Beam Section Library

Figure 1 Beam Section Dialog Box

One of the parameters which is required to set-up the Finite Element model for computing the distribution factors using the refined methods of analysis for I-girders is the Iyy value, which is the moment of inertia around y axis. Iyy, is used as input data in computing J value (J is the St. Venant's Torsional Constant). For the rest of the sections Iyy is not used.

The LRFD Specification Eq. C4.6.2.2.1-2 specifies:

"In lieu of more refined information, the St. Venant Torsional Constant, J, may be determined as:

for stocky open sections, e.g. prestressed I-beams, T-beams, etc. and solid sections:

where

Ip = Ixx + Iyy (polar moment of inertia) (in4)

A = area of cross-section (in2)

To clarify the input expected, the old term for Moment of Inertia (MI) was replaced with the term Ixx, which is the moment of inertia about the x axis or the horizontal axis. The coordinate axis is also drawn in the schematic in the section dialog.

The program helps you to complete the Section Properties dialog, by inputting the correct value of Iyy if you choose to simply leave the Iyy value as 0 in edit box and then click on the OK button in Beam Section Library.

Figure 2 Spread Box Beam Section Dialog Box

You will be prompted with a message giving you a couple of options:

Click YES to introduce Iyy based on the dimension you input;
Clicking NO to introduce your own value for Iyy (must be a non-zero value)