Damage Mechanism And Performance Assessment Of Concrete-filled Double-skin Steel Tubular Columns Subjected To Blast Loads

The number of terrorist bombing attacks has been increased dramatically all around the world over the past few decades,which results in an increasing threat on bridge structures from being subjected to such terrorist attacks.On the other hand,with the rapid development of economic and urbanization,accidental explosion events that caused significant damages to bridge structures have been more and more frequently reported around the world.The bombing attacks and accidents will not only cause tremendous casualties but also lead to severe damage of bridge structures nearby the source of explosion which will further intensify the severity of such disaster.Pier columns are the key load-bearing members in bridge structures,and are probably the most vulnerable structural components for protection against bomb threats.Failure of critical columns can lead to partial or even global collapse of bridge structures and loss of functionality.However,there is still a lack of design specifications and guidelines with regards to blast-resistant and protective design on bridge structures.Consequently,the response analysis and protective techniques of bridge structures against blast loadings have gained increasing attentions in the recent past and show immense social and economic significance.Past studies have demonstrated that concrete-filled double-skin steel tubes(CFDSTs)are a promising multihazard-resistant structural system exhibiting desirable performance for different types of extreme events such earthquake,impact and fire.However,the current knowledge on the blast-resistant performance of CFDST columns is still limited.The objective of this research is to understand the dynamic response,damage mechanism and residual load-bearing capacity of CFDST columns subjected to close-in and contact explosions through experimental and numerical studies.The primary work and achievements are summaried as follows:(1)The terrorist bombing threats to bridge structures were classified and the corresponding TNT equivalency for blast-resistant design of bridges against terrorist bombing attacks was proposed.Air burst explosion test was conducted on CFDST columns and the results indicated that the peak overpressures on the square column were generally greater than those on the circular columns.The blast load on the column surface generated by cubic TNT explosive charge cannot be accurately predicted by the CONWEP model.A high-fidelity physics-based(HFPB)numerical model,which consists of a CFDST column,TNT explosives and surrounding air domain,was developed utilizing the Arbitrary-Lagrangian-Eulerian(ALE)formulation coupled with Fluid-Structure Interaction(FSI)algorithm available in the nonlinear dynamic analysis program LS-DYNA.The HFPB model was then used to simulate the air explosion test and the obtained results were compared with the test results.It was demonstrated that the developed numerical model can reasonablely simulate the propagation of the shock wave from the high explosive through air and its interaction with the column.(2)An experimental program was carried out to study the blast-resistant performance of CFDST columns.The tests were performed in two phases: Firstly,field blast tests on column specimens under close-in and contact explosions were conducted.In the second phase,axial compression tests were conducted on the blast-damaged columns and the residual axial bearing capacities of the columns were evaluated.The results from blast tests demonstrated that the damage phenomenon is predominantly local and global response is negligible in case of both close-in and contact explosions.The axial compression tests demonstrated that the blast-damaged columns can still retain a high performance under axial compressive loading.The contact explosion with a significantly small amount of explosive mass can initiate more severe damage in a CFDST column as compared to close-in explosion.Columns with a circular cross-section exhibit better blast-resistant performance,residual axial bearing capacity and ductility in comparison to that having square cross-section.(3)A HFPB numerical model was developed with the nonlinear dynamic analysis program LS-DYNA and was validated against the experiment results.With the HFPB model,the dynamic response,damage mechanism and failure modes of CFDST columns subjected to close-in and contact explosions were further investigated.The results indicated that the typical failure mode of CFDST columns under close-in explosion is dominated by localized denting of the cross-section directly facing the explosion with minor global deformation of the column.Under contact explosion,the typical failure mode of CFDST columns is cratering around the detonation point,fracture or rupture failure of the outer steel tube,and breaching failure in the front sandwich wall.(4)The energy absorption mechanism of the CFDST columns subjected to close-in and contact explosions was analyzed with the numerical simulation.The results showed that the concrete core dissipated a significant portion of the blast energy imparted to the CFDST columns under close-in and contact explosions,which demonstrated that the severe plastic damage of concrete core plays a significant role in the energy absorption mechanism of CFDST columns under close-in and contact explosions.The confinement effect provided by the steel tubes can help to fully develop the energy absorption capacity of concrete.In addition,the presence of the outer steel tube can effectively prevent the concrete spall damage thus mitigating the potential damage to personnel and equipment caused by ejected fragments.(5)A four-stage analysis procedure was proposed for the evaluation of the residual axial load-bearing apacity of the damaged CFDST columns under close-in and contact explosions and was validated against the test results.Parametric studies were conducted to investigate the effect of critical parameters on the residual axial load-carrying capacity of CFDST columns after close-in and contact explosions.The results demonstrated that increasing the cross-section area and structural steel ratio of CFDST columns can significantly improve the blast resistance as well as post-blast residual axial capacity of CFDST columns.The existing of axial compressive load can be beneficial to improve the redidual axial load-carrying capacity of CFDST columns subjected to close-in explsions,while it has limited influence on the blast performance of CFDST columns under contact explosions.(6)Based on the parametric analysis results,empirical formulae were derived with multivariable regression fitting method for evaluating the residual axial load-carrying capacity of CFDST columns after being subjected to close-in and contact explosions,respectively.The empirical formulae can be used for rapid assessment of the residual axial load-carrying capacity of blast-damaged CFDST columns after close-in and contact explosions.The obtained results can help to facilitate the adoption of CFDST columns in blast-resistant and multi-hazard resilient bridge designs.