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Study on Damage Effects and Assessments Method of Reinforced Concrete Structural Members under Blast

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Tutor: LuFangYun
School: National University of Defense Science and Technology
Course: Mechanics
Keywords: Blast load,Reinforced concrete structure,Dynamic response,Damagemodes,Single deg
CLC: TU375
Type: PhD thesis
Year:  2012
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Abstract:
Damage effects analysis and assessment of buildings under blast loading is animportant problem concerned by the area of explosion accident analysis, blast-resistantdesign, anti-terrorist and military weapon design. When designing of blast-resistantbuildings or assessing and analyzing of damage effects of buildings under blast loading,the relationship of blast damage effects and explosive source should be found first. Themost often used method is analysing the data obtained in the blast scene and validatednumerical simulation to find the relationship of blast damage effects and explosivesource. But due to the work related to blast mechanics, dynamics of structures andcompute mechanics and so on related contents, damage effects analysis and assessmentis a complex and time-consuming work. This made the related assessment workdepended on few special men to do. So there is much need to find a fast and exactassessment method to accomplish this work, and the related assessment method is ofdirect military value in weapon damage efficiency and forecast.Several main problems in research of dynamic response and damage mechanism ofbuilding structures under blast loading are studied in this dissertation£®They are:simulmion of interaction between blast wave and structural element and derivation ofthe formulae to estimate blast loads acting on the elements; dynamic response anddamage modes of reinforced concrete (RC) structural members under blast loading;single degree of freedom (SDOF) method of structure elements under blast loading andthe corresponding pressure-impulse (P-I) damage evaluation method. The primary workand achievements are as follows£º(1) Simulation of blast wave propogation and its interaction with structuralmembers are studied. A method to simulate the interaction between blast wave and aalone structural members is established using Hydrocode AUTODYN£®Parametricstudies are carried out to study the influence of structure parameters on the blastwave-element interaction and the blast loads acting on different points of the structuralmembers£®It is found that the width of the structural element has a positive effect on theimpulse of the blast loads acting on the structural element£®However, it has nosignificant effect on the peak pressure of the blast loads on the structural element. Basedon the numerical results of the parametric studies, some formulae are proposed toestimate the blast overpressure and impulse of the characteristic points on the frontsurfaces of any standalone structural elements. A common method is established toestimat the blast loads¡¯ parameters of any points on the structural element.(2) The damage mechanism of RC structure member under close-in blast loading isinvestigated by experiments. The damage modes and damage levels of RC slabs and beams are studied under different blast loads. The results show that one-way RC slabsare prone to be damaged by flexure and spallation on the back surface of the slabs, andthe spallation area increases with the increase of the scaled distance. The concretebeams are prone to be damged in flexure with concrete crushed on the front face,concrete spllsation on the back surface and concrete flake off on the side surface. Thescaling of the dynamic response of one-way square reinforced concrete slabs and beamssubjected to close-in blast loadings are also studied. The test results show that themacrostructure damage and fracture in the experiments are almost with similarity. Butthe local damage of concrete slabs with smaller specimen has been reduced a little ascompared with that of concrete slabs with larger specimen. Based on the results,empirical equations are proposed to correct the scaling model of center deflection toheight ratio when scaling the results from the model up to the prototype.(3) A method for simulating the dynamic response and damage of RC slabs underblast loading is established. The damage mechanism and damage features are alsostudied by numerical simulation, and an emperial damage criterion is established fordifferent damage levels of concrete slabs. The results show that the initial compressiveshock wave generated by the blast passes through the concrete slab and is reflected offthe free surface, the shock wave is converted into a tensile wave, resulting in high levelsof cracking and spallation damage. The damge modes are changed from intial wholeflexural damge to local punch shear failure in the center of the slabs. The damge modesof the slabs under uniform blast loading are also studied. The results indicated that whenthe slab is subjected to impulsive blast load, the slab is inclined to be damaged by shear;however, the slab is likely damaged by flexural mode in the quasi-static region; and inthe region of dynamic loading, the failure of the slab might be a combination of shearand flexural damage.(4) The single degree of freedom (SDOF) method of concrete structute membersunder close-in blast loading is studied. A new effective model for calculation of theequivalent uniform blast load for non-uniform blast load such as close-in explosion ofRC strucutre members is proposed. The model is then validated using SDOF systemwith the experiments and blast tests for square slabs, rectangle slabs and beams. Twocoupled SDOF systems used to model the flexural and direct shear responses of RCmembers subjected to explosive loading are investigated. The equations of shear SDOFdynamic shear force of RC slabs under nonuniform blast loading are derived. The twocoupled SDOF systems are validated with numerical tests. The numerical results showthat the SDOF systems are accurate in predicting the failure mode of the RC membersunder blast loads.(5) Pressure-impulse (P-I) diagram methods with the two coupled SDOF systemsfor evaluate the damage degree of RC members under blast loading are studied£®The influence of blast load shapes and the RC structure parameters on the different damagelevel P-I curve quasi-static asymptote and impulse asymptote is investigated. The resultsshown that the blast load shape influences the pressure-impulse shape in the dynamicdamage region for all damage levels; the curves of rectangular load are the lowest in thediagrams and the curves of exponential load are the highest. The impulsive asymptoteand the quasi-static asymptote are almost the same for the three blast load shapes. Thevalues of flexural damage P-I curve¡¯s impulsive asymptote and the quasi-staticasymptote decrease with the increasing of members¡¯ length, however the values of sheardamage P-I curve¡¯s impulsive asymptote and the quasi-static asymptote increase withthe increasing of members¡¯ length. The values of both flexural and shear damage P-Icurve¡¯s impulsive asymptote and the quasi-static asymptote increase with the increasingof the concrete strength and reinforced ratio. Based on the results, an empirical equationof the P-I curves of different damage modes and blast load shapes is established and asimplified numerical method to generate P-I diagram for RC members is proposed£®
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