Concrete structural elements subjected to severe dynamic loads such as explosions at close range may cause shear failures. In the Oklahoma City bombing in 1995 two concrete columns on the ground level were reported to have failed in shear. Such shear failures have also been reported to occur in several experimental investigations when concrete beams and slabs were subjected to blast or impact loads. The dynamic shear mechanisms are not yet fully understood and it is therefore of research significance to further investigate these mechanisms. The main objective of the research presented in this thesis is to experimentally and theoretically analyse shear failures of reinforced concrete elements subjected to uniformly distributed dynamic loads.

The experimental work consisted of concrete beams of varying concrete grades and reinforcement configurations subjected to blast loads. One series involved testing of steel fibre reinforced concrete (SFRC) beams and the other series involved tests with concrete beams reinforced with steel bars. The former investigation showed that SFRC beams can resist certain blast loads. In the latter investigation, certain beams subjected to blast loads were observed to fail in flexural shear while the same beams exhibited flexural failures in the static tests. Such shear failures specifically occurred in beams with relatively high reinforcement contents. With these experiments as reference, numerical simulations with Ansys Autodyn were performed that demonstrated the ability to predict flexural shear failures.

A direct shear failure mode has also been observed in experiments involving concrete roofs subjected to intense distributed blast loads. In several cases, the roof slabs were completely severed from their supporting walls along vertical or near-vertical failure planes soon after the load had been applied. Theoretical analyses of the initial structural response of beams subjected to distributed loads were conducted with the use of Euler-Bernoulli beam theory and numerical simulations in Abaqus/Explicit. These analyses show that the initial structural response consists of shear stresses and bending moments developing at the supports. The remaining parts of the beam will be subjected to a rigid body motion. Further simulations with Abaqus shows that that dynamic direct shear failure appears to be due to a deep beam response with crushing of the compressive struts at the supports, and therefore differs from a static direct shear mode. The results also showed that parameters such as element depth, amount of reinforcement, load level and load duration played a role in developing a dynamic direct shear failure.

Byggnadselement i betong utsatta för stora dynamiska laster som explosioner på nära håll kan förorsaka skjuvbrott. I bombådet i Oklahoma City 1995 rapporterades att två betongpelare i marknivå gick till skjuvbrott. Sådana skjuvbrott har observerats i flera experimentella undersökningar med betongbalkar eller plattor som utsattes för explosionslaster eller anslag från fallande föremål. Mekanismerna bakom dynamisk skjuvning är ännu inte helt klarlagda och det är därför av intresse att utforska dessa mekanismer. Huvudsyftet med forskningen i föreliggande avhandling är att experimentellt och teoretiskt analysera skjuvbrott i armerade betongelement utsatta för jämnt utbredd dynamisk last.

Den experimentella delen av forskningen bestod av betongbalkar med varierande betonghållfasthet and armeringsutformning utsatta för explosionslaster. En försöksserie omfattades av stålfiberarmerade balkar och den andra av betongbalkar med armeringsstänger. Den förra undersökningen visade att de fiberarmerade balkarna kan bära en viss explosionslast. I den senare undersökningen observerades att de balkar som utsattes för explosionslast och gick till böjskjuvbrott medans samma balkar gick till böjbrott i de statiska försöken. Skjuvbrotten uppstod i balkar med relativt höga armeringsinnehåll. Dessa balkar användes senare som referensbalkar för numeriska simuleringar med Ansys Autodyn där simuleringarna visade på möjligheten att förutsäga böjskjuvbrott.

Även direkt skjuvning har observerats i experiment med betongtak utsatta för höga explosionslaster. I flera fall separerades taken från de stöttande väggarna längs vertikala eller nära vertikala brottytor kor tid efter pålastningen. Teoretiska analyser av den tidiga strukturresponsen för balkar utsatta för utbredda laster genomfördes med Euler-Bernoulli balkteori och numeriska simuleringar med Abaqus/Explicit. Dessa analyser visar att den initiala strukturresponsen består av skjuvspänningar och böjande moment som uppstår vid stöden. Områdena på balken från nära stöd mot balkmitt rör sig i form av en stelkropp. Vid ytterligare simuleringar med Abaqus förefaller ett dynamiskt direkt skjuvbrott vara resultatet av en respons likt en hög balk med krossning av de tryckta strävorna vid stöden, och därmed skiljer sig från statisk direkt skjuvning. Resultaten visar även att balkhöjd, armeringsinnehåll, lastnivå och lastens varaktighet är parametrar som påverkade utvecklingen av ett dynamiskt direkt skjuvbrott.

Shear failures in reinforced concrete structures under intense dynamic loads are brittle and limit the structure's energy-absorbing capabilities. This paper comprises a review of the literature dealing with the problem of dynamic shear of reinforced concrete elements, with a focus on parameters that control flexural shear and direct shear. In this context, dynamic loads refer to intense events due to explosions and impacts. For this reason, the initial response is also highlighted. Experimental investigations and calculations show that shear force and bending moment distributions in dynamic events are initially significantly different from the distributions under slowly applied loads. Therefore, structural wave propagation, geometrical properties of elements, strain rate effects and dynamic load characteristics need to be considered when analysing shear. The review also indicates that arch action in the shear span soon after the load has been applied has a large influence on the shear capacity of an element. This action is of particular importance in intense loading events. Finally, suggestions for further research are identified.

Numerical analysis is a suitable tool in the design of complex reinforced concrete structures under extreme impulsive loadings such as impacts or explosions at close range. Such events may be the result of terrorist attacks. Reinforced concrete is commonly used for buildings and infrastructures. For this reason, the ability to accurately run numerical simulations of concrete elements subjected to blast loading is needed. In this context, reliable constitutive models for concrete are of capital importance. In this research numerical simulations using two different constitutive models for concrete (Continuous Surface Cap Model and Brittle Damage Model) have been carried out using LS-DYNA. Two experimental benchmark tests have been taken as reference. The results of the numerical simulations with the aforementioned constitutive models show different abilities to accurately represent the structural response of the reinforced concrete elements studied.

The structural behaviour of concrete beams subjected to air blast loading was investigated. Beams of both high-strength concrete (HSC) and normal-strength concrete (NSC) were subjected to air blasts from explosives in a shock tube and for reference were also loaded statically. Concrete with nominal compressive strengths of 40, 100, 140, 150 and 200 MPa were used and a few beams also contained steel fibres. Furthermore, beams with two concrete layers of different strength were tested. All beams subjected to static loading failed in flexure. For some beam types, the failure mode in the dynamic tests differed from the failure mode in the corresponding static tests. In these cases, the failure mode changed from a ductile flexural failure in the static tests to a brittle shear failure in the dynamic tests. Beams without fibres and with high ratio of reinforcement exhibited shear failures in the dynamic tests. It was observed that the inclusion of steel fibres increased the shear strength and the ductility of the beams. The investigation indicates that beams subjected to air blast loading obtain an increased load capacity when compared with the corresponding beams subjected to static loading.

The results from this investigation demonstrate the ability to perform numerical simulations of dynamic structural response of concrete elements subjected to air blast loading. Beams of both high-strength concrete (HSC) and normal-strength concrete (NSC) were studied. Also beams with two concrete layers of different strength were simulated. It is of particular interest to investigate the use of material models for implementation with software for the explicit analysis of non-linear dynamic events. The influences of concrete strength, amounts of reinforcement, the bond between concrete and reinforcement, bi-linear strain softening of concrete, the strain rate dependence of reinforcement and boundary conditions at the supports were studied. The simulations were performed with the text data as reference through comparison between numerical examples and experimental test results. It was possible numerically to analyse the dynamic behaviour of beams tested in situ and to describe the observed failure modes of these beams. The analysis tool will be used for evaluating the dynamic strength of future protective structures of HSC, possibly with parts consisting of NSC elements.

This paper involves testing of steel fibre reinforced concrete(SFRC) beams subjected to static and dynamic loads. Thedynamic load was generated by a detonating explosive charge.The work focused upon studying the mechanical behaviour ofthe beams. The concrete compressive strength varied between36 MPa and 189 MPa with a fibre content of 1.0 percent byvolume. Two different fibre lengths having constant length-todiameterratio were employed. The experimental results indicatethat the toughness is reduced when increasing the compressivestrength and the dynamic strength is higher than thecorresponding static strength.