ProtaStructure : Overview of Axial Load Comparison Report (Detail Example)

Overview of Axial Load Comparison Report (Detail Example)






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

Beam Size  : 250mm (width) x 500mm (depth) 
Column Size : 250mm x 250mm
Column Height : 3000mm
Slab Thickness : 150mm - therefore self-weight = 3.6 kN/m2
Loading on slab : Self-Weight (calculated automatically based on thickness of slab)
: SDL = 1 kN/m2
: Total Dead Load = 4.6 kN/m2
: Imposed Load = 0 kN/m2

The following report is produced by ProtaStructure for this model. Note that there are 3 main sections (tables) of information the headings of which have been emboldened. 




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: Total Loads (Based on Slabs)


This table shows the sum of:
  1. Member self‐weights
  2. Applied loads on columns/walls
  3.  Applied loads on beams
  4. 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: Total Loads (Decomposition to Beams)

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 Decomposition


Once again, this table shows the sum of:
  1. Member Self‐weights
  2. Applied loads on columns/walls
  3. Applied loads on beams 
  4. Applied loads on slabs

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:

Decomposed Slab Load    = (A1 + A2 + A3 + A4) kN/m2 <Please refer to diagram> - adjustment loads
                                           = 4 x 28.75 kN - 9 kN
                                           =  106 kN

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:

                               Columns Self Weight      = UNCHANGE
                                                                         = 18 kN


                               Beams Loads                   = 57 kN + 106 kN
                                                                         = 163 kN

Using Finite Element Decomposition 

If 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.


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.
Notes
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 or "Include Slab in Building Model" must be used and the discrepancy between the totals in Tables 1 and 2 should be seen as an  indicator of this requirement.
Notes
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 undecomposed load in Table 1.

Table 3: Building Analysis Column and Wall Axial Loads



The figures in table above 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.

Axial Load Comparison Warnings

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:
  1. If there were warnings during the building analysis, the warnings should not be ignored and corrective action should be taken.


This error normally due to some common building modelling mistakes. To resolve this issue: 
  1. Check whether the discontinuous columns and beams are properly supported.
  2. Use "Load Decomposition by FE" instead of "Yield Line Method" to ensure loading on slab (ie. nodal load, UDL). For more detail, please refer to Yield Line and FE Load Decomposition with Example.
  3. Make sure "Similar Storey" set in "Edit Storey" should have the same storey height. For example, if 2nd storey with 4m height and 3th storey with 3m height set as similar storeys, this will causes axial load comparison error. Exclude the storey which has different height will solve this issue.
Info
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. Alternatively. mesh the floor slabs by ticking the option "Include Slabs in Building Model" in Building Analysis menu.

Axial Load Comparison Warning Limitation

A false warning may pop out when: 
  1. Too many (i.e. 4 or more) "Plane Definition" created at the same storey. This will affect the loading captured in table 1.
  2. "Plane Definition" created on top of a slanted slab (i.e. Ramp). This will affect the loading captured in table 1.
  3. “Support Type” is defined in beam. This will affect the loading captured in table 3.
  4. Loadings applied on sag rods. 
  5. Dome applied on a similar storey floor. This will affect the dome loading captured in table 1 and 2. Reset the similar storey to resolve this warning.
  6. Loading applied onto rigid zone (red). This loading will not be captured in table 1 and table 2 but correctly captured by table 3 (Building Analysis Result).

Warning
If this happens, user may choose to ignore the Axial Load Comparison Warning upon self checking and verifying the behaviour and the forces of the Analytical Model.

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