Keywords: infill reinforced concrete frames, cyclic response of reinforced concrete frames, non-linear analysis.

The numerical cyclic response of reinforced concrete (RC) frames with and without masonry infills are evaluated using a commercial finite element method (FEM) package. The numerical results are compared and calibrated with experiments [1] carried out in the National Laboratory of Civil Engineering (LNEC), Lisbon, Portugal, to assess the accuracy of the distinct modelling of the highly non-linear behaviour of RC frames for large deformations. The test frame was subjected to a constant vertical load at the top of each column and a lateral increasing cyclic load/displacement pattern at beam level.

Numerical models of RC frames without infill are based on the inelastic hinge method and on the fibre model. Successively higher complexity of the hinge constitutive laws, permitted the verification of the suitability of hinge models in comparison with the experimental results. In this work four different models related with concentrated and distributed plastic hinges were considered [2,3]: bilinear Clough's model; tri-linear and tetra-linear Takeda's model, and also the fibre model.

To understand the performance of each model with respect to the bare RC frame, a comparison was made of the cumulative deformation energy stored in the structure. The fiber model and the tetra-linear Takeda's model are those that numerically better represent the experimental structure behaviour. The fibre model is the more elaborate model and the results obtained demonstrate a good approximation to the experimental model; however, the use of this methodology requires a deep knowledge of the materials involved and the refining of the parameters can be significantly time consuming. The tetra-linear Takeda's model is revealed to be a very good model for the behaviour and performance of the bare RC frame.

For the model of the behaviour of the infilled RC frames a non-linear simplified approach was used in this study for the infill panel, to allow the addition of this element to the structural model [4]. To represent the masonry panel four rigid struts with linear elastic behaviour were used, that give support to a fifth central element where the hysteretic non-linear behaviour of the masonry infill panel is concentrated; this central element macro-model is characterized by universal rules that reproduce the load history and depend on the law of the material behaviour imposed.

Comparing the envelopes of the load-displacement curves for the bare and for the infill RC frames, the scaling of the associated carrying capacity of sustained loads indicated that there is a significant increase in stiffness and strength [5] of the infill RC frame before reaching the deformation demand capacity of the infill masonry.

[1]

F. Pires, "Influência das paredes de alvenaria no comportamento de estruturas reticuladas de betão armado sujeitas a acções horizontais", LNEC, 1990.

[2]

M.B. Cesar, D.V. Oliveira, R.C. Barros, "Cyclic Experimental Response of Reinforced Concrete Frames: validation methodology", Proc. 13th Int. Conf. Mechanika 2008, Paper 119, pp. 1-3, Kaunas, Lithuania, 2008.

[3]

M.B. Cesar, D.V. Oliveira, R.C. Barros, "Validação Numérica da Resposta Cíclica Experimental de Pórticos de Betão Armado"; Submitted for publication in Mecanica Experimental, pp. 1-13, Lisbon, APAET, 2008.

[4]

S. Brokken, V.V. Bertero, "Studies on effects of infill in seismic resistant RC construction", Report UCB/EERC, University of California, Berkeley, 81-12, 1981.

[5]

M.N. Fardis, T.B. Panagiotakos, "Seismic design and response of bare and mansory-infilled reinforced concrete buildings. Part II: Infilled structures", Journal of Earthquake Engineering Vol. I, No 3, 475-503, 1997. doi:10.1142/S1363246997000180

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