Section 11.6 Beam Torsion

Only the "core" of a cross section is used for torsion design.
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 lesser of the top cover and the bottom cover.
A_cp and p_cp only consider the cross section "core".
A_o is assumed to be equal to 0.85 A_oh per 11.6.3.6.
θ in equations 11-21 and 11-22 is always taken as 45°.
The balance loading axial force and the entire cross section area are used to determine f_cp.
The minimum f’_c of the cross section is used in the unusual situation where a cross section contains multiple concrete mixes.
Torsion reinforcement is limited to 60 ksi per 11.6.3.4.
Longitudinal Reinforcement:
- By rearranging code equations 11-21 and 11-22, the longitudinal reinforcement can be calculated as follows:
  A₁f_y1 = T_n(p_h/2A₀)cotθ
- By rearranging code equation 11-24, the minimum longitudinal reinforcement can be calculated as follows:
  $A_{1} f_{y 1} = 5 \sqrt{{f^{'}}_{c}} A_{c p} - (25 psi) p_{h} b_{w}$
- Longitudinal Reinforcement is designed in Pass 1.
- Longitudinal Reinforcement is added to the bending reinforcement and reported as being due to both designs:

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.
- Section 11.6.3.1 (equation 11-18) is implemented such that shear capacity is reduced by torsion. For very high torsions, this can make shear capacity negative.
- The spacing of transverse reinforcement is determined by 11.6.6.1.
- The area of transverse reinforcement is determined by 11.6.3.6
- Minimum transverse reinforcement is determined by 11.6.5.1 and 11.6.5.2
- Torsional longitudinal reinforcement is considered along with other longitudinal reinforcement when determining effective depths and other bending parameters that affect shear design.