Resistance Of Planar Steel-framed Substructures In Falling-debris-impact Scenario

Progressive collapse of building structures result in serious consequence,i.e.,heavy casualties,huge property damage and negative social influence.To date,many studies have been carried out to improve the progressive collapse resistance of building structures.However,most studies on progressive collapse based on the column-removal scenarios.Limited studies focus on the falling-debris-impact scenario,which may be a primary reason causing progressive collapse of structures.The falling-debris-impact scenario assumes that the structural and non-structural elements drop down due to abnormal load and hit the remaining structures below.In the two well-known events,i.e.the partial collapse of Ronan Point and the disaster of World Trade Center,the falling-debris impact is the critical reason that caused the final collapse.Thus,it is important to carry out corresponding studies to investigate the resistance of structures subject to falling-debris impact.This thesis focuses on the impact resistance of steel framed structures under falling-debris-impact scenario,the primary contents are shown as follows:(1)Preanalysis for design of impact tests was carried out using numerical simulations.The finite element models of steel beam with fin-plate connections were established using ANSYS/LS-DYNA.The accuracy of finite element models was validated against previous experimental results.Based on the verified numerical models,the dynamic behaviour of steel beam with fin-plate connections were investigated by parametric analyses.Six groups of numerical simulations were conducted to study the influence of different parameters,i.e.,impact energy,impacted location,strength of material,imposed load,distance of bolt holes and thickness of fin plate,on dynamic behaviours including energy absorption,maximum displacement,distribution of plastic deformation and internal force in steel beams and joints.The numerical simulations show that impacted location siginificantly affects the impact resistance.The numerical results provided suggestion on choosing parameters for the design of impact tests.(2)Steel framed specimens with five types of beam-column connections were tested to investigate the dynamic response and resistance under falling-debris impact.The five connections included Welded Unreinforced Flange-Bolted web connections(WUF-B),Reduced Beam Section connection(RBS),Fin Plate connection(FP),Reverse Channel connection with Flush End Plate(RC-FEP)and Reverse Channel connection with Extended End Plate(RC-EEP).Impact loads were applied by a drop weight impact testing machine.Mid-span of the steel beam was chosen as the loading position under the greatest flexural damage.A typical impact process was divided into 3 stages according to impact time-histories and displacement time-histories obtained from the experimental tests.Energy conservation and momentum conservation were employed to calculate peak impact force and duration of impact process.According to load-resisting mechanism,the applied load was primarily resisted by flexural action.Catenary action effectively provided load-carrying capacity under large deforamation.Most of impact energy was absorbed by flexural deformation.(3)Five beam-end-impact tests were carried out using the specimens with the same geometric size and beam-column connections as the mid-span-impact tests.The dynamic behaviours of the specimens with various beam-column connections were obtained from the beam-end-impact tests.Load-resisting mechanism under beam-end impact was proposed according to the beam-end-impact tests.Dynamic response,failure modes and impact resistance under various impacted locations were comparing based on experimental results.Changing the impacted location from mid-span to beam-end,the load-carrying capacities of specimens increased while the deformation capacity of specimens decreased except specimen with fin-plate connections.Both the load-carrying capacity and the deformation affected the energy absorption capacity of specimens.The failure modes of specimens showed flexural and tensile modes under mid-span impact and showed shear modes under beam-end impact.(4)Finite elements models of all the 10 experimental specimens were established by ANSYS/LS-DYNA.The accuracy of the finite element models was validated against to the impact tests.Using the verified models,the dynamic responses of steel framed specimen were compared between single impact and multiple impacts with identical impact energy.The results show that single impact is the most dangerous condition comparing to multiple impacts.The finite element models of RC-EEP were employed to carry out parametric analyses.The effect of an important design parameter,Span-Depth Ratio(SDR)of beam,on impact resistance was investigated under both the mid-span impact and the beam-end impact.Failure displacement,maximum load-carrying capacity and energy absorption of specimens with various SDRs were compared with unchanged steel beam section.In both the mid-span-impact scenario and the beam-end-impact scenario,both the load-carrying capacities and the energy absorption capacities decreased with the increase of SDR.Smaller SDR was suggested in the anti-impact design.(5)A simplified approach to effectively assess impact resistance was proposed based on specimen with reverse channel connection with extended end plate.In the simplified approach,energy transfer,energy conservation,axial-bending state of specimens under large deformation were and plastic flow law were taken into consideration.The accuracy of the proposed approach was validated against the experimental and numerical results.