R.C. Fenwick

Detailing of Plastic Hinges in Seismic Design of Concrete Structures

Recent changes of New Zealand structural codes for seismic design revised the way in which the level of detailing is determined for potential plastic hinges. Previously, the level of detailing was based principally on the structural ductility factor, which is broadly similar to the reduction factor used in U.S. practice. The level of detailing in the revision is based on the predicted magnitude of curvature that a plastic hinge is required to sustain in the ultimate limit state. This paper explains why the structural ductility factor does not give a reliable guide to the deformation sustained in an individual plastic hinge. It is proposed that nominal curvatures in potential plastic hinges, calculated by simplified rules, be used as an index to define the level of detailing required in the plastic hinges. Design curvature limits are proposed for different categories of plastic hinge based on test results of beams, columns, and walls. The proposed material strain limits provide a high margin of safety against failure in the ultimate limit state earthquake and an adequate margin of safety against collapse for the maximum credible earthquake.

Multispring Hinge Element for Reinforced Concrete Frame Analysis

AbstractThis paper describes the development and validation of an analytical multispring plastic hinge element that can predict elongation of ductile RC plastic hinges together with its flexural and shear responses. The element consists of layers of longitudinal and diagonal springs that represent the behavior of concrete, reinforcing bars, and diagonal compression struts. Beam tests reported in the literature, for which elongation of plastic hinges was measured at different stages of the lateral cyclic loading, were used to validate the effectiveness of the newly developed plastic hinge element. Comparisons of the analytical predictions with experimental results show that the proposed element predicts elongation of plastic hinges satisfactorily. The ability of the model to predict elongation of a plastic hinge together with its flexural and shear deformations offers a significant advancement in seismic performance assessment of RC structures.

Seismic performance of reinforced concrete frames with precast prestressed flooring system

In a current research project the influence that floor slabs containing precast prestressed units have on the seismic performance of plastic hinges in beams of moment resisting frames is being examined. A three dimensional, approximately half scale, one storey, two bay moment resisting frame together with adjacent floor slab has been constructed and tested. The experimental results show that the building code, ACI 318-05, underestimates the flexural strength of beams by a considerable margin. An analytical model has been developed which predicts the flexural, shear and elongation response of plastic hinges in beams subjected to inelastic rotation history and varying axial load levels. This model can be used to predict the interaction between beams in moment resisting frames and floor slabs containing precast prestressed units. Analysis using this model shows good agreement with the experimental results.