Quence
Structural
- Jul 16, 2018
- 84
In diagonal shear crack.. when this happens.. the stirrups were being engaged so you still have shear capacity by Vc + Vs.. Is it not torsion cracks are similar where after it occurs.. torsion is resisted by the stirrups? But in ACI 318 there is this passage "At torsional cracking, however, a large twist occurs under an essentially constant torsional moment, resulting in a large redistribution of forces in the structure (Collins and Lampert 1973; Hsu and Burton 1974)"
Were they referring to beam without any torsion stirupps or reinforcement such that a large twist can still occur or does this happen even with stirrups? How? In diagonal shear crack, you don't have happening because the stirrups resisted them.
Let's review the concept of Torsion cracks from Mcgregor textbook:
"Pure Torsion
When a concrete member is loaded in pure torsion, shearing stresses, and principal stresses develop as shown in Fig. 7-8a and b. One or more inclined cracks develop when the maximum principal tensile stress reaches the tensile strength of the concrete. The onset of cracking causes failure of an unreinforced member. Furthermore, the addition of longitudinal steel without stirrups has little effect on the strength of a beam loaded in pure torsion because it is effective only in resisting the longitudinal component of the diagonal tension forces.
A rectangular beam with longitudinal bars in the corners and closed stirrups can resist increased load after cracking. Figure 7-12 is a torque-twist curve for such a beam. At the cracking load, point A in Fig. 7-12, the angle of twist increases without an increase in torque as some of the forces formerly in the uncracked concrete are redistributed to the reinforcement. The cracking extends toward the central core of the member, rendering the core ineffective. Figure 7-13 compares the strengths of a series of solid and hollow rectangular beams with the same exterior size and increasing amounts of both longitudinal and stirrup reinforcement [7-4]. Although the cracking torque was lower for the hollow beams, the ultimate strengths were the same for solid and hollow beams having the same reinforcement, indicating that the strength of a cracked reinforced concrete member loaded in pure torsion is governed by the outer skin or tube of concrete containing the reinforcement.
After the cracking of a reinforced beam, failure may occur in several ways. The stirrups, or longitudinal reinforcement, or both, may yield, or, for beams that are overreinforced in torsion, the concrete between the inclined cracks may be crushed by the principal compression stresses prior to yield of the steel. The most ductile behavior results when both reinforcements yield prior to crushing of the concrete."
Were they referring to beam without any torsion stirupps or reinforcement such that a large twist can still occur or does this happen even with stirrups? How? In diagonal shear crack, you don't have happening because the stirrups resisted them.
Let's review the concept of Torsion cracks from Mcgregor textbook:
"Pure Torsion
When a concrete member is loaded in pure torsion, shearing stresses, and principal stresses develop as shown in Fig. 7-8a and b. One or more inclined cracks develop when the maximum principal tensile stress reaches the tensile strength of the concrete. The onset of cracking causes failure of an unreinforced member. Furthermore, the addition of longitudinal steel without stirrups has little effect on the strength of a beam loaded in pure torsion because it is effective only in resisting the longitudinal component of the diagonal tension forces.
A rectangular beam with longitudinal bars in the corners and closed stirrups can resist increased load after cracking. Figure 7-12 is a torque-twist curve for such a beam. At the cracking load, point A in Fig. 7-12, the angle of twist increases without an increase in torque as some of the forces formerly in the uncracked concrete are redistributed to the reinforcement. The cracking extends toward the central core of the member, rendering the core ineffective. Figure 7-13 compares the strengths of a series of solid and hollow rectangular beams with the same exterior size and increasing amounts of both longitudinal and stirrup reinforcement [7-4]. Although the cracking torque was lower for the hollow beams, the ultimate strengths were the same for solid and hollow beams having the same reinforcement, indicating that the strength of a cracked reinforced concrete member loaded in pure torsion is governed by the outer skin or tube of concrete containing the reinforcement.
After the cracking of a reinforced beam, failure may occur in several ways. The stirrups, or longitudinal reinforcement, or both, may yield, or, for beams that are overreinforced in torsion, the concrete between the inclined cracks may be crushed by the principal compression stresses prior to yield of the steel. The most ductile behavior results when both reinforcements yield prior to crushing of the concrete."