Stirrups are the shear reinforcements which hold longitudinal
reinforcements and also resists shear and diagonal tensions on the
structure.
How strirrups resist parallel forces can be answered with 2
reasons.
- Actually Shear force never acts directly onto the stirrups
instead it gets transmitted through tensile and compression
reinforcements when it acts on the structure.
- Due to other forces acting, crack profile generated due to
shear force on the structure (in case of beam) may not be
absolutely vertical and due to this ,the crack shall be inclined
and if stirrups are there, the crack profile passes through the
stirrups and can be resisted.