Seismic behaviour of reinforced concrete structures with masonry infills
Thesis DisciplineCivil Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This thesis focuses on the seismic behaviour of reinforced concrete structures with masonry infills, with particular interest in the development of rational procedures for the design and analysis of this type of structure. The properties of masonry and its constitutive materials were reviewed, giving special emphasis to those aspects which contribute to a better understanding of the strength mechanism. Theoretical procedures were developed for the rational evaluation of the strength of masonry subjected to compressive and shear stresses. A large amount of experimental work related to the behaviour of infilled frames was also reviewed. The main characteristics of the response under lateral loading were discussed for different types of infilled frames and a comprehensive classification of the modes of failure, for both the masonry panel and the surrounding frame, was conducted. In addition, the influence of several parameters which can affect the structural response was evaluated. Two theoretical procedures, with different degree of refinement, are proposed in this study for the analysis of infilled frames. The first procedure is a simple approach, based on the equivalent truss mechanism, which allows the evaluation of the lateral resistance of the infilled frames, considering two different types of failure in the masonry panel, namely, shear-friction and diagonal tension failure. The compressive strength of the diagonal strut is assessed by transforming the shear failure envelope obtained from the modification of the Mann and Muller's theory. This transformation takes into account the inclination of the diagonal strut and neglects the effect of the tensile principal stresses acting on the masonry panel. The second procedure is a refined macroscopic model based on a multi-strut formulation, which is intended to represent more accurately the effect of the masonry panel on the surrounding frame. Since debonding of the mortar joints is the most common type of failure observed in the masonry panel, the formulation of the procedure is specifically developed to represent this situation. The model accounts separately for the compressive and shear behaviour of masonry using a double truss mechanism and a shear spring in each direction. Recommendations are also given for the analysis of infilled frames when a failure due to diagonal tension or crushing of the corners is expected in the panel. A test programme was implemented to investigate the seismic response of infilled frames. The main criterion followed for the design was that the reinforced concrete columns should yield in tension in order to obtain a reasonable ductile response under lateral loading. New reinforcing details were provided in one unit, aimed at enhancing the structural response. These details consisted in tapered beam-column joints with diagonal reinforcement, and additional longitudinal reinforcement in the frame members. The additional bars placed in the columns were not anchored to the foundation in order to produce a weak region at the base of the columns, where most of the plastic deformations were expected to occur. The most important conclusion of the experimental programme is that the response of reinforced concrete frames with masonry infills can be significantly improved by a rational design aimed at reducing the distortion of the masonry panels while plastic deformations arc concentrated in selected regions of the structure. A new design approach is proposed for infillcd frames, in which two cases are considered: cantilever and squat infillcd frames. In the first case, the ductile behaviour is achieved by yielding of the longitudinal reinforcement, which is limited to occur only at the base ofthe columns, and by avoiding large elongations of the remaining parts of the surrounding frame. A pre-cracked connection is induced between the infilled frame and the foundation, where plain round dowels can be placed to control shear sliding. In the second case, ductility is conferred to the structure by allowing controlled sliding of the infillcd frame over the foundation. The applicability of this approach is limited to those cases where the total shear force exceeds the frictional strength of the pre-cracked connection. The effectof pinching of the hysteresis loops in the response of infilled frames subjected to earthquakes was investigated. A parametrie study was conducted using a one-degree-of-freedom oscillator subjected to ground accelerations recorded in five different earthquakes. Results obtained from the dynamic nonlinear analyses indicated that the effect of pinching and the damping model used can significantly influence the response of infilled frames, which normally exhibit a short to medium initial period of free vibration. Therefore, the displacement demand imposed by the earthquake can be larger than that assumed by the seismic codes if they are based on the concept of equal displacement.