 # G.6.1 Prismatic Properties

The following prismatic properties are required for analysis:

• AX = Cross sectional area
• IX = Torsional constant
• IY = Moment of inertia about y-axis.
• IZ = Moment of inertia about z-axis.

In addition, you may choose to specify the following properties:

• AY = Effective shear area for shear force parallel to local y-axis.
• AZ = Effective shear area for shear force parallel to local z-axis.
• YD = Depth of section parallel to local y-axis.
• ZD = Depth of section parallel to local z-axis. ### Prismatic property nomenclature

For T-beams, YD, ZD, YB & ZB must be specified. These terms are:

• YD = Total depth of section (top fiber of flange to bottom fiber of web)
• ZD = Width of flange
• YB = Depth of stem
• ZB = Width of stem

For Trapezoidal beams, YD, ZD & ZB must be specified. These terms, which too are shown in the next figure are:

• YD = Total depth of section
• ZD = Width of section at top fiber
• ZB = Width of section at bottom fiber

The top & bottom are defined as positive side of the local Z axis, and negative side of the local Z axis respectively.

STAAD.Pro automatically considers the additional deflection of members due to pure shear (in addition to deflection due to ordinary bending theory). To ignore the shear deflection, enter a SET SHEAR command before the joint coordinates. This will bring results close to textbook results.

The depths in the two major directions (YD and ZD) are used in the program to calculate the section moduli. These are needed only to calculate member stresses or to perform concrete design. You can omit the YD & ZD values if stresses or design of these members are of no interest. The default value is 253.75 mm (9.99 inches) for YD and ZD. All the prismatic properties are input in the local member coordinates.

To define a concrete member,you must not provide AX, but instead, provide YD and ZD for a rectangular section and just YD for a circular section. If no moment of inertia or shear areas are provided, the program will automatically calculate these from YD and ZD.

The following table is provided to assist you in specifying the necessary section values. It lists, by structural type, the required section properties for any analysis.

Note: For the PLANE or FLOOR type analyses, the choice of the required moment of inertia depends upon the beta angle. If BETA equals zero, the required property is IZ.
Table 1. Required Section Properties
Structure Type Required Properties
TRUSS AX
PLANE AX, IZ, or IY
FLOOR IX, IZ or IY
SPACE AX, IX, IY, IZ