Investigation of the optimum design in the strengthening of reinforced concrete beams with insufficient shear capacity using mechanical steel stitches angled at 45 degrees

In this study, beams with insufficient shear capacity were strengthened with U-shaped Mechanical Steel Stitches (MSS), which is a practical, economical and innovative method. MSSs were applied on both faces of the beam web at a 45-degree angle along the shear span. In this context, a total of eight shear beam specimen, one of which was a reference specimen and the other seven with different MSS intervals, were tested under vertical loads. The diameter (Ø6), anchorage depth (10Ø) and mechanical properties of the MSS, as well as the geometry of the reinforced concrete beam (125 mm × 250 mm × 2500 mm), longitudinal (3Ø16 tension and 2Ø12 compression) and transverse (Ø6/350 mm) reinforcement were kept constant. By changing the MSS intervals applied to the beam (ranging from d/5 to d), the changes in strength, ductility, stiffness and energy consumption capacities were examined. As a result of the study, a 53.74% increase in shear capacity was observed in the beam with the most tightened MSS interval (d/5). However, the nominal yield and total energy consumption capacity increased by 144% and 366%, respectively, compared to the reference beam. According to the damage analysis conducted on beams, in the most frequently encountered MSS application with a spacing of (d/5), splitting damage is observed. However, as we move towards the less frequent spacing of d/2, the damage transitions to a more sudden and brittle phenomenon known as diagonal tension failure. As we move from the d/2 spacing to the sparsest d, the behavior is now dominated by diagonal tension damage alone. Finally, as the MSS spacing in beams increases from d/5 to d/2, the nominal yield stiffness of the beams tends to decrease between 2.1% and 21.2% compared to the reference beam; however, surpassing d/2 leads to a change in the failure mode, resulting in the beam yield stiffness returning to the reference beam level (approaching from -2.5% to 2%) due to the change in the failure mode associated with exceeding the d/2 spacing. Therefore, it can be concluded that this new technique, which stands out with its low cost and easy applicability, is very effective in strengthening beams.