Overview of the slab model

Tekla Structural Designer
2021
Tekla Structural Designer

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
Slab on beams
  • Reinforced concrete
  • Post tension
  • One-way
  • Two-way
Flat slab
  • Reinforced concrete
  • Post tension
  • Two-way
Precast
  • Precast concrete planks
  • One-way
Composite slab
  • Profiled metal decking
  • One-way
General
  • Steel plate
  • Timber
  • General
  • One-way
  • Two-way
Foundation mat
  • Reinforced concrete
  • Post tension
  • Two-way

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.

Note: If you amend other properties, such as material grades or general design parameters you will find they are updated not just for the individual slab item, but for all other slab items within the parent slab.

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
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

See: Create slab items

Decomposition

• 1-way

• 2-way

2-way 1-way 1-way

• 1-way

• 2-way

See: One-way and two-way load decomposition

Diaphragm action

• rigid

• semi rigid

• none

• rigid

• semi rigid

• none

• rigid

• semi rigid

• none

• rigid

• semi rigid

• none

• rigid

• semi rigid

• none

See: Overview of diaphragm action in roof panels and slabs

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

Modification factors for 2-way spanning slabs yes yes n/a n/a yes

See: Analysis Settings>Modification factors

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

Meshed in FE Chasedown analysis

• 1-way: no

• 2-way: yes

yes no no

• 1-way: no

• 2-way: yes

See: FE meshed slabs

Designed in Tekla Structural Designer

• 1-way: beyond scope

• 2-way: yes

yes beyond scope beyond scope beyond scope

See: Design slabs and run punching shear checks

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:

  • T.O.S. (Top Of Steel): the base of each slab item aligns with the level.

  • S.S.L. (Structural Slab Level): the top of each slab item aligns with the level.

  • 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 openings

You can create openings of any shape within slabs. These are only considered when the slab is meshed for load decomposition or analysis.

See: Create slab or mat openings

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.

See: Split and join slabs and mats

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