Concrete slab design fundamentals

Tekla Structural Designer
Tekla Structural Designer

Concrete slab design fundamentals

Slab panels

Following analysis, the area of steel required for a given panel is determined from the analysis results.

For each panel the basic design check is performed in all areas where no patches exist:
  • Is A s,prov > As,reqd ?

Slab patches

Patches are used during the design of concrete slabs as a way of managing the physical and design data. Each patch defines a rectangular area of slab within which FE analysis results are collected, enabling Tekla Structural Designer to perform the patch design process. Design moments are calculated along result strips, embedded within each patch. Depending on the patch type, a patch can contain up to 6 result strips, catering for up to 3 strips of reinforcement in each of two perpendicular directions.


Punching checks can be requested at any

  • column stack/slab (or foundation mat) intersection
  • wall stack/slab (or foundation mat) intersection
  • applied slab point load

For further details, see: Punching shear check fundamentals

Concrete type

While you can apply both normal and lightweight (LW) concrete in the slab properties, slab design using lightweight concrete is only available for Eurocodes.

When using other regional codes slabs can only be designed using normal weight concrete.

LW density classes and grades

6 Density classes (1.0, 1.2 ….. 2.0) are available and 15 default grades are provided; 5 in each of the density classes: 1.6, 1.8 and 2.0.
  • For example the grade name “LWAC30/37-DC1.8” denotes; LWAC = Lightweight aggregate concrete; 30/37 = Strength class; DC1.8 = density class.
  • Custom LW grades can be added for which note that new LW-specific property η1 must be specified.
Note: LW grades can be reviewed, edited and applied via Review View > Show/Alter State Material Grade Attribute.

How the load decomposition method affects slab design

Only slabs that have been specified with two-way load decomposition are designed.

One-way load decomposition in Tekla Structural Designer is a simple procedure that does not determine slab design forces. When a slab's decomposition is set as one-way it is assumed that it is some form of precast slab (presumably designed by safe load tables).

  • A flat slab panel always uses two-way load decomposition.
  • A slab on beams panel can either be specified to use one-way or two-way decomposition - however if it is specified as one-way it cannot then be designed in Tekla Structural Designer.

It should be noted that any in-situ slab is capable of two-way decomposition:

  • When a slab is set as two-way it will only effectively span in 2 directions if its proportions and support conditions mean that there will be a two-way effect.
  • For example - If a slab that has a span of 6 units in one direction and 50 units in the other is set to two-way decomposition, then although it is two-way the FE analysis will still inherently take the load one-way.

Two-way spanning slab panel design moments

When a slab panel is specified with two-way decomposition, a general FE based approach is used to determine the design moments. If you have elected to mesh 2-way slabs in the analysis, the 3D Analysis results are also considered:

  • The worst design moment (per unit width) is found in each direction of the slab - if the design moment is zero in one of the directions then the analysis has shown that the slab is effectively spanning one-way and the supplied reinforcement in this secondary direction will be selected to suit the minimum requirements of secondary reinforcement.
  • Note that this FE based approach inherently caters for point loads, line loads, openings, etc and for the possibility of variable adjacent span lengths in a continuous "1-way" slab (and of course it can still be applied to the simple case of a "1-way" slab with a uniform UDL applied and uniform span lengths).

Reinforcement design settings

Prior to running slab design you should review the design settings by selecting Design > Settings > Concrete > Slab.

These settings are used to control:
  • Reinforcement layout, e.g.
    • Minimum and maximum user bar sizes
    • Minimum and maximum user spacing values
    • Maximum bar lengths
    • Auto selection of outer bars
    • Use of mesh
  • Patches
    • Default patch size
    • Use of mesh
  • General parameters (depending on the regional code being designed to)

For further details, see: Slab design settings

Slab panel reinforcement

In a slab you have two surfaces: Top (T) and Bottom (B).

In each surface you have two layers of steel in orthogonal directions - X direction steel and Y direction steel. Layer 1 is the outside layer - the one closest to the surface. Layer 2 is the inside layer. Which direction is the outer layer is controlled by this slab panel setting: Outside layer in X direction.

