Seismic retrofit of low-rise masonry and concrete walls by steel strips.
Creators: name: Taghdi, Mustafa.
Publisher: name: Université d'Ottawa / University of Ottawa
Year: 1998
Subjects: subject: Engineering, Civil.

description: A large number of low-rise non-ductile masonry (unreinforced and partially reinforced) and reinforced concrete shear walls were built before seismic provisions have been introduced in modern building codes. During seismic evaluation of existing buildings, such walls are frequently found to be in need of seismic retrofit. This research investigates the in-plane cyclic inelastic behaviour of such low-rise walls, as well as that of comparable walls retrofitted using steel strips. Four concrete block masonry and two reinforced concrete walls of large scale were subjected to combined constant gravity load and incrementally increasing lateral deformation reversals. All the walls had an aspect ratio of 1.0 and a height of 1800 mm. Masonry walls had a thickness of 190 mm, and concrete walls 100 mm. They were designed to simulate walls built using provisions in effect decades ago, before the enactment of earthquake-resistant design provisions. The first pair of masonry walls were unreinforced (URM), and the second pair were partially reinforced masonry (PRM). Three #15 reinforcing bars, one in each exterior cell and one in the central cell, were used in the vertical direction of the PRM walls. One truss type bed joint reinforcement, No. 8 gauge, was placed in mortar joints every two courses. Each concrete wall was reinforced using 3 pairs of 9.5 mm reinforcing bars, uniformly distributed along the length, in both directions. One wall from each pair was retrofitted using a steel strip system. The steel strip system consists of diagonal (220 x 3.81 mm) and vertical (80 x 3.81) strips attached to the walls using through-thickness bolts. Stiff steel angles and anchor bolts were used to connect the steel strips to the foundation and the top loading beam. The unretrofitted walls had a relatively low shear strength. However, the URM wall exhibited a very stable rocking response at an in-plane load of 61 kN. The PRM wall exhibited a non-ductile shear failure at a maximum load of 120 kN. The reinforced concrete wall showed flexural failure at 171 kN with buckling of the end re-bars. The ultimate strength of the retrofitted URM, PRM and R/C walls were respectively 7, 3.8 and 2.9 times that of the unretrofitted walls. These tests show that the steel-strip system is most effective to significantly increase the in-plane strength and ductility of low-rise unreinforced and partially reinforced masonry walls, and lightly reinforced concrete walls. The tests also demonstrated that the desirable in-plane rocking behaviour, observed in unreinforced brick masonry walls by other researchers, can also develop in concrete block walls. In the analytical part of this research, a truss model was developed for analysis of retrofitted low-rise walls. This model consists of an indeterminate truss having five members each representing part of the retrofitted wall. A step-by-step calculation procedure was used to find strength and deformation of each wall. The results of this analytical model were in good agreement with experimental results reported in this study. Based on this model a displacement based design procedure is proposed as a simple design procedure that may be used for retrofitting existing non-ductile low-rise walls. This design procedure proved to be easy and effective.