Strut and tie model for estimating shear strength of RC exterior beam-column joints without transverse reinforcement

A new formula based on strut and tie model to determine the shear strength of exterior beam-column joints without transverse reinforcement (stirrup) in the joint region under monotonic and seismic lateral loading is proposed in this paper. Based on the accomplished topology optimization to determine directions of the principal stresses of the joint region, the main shear transfer mechanism in beam-column joints without transverse reinforcement in the joint region is diagonal strut mechanism; therefore, the proposed strut and tie model is made up of a concrete bottle-shaped diagonal strut. Formulae available in published literature for calculating the diagonal strut angle of inclination was checked by finite element analysis results of two exterior joints, the results indicate that all formulae have good agreement. Various experimental data set of this kind of joints are collected from previous literature based on consistent and specified criteria for data selection. The accuracy of the proposed procedure and two approaches of determining the diagonal strut width was checked by comparing calculated shear strengths with experimental data. Based on the two introduced approaches for calculating the diagonal strut width, approach 1 which does not take into account the column axial load, has the best performance in the proposed model. The proposed formula, in addition to the specified compressive strength of concrete which is included in Iranian and American concrete codes, considers other effective parameters on exterior joints behavior such as the beam longitudinal tension reinforcement ratio and its strength, the intermediate column bars and the joint aspect ratio. Therefore, the proposed formula is able to estimate accurately the shear strength of exterior beam-column joints without transverse reinforcement in the joint region.