective properties in the specimens having a large diameter of openings, D > 0.6 h , increased with an increase in D. The recommended distance from the edge of the first opening to the surface of the column is equal to the depth h of the beam section. A large value of spacing B between the openings has a minor influence on the bearing resistance in the collapse analysis. [ABSTRACT FROM AUTHOR]

A link element incorporating a nonlinear axial-shear-flexural (PVM) yield function is established for idealized beams with reduced web section (RWS), focusing on the static response when a column loss is triggered. The proposed model is considered in terms of the lowest level of model reduction, that is, axially restrained beams subjected to mid-span point loading (MPL) with boundary springs. A monotonic loading test was conducted on the perforated beams to calibrate the proposed model. The main features of the new approach include the consideration of the multi-surfaces concept and an anisotropic hardening rule is employed in the plastic hinge. The proposed model can predict the load–displacement relationship before the failure limit. The internal force evolution of the inner hinge is analyzed to determine the change in the load transfer mechanism of the RWS connection with rectangular openings. The numerical results show that the traditional equivalent rectangular opening method should be improved to reflect the large deformation response of the circular web opening beams. The proposed model can offer a practical tool for the progressive collapse design of steel frames with web opening beams under column removal scenarios. [Display omitted] • PVM link element for perforated beams. • Incorporates axial-shear-flexural yield function. • Anisotropic hardening rule • Equivalent rectangular web opening method [ABSTRACT FROM AUTHOR]

In this study, beam–column connections with various web openings were prepared to improve the mechanical characteristics of welded steel frames. First, several specimens were constructed and tested after adding various web openings, including circular, square, hexagonal, semicircular, and elliptical web openings. We mainly focused on analysing failure modes, deflection shapes, load–displacement responses, and deformation capacities. The height, width, and position of the web openings were further studied through numerical simulations following experimental verification, and the results were compared for specimens with and without various forms of web openings. The catenary internal energies and contribution coefficients of the specimens with web openings were found to be greater than those of the base specimens; the values of the hexagonal and semicircular web openings were greater than those of commonly used circular openings. The bearing and deformation capacities initially increased and then decreased with increasing opening height. When only the opening distance b was changed, the load–displacement responses of the specimens overlapped in the elastic state; the bearing and deformation capacities increased with increasing b. We also found that the vertical loads of circular or near-circular web openings were smaller than those of polygonal openings with fewer edges. • Beam–column substructures with web openings are analyzed. • Progressive loading is applied to various test specimens. • Finite element models are developed based on the test results. • Various web opening parameters are analyzed in terms of vertical load. • Circular and nearly-circular openings are determined to be suboptimal. [ABSTRACT FROM AUTHOR]