Overview of the slab model
Slabs allow you to decompose loads placed on a floor back to the supporting structure. You can create slabs in either levels or inclined planes.
Slabs and slab items
Tekla Structural Designer stores slab data in the form of parent slabs, each consisting of one or more individual panels, called slab items. Slab items can be either connected or separated from each other. However, they must be on the same level.
Every parent slab has a unique name. A slab name used at one level can not be re-used at a different level.
Every parent slab has general properties of slab type, associated deck type and decomposition as follows:
Slab Type | Deck Type | Decomposition |
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Slab on beams |
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Flat slab |
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Precast |
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Composite slab |
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General |
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Foundation mat |
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Slab items initially inherit their properties from the parent slab. However, once the slab items have been created, you can modify them to amend certain properties individually: for example you can change the rotation angle, override the slab depth, or exclude a panel from the diaphragm. Once you have overridden the slab depth, you can also apply a vertical offset in order to model a slab step.
The relationship between slabs and slab items means that at the initial modeling stage you can create slabs over a wide area. While creating slabs, you do not have to consider sub-sections of the slab - you can simply create one big expanse of slab.
When it comes to design, you need to conceptualize the slab as a series of design panels, (i.e. slab items). Each slab item will have its own design settings and its own design results. You can select different reinforcement in different slab items. You also have to consider pattern loading, where some panels are loaded, and others not.
When results are later presented in calculations and drawings, you can specifically reference the design slab items.
Comparison of slab types
The different slab types available in Tekla Structural Designer are compared in the below table.
Slab on beams | Flat slab | Precast | Composite | General | |
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Deck Type |
• Reinf. concrete • Post tension |
• Reinf. concrete • Post tension |
Precast concrete planks | Profiled metal decking |
• Steel plate • Timber • General |
Note: Design of post tension slabs is beyond scope |
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See: Create slab items |
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Decomposition |
• 1-way • 2-way |
2-way | 1-way | 1-way |
• 1-way • 2-way |
Diaphragm action |
• rigid • semi rigid • none |
• rigid • semi rigid • none |
• rigid • semi rigid • none |
• rigid • semi rigid • none |
• rigid • semi rigid • none |
Load | all panel load types | all panel load types | all panel load types | all panel load types | all panel load types |
See: Apply panel loads |
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Modification factors for 2-way spanning slabs | yes | yes | n/a | n/a | yes |
Meshed in 3D analysis and grillage chasedown analysis |
• 1-way: no • 2-way: optional |
optional | no | no |
• 1-way: no • 2-way: optional |
See: Define whether slabs are meshed for 3D building analysis and grillage chasedown analysis |
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Meshed in FE Chasedown analysis |
• 1-way: no • 2-way: yes |
yes | no | no |
• 1-way: no • 2-way: yes |
See: FE meshed slabs |
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Designed in Tekla Structural Designer |
• 1-way: beyond scope • 2-way: yes |
yes | beyond scope | beyond scope | beyond scope |
Vertical alignment
The vertical alignment of all slabs in a given level is determined by the construction level type specified in the Construction Levels dialog. The possible types are:
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T.O.S. (Top Of Steel): the base of each slab item aligns with the level.
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S.S.L. (Structural Slab Level): the top of each slab item aligns with the level.
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T.O.F. (Top Of Foundation): the top of each slab item aligns with the level.
Vertical offsets
A vertical offset can be specified after checking ‘Override slab depth’ in the slab item properties.
These offsets are not structurally significant in slabs as they have no effect on the analysis mesh position relative to the top of the slab in the solver model. However, the mesh properties would reflect any change in the slab thickness.
Slab edges
Slab edges can be either straight or curved.
Slab openings
You can create openings of any shape within slabs. These are only considered when the slab is meshed for load decomposition or analysis.
Column drops
Column drop panels are slab items with an increased thickness. In flat slabs, at points where the slab is supported by columns, you can use column drops to thicken the slab.
See: Create column drops
Panel sub-division
Regardless of how the slabs and slab items are initially created, you can further divide or re-form them with the Slab Split and Slab Join commands. There are several reasons why you may choose to use the previously mentioned commands, related to adding steps, loading patterns, and designing panels.
For flat slabs in particular, the way that slabs are split for the purposes of pattern loading is a matter of engineering judgement. The views below view two options that two different engineers might both justifiably choose for the same slab perimeter.