PGSuper 7.0
Precast-prestressed Girder Bridges
St. Venant Torsion Constant for type f and g sections

This section describes how the St. Venant torsion constant, J, is computed for type f and g sections from LRFD Table 4.6.2.2.1-1.

Type f and g sections are considered to be closed thin-walled shapes. The St. Venant torsion constant is given by LRFD Equation C4.6.2.2.1-3,

\[ J = \frac{4 {A_o}^2}{\sum{\frac{s}{t}}} \]

The beam section is discritized as shown in the image below:

Basic Voided Slab

The dimensions for a basic voided slab torsion constant are:

Element s t
1 Top W - t_ext (H - Void Diameter)/2 + Structural Slab Depth
2 Bottom W - t_ext (H - Void Diameter)/2
3 Exterior H + Structural Slab Depth - t_top/2 - t_bot/2 [W - (# of voids - 1)(void spacing) + (void diameter)] / 2
4 Interior H + Structural Slab Depth - t_top/2 - t_bot/2 (void spacing) - (void diameter)

Advanced Voided Slab

The dimensions for an advanced voided slab torsion constant are:

Element s t
1 Top W - 2*Max(c1,c3) - t_ext Min of (H - H1 - D1/2) and (H - H2 - D2/2) + Structural Slab Depth
2 Bottom W - 2*Max(c1,c3) - t_ext Min of (H1 - D1/2) and (H2 - D2/2)
3 Exterior H + Structural Slab Depth - t_top/2 - t_bot/2 W - 2*Max(c1,c3) - (2*S1 - (number of voids - 3)S2) - D1/2
4 Interior between Exterior and Interior Void H + Structural Slab Depth - t_top/2 - t_bot/2 S1 - D1/2 - D2/2
5 Interior H + Structural Slab Depth - t_top/2 - t_bot/2 S2 - D2

For exterior beams, c1 is taken to be zero on the exterior face.