Difference between Building Analysis and FE Floor Analysis
This article explains the differences between Building Analysis and FE Floor Analysis & which method is suited for different structural system.
The fundamental differences of the analytical model created are shown in the below diagram.
For detail explanation of the entire modeling process refer to : Modelling, Analysis & Design Flowchart
Building Analysis without floor slab meshed
With reference to the above figures, for building with only beam / slab layout, i.e. no flat slab, the fastest and most efficient method is to use Building Analysis without floor slab meshed :
- The analysis only considers beam, columns & walls (i.e. frame elements only) in the analytical model.
- The physical slab itself is excluded in the analytical model, i.e. the stiffness of the slab is completely ignored.
- The slab load is automatically calculated (decomposed) onto the supporting beams & applied as external loadings onto the beams in the analytical model.
- For details, refer to Yield Line and FE Load Decomposition with Example
As such Building Analysis without meshing the floor slab cannot be used for flat slab/partial flat slab system, as there will be no result for the flat slab & the analysis result will be wrong.
Flat Slab Model
If you are modelling flat slab or partial flat slab system, there are 2 options with reference to the figure above :
1. Building Analysis with floor slab included, i.e. floor slabs of selected storeys will be considered by meshing slabs with finite element shells in Building Analysis.2. FE Floor Analysis with Gravity Load Chase down.
Method 1 : Building Analysis with floor slab included
- For structure that has transfer beams and transfer slab, supporting discontinuous columns or walls, ensure that Building Analysis is always used. Since FE Floor Analysis can only cater to gravity load, the analysis of these transfer structure will not be accurate, as lateral load cannot be considered in a single analytical model.
- For more guidance on transfer structure, refer to How to model a Transfer Beam & How to model a Transfer Slab.
- Building Analysis will inherently consider pattern / load arrangement for multi-span beams, if pattern load cases & combination are generated.
- Building Analysis is highly recommended for partial flat slab system, where there is mixture of beam-slab & flat slab :
- A single analysis can be run to obtain all the results of the frame members and flat slab, which is faster & includes pattern loading on beams.
- Ensure to mesh the slabs for floors where flat slab exists, by checking option "Include Slabs in FE Model" in Building Analysis menu > Model Options > Slab Model tab.
Method 2 : FE Floor Analysis
- In FE Floor analysis all frame elements (beams, columns & walls) as well as slabs by meshing are considered.
- FE Floor analysis ignores beam pattern loading or load arrangement, which is a requirement of most code of practice, i.e. it only assumes all beams are fully loaded. This may result in under-design of multi-span continuous beams, if load patterning results in more critical forces.
- If this is a concern, do not use FE Floor Analysis for models with beams, i.e. used Method 1 : Building Analysis.
- Alternatively, if you wish to use FE Floor Analysis for the analysis of the flat slab only, but not for beam design, you can uncheck “Merge Beam Result with Building Analysis result” in the FE Floor Analysis dialog.
- This means the beam design will use Building analysis results, which considers pattern loading, if pattern load cases & combination are generated.
- Since FE Floor Analysis only considers one floor at a time, it can be used to eliminate or minimize full 3D analysis effects such as differential axial deformation effect and natural sway of tall models. For more details refer to :
FE Floor Analysis & Gravity Load Chasedown
Since FE Floor Analysis can only consider one floor at a time (subframe analysis), in order to correctly accumulate the column & wall loads, a gravity load chasedown process from top to bottom storey is required.
Firstly, Building Analysis must be successfully run to obtain the horizontal (lateral) load results since FE Floor Analysis only deals with vertical loads.
Then proceed to perform a FE Gravity Load Chase from top to bottom storey.
The complete results for FE Floor Analysis will be the combination of FE Floor Analysis for vertical loadcases (G+Q) and from Building Analysis for horizontal load case (e.g. notional horizontal load / wind load).
For FE Gravity Chasedown procedure, please refer to Finite Elements Floor Analysis
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