There are optional methods for idealising walls within Orion. This subject is discussed at length in the chapter on Wall Modelling Considerations.
3D Effects
In general our traditional engineering expectations are developed from considering simplified sub-models for analysis and design. When a full 3D model is created unanticipated effects sometimes creep in, is this wrong?
Some simple examples will help demonstrate these effects.
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Continuous Beams
Traditionally continuous beam lines are analysed and designed in isolation. The modelling of the support conditions in such cases is often unsophisticated. In the example below a series of 3 span secondary beams are supported at different points on primary beams and also directly by columns.
When analysed in 3D using unadjusted gross member properties the bending moments for a general UDL case are as shown below.
Note how the end span moments for the internal beams (which all carry the same load) vary from 38.94 to 49.54 kNm. The smallest value occurs on the central column line and largest occurs where the beam is supported at the centre of the primary beam. The variation is not too significant but does start to indicate the importance of all the relative stiffnesses in a 3D model.
However, note that hogging moments develop at the extreme ends of the secondary beams where they are supported by a primary beam. Investigation would show that these develop because torsional forces are developing in the primary beams. A traditional 2D continuous beam line analysis will generally model this support as a pin and no hogging moment will develop. In fact torsion is only usually considered in traditional hand calculations where it’s development is essential to the local stability of the structure.
In generating the above results the default unadjusted properties of the rectangular beam sections were used in the analysis. These results are exactly what you would get from any general 3D analysis package. In Orion you have options to apply adjustment factors to the default properties of groups of elements as shown below.
These adjustments are discussed further in the next section, but it is worth noting here that by default Orion suggests adjusting the torsional stiffness factor of beams by a factor of 0.01 (i.e.
reducing to 1%). This is suggested because in most cases this means that the results of a 3D analysis will be more compatible with the tried and tested analysis/design achieved using older 2D idealisations.
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When the model above is reanalysed using this setting the results are as shown below.
The hogging moments have disappeared and the end span moments have increased and become more consistent at around 67 kNm.
Effects of one Member on Another
The most likely 3D effect is one where the design forces generated are not what you expect.
This sort of effect could happen in many different ways and becomes more likely as the structural arrangement becomes more complicated and irregular. However, it can be demonstrated with the very simple model shown below.
A beam is selected along grid A at the front of the model. The loading, shear and moment diagrams for this beam are shown below.
The loads are slightly offset due to the triangular area of slab being supported, but the bending moment diagram shows an (unexpected?) sagging moment at the left end support and an (expected?) hogging moment at the right end support. Can this be correct?
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The explanation relates to the diagonal beam across the floor. It has been loaded with an extra UDL. When the structure is examined in the building analysis post processor we can see how it deflects and bends.
The heavily loaded diagonal beam is putting a big moment into the supporting columns and the joint is clearly rotating. This rotation cannot happen without some effect on the other connected beams. When viewed in this way the design moments in the beam do not seem unreasonable.
Innumerable examples of this nature could be developed. The point is that if the design forces in a member seem wrong you cannot assume that the analysis is wrong. You need to review the results carefully and in particular look at the nature of deflections.
Sway Effects
Many structures will undergo a natural sway under purely vertical (gravity) loads. These sways can sometimes introduce significant changes to the expected moment diagrams in beams in much the same way as is shown in the example above.
In such cases it is important to ensure that checks are made for combinations where notional load cases are applied in sympathy with the natural sway of the structure.
In such situations where you might like to check for differences exposed by a 3D analysis of a floor in isolation, the chapter Analysis and Design using FE introduces this option.
In more extreme cases buildings stabilised by shear and core walls will sway significantly under purely vertical load, this topic is discussed in more detail in the chapter Wall Modelling Considerations.
Transfer Beams
The way in which load accumulates in columns that are in turn supported on transfer beams in a full 3D analysis is often at odds with initial/traditional expectations. This subject is specifically dealt with in the series of chapters dealing with transfer levels and in particular the section Discussion of Frame Analysis Results in the chapter Transfer Beams – General Method.