A NEW PREDICTIVE EQUATION FOR PUNCHING SHEAR STRENGTH OF REINFORCED CONCRETE FLAT SLABS BASED ON NUMERICAL PARAMETRIC STUDY

The paper proposes a new equation for the prediction of punching shear strength of reinforced concrete flat slabs. The basis of the new predictive equation is a detailed numerical parametric study conducted using the nonlinear 3D finite element analysis using FE software MASA. For this, results of the previously tested flat slabs from literature are used as reference for validation of the numerical model. The numerical modelling procedure is validated with two previously tested slabs, one failing in pure punching prior to yielding of flexural rebar, and the second failing in flexure-punching which resulted in yielding followed by punching. The result shows that the load-displacement behavior, failure modes and the crack pattern are captured well by the analysis. Following the validation, a detailed parametric study is performed to investigate the influence of slab depth, concrete strength, longitudinal reinforcement ratio, column size and effect of reinforcement spacing. From the evaluation of results, it is observed that the punching resistance increases with slab depth but at a decreasing rate (size effect). The punching shear strength also increases with increasing longitudinal reinforcement ratio, concrete strength as well as the column area. All the reinforcing bars placed within a distance of 3.5 times the effective depth of the slab from the column center contributes significantly towards dowel action. With increasing column size, the deformation at the peak load also increases. Based on the evaluation of the results of the analyses, an empirical equation for the prediction of punching shear is derived. The results of the equation are compared with the results of a large experimental database of 235 tests, and it is shown that the proposed equation leads to better agreement with the test results compared to the equations given in the current codes (ACI, Canadian, Eurocode, Japanese code). The comparison shows that generally the predictions by existing equations in the codes tend to be unconservative for large slab with low reinforcement ratio

Size effect in anchorage behavior

The results of the numerical analysis for pull-out tests on headed anchors with axis-symmetric finite elements and nonlocal microplane model for concrete are presented. Based on the results of the numerical analysis and Bazant's size effect law a failure load formula is derived and compared with empirical failure load equations. Numerical results are in good agreement with experimental evidence and indicate a strong size effect which should be taken into account in current design practice

Fastening elements in concrete structures - numerical simulations

Anchoring elements such as headed and expansion studs and grouted or undercut anchors, are often used for local transfer of loads into concrete members. In order to better understand the failure mechanism, a large number of experiments have been carried out in the past. However, due to the complicated three-dimensional load transfer a very few or no numerical studies have been performed for a number of different fastening situations i.e. influence of the embedment depth, crack-width inftuence (fastening in cracked concrete), influence of the edge distance etc. Therefore, in the present study some results of the axisymmetric and three-dimensional numerical analysis of the headed studs embedded in plane concrete block are presented. Influence of different geometrical and material parameters have been studied employing finite element method and nonlocal microplane model. Comparison between experimental and numerical results indicate reasonable good agreement. Generally it has been observed that the failure mechanism is governed by fracture energy rather than by tensile strength of concrete. As a consequence, the size effect is strong and close to linear elastic fracture mechanics

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

Influence of crack width on the concrete cone failure load

In the present paper the influence of the crack width on the concrete cone failure load of headed anchors embedded in concrete is analyzed. The analysis is carried out on a reinforced concrete thick plate specimen using three-dimensional finite elements and the non local microplane model. In order to introduce precracking into the specimen before loading the headed anchor, the specimen is loaded in longitudinal direction by applying tension forces through reinforcement. At different crack levels pull-out of the fastening element is performed. Results of the analysis show that the concrete cone failure load is decreasing with increasing crack width up to ω ~ 0.15 mm to approximately 70 % of the failure load obtained for non-cracked concrete. Further increase of the crack width does not cause further decrease of the failure load. Comparison between numerical and experimental results indicates good agreement

Numerical analysis of headed studs embedded in large plain concrete blocks

Anchoring elements such as headed studs, expansion, grouted or undercut anchors are used for local transfer of loads into concrete members. Parameter study of the behavior of headed stud anchors with embedment depth h v= 130 mm and failing by pulling out a concrete cone, is performed through numerical analysis. Compression and tension strength, fracture energy and the head diameter are varied. Numerical analysis is performed using nonlocal microplane model and axisymmetric finite elements. Results of the analysis are compared with experimental results

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 häufig eingesetzt, um Lasten in Stahlbetontragwerke einzuleiten. Versuche zeigen, dass das Versagen bei ausreichend hoher Stahltragfähigkeit durch die Ausbildung eines Betonausbruchkegels hervorgerufen wird. Die Bruchlast hängt nach den Ergebnissen der numerischen Untersuchungen hauptsächlich von der Betonbruchenergie ab. Es wird diskutiert, dass in diesen Anwendungsfällen die Betonzugfestigkeit in Anspruch genommen werden kann, ohne dass ein Sicherheitsrisiko besteht