Josko Ozbolt

Measurement of time-dependent strains of concrete: Prepared by subcommittee 4: Standardized test methods for creep and shrinkage

1359-5997/98

THREE-DIMENSIONAL FE ANALYSIS OF HEADED STUD ANCHORS EXPOSED TO FIRE

In the present paper a transient three-dimensional thermo-mechanical model for concrete is presented. For given boundary conditions, temperature distribution is calculated by employing a three-dimensional transient thermal finite element analysis. Thermal properties of concrete are assumed to be constant and independent of the stress-strain distribution. In the thermo-mechanical model for concrete the total strain tensor is decomposed into pure mechanical strain, free thermal strain and load induced thermal strain. The mechanical strain is calculated by using temperature dependent microplane model for concrete (Ožbolt et al., 2001). The dependency of the macroscopic concrete properties (Young’ s modulus, tensile and compressive strengths and fracture energy) on temperature is based on the available experimental database. The stress independent free thermal strain is calculated according to the proposal of Nielsen et al. (2001). The load induced thermal strain is obtained by employing the bi-parabolic model, which was recently proposed by Nielsen et al. (2004). It is assumed that the total load induced thermal strain is irrecoverable, i.e. creep component is neglected. The model is implemented into a three-dimensional FE code. The performance of headed stud anchors exposed to fire was studied. Three-dimensional transient thermal FE analysis was carried out for three embedment depths and for four thermal loading histories. The results of the analysis show that the resistance of anchors can be significantly reduced if they are exposed to fire. The largest reduction of the load capacity was obtained for anchors with relatively small embedment depths. The numerical results agree well with the available experimental evidence.

Dynamic Fracture of L and CT Concrete Specimens: Numerical and Experimental Studies

To confirm the findings of recent numerical studies and to obtain the experimental evidence on dynamic fracture of concrete, experimental tests were performed on Land CTspecimens. The experiments fully confirmed the results of previously performed numerical studies. It is shown that inertia effects are responsible for progressive increase of resistance, crack branching and rate dependent crack propagation. The presented, relatively simple, tests can be used to check whether numerical model is able to realistically predict complex phenomena related to dynamic fracture of concrete.

Dynamic Fracture of Concrete Compact Tension Specimen: Mesh Sensitivity Study

It is well known that without a proper regularization scheme the finite element analysis of softening materials leads to mesh dependent response. The results of the analysis should be as less as possible dependent on the choice of the finite element type and on the finite element discretization. In the present article the mesh sensitivity study of the compact tension specimen loaded under quasi-static and dynamic loading conditions is carried out. It is shown that relatively simple regularization scheme based on the element size independent energy dissipation (crack band method) leads to mesh insensitive results. In the analysis the constitutive law based on the rate sensitive microplane model for concrete is used. In the model the tensile response is based on the bi-linear crack-opening softening law. It is shown that, in spite of the simple regularization scheme, the rate dependent peak load, failure mode, crack branching and crack velocity are realistically predicted for different types and sizes of finite elements.

3D Smeared Fracture FE-Analysis of Concrete Structures: EDF Benchmark Examples

ABSTRACT In the present paper, a finite element code based on the microplane model for concrete is used for the analysis of recently proposed EDF benchmark examples. Four numerical cased are considered: resonnse of concrete constitutive law under uniaxial cyclic loading, mixed tension-shear test (Willams test) at the constitutive level, shear failure of reinforced concrete beam with and without shear reinforcement and mixed mode failure (Nooru-Mohamed test). In the numerical studies the three-dimensional eight node solid finite elements with eight integration points are employed. As a regularisation procedure the crack band method is used. The numerical results are compared with available experimental data that were only partly known before the study was performed.

Numerical simulation of cyclic bond-slip behavior

In the present paper the pull-out of deformed steel bar embedded in a concrete cylinder and pulled out by monotonic and cyclic loading is analyzed and discussed. The analysis is carried out by the use of axisymmetric finite elements and general microplane model for concrete. Current version of the model was not able to predict damage in shear due to cycling loading correctly. Therefore, the model is further improved and extended in a more general form. In the present numerical case study, instead of the classical interface element approach, a more general approach is used in which the geometry of the ribs of the deformed steel bar are exactly modeled. In the present numerical case study, the pull-out failure mechanism is analyzed and compared with experimental observations. The comparison indicate qualitatively good agreement. Predicted failure load is in good agreement with experimental results, however, calculated displacement are much smaller than measured in tests . The present approach is able to correctly predict the monotonic as well as cyclic behavior including friction and degradation of pull-out resistance caused by previous damage.

Use of the tensile strength in anchorage to concrete

Anchoring elements, such as headed studs, are used to transfer concentrated loads into reinforced concrete members. From expenmental evidence it is known that, provided the steel strength of the stud is high enough, failure occurs by pulling out a concrete cone formed by circumferential crack growth in the so-called mixed mode, with significant size effect. Numerical analyses indicate that the dominant influence factor on the failure load is the concrete fracture energy. It is shown that in such applications, the load transfer from the anchor into the concrete may safely be done by using the concrete tensile strength. Befestigungselemente wie Kopfbolzen werden haufig eingesetzt, um Lasten in Stahlbetontragwerke einzuleiten. Versuche zeigen, dass das Versagen bei ausreichend hoher Stahltragfahigkeit durch die Ausbildung eines Betonausbruchkegels hervorgerufen wird. Die Bruchlast hangt nach den Ergebnissen der numerischen Untersuchungen hauptsachlich von der Betonbruchenergie ab. Es wird diskutiert, dass in diesen Anwendungsfallen die Betonzugfestigkeit in Anspruch genommen werden kann, ohne dass ein Sicherheitsrisiko besteht.