Building Analysis
Having modeled your structure and defined the parameters / loading to be applied, you are now ready to generate the member design forces. These are determined by performing a 3D analysis of the whole building.
To begin analysis, go to Analysis top ribbon > click "Building Analysis" to launch the analysis form.
On this page options are also provided to run a Building Model Check, to perform an Eigenvalue Analysis, to run batch Column/wall and Beam designs, batch Steel Member Check and to display the Axial Load Comparison Report.
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.
If there are any errors or warnings, you have to return back to the Graphic Editor and modify the members that cause the problem.
Building Model Validity Check option can be applied to the "Current Storey" or "All Storeys".
For a more detailed article, please follow this link: 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.
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 then be used for seismic design purposes and can also be of value if wind tunnel tests are required.
Controlling the Storey Mass for Eigenvalue Analysis
The storey mass for Eigenvalue analysis is always based on the dead load G plus a fraction of the live load Q. The degree to which the live load is assumed to participate is controlled by the participation factor (n), which can be specified on the "Lateral Loading" tab of the "Building analysis" > "Parameters" dialog.
Note: For Lateral Load calculations, the storey weight can be based on G or Q or G+nQ. The Live Load Participation Factor, (n) does not affect the Notional Load Calculation unless the G+nQ option 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 “Settings” tab of the Building Analysis “Model Options" dialog.
The mass/weight determined for the chosen option can be reviewed by hovering the cursor over the Center of Gravity indicated on the form plan after running a Building Analysis.
If all slabs transfer their loads to beams or walls, either choice should produce a similar mass/weight. 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. In such models it is important that the latter option 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 “Model Options/Stiffnesses” tab of the 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 "Model and Analysis Results Display" (Building analysis > Post analysis tab) > activate the “Displacements” button (in the "Results" tab) and select the mode shape required from the Loading menu (on the bottom of right panel). Animation can be activated if required from the "Displacements" menu.
Numerical Results
Numerical output from the analysis consisting of frequencies and mass participation can be accessed from the “Report” tab > “Eigenvalue Results Report” Button.
Column/Wall and Beam Reinforcement Design
Check these boxes if you want the program to automatically run batch designs of the columns/walls and beams immediately following the analysis.
For a bigger model, we suggest you do not check this option. The columns/walls and beams can be designed in Run > Column Section Design and Run > Beam Reinforcement Design > Storey Beams, respectively.
Re-select Steel Bars
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.
Steel Member Design Check
Check this box if you want the program to automatically check the steel members immediately after the analysis.
All steel beams, columns, bracings, girts, purlins, and trusses will be checked if this option is checked.
It is highly recommended that users leave these boxes unchecked in the first instance so that they can check their analytical model before design is being performed. Users can check these options in accordance with their requirement after performing minor or no changes to their model after checking is done.
Axial Load Comparison Report
The "Axial Load Comparison Report" provides a verification for the total dead and live load applied to the building. It can also can be used to cross check:
- slab loads have been correctly decomposed on to supporting members
- gravity load applied matches the building analysis total vertical reaction
- 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:
- 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.
- 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:
- 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.
- Ignore Table 2 and Table 3 and the associated warnings because these tables are only applicable to beam slab model
For a detail example of Axial Load Comparison refer : Overview of Axial Load Comparison Report (Detail Example)
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