Numerical Modeling of the In-Plane Seismic Behavior of Unreinforced Masonry Wall Retrofitted with Bed Joint Reinforcements

Abstract

Gas-induced earthquakes are a major problem in the north of the Netherlands due to many unreinforced masonry (URM) buildings being located in this area. To improve the seismic performance of the URM buildings, existing structures must be retrofitted and if the aesthetic of the building is important, the implemented retrofitting technique must not alter the appearance of the property. An experimental campaign was conducted at Delft University of Technology to investigate whether bed joint reinforcement (BJR) can be used for seismic retrofitting of URM walls. In this thesis, the influence of BJR, diagonal anchors and reinforcement layout on the in-plane seismic behavior of the retrofitted wall is studied. To achieve this goal several nonlinear static analyses using DIANA software were performed. First, the case experimentally tested is adopted as a benchmark and a validation of the numerical model is performed. Afterwards, the validated numerical model is adopted to perform a parametric study considering different reinforcement layouts. The numerical study showed that BJR was able to improve the seismic performance of the retrofitted wall. The Peak-load was increased slightly (13%) in the retrofitted wall compared with the URM wall. BJR acts in tension to restrains the crack opening so, the maximum crack width is another affected factor: a difference of -110% in maximum crack width was observed after retrofitting. Finally, the crack pattern and failure mechanism of the structure was changed due to BJR. The sensitivity analysis showed that the results were sensitive to variation in tensile fracture energy and modulus of elasticity, while tensile strength, compressive strength, and compressive fracture energy were the other variations that did not affect the results considerably. According to an extensive parametric study, it was concluded that double BJR was slightly more effective than single BJR. By applying only 4 layers of double BJR almost the same peak-load and maximum crack width were obtained comparing the results of the retrofitted wall with the original layout (with 12 BJR layers). The location of the BJR, however, was the governing concern. In the URM wall cracks mostly developed diagonally from the window corners and by applying horizontal bars below and above the window opening, the diagonal cracks were restricted and failure modes of the wall changed. The absence of single BJR next to the opening (in piers) might lead to the shear mechanism of the piers. The number of BJR is a function of the opening’s dimension. According to a proposed reinforcement layout it is believed that the mentioned 4 layers of double BJR (above and below the opening) and 3 layers of single BJR (next to the opening) were a wise choice for retrofitting of the wall. Furthermore, diagonal anchors could be ignored as they did not affect the performance of the wall. Conservatively, a layer of double BJR far above and below the window level could be applied. In this way reinforcement was decreased by 30%, however, the results were comparable with the retrofitted wall with original reinforcement layout.