Rigid zones deal with two aspects of the analysis and design of concrete frames:
1. Design codes generally allow you to design for forces at the face of a support.
2. From an analytical modelling point of view it is commonly accepted that a simple centre-line model does not properly idealise the physical size of members. By modelling rigid zones we account for the extra stiffness that exist within the 3D block where the members physically interact with one another.
The chapter dealing with Wall Modelling Considerations shows examples that take this second point to a more extreme level. Beams attach to the ends of walls (not the centre lines) and we are happy to models walls and beams in this way. Why would the same not be true where beams attach to columns?
To demonstrate the effect of rigid zone modelling the following simple example will be used.
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A simple symmetrical structure is shown in the rendered view above. When running the building analysis you have a choice of 3 model options relating to rigid zones, none, reduced by 25%, and maximum as shown below.
The sections below describe what each of these options will do.
Rigid Zones – None
The analysis results are as shown above. The beam bending moment diagrams extend right to the centre of the column, the beams will be designed for a support moment of 77.91 kNm and a span moment of 86.56 kNm.
This is the most simplistic analysis model and is likely to be the most similar to any models you may have created in general analysis packages.
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Rigid Zones – Reduced by 25%
The analysis results are as shown above. Note that the beam bending moment diagrams do not extend right to the centre of the column, they stop at the face of the column. The column moment diagram also stop at the underside of the beams. No diagrams are shown on the rigid zone lengths.
Importantly, for this model type, although the bending moment diagram starts at the face of the column the actual rigid length within the rigid zone only extends to 75% of this length (i.e.
it is reduced by 25%).
Comparing the moment diagrams to the case where no rigid zones are used we see that:
1. The span moment is reduced from 86.56 kNm to 76.44 kNm. Adding rigid zones at the ends of beams generally lifts the bending moment diagrams and hence will slightly reduce span moments.
2. Although the diagram has been lifted the support moment of 77.91 kNm has reduced slightly to 75.12 kNm because the moment is now being taken at the face rather than the centre-line of the column.
3. The column design moments are also reduced from 77.91 to 69.59 kNm.
Rigid Zones – Max
The analysis results are as shown above. Once again the beam bending moment diagrams do not extend right to the centre of the column, they stop at the face of the column. The column moment diagram also stop at the underside of the beams. No diagrams are shown on the rigid zone lengths.
Importantly, for this model type, the bending moment diagram starts at the face of the column AND the actual rigid length within the rigid zone extends to 100% of this length (i.e. it is max).
Comparing the moment diagrams to the cases where no rigid zones and reduced rigid zones are used we see that:
1. The span moment is further reduced from 86.56 kNm to 76.44 to 73.2 kNm. Adding full rigid zones at the ends of beams increases the lifting of the bending moment diagrams.
2. In this case the diagram has been lifted so much that the support moment of 77.91 kNm which reduced slightly to 75.12 kNm using reduced rigid zones has now increased to 78.36 kNm.
3. The column design moments which reduced from 77.91 to 69.59 kNm have also increased again slightly to 72.28 kNm.
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Discussion
It should be noted that the simplistic model used above to demonstrate rigid zones does not deal with the complexities of continuous beams and pattern loading and may to some degree exaggerate the effects of rigid zone modelling. A more typical four span continuous beam example is considered below.
Above – results when no rigid zones are used.
Below – results when rigid zones are reduced by 25%, span moments reduced by 12% in end span, support moments reduced by around 10%.
Below – results when maximum rigid zones are used, span moments reduced another 2–3%
and support moments increase again by 2–3% but are still less than the moments when no rigid zones are used.
In general the use of rigid zones reduced by 25% will result in the maximum reduction in support moments combined with a less extreme reduction in the span moment. It is
considered that this option will give maximum efficiency and will be the preferred option for most designers using Orion.
NOTE: Although this option can often result in reduced support and span design moments, it should not be confused with moment redistribution. In theory there is nothing to these design forces being further reduced by the use of moment redistribution.