Application of Reinforcement Concrete Layer Method for Retrofit of Slant Jack Arch Roofs in Masonary Buidlings

Jack arch masonry slab, developed in the 19th century in Britain has been used widely to floor and roof industrial and residential masonry buildings in many parts of the world. It is still in use in parts of Europe, the Middle East and Indian subcontinent. Taking into consideration the widespread use of the jack arch flooring and its ease of constructing compared to the more modern concrete-based slabs, it is rather surprising that there is no mention of the system in codes of practical installation . Most of these roofs are built in traditional ways and little control is applied on their method of construction.
Collapse of a large number of these composite slabs during past earthquakes pointed out the weakness of this type of flooring to seismic loading. It has also highlighted the need for developing appropriate retrofitting schemes sience Statistics has showed that over half of the slabs used in buildings in Iran are jack arch roofs. The point is that these slabs (specially the slant type that is widely used in buildings of northern area in Iran) do not show appropriate seismic performance in severe earthquakes. Therefore rehabilitation of them needs to be considered. One of the effective methods is to add a thin layer of reinforced concrete over the slab. The retrofitting procedure includes; removing the slab flooring finish, then placing over the slab a mesh of reinforcement bars and finally covering the mesh with a thin layer of concrete. The effectiveness of such a method needs more investigations. To further investigate the seismic behavior of these roofs, response modification factor can be utilized as a well-known seismic parameter.
This study investigates the seismic performance of masonry buildings with slant jack arch slabs retrofitted by the method of adding a layer of reinforcement concrete. Two groups of one story masonry buildings with jack arch masonry slabs are designed including roofs with slopes of 0, 10, 15 and 20 degrees with and without concrete layer for roof retrofitting. Static nonlinear (pushover) analysis is carried out. Nonlinear analysis program “ANSYS” is employed for the analyses. The load–displacement curves for both types of models are obtained and variations of strength, ductility factor, stiffness and rigidity of roofs n both types of models are investigated. Response modification factor of two groups are calculated and results are compared. Results show that according to standard no. 2800 criterion, slant jack arch masonry slabs are classified as semi-rigid roofs and by retrofitting them, their rigidity can be enhanced. Increasing the Slope of roofs inversely affects the Response modification factor (R), strength and elastic stiffness of structure. Finally For the consideration of economics factors, a cost analysis based on the tariffs of the Iranian Management and Programming Organization is carried out on three conventional methods of roof retrofitting (method of adding a concrete layer on the roof, steel grid method and tie-bracing method recommended by standard no. 2800). The obtained results indicate that method of adding a concrete layer is the most cost-effective method for jack arch retrofitting.