Consider the following 2 span beam supported on columns and loaded with a udl:
Rigid Zones Not Applied
The analysis model is simply constructed from the solver elements with rigid offsets applied as necessary to connect the beam solver elements to the column solver elements.
The resulting beam bending moment diagram is as follows:
Rigid Zones Applied (default 50%)
Rigid Zones are activated via Model Settings, and this is also where the percentage of rigidity of the zones can be specified. Initially for this example it will be left at the default of 50%.
The revised solver model is as shown below, note the rigid zones that have been formed where the columns and beams connect:
The beam bending moment diagram for the revised model is as shown below.
Note: The above diagram was generated from a Results View to illustrate that a “gap” is formed in the diagram where there are rigid zones. It should be noted that when the same result is displayed in a Load Analysis View the gap is removed, leaving only the non-rigid length of the member displayed.
We might expect the extra stiffness introduced at the supports to increase the hogging moments and reduce the sagging moments, however because the element end forces are now reported at the rigid zone boundaries (rather than the ends of the solver elements) - in this example the main effect is that the hogging moment over the central column support is substantially reduced.
Rigid Zones Applied (100%)
To investigate the effect of the percentage rigidity an additional run is made with the percentage rigidity increased to 100%. The bending moment diagram that results is shown below:
As expected the extra stiffness introduced at the supports increases the hogging moments and reduce the sagging moments in comparison to the run at 50%.
Rigid Zones Applied (0%)
If the percentage rigidity is reduced to 0% the bending moment is as shown below:
If this result is compared to the run in which rigid zones were not applied, it is clear that although the sagging moments are identical, the hogging moments that are reported are not the same. This is because, although the total elastic length of a member is the same in the two models, the position of the start and end of design members is different (being taken at the rigid zone boundaries when applied).