Section 11.5 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 greater 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 11.5.3.6.

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.

For nonprestressed members, axial force is accounted for according to section 11.5.1(c).

The minimum f ’c of the cross section is used to calculate strength capacity or other maximum requirements (e.g. maximum reinforcement spacing) in the unusual situation where a cross section contains multiple concrete mixes. The maximum f’c is used when calculating minimum requirements (e.g. minimum reinforcement area) that are determined with f’c.

Torsion reinforcement is limited to 60 ksi per 11.5.3.4.

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)cot θ
By rearranging code equation 11-24, the minimum longitudinal reinforcement can be calculated as follows:
$A_{1} f_{y 1} = 5 \sqrt{{f^{'}}_{c}} A_{c p} - (\frac{A_{t}}{s}) p_{h} \cdot \dot{f} y v$

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 11.5.3.1 (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 11.5.6.1.

The area of transverse reinforcement is determined by 11.5.3.6

Minimum transverse reinforcement is determined by 11.5.5.1 and 11.5.5.2. An option to automatically design minimum torsion reinforcement even if not required by code is available in the Span Segment Property and Design Section Property dialogs.

Torsional longitudinal reinforcement is considered along with other longitudinal reinforcement when determining effective depths and other bending parameters that affect shear design.