In the design of structures to resist the effects of air blast loading or other severe dynamic loads it is vital to have large energy absorbing capabilities, and structural elements with large plastic deformation capacities are therefore desirable. Structures need to be designed for ductile response in order to prevent partial or total collapse due to locally failed elements. The research in this thesis considers experimental and theoretical studies on concrete beams of varying concrete strength. The nominal concrete compressive strength varied between 30 MPa and 200 MPa. A total of 89 beams were tested of which 49 beams were reinforced with varying amounts of tensile reinforcement. These beams were also reinforced with stirrups and steel fibres were added to a few beams. The remaining 40 beams were only reinforced with steel fibres with a fibre content of 1.0 percent by volume. Two different fibre lengths having constant length-to-diameter ratio were employed. The tests consisted of both static and air blast tests on simply supported beams. The blast tests were performed within a shock tube with a detonating explosive charge. All experimental research focused on deflection events, failure modes and loads transferred to the supports. The dynamic analyses involve single-degree-of-freedom (SDOF) modelling of the beam response and the use of iso-damage curves. Also, the dynamic support reactions were calculated and compared with test results.

For beams with tensile reinforcement, the failure mode of some beam types was observed to change from a flexural failure in the static tests to a flexural shear failure in the dynamic tests. Beams with a high ratio of reinforcement and not containing steel fibres failed in shear, whereas beams with a lower ratio of reinforcement failed in flexure. The introduction of steel fibres prevented shear cracks to develop, thus increasing the shear strength of the beams. The presence of steel fibres also increased the ductility and the residual load capacity of the beams. Beams subjected to air blast loading obtained an increased load capacity when compared to the corresponding beams subjected to static loading. The SDOF analyses showed good agreement with the experimental results regardless of concrete strength and reinforcement amount. The results of using iso-damage curves indicate conservative results with larger load capacities of the beams than expected. The theoretical evaluations of the dynamic reactions were in agreement with the measured average reactions, both in amplitudes and in general shape.

The experimental results with steel fibre reinforced concrete beams indicate that the dynamic strength was higher than the corresponding static strength and that the toughness was reduced when increasing the compressive strength. Beams of normal strength concrete failed by fibre pull-out while a few beams of high strength concrete partly failed by fibre ruptures. It may be favourable to use shorter fibres with smaller aspect ratios in structural elements of high strength concrete and subjected to large dynamic loads.

Further research should involve studies on the size effect, on different boundary conditions, on different types of structural elements and on the combination of blast and fragment loads. The theoretical work should involve analyses both with the use of SDOF modelling and finite element analysis.

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.