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.
Acp and uc only consider the
cross section "core".
Where the design yield strength of torsion reinforcement,
fsy.f is used in calculations, it is limited to 500
MPa per Table 3.2.1.
Torsion reinforcement consists of longitudinal reinforcement and closed fitments
perpendicular to the axis of the member according to 8.2.5.4 through 8.2.5.6.
Equation 8.2.1.2(2) is used to calculate Tcr, with
σcp taken as the balanced axial compression at
the centroid of the cross section divided by the cross sectional area.
Veq* is calculated using equation 8.2.1.2(3).
Ao is assumed to be equal to 0.85
Aoh.
Equation 8.2.3.4(3) is implemented such that the torsion demand reduces the shear
capacity. For very high torsions, this can make the shear capacity negative.
Where minimum torsion reinforcement is required according to 8.2.1.6(2), the
quantity is taken as the maximum of equation 8.2.1.7 and 8.2.5.5 (a) and the
fitments are required to be closed.
RAM Concept calculates the longitudinal strain parameter,
εx using the approach outlined in Section 8.2 Shear
Design, but adding the torsion tension component in equations 8.2.4.2.3. The total
shear/torsion tension component is taken as the square root of sum of the squares as
indicated in the equations. Pv is taken as zero.
The area of closed torsion reinforcement is determined by equation 8.2.5.6.
The maximum torsion spacing of closed torsion fitments is calculated as the lesser
of 0.12ut and 300 mm. The term
ut does not appear to be defined in AS 3600-2018, so
the definition from AS 3600-2009 is used.
Torsional and shear longitudinal reinforcement is considered along with other
longitudinal reinforcement when determining effective depths and other bending
parameters that affect shear design.
