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3D pre analysis processes
3D pre analysis processes
Pre-Analysis consists of a number of processes, such as:
- Decomposing slab and wall loads
- Preparing loadcases and combinations
- Meshing and diaphragms
- First-order gravity analysis
- Resolving vertical loads for application of global imperfections
- Generation of pattern loading
The actual pre-analysis processes performed will vary depending on the specific model that has been defined.
Overview of slab load decomposition
Decomposition of slab loads on to supporting members is automatically performed where necessary during pre-analysis.
Decomposition is not just performed for beam and slab models, the program may also need to decompose flat slab loads onto supporting columns and walls.
In Tekla Structural Designer the term "Decomposed Loads" refers to the loading on beams, columns, and walls that comes from slabs
Whether load decomposition is performed or not will depend on the analysis model, the slab properties and the Mesh 2-way slabs in 3D Analysis setting as follows:
Decomposition method specified in Slab properties | 3D Analysis and Grillage chasedown models | FE chasedown model | |
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Two-way | Mesh 2-way slabs in 3D Analysis option not selected (default):
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| |
Mesh 2-way slabs in 3D Analysis option selected:
| |||
One-way |
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|
Potential Load Decomposition Methods
Traditionally 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.
Overview of global imperfections
Equivalent notional horizontal loads are determined and applied during pre-analysis to cater for global imperfections (additional second order effects due to the structure not being built plumb and square). These loads are also used in seismic design to establish the base shears.
Following a first-order analysis of all gravity loadcases, the forces at the nodes at the top/bottom of each column stack/wall panel are resolved vertically. A proportion of the vertical load is determined which gives the value of the horizontal load at each point. The proportion is code dependent.
These horizontal loads are applied to the nodes in a particular direction (Direction 1 or Direction 2 or both) as specified in an individual design combination.
Overview of load reductions
Live load reductions are established during pre-analysis for use in subsequent column and wall design, and when the head code is set to ACI/AISC - beam design also.
- For head codes other than ACI/AISC, the level of live load reduction to consider for beams can be set manually in the beam properties. This is especially useful for more economic design of transfer beams supporting a number of floors.
Reductions are only applied to those live load cases that have had the Reductions box checked in the Loading dialog.
The reduction percentage to be applied is specified in Model Settings. This percentage can differ depending on the number of floors being supported.
To cater for additional floors that are carried but that have not been included in the model an Assume extra floors supported value can be specified in the column and wall properties.
The methodology for live load reduction differs between national codes of practice:
Head Code: EC or BS
Levels can be designated either as "to be" or "not to be" included in the determination of the load reductions through Count floor as supported check boxes for each level in the column and wall properties. This feature enables what appears to be a roof to be counted as a floor, or conversely allows a mezzanine floor to be excluded from the number of floors considered for any particular column or wall. The moments from fixed ended beams framing into a column or wall are never reduced.
Head Code: ACI/AISC
Before undertaking member design, Live and Roof Live loads are multiplied by a reduction factor R for roof live loads and other live loads independently. This reduced load is then used in combination to create design forces. The reduction factor is related to the tributary area of load carried by the particular member and also the KLL factor, where KLL comes from Table 4-2 in ASCE7-05/ASCE7-10.
Essentially:
- Interior and exterior cols (no cantilever slabs) KLL = 4
- Edge and interior beams (no cantilever slabs) KLL = 2
- Interior beams (with cantilever slabs) KLL = 2
- Cantilever beams KLL = 1
- Edge cols (with cantilever slabs) KLL = 3
- Corner cols (with cantilever slabs) KLL = 2
- Edge beams (with cantilever slabs) KLL = 1
As it is not possible to assess where cantilever slabs are and what they are attached to - you are able to change the KLL factor for all column/wall stacks and beam spans as required in the Properties Window.
The default settings are:
- Columns/walls = 4
- Beams = 2
- Cantilevers = 1
Head Code: AS
Before undertaking member design, imposed loads are multiplied by a reduction factor ψa.
This reduced load is then used in combination to create design forces. The reduction factor is related to the tributary area of load carried by the particular member.
Overview of pattern loading
If combinations of pattern load exist then the pattern loading is automatically generated prior to analysis. See: Manage load patterns