Any of the four layers can be set to "none" if required.

Alignment of slab panel reinforcement

X direction reinforcement is aligned to the panel span, which is determined by the slab's rotation angle property. Y direction reinforcement is always perpendicular to the X direction reinforcement.

For further details, see: Rotation angle for slab items and panels

Slab patch reinforcement

Tekla Structural Designer contains the following patch types:

  • Column patch: can be placed at column stack heads
  • Beam patch: can be placed along beams
  • Wall patch: can be placed along walls
  • Panel patch: can be placed at a specified position within the panel boundary
    • not restricted to a centralized position or to existing within one panel
    • can be positioned under loads

Patches can be either on the top or the bottom of the slab and may or may not have reinforcement defined in them. If you have not defined any reinforcement, Tekla Structural Designer uses the background reinforcement.

If you have defined reinforcement, then it will either be used on its own, or if you select the "Combine with Panel Reinforcement" option, the sum of the background + patch reinforcement will be used. Note that this option is only selectable when the "Align to Panel Reinforcement" option is also selected since combining in this way would only be valid provided the reinforcement is reasonably aligned. Choosing the reinforcement to be combined also forces the "Cover as Panel" option to be selected as the program assumes the reinforcement to be in a single layer. If the reinforcement is not combined you can specify the cover to the patch reinforcement by turning off the "Cover as Panel" option.

Note that patches may overlap on the plan view, and there is no restriction on this, even patches relating to the same layer of reinforcement are allowed to overlap. This situation is handled conservatively during design by simply ignoring the overlap.

Each patch manages reinforcement and the reinforcement design using a number of slab design strips. Some key points to bear in mind considering patch design are:

  • A patch only manages data in the top or the bottom layers of a slab, not both.
  • There can be up to 3 design strips in each direction of a patch
  • There is no requirement to have design strips in both directions - there can be one design strip in one direction and none in the other.
  • Within any strip there might be patch reinforcement to consider but note that:
    • The underlying panel reinforcement can be none
    • The added patch reinforcement can be none.
  • If there is patch reinforcement to consider this can be considered instead of, or in addition to, relevant slab reinforcement

Slab patch strip design

For the strip designs within each patch it is necessary to establish which bar layer is to be designed and work out if and how the patch reinforcement combines with the panel reinforcement.

As shown above there are 2 distinct options:

  1. Patch aligned to panel
  2. Patch NOT aligned to panel:

In both cases the reinforcement that is determined for use in the design checks is some combination of the panel and patch reinforcement. Expanding upon this the cases considered are:

Patch aligned to panel:

  1. Patch reinforcement type = NONE then the panel reinforcement is used for all layers
  2. Combine with Panel Reinforcement = No Then the patch reinforcement is used in the patch surface and the panel reinforcement is used in the opposite surface.
  3. Combine with Panel Reinforcement = YES Then the patch reinforcement is combined with panel reinforcement and used in the patch surface and the panel reinforcement is used in the opposite surface.

Patch NOT aligned to panel:

Patch reinforcement is used in the surface to which the patch applies. Reinforcement in the other surface is taken from the panel using the most aligned possibility.

Patches to both surfaces

As stated above, patch reinforcement is only ever used in the surface to which a patch applies, reinforcement in the other surface is taken from the panel.

In rare situations you may have separate patches on both surfaces; in which case you would want the patch reinforcement from the top patch to be considered on the top surface and patch reinforcement from the bottom patch to be considered on the bottom surface.

In this specific situation, for both patches the other surface doesn't necessarily need to be designed to only consider reinforcement in the panel; you can avoid this by selecting the patch property Consider patch surface moments only.

Span effective depth checks for irregular shaped panels

Only slabs that have been specified with two-way load decomposition are designed.

In order to perform span - effective depth checks for irregular shaped beam and slab panels, they have to be converted to idealized four sided rectangular panels.

For further details, see: Slab on beam idealized panels

Was this helpful?