Seismic retrofit of unreinforced masonry infills in non-ductile reinforced concrete frames using engineered cementitious composites [electronic resource]

Masonry infills in non-ductile reinforced concrete frames can be found in many places around the world, such as the western United States, China, and in countries of the Mediterranean region. There is strong laboratory and field evidence that masonry infills can improve the performance of reinforced concrete structures, as demonstrated in the 1994 Northridge earthquake. However, many masonry-infilled reinforced concrete buildings suffered catastrophic failures in recent earthquakes such as in the 1999 Kocaeli earthquake in Turkey and the 2008 Sichuan earthquake in China, causing the death of hundreds of people and affecting the lives of millions. A new seismic retrofit technique specifically for unreinforced masonry infills in non-ductile reinforced concrete frames has been developed. The technique uses a sprayable, ductile fiber-reinforced cement based material referred to as Engineered Cementitious Composites, or ECC. Small-scale component experiments including compression tests of masonry prisms and flexural tests of masonry beams retrofitted with different ECC retrofit schemes were conducted to investigate the impact of the retrofit on the performance of masonry in terms of strength, stiffness and ductility. The findings of the small-scale component experiments were used for the development of the ECC retrofit design. Four 1/5-scale masonry infilled non-ductile reinforced frames - one with an unretrofitted masonry wall and three with retrofitted masonry walls - were subjected to quasi-static, in-plane cyclic loading to validate the retrofit design. It was found that when a thin layer of ductile cement-based retrofit is applied to the masonry wall and properly tied into the concrete frame, the deformation capacity of the infilled frame is increased 10 times through a rocking motion. In collaboration with researchers from The University of Colorado, at Boulder and The University of California, San Diego, the retrofit design was evaluated through a quasi-static, in-plane cyclic test of a 2/3-scale masonry infilled non-ductile reinforced concrete frame and a dynamic shake-table test of a 2/3-scale, three-story, two-bay masonry infilled non-ductile reinforced concrete frame. These tests demonstrated that the ECC retrofit can significantly improve the performance of this type of structure under cyclic loads and seismic excitation. Existing analytical models for the prediction of the lateral strength and failure mode of masonry infilled reinforced concrete frames subjected to in-plane lateral load are evaluated through a limit analysis method. New analytical models for such structures with ECC retrofitted masonry walls are proposed. Two-dimensional non-linear finite element analyses using two different micro-modeling approaches are used to simulate unreinforced masonry beams retrofitted with a thin layer of ECC under four-point bending. In a detailed approach each material is modeled independently and in a simplified approach, expanded brick units with zero thickness mortar elements are used. The adequacy of these models to capture the experimental response of ECC strengthened masonry beams under out-of-plane bending is examined. The impact of the amount and location of reinforcement in the ECC layer, and of the use of anchors as a method of improvement of the bond between the ECC layer and the masonry surface is also examined through simulation. Two-dimensional non-linear finite element analyses are conducted to asses the ability of various smeared and discrete crack modeling approaches in a commercial finite element program for capturing the response of masonry infilled non-ductile reinforced concrete frames. A methodology for simulating this type of structures with ECC retrofitted walls in two dimensions is also presented.

Advisor: Billington Sarah L. (Sarah Longstreth)
Deierlein Gregory G. (Gregory Gerard)
Krawinkler Helmut advisor. Thesis advisor