Experimental Research And Analysis On The Vertical Collapse Mechanism Of Reinforced Concrete Spatial Frame Structures

Some accidental loadings occurring on the structures during the whole structures’service period, namely explosions, terrorist attacks, fire and impact, are of short durationand catastrophic damage. Once occurs, it will lead to progressive collapse of the structures,causing serious casualties and great loss of properties. To carry out the study on theprogressive collapse performance of the structures is of greatly academic, economic andsocial significance. As one basic progressive collapse formation, the vertical progressivecollapse subject is attracting lots of scholars’ interests. Reinforced concrete frame structurewas taken as the research subject, with universality, contemporaneity and necessity.Five key subjects on the vertical collapse of the RC spatial frame structure are carriedout via the numerical simulation, experimental verification and theoretical analysis in thepaper. The five key subjects include the development of the microcomputer controlhydraulic loading device involved in the experiment, to reveal the failure mechanism ofvertical progressive collapse, the anti-slump performance of the cast-in-situ slabs, thetheoretical method for lateral deformation of the vertically still column and the theoreticalmethod for the critical displacement of the removed column, to reveal the restraint effect ofcast-in-situ beams and slabs on the removed column, to reveal the effect of cast-in-situbeams and slabs on slump influenced area. The results obtained from the researches arediscussed as follows.1. The microcomputer control hydraulic loading device was developed to lay thefoundation for the vertical progressive collapse experiment of RC spatial frame structure.Based on the practical engineering, the experimental model was established in considerationof the similarity test theory. Based on the numerical simulation, the loading system anddetection programs were finally set up in consideration of the purpose of the experiment.2. The whole vertical collapse process of RC spatial frame structure was identified andclassified, including extrapolation phase (arch action stage), adduction phase (catenaryaction phase) and total collapse phase (the stacked cake style collapse). The singleparameter index of the collapse damage D was proposed to quantitatively assess ofstructural collapse damage status with accuracy and speed, and to provide solid foundationfor the computational model of the damaged structure. The effect of the cast-in-situ slabs on the beams hinge was quantitatively analyzed, and it’s effect varies with the specificconstraints condition of slabs on the beams. The concepts about the collapse，namely theslumping area and the slump influenced area, were raised. The details about the collapsewere identified, laying bases for the confirmation of the numerical modal. The mechanismfor the anti-slump performance of the cast-in-situ slabs was elaborated. That is to say that onthe extrapolation phases the constraints effect of the cast-in-situ slabs on the removedcolumn is significant, and on the adduction phases the slabs play the part of tension on theremoved column. The tension, namely catenary action, is relative with the slumping area.3. The theoretical method for lateral displacement of the vertically still column wasraised, and was check by the experimental results. The theoretical computational model withsimplified method for the critical displacement of the removed column was presented, withthe accuracy of the formula verified by the experimental results. Based on experimental andtheoretical results, the evaluation method for the ratio of the vertical displacement to thespans was proposed, with the vertical displacement corresponding to the ratio of the verticaldisplacement to the spans of0.2confirmed as the critical slump displacement. The structuralmeasures to improve structural resistance to progressive collapse were presented in detail.4. From the prospect of the force-displacement and the energy, the restraint effect ofthe cast-in-situ slabs and beams on the removed column was analyzed respectively. Fromthe prospect of the force-displacement, the relationship between the force (the restrainteffect of beams and slabs on column) and displacement (the vertical displacement) waselaborated based on specific cases. The restraint effect on the columns was compared withdifferent cases. From the prospect of energy the original concept, namely the anti-slumpenergy, was put forward. Based the obtained results, the constraint effect of the cast-in-situslabs on the column is remarkable.5. The effect of cast-in-situ slabs and beams on the lateral displacement anddeformation modes of the vertically still columns on the slump influenced area waselaborated as follows. The existence or not and the locations of the cast-in-situ slabs havefine influence on the lateral displacement of the vertically still columns. On the initial stageof the collapse, the extrapolation exhibits on the symmetry direction. However, theextrapolation was skipped and the structure elapses into the adduction phase on the non-symmetry direction. The slump influenced areas generally involve two spans from theslumping beam. Peripheral components can significantly mitigate the lateral deformation ofthe vertically still columns. The vertically still columns on the non-symmetry area exhibitedrelatively less displacement. The lateral deformation of the vertically still columns on two- dimensional framework varies greatly with that on three-dimensional framework. Thestrength of the material had little effect on lateral deformation of the vertically still columns.The innovation of this paper is discussed as follows.1. The failure mechanism for vertical progressive collapse of RC spatial framestructures and the mechanism for the anti-slump performance of the cast-in-situ slabs wereelaborated; meanwhile the single parameter index of the structure collapse damage D wasput forward. What’s more, the effect of cast-in-situ slabs on the plastic hinges wasquantified.2. The theoretical method for lateral deformation of the vertically still column and themethod for the critical displacement of the removed column were raised respectively, andthe ratio of displacement to the span evaluation method can be utilized to evaluate thedamaged status with speed and accuracy.3. The effect of cast-in-situ slabs and beams on the lateral displacement anddeformation modes of the vertically still columns on the influenced area and the removedcolumn was elaborated respectively. Meanwhile, the anti-slump energy concept was raised.