ProtaStructure : Analysis

Building Analysis

Having modeled your structure and defined the parameters / loading to be applied, you are now ready to perform 3D building analysis of the whole building. 

To begin analysis, go to Analysis top ribbon > click "Building Analysis" to launch the analysis form. 


  1. If there is no previous analysis, a red cross will be shown next to the Building Analysis icon
  2. If analysis is done, a green tick will be shown next to the Building Analysis icon.  Analysis Date will also show date time stamp. 
The various functions and options are explained in the following sections. 

Building Model Check

Before making the building analysis you can use the "Building Model Check" button to make a final model check on the model you have created.  This will expose the common mistakes and errors in the model. 

If fact, it is recommended to you perform  "Building Model Check"  progressively as you build the model to trap problems early and to avoid errors being propagated or copied to other areas or floors. 
NotesFor more details refer to main article : Building Model Check

Perform Building Analysis

In the Building Analysis dialog, go to Analysis tab > click "Building Analysis" to start the analysis. 

A linear elastic static analysis is performed for every unstaged load case that has been defined and a staged construction analysis is performed for every staged construction load case.

The results for both the unstaged and staged load cases can be examined in the Analytical Model view. 

The unstaged and staged combinations are both used for design purposes.

Member Design options

When Building Analysis button is pressed, the design options dialog will appear : 


Check these boxes if you want the program to automatically run batch designs members immediately following the analysis. 

Re-select Steel Bars of RC members

Any previously designed steel will be checked using the latest analysis results, unless you check the option to re-select steel bars – in which case the program will attempt to design new steel for the latest analysis results.
InfoIdea
We recommend that for 1st and early stages, do not perform design of members this way.  The main reason is the results should be verified to be correct and reasonable before performing design, i.e. if analysis results are in doubt, the design will also be wrong. 
Further, the batch design this way does not show any details of which member is passing or failing.  For bigger models, the design process will likely be long, in which case it,  will be much for flexible and effective to design members by batches via the separate member design dialog via the Design tab. 

Perform Eigenvalue Analysis

An “Eigenvalue Analysis” can be performed as part of the Building Analysis in order to calculate natural frequencies and mode shapes, (which will be dependent on storey mass and model stiffness). No loading is used in the analysis. The Eigenvalue Analysis results can be of value if wind tunnel tests are required.

If a seismic code is selected,  the Eigenvalue Analysis will automatically be performed, as the natural frequencies & mode shapes are integral to seismic analysis & design. Hence this option will be hidden. 

After analysis is completed, go to Reports tab > click Eigenvalue Results to generate the report.

Controlling the Storey Mass for Eigenvalue Analysis

If a seismic code not selected, the storey mass for Eigenvalue analysis is always based on the dead load G only (exclude any live load, Q). 
If seismic code is selected, the storey mass for Eigenvalue analysis is always based on the dead load G plus a fraction of the live load Q, i.e. :  
  1. Storey weight = G+nQ :   where "n" is generally know as participation factor or simultaneous presence factor. 
  2. This factor is set for each storey under Edit Storey menu under column "Simultaneous Presence" - whose field is only visible when a seismic code is selected. Example below is when EuroCode 8 seismic code is selected. 

The G and Q components of the Storey Mass/Weight can be derived from the Decomposed Beam Loads, or the Undecomposed Slab Loads, the choice of option being controlled via the Building Analysis menu > Model Options tab > Settings tab > Storey Weight and Center of Gravity Calculations :
  1. If all slabs transfer their loads to beams or walls, either choice should produce a similar mass/weight.
  2. However in flat slab models this is often not the case - the mass determined using the decomposed beam loads option is likely to be significantly smaller than that from the undecomposed slab loads option.
  3. In such models it is important that the latter option Use Undecomposed Slab Loads is always selected.

Model Stiffness for Eigenvalue Analysis

The engineer should use section properties that are appropriate for the Eigenvalue Analysis. For columns and walls this could involve making global stiffness adjustments to model cracked section properties. The ACI code may be referred to for some guidance in this regard. These adjustments can be made via the Material and Section Effective Stiffness Factor table in Building Analysis form.

Controlling the Number of Mode Shapes Required

These are set on the Lateral Loading tab of the "Building analysis" > "Settings Center" dialog.

Graphical Results

To view a mode shape, launch the Analytical Model view after completing Building Analysis.


Go to Analysis tab >  Analytical Model > Building Analysis Model (as below)


Analytical Model view will be created as shown below for example quick start guide model. 



  1. In the Results tab, ensure Displacement is activated. 
  2. Click Animation to clearly visualize the vibration mode of the building.
  3. Click " + Increase Scale " or "Auto Scale"  if the vibration is not visually significant.
  4. Select Model Shapes in the right pane. 
The below shows the animation of the first 3 modes : 



Numerical Results

Numerical output from the analysis consisting of frequencies and mass participation can be accessed from the Report tab > “Eigenvalue Results Report”  button.

Display Analytical Model Before Analysis

Checking this option enable you to review the underlying Analytical Model before the analysis proceeds further to work out the members forces, & other results such as deflections and design data. 

The 3D model you built is a physical model with actual member size rendered visually.  The analysis converts the physical model into the analytical model (as shown below). 
  

The analytical model is essentially a centre-line wireframe model. For members to be connected properly, the analytical wire frame must meet a common joint. Joints are insertion points created at intersection of axes. 
Notes
Enabling this option is useful as you can quickly check the validity of the analytical model before wasting precious time before continuing the analysis which requires time to complete.  If you find problems in analytical model, you can stop the analysis by clicking "Cancel" in the analytical model preview and then go back to the model to take correction action.

Refresh the Connectivity Information of All Members

This option refresh or regenerate the connectivity date of members before analysis. We highly recommend this option is checked by default. 

Save Foundation Column/Wall Results for Multi-block Combined Foundation Design

This option ensure the foundation column & wall results are properly saved for multi-block combined foundation design. For more details refer to :  Linked Model Manager with Examples

Axial Load Comparison Report

After running building analysis successfully, the Axial Load Comparison Report can generated. This report provides a verification for the total dead and live load applied to the building. It can also can be used to cross check:
  1. slab loads have been correctly decomposed on to supporting members
  2. gravity load applied matches the building analysis total vertical reaction
  3. gravity load applied matches the FE chasedown total vertical reaction
This report sums all of the dead and live load applied at each storey and displays the axial forces in the columns and shear walls. These values need to agree with each other within a tolerance limit (5% by default). If they do not, it may indicate loads are lost due to possible error in modelling. Hence, the reason for the discrepancy should be investigated.

Table 1 : TOTAL LOADS (Based on Slab) is sum of dead and live loads of all members with the slab load not yet decomposed or calculated on the beam. You can take this as the input weight of the structure.

Table 2 : TOTAL LOADS (After Decomposition of Beam) takes into account the decomposition of the slab load onto the beams based on either yield-line or FE Decomposition. The beam load now includes the slab loads (and hence zero values are shown under the slab column).

Table 3 : BUILDING ANALYSIS COLUMNS AND WALL AXIAL LOADS sums up the actual column and wall axial loads after building analysis (for regular beam/slab model)

Table 4 : FINITE ELEMENT ANALYSIS COLUMN/WALL AXIAL LOADS sums up the actual column and wall loads after Finite Element Chasedown process from top to bottom storey (required for flat slab models)

For beam slab model (all slabs supported properly by beams), check the following:
  1. Firstly, check Table 1 total values are similar to Table 2. This ensures that all slab loads are accurately captured by beams, i.e. no slab loads are lost.
  2. Then verify Table 2 total values are similar to Table 3. This ensures that all the superstructure weight are completely captured by the columns and walls down to the foundation.
For flat slab models, check the following:
  1. Check Table 1 total values are similar to Table 4. This ensures that all the superstructure slab weight are completely captured by the columns and walls down to the foundation.
  2. Ignore Table 2 and Table 3 and the associated warnings because these tables are only applicable to beam slab model
Notes
For a detail example of Axial Load Comparison refer : Overview of Axial Load Comparison Report (Detail Example)

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