Section 11.3 Torsion Design
Included code sections - 11.2.9.1, 6.3.2
Excluded code sections - 11.2.9.2
Only the "core" of a cross section is used for torsion design. See "Concrete "Core" Determination".
If the core consists of multiple ribs, then the torsion calculations are performed for an average rib:
- rib width = total core width / num ribs
- with ultimate forces scaled down by the number of ribs (/ num ribs) and capacity and reinforcement scaled back up by the number of ribs (* num ribs).
- To get a more detailed and exact calculation, use a separate design section or design strip for each rib.
The side cover is assumed to be equal to the greater of the top cover and the bottom cover.
Torsion properties are calculated in accordance with 11.3.10.
Torsion reinforcement consists of longitudinal reinforcement and closed ties perpendicular to the axis of the member according to 11.2.6(a).
Acp and pcp only consider the cross section "core".
Ao is assumed to be equal to 0.85 Aoh per 11.3.6.
11.3.10.4 equation 11-19 is implemented such that the torsion demand reduces the shear capacity. For very high torsions, this can make the shear capacity negative.
The longitudinal torsion tension demand is satisfied by calculating a torsion tension in accordance with equation 11-21, incorporating it with the shear tension and then adding these forces to the section forces and then performing a bending/axial desing in Pass 1.
Transverse Reinforcement:
- Transverse reinforcement is designed in Pass 2.
- Stirrups/links are assumed to be closed hoops. RAM Concept will report the reinforcement in terms of the number of legs specified (by the user), but the calculations assume a hoop shape. The link detailing reported by RAM Concept will be difficult to decipher if the number of legs specified by the user is not 2.
The area of transverse reinforcement is determined by equation 11-17.
Torsional and shear longitudinal reinforcement is considered along with other longitudinal reinforcement when determining effective depths and other bending parameters that affect shear design.