CAPACITY DESIGN OF INFILLED FRAME STRUCTURES

DESIGN CRITERIA

Background Experimental data reported by different researchers  indicate that a reasonably ductile response can be obtained if the infilled frame is properly designed and constructed. In the inelastic range, the surrounding frame is able to resist large deformations, whereas the masonry panel cracks and fails under relatively low distortions. Consequently, they can be considered as structure of limited ductility (global ductility factor between 2.0 and 3.0), provided that brittle modes of failure are avoided or adequately controlled to delay their effect on the response. Infilled frames are usually designed according to simplified procedures, in which the lateral resistance of the structure is evaluated taking into account the shear bond strength of masonry and the applied vertical load. The surrounding reinforced concrete frame is usually designed to resist the axial forces resulting from the truss mechanisms, without taking into account the particular aspects resulting from the interaction between the panel and the frame. Other aspects, such as adequate design of the floor beams and the beam-columns joints, and control of potential sliding shear failure of the columns, should be also considered. In the last years, significant efforts have been made to improve the response of infilled frames by providing horizontal reinforcement in the mortar joints of the panel. This reinforcement is properly anchored in the surrounding columns, and design recommendation based on this criterion has been proposed.  Paulay and Priestley  and San Bartolomé et al.  suggested that infilled frames can be designed to fail in a "flexural mode" by yielding of the tension column in order to obtain a reasonably ductile behaviour, avoiding other brittle types of failure. However, yielding of the tension column could produce lack of restraint and stability problems. When the columns of the infilled frame are subjected to tensile forces, numerous nearhorizontal cracks cross the width of the column. Increasing forces will produce yielding of the longitudinal reinforcement, if the masonry panel is able to resist the diagonal compressive force resulting from the equivalent truss mechanism. Since the tensile force, and consequently the strain, is approximately constant along the column, yielding of the reinforcement results in a significant elongation of the member, which is usually incompatible with the brittle characteristics of the masonry panel. The elongation of the columns reduces or eliminates the beneficial effect of the frame in restraining the masonry panel and jeopardizes the stability of the panel against out-of-plane actions.