Nonlinear behaviour of coupled shear walls strengthened with externally bonded carbon fibre reinforced polymer composite under seismic loadings

Reinforced concrete (RC) coupled shear walls (CSWs) with adequate strength and stiffness can be an effective system to resist lateral forces such as wind and earthquakes. CSWs are generally used for medium-high rise buildings. They resist lateral forces not only through the shear and moment resistance of their wall segments, but also through the shear action of their coupling beams (CBs). In CSWs, the shear forces are transferred through the CBs, and the overturning moment is partially resisted by an axial compression-tension that is coupled across the walls. A properly designed CSW should ensure that: (i) plastic hinging occurs in the CBs before it does in the walls; (ii) the CBs do not show major strength or stiffness degradations with load reversal; and (iii) the CBs should function as the primary energy-dissipation elements by providing stable energy-absorbing hysteresis loops without pinching. However, designing and detailing CBs with all these important features were not possible before the 1970s. Therefore, the old existing CSWs are potentially at risk of suffering severe damage under moderate to severe earthquakes. Many conventional techniques have been used with some success to retrofit and strengthen deficient structures and thereby improve their seismic performance. In the last few years, the use of externally bonded (EB) fiber reinforced polymer (FRP) composites has proven to be an innovative, reliable and a cost effective retrofit method for RC structures. This study presents results of an experimental and analytical investigation on the seismic behavior, evaluation, and retrofit of reinforced concrete CSWs designed according to codes and Standards before the 1970s. In the experimental part of this study, a new retrofit method using carbon FRP (CFRP) sheets was proposed to enhance the seismic performance of deficient reinforced concrete CBs. To that end, two coupled shear wall specimens with conventionally reinforced CB, representative of CSWs designed according to Canadian codes prior to the 1970s were considered as follows: One as a control specimen and the other strengthened using EBCFRP. Both specimens were tested under reversed cyclic loading to assess the efficiency of the proposed retrofit method. In addition, one more specimen was designed according to modern Canadian design code and Standard with a diagonally reinforced CB. This specimen was submitted to cyclic loading up to failure simulating a seismic loading. Thereafter, the damaged specimen was repaired using EB-CFRP sheets and retested under reversed cyclic loading to investigate the effectiveness of the proposed repair technique. Experimental results revealed the ability of the CFRP retrofitting system to increase load carrying, ductility and energy dissipation capacity in deficient CB specimen designed according to old codes. In addition, the proposed repair method is also capable of recovering the initial strength, stiffness, ductility and energy dissipation capacity in severely damaged CB specimen with diagonal reinforcement. The numerical part of this research study dealt with nonlinear time history analysis of a prototype 20-story coupled shear wall structure located in Canada. Four CSW prototype structures including two identical CSWs designed according to old National Building Code of Canada (NBCC) before 1970s, and two CSWs designed according to new design code (NBCC 2015) and Standard (CSA A23.3-14) located in Montreal and Vancouver as representative of East and West of Canada, respectively. Nonlinear dynamic analyses using RUAUMOKO were conducted under scaled earthquake accelerations to investigate the CSW structural behavior, adequacy of the design, and the deficiencies of old designed CSWs. An EB-CFRP retrofit method was proposed for deficient CSW structures to be in conformity with the new seismic design code. The beneficial effects of this retrofit method on seismic behavior of such CSWs was investigated through nonlinear time history analyses. The results of this analytical study revealed that the CFRP retrofitting technique performed very well since it resulted in improved sequence of yielding in CBs and walls, and reduced story displacement, inter-story drift, CBs rotation, and ductility demand. It was also found that unlike old NBCC, the requirements prescribed by CSA A23.3-14 and NBCC 2015 for the capacity design of ductile coupled walls are acceptable in approximating seismic demands