STAAD - Property Definition - General Prismatic

[HorseBridge]

Hello,

Just wondering why there is "Ax Ay Az"...can a beam have a cross sectional area in more than one direction?

Same question for "Iz Ix Iy"....I would expect Ix Iy and J for torsion.

Also, STAAD doesn't seem to complain if you enter an area that is not compatible with rectangular dimensions. I entered 100 in^2 for area and 1 in by 1 in for YD and ZD. STAAD ignored these rectangular dimensions except for the rendered view.

Thank you,
HorseBridge

 

[SethGuthrie]

Think of a wide flange for example. We often assign a major axis shear area (AY) = beam depth * web thickness which is less than total section area (A). You can do the same thing in STAAD with Prismatic Properties. When it comes to torsional moment of inertia, STAAD uses IX just to be consistent with the local axis convention.

 

[HorseBridge]

Thanks Seth,

Part of the problem was I just see colors for the local axes labels, not "X Y X". But I found the answer to my question in the help contents:

1.7.1 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, the user 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.
For T-beams, YD, ZD, YB & ZB must be specified. These terms, which are shown in the next figure 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
Top & bottom are defined as positive side of the local Z axis, and negative side of the local Z axis respectively.

STAAD 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.

Figure 1-34: Prismatic property nomenclature for a T and Trapezoidal section




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.

Table 1.1 is offered to assist the user in specifying the necessary section values. It lists, by structural type, the required section properties for any analysis. 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 1-4: Required Section Properties Structure Type Required Properties
TRUSS structure AX
PLANE structure AX, IZ, or IY
FLOOR structure IX, IZ or IY
SPACE structure AX, IX, IY, IZ