How slab properties and features impact on meshing

Tekla Structural Designer
2021
Tekla Structural Designer

How slab properties and features impact on meshing

The slab mesh is always