When a beam is specified as RBS it is assumed that there is a "dogbone" at both ends. The engineer can specify that the reduced section either be considered or ignored in the analysis.

The reduced beam section properties can be defined by clicking RBS Properties.

When a RBS connection type is applied to a beam and the Use Reduced Section Properties in Analysis option is chosen, the program creates four finite elements at the end of the beam: one for section "a" and three for section "b". Stiffness properties for section "a" are the same as the beam. Stiffness properties for section "b" are calculated in such a way that the section is first divided into 3 elements and then section properties for each element is calculated at its mid-length (see the following figure).

Reduced Beam Section Modeling

Reduced Beam Connection Zone Elements

In the above figure, it is shown that the zone "a" is modeled with one element and the zone "b" is modeled with three elements. The length of each elements depend on values defined in Reduced Beam Section Properties Dialog (see the figure above).

If the Use Full Section Properties in Analysis option is invoked, the above mentioned internal elements defined at the end of the beams are removed (also internal nodes created for these elements removed). In other words, this is the case that the full stiffness properties is used everywhere along the beam.

Design check is performed at the mid-length of Section "b".

Member Forces and Modal data report provides more information about elements created for reduced beam section connection zone (element length, stiffness properties, etc…)