Retrofitting of Unreinforced Masonry with Timber Elements

Abstract

Due to gas extraction, seismic activity has become a major problem in the province of Groningen. Existing buildings are not designed to withstand seismic loads and are likely to be structurally unsafe. Two storey terraced houses with concrete floors and large openings in the ground floor façade walls are onsidered vulnerable, since the in-plane behaviour of these walls is weak. This research aims to investigate the possibilities of enhancing the seismic in-plane performance of this building type with the application of timber elements. Non-linear static pushover analyses are performed to assess the in-plane behaviour according to the Dutch guidelines for the Near Collapse limit state using a macro-element modelling approach in ETABS, a 3D non-linear analysis software. Variants of the typology are studied to investigate the sensitivity to certain geometrical and structural parameters, such as the height, width and depth of a building and the applied masonry type. Based on the results, a timber strengthening design is proposed and the effect on the behaviour is studied. The majority of the un-strengthened buildings presents rocking behaviour caused by the slenderness of masonry piers due to the large openings in the façade walls. This ductile behaviour ensures relatively large lateral displacement capacities, often resulting in the satisfaction of the safety standards. Moreover, results show that structures with wide masonry piers, calcium silicate element masonry and extremely large openings at ground floor level (up until 70% of opening) are likely to be unsafe. Therefore, strengthening is required. Global capacity of the numerical models depends heavily on prescribed drift limits. Therefore, establishing the appropriate limits for each model is essential. The analysis indicates that assessment according to other guidelines, can lead to different outcomes. The proposed retrofit design consists of a timber framework connected to the inner masonry piers, with an OSB panel nailed on top of it to increase the stiffness. The retrofit is attached to the foundation by tension anchors. Gravity loading of the structure is mainly carried by the masonry elements. Implementation results in stable rocking behaviour of the piers, which ensures higher displacement capacities. Analysis demonstrates that the application of the timber retrofit leads to the satisfaction of the Near Collapse limit state, when higher drift limits can be prescribed. Furthermore, applying the reinforcement results in a change from unfavourable shear behaviour to rocking behaviour. However, increase of strength is limited, since the resistance of the anchors to the lateral forces is largely depending on the design of the foundation and structural elements to which they are attached. Therefore, the retrofit is not suitable for significantly increasing lateral resistance, which is required for structures with extremely large openings in the façade walls. Experimental testing is recommended to quantify the increase of drift limits. An overall consistent definition of capacity values and corresponding drift limits for each possible failure mechanism is essential for reliable evaluation of the in-plane response, especially when buildings are assessed using non-linear analyses.