Orion Documentation page 102 Chapter 5 : General Building Analysis
A simple model can be created for a simple illustration of the Axial Load Comparison Report.
The 3D view of the model is as above. The plan view of the model is as below.
General information regarding the model.
The following report is produced by Orion for this model. Note that there are 4 main sections (tables) of information the headings of which have been emboldened.
• Beam size: 250 mm (width) x 500 mm (depth)
• Column size: 500 mm x 250 mm
• Column Height: 2800 mm
• Slab Thickness: 175 mm – therefore self-weight = 4.2 kN/m2
• Loadings on slab: self-weight (calculated automatically based on thickness of slab) SIDL = 1 kN/m2
Total Dead Load = 5.2 kN/m2 Imposed Load = 0 kN/m2
AXIAL LOAD COMPARISON REPORT
SUM OF APPLIED LOADS (Using Un-Decomposed Slab Loads):
SUM OF APPLIED LOADS (After Decomposing Slab Loads):
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The purposes and validity of the comparisons that can be made between the sections of this table are discussed in the following sections.
Table 1
SUM OF APPLIED LOADS (Using Un-Decomposed Slab Loads):
This table shows the sum of:
• Member self-weights
• Applied loads on beams
• Applied loads on slabs
The slab loads are calculated based on the area as seen on plan – i.e they are not yet decomposed onto beams.
The detailed calculations are therefore as follows:
Table 2
Differences between this table and table 1 are specifically intended to expose problems in slab load decomposition. Since there are two methods of slab load decomposition supported we will look at each of these separately.
Using
Yield Line
DecompositionSUM OF APPLIED LOADS (After Decomposing Slab Loads):
Once again this table shows the sum of:
• Member Self-weights
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The key difference in this table is that the slab loads are now decomposed and thus counted as UDL’s, VDL’s etc. on the supporting beams. Therefore, you will find that the slab loads become zero but the beam loads increase accordingly.
Decomposed Slab Loads – Consider again the model and the decomposed slab loads shown below.
The plan view shows the yield lines, strictly speaking these are really just load decomposition lines which are used to show the area of slab loading that will be attributed to each beam. This method of area load decomposition is commonly known as the Yield Line Method. Looking at the triangular load distribution generated on the above beam, the beam load calculation effectively becomes:
The beam loading profile and the above calculation clearly get much more complex when more irregular slab arrangements are used.
The calculations reported in table 2 are therefore as follows:
Decomposed Slab Load = (A1 + A2 + A3 + A4) kN/m2 <Please refer to diagram>
= 4 x 29.33 kN
= 117.3 kN
Columns Self Weight = UNCHANGED
= 33.6 kN
Beams Loads = 65 kN + 117.3 kN
= 182.3 kN
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Using
Finite Element
DecompositionIf slab loads are decomposed using the alternative FE method, the loadings from the slab are applied to the beam as a complex beam load as shown below.
SUM OF APPLIED LOADS (After Decomposing Slab Loads):
Comparisons between tables 1 and 2
Comparison between these 2 tables provides an indication of the success of the slab load decomposition. If the totals in table 2 are less than those in table 1 this would give an indication that loads have gone missing during the decomposition process in which case you should:
1. Check that slab loads are applied within slab boundaries
2. Visually Check Yield Lines – if they do not look right on the plan the decomposition is probably not right.
3. If Yield Lines are wrong, consider swapping to the FE Load Decomposition method.
Note Both FE and Yield Line decomposition decompose slab loads to beams. If there are areas without beams (where slabs are supported directly by columns – i.e.
Flat Slabs) the decomposition process is guaranteed to lose load. In such cases full FE analysis must be used and the discrepancy between the totals in tables 1 and 2 should be seen as an indicator of this requirement.
Note In all but the simplest of models, there will always be a small discrepancy between table 1 and 2. However, this can be ignored especially if the decomposed load in table 2 is slightly higher than the un-decomposed load in table 1.
Table 3
The figures in this table reflect the results of the building frame analysis. The building analysis is a frame analysis where the beams are loaded with all the decomposed slab loads. Therefore the input is based on either the yield line or FE Load Decomposition method; whichever has been selected.
When the analysis is complete the accumulated column loads on each storey are shown in the table.
It is therefore appropriate to compare table 2 with table 3 which is in effect a comparison of analysis input with analysis results. If the totals are different the building analysis is incorrect in some way in which case you should check:
• If there were warnings during the building analysis, have you ignored them and is it OK to ignore them?
• If transfer levels exist, check whether the discontinuous columns are properly supported.
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Note Flat slab models are a good example of the case where there will be discrepancies and loads are lost, but this can all be ignored since Finite Element Analysis load Chase Down is required – see next section.
Table 4
Note This table is only available if an FE Chase Down (finite element analysis load chase-down) has been performed:
When performing the FE Floor Analysis, there are 2 options available.
Option 1: Include Slab Plates in FE Model
When this option is active as shown above, a meshed up FE model is created as shown below.
Slab loads are applied to all the individual shells and so the slab load decomposition is effectively inherent in the FE analysis of the floor.
After analysis the total axial loads in columns and walls are derived at each level and reported in table 4 as shown below.
The total loads and self-weights reported will always be slightly different in FE analysis because this is a true centre-line model. In this simple example the loads effectively total up as follows:
Note The area of the slab used in the FE model is different (greater) than the area used in tables 1 and 2 because the slab extends to the centre-line (not the edge) of the beam. This has two effects:
• The self weight if the slab in the zone that overlaps with the beam is double counted – small extra load.
Column Self Weight = 0.25 m x 0.5 m x 2.8 m(height) x 24 kN/m3 x 4(nos)
(i.e. self weight only – finishes loads are not applied to the beam)
Slab Self Weight = (5 m x 5 m) <area of meshing plan> x 5.2 kN/m2
= 130.0 kN (see note below)
Total = 60 kN + 33.6 kN + 130.0 kN
= 223.6 kN
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• The finishes and imposed loads that apply on the outer edge of edge beams will not be counted – small loss of load.
When this option is used it is appropriate to compare the totals in table 1 with table 4 (Load before decomposition and FE analysis results which embody both decomposition and analysis). Some degree of difference must be accepted for the following reasons:
• In all models, there will be a slight difference for the reasons noted above.
• The FE method makes much better allowance for holes in slabs.
If totals returned from FE Analysis (table 4) are significantly less than totals from table 1, the FE Analysis model may be incorrect. In such cases the following possibilities should be checked:
• Are slab line loads applied outside the building perimeter or over voids.
• If the option to include column and wall sections in the model has been used, are slab line loads applied through columns or walls (the load inside the column or wall boundary is included in table 1 but is not taken into account in the FE analysis because the load could not physically exist.)
• As above, but checking for point loads applied inside column boundary.
• Incorrectly modelled cantilever slabs.
Note The totalling of loads in table 4 will be incorrect if some columns are len(storey) 2 or more, but the results themselves are actually correct.
Option 2: Exclude Slab Plates in FE Model
In this case the FE analysis is simply a 3D frame analysis where the decomposed slab loads are applied to the beams.
In this case it is valid to compare table 2 with table 4 since this is checking that the
decomposed loads which were intended to be applied to the FE models have been successfully applied.
Orion Documentation page 114 Chapter 6 : Eigenvalue Analysis
Chapter 6 Eigenvalue Analysis
Introduction
An Eigenvalue Analysis can be performed as part of the Building Analysis in order to calculate natural frequencies and mode shapes; these will be dependent on storey mass and model stiffness. The Eigenvalue Analysis results can then be used for seismic design purposes and can also be of value if wind tunnel tests are required.