Concept of member offset in STAAD.Pro

 Applies To 
 Environment: N/A
 Area: Modeling Solutions
 Subarea: Specifications
 Original Author:Bentley Technical Support Group

Can you please explain the concept behind member offsets?  

 When creating a model consisting of beams and columns, generally, the START or END face of the member is assumed to be located at the nodal point. In other words, the distance from the respective node to the start or end face of the member is treated as zero. Thus, for example, if member 47 is defined as being connected between nodes 12 and 13, then, the start face of the member is located at node 12, and the end face at node 13.


 This assumption may not always reflect the true physical condition on the structure. For example, when a beam meets a column, the common node between the beam and column is usually defined as being at the shear center (centerline for symmetrically shaped) of the column.

But, physically, the start face of the beam is not at that node, but at half the column depth away from the node. One may choose to ignore this "shift" if the column depth is negligible in comparison to the span of the beam. However, if one wishes to take advantage of the high stiffness that the half-depth region of the column offers, he/she may consider this using the member offset command.

The member offset is a way of declaring that the region, whose length is defined by the offset, is a rigid zone. Hence, if the offset values in X, Y and Z coordinates are a, b and c, the length of that region is d=sqrt(a*a + b*b + c*c). The face of the member is then assumed to be "d" away from the node.

The member end forces that STAAD reports are at the face of the member, not at the node, when an offset is specified. If the offset is applied at the base of a column, then the member end force may not be equal in magnitude to the corresponding support reaction terms. If one is interested in checking static equilibrium based on the free body diagram at that support, the member end forces must be transferred from the member face to the support node taking into consideration the rigid link defined by the offset.