Verification of Typical Retrofit Scheme for Two-Story URM School ‎‎Buildings Retrofitted by Shotcrete Method

Unreinforced masonry buildings are quite popular in many countries such as Iran, even though they are prone to significant damage against even moderate seismic excitations. In 2003, 26000 people lost their lives and 30000 were injured in southeastern Iran during Bam earthquak. The regional investigations showed an almost total overall collapse of all adobe and masonry buildings. Even to this day, notable number of buildings in Iran is masonry buildings due to low costs, especially in rural areas. Hence, the popularity despite the poor performance has sought researchers to study efficient ways to improve the URM buildings. Generally speaking, masonry buildings are classifed into unreinforced and reinforced; the latter usually beneft from horizontal and vertical steel bars which signifcantly improve strength, ductility and energy dissipation capacity of such walls. Masonry buildings can also be categorized as confned and unconfned. Using horizontal and/or vertical ties usually in the form of lightly reinforced concrete members at the perimeter of walls, their intersection with perpendicular walls and if necessary, around large openings can considerably enhance their ductility and in some cases their strength. Unlike other masonry construction types, the behavior of Confned unreinforced Masonry (referred to CM hereafter) buildings has not yet been fully formulated. This is mainly because of more sophisticated behavioral characteristics of CM walls compared to unconfned Unreinforced Masonry (referred to URM hereafter) buildings. CM is the only masonry system that has been allowed practicing by the Iranian Code of Practice for Seismic Resistant Design of Buildings (Standard 2800) in seismic-prone areas. When it comes to seismic retrofitting of masonry buildings, there are a rather wide variety of available options, e.g. shotcrete, adding steel or FRP sheets on the wall, adding Reinforced Concrete (RC) ties, changing the arrangement and size of the openings, and using Near Surface Mounted (NSM) rods on the walls. Considering the significant number of two-story URM school ‎‎buildings in Iran, there is a need to provide instructions for ‎the ‎type of improvement plan of these buildings. In response ‎to this ‎need, the Organization of Renovation, Development ‎and ‎Equipping of Schools of Iran developed a guideline for ‎the ‎purpose of typifying the improvement plan of these ‎buildings ‎with the shotcrete method in 2021.‎ In this research, based on the results obtained from five two-‎‎story URM schools, the accuracy of this guideline is ‎evaluated. ‎For this purpose, the investigated buildings in the ‎two cases of ‎flexible roof without ties and rigid roof with ties ‎in ETABS ‎software using shell elements and spectral dynamic ‎analysis ‎method, once based on the requirements of the ‎guideline and ‎once based on the Code 360 was evaluated and ‎retrofitted using ‎Shotcrete.‎ The results of this research show that the guideline ‎in the case ‎‎of flexible roofs leads to ‎minimum vulnerability of the walls of ‎‎the studied buildings. In the case of a rigid roof, the ‎results ‎‎show that according to this guideline, some of improved and ‎‎unimproved walls are ‎vulnerable in both direction; In such a ‎‎way that the ‎average percentage of vulnerable walls in terms ‎of ‎‎relative length for the first floor is 63% in the ‎longitudinal ‎‎direction and 38% in the transverse ‎direction, and for the ‎‎ground floor in the ‎longitudinal direction is 56% and in the ‎‎transverse ‎direction is 17%.‎