One-way and two-way load decomposition
Apart from when slabs are set to be meshed in the analysis, any panel loads applied to slab panels and roof panels have to be decomposed on to supporting members prior to analysis.
Potential load decomposition methods
In traditional hand calculations a "tributary area" (sometimes called "yield line") loading approach would have been adopted to determine the decomposed loads, but this has limitations when dealing with complex geometry such as:
- Slabs not supported on all edges
- Complex panel shapes
- Panels with openings
Also the "tributary area" approach can only approximately handle point, line, and patch loads (by converting them to area loads).
Because of these limitations the "tributary area" method is not used in Tekla Structural Designer - instead a method referred to as FE decomposition is applied instead. This is based on finite element analysis.
FE decomposition model
The FE decomposition model can be demonstrated using the following two-way slab on beams example.
For this type of structure:
- A separate FE decomposition model is created for every floor (or sloped plane).
- Beam column and wall nodes in each FE decomposition model (shown selected in pink below) are all rigidly supported.
• Each FE decomposition model is analysed and the reactions at the rigidly supported nodes are turned back into VDLs along beams and walls, and point loads on columns.
A common question
So if the slabs have to be meshed for the FE decomposition during pre-analysis, why not do away with decomposition entirely and just mesh the slabs when performing 3D analysis of the entire model instead?
Because this gives results that you don't like....
In the first run of the model below the slabs are left unmeshed, this requires the loads on the two-way slabs to be FE decomposed prior to analysis. The bending moments from the resulting 3D Analysis are as shown, (max hogging -198, max sagging 165):
The above results can be compared against a second run of the model in which the 2-way slabs are set to be meshed in the 3D analysis. FE decomposition is no longer required and the bending moments from the resulting 3D Analysis are as shown, (max hogging -89, max sagging 82)
In this second run, because the slabs are included in the 3D analysis model, some of the load is being transferred directly to the supporting columns and walls via the slabs themselves. While this is not wrong, it goes against the engineer's expectation - which would be to design the beams on the basis that they transfer all the load to the supporting members.
Controlling decomposition via the 'Mesh slabs in 3D Analysis' level property
As stated in the previous section, the engineer's expectation would be to design the beams on the basis that they transfer all the load to the supporting members. This is achieved in Tekla Structural Designer because (by default), slabs are not meshed in the 3D analysis and grillage chasedown solver models.
Note however that by switching on the Mesh slabs in 3D analysis level property the engineer can allow some of the load to be transferred directly to the supporting columns and walls via the slabs.
| Solver model under consideration | |||
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| Decomposition method in slab properties | 3D Analysis and Grillage chasedown models | FE chasedown model | |
| Two-way |
Mesh slabs in 3D analysis OFF (default):
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Mesh slabs in 3D analysis ON:
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| One-way (with Analyze slab OFF) |
Mesh slabs in 3D analysis OFF
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Mesh slabs in 3D analysis ON
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| One-way (with Analyze slab ON) |
Mesh slabs in 3D analysis OFF
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Mesh slabs in 3D analysis ON
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1 - Beams designed on the basis that they transfer all the load to the supporting members 2 - Beams designed on the basis that slabs transfer some of the load to the supporting members |
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Decomposition rules for the different types of panel and overlapping panels
The way in which Tekla Structural Designer decomposes panel loads depends on how the slabs/panels are modeled and how they are spanning.
| Slab/Panel type | Decomposition |
|---|---|
| One-way spanning slab item |
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| Roof panel |
Panel loads applied to the roof panel are 1-way decomposed directly on to supporting members/two-way spanning slab edges before performing 3D analysis. The rotation angle of the roof panel determines the decomposition direction. |
| Two-way spanning slab item |
Note: Where openings
have been defined, any portion of a panel load that lies within
the opening will not be decomposed.
|
| Roof panel overlapping a one-way spanning slab item |
Panel loads are 1-way decomposed directly on to supporting members/two-way spanning slab edges before performing 3D analysis. The rotation angle of the slab item (and not the roof panel) determines the decomposition direction. Note: Any openings in
1 way slabs are ignored and thus have no impact on load
decomposition.
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| Roof panel overlapping a two-way spanning slab item |
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How to view the solver model used for load decomposition
- Open a 2D view of the required level.
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Display the Level properties in the Properties window
- A separate FE decomposition solver model will only be used provided you have not chosen Mesh 2-way Slabs in 3D Analysis
- Furthermore, you will only be able to view this FE decomposition solver model provided Keep solver model has been selected.
- If necessary, re-run the analysis to create the FE decomposition solver model.
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Set the view regime to
Solver View
- From the right-click context menu select Solver Models > Solver Model used for Load Decomposition