Applies To
	Product(s):	STAAD.Pro
	Version(s):	All
	Environment:	N/A
	Area:	General Solutions
	Subarea:	Configuration Settings
	Original Author:	Bentley Technical Support Group

I am analyzing a simply supported beam, which is 20 feet long. There is a concentrated force acting at the mid-span point of the beam. In one case, I model it as a single member and apply the load at midspan using the member load option. In another case, I model it as 2 separate members, each 10 ft long, and apply the load at the central node using the JOINT LOAD option. The member cross section is a W12X26 from the American steel table. When I look at the deflection at the 10 ft point, using the PRINT SECTION DISPLACEMENT command for case (a) and the PRINT JOINT DISPLACEMENT command for case (b), the values do not match. Why?

The difference is due to shear deformation.

When STAAD computes the displacement at a node using the stiffness method, the bending stiffness coefficients in the stiffness matrix include the contribution from

1. flexural deformation

2. shear distortion if the shear areas are part of the user specified member property input.

As a result, the joint displacements consist of the pure bending component plus the shear deformation component.

When you ask for intermediate section displacements along a member span, STAAD calculates these using the moment area method from

* the joint translation and rotation (which includes the shear deformation component as explained above)

* flexural deformation of the member (the shear deformation component is not considered here)

In other words, in the implementation of the moment area method, only the term (a) contains shear deformation where as term (b) does not. This is an error in the program, albeit a small one.

If shear deformation is eliminated from all of the above calculations, this discrepancy will be removed. Shear deformation is part of the above calculations only when the shear areas AY and AZ are non-zero values. When member properties are specified using methods such as defining sections from the built-in steel tables, or by specifying them as PRISMATIC sections with a DEPTH and WIDTH value, STAAD internally calculates the shear area before proceeding with the stiffness matrix assembly.

The user may nullify the effect of shear deformation by doing one of the following :

* Provide the properties using the PRISMATIC attribute, and specify just AX, IX, IY and IZ, as in,

1 PRI AX 21 IX 35.4 IY 45.3 IZ 85.75

In this case, since AY and AZ are not specified, shear deformation will not be calculated.

* Provide a very large value for the shear areas AY and AZ, thereby minimizing the shear deformation, as in

1 PRI AX 21 IX 35.4 IY 45.3 IZ 85.75 AY 1E10 AZ 1E10

In this case, the large shear area will result in negligible shear deformation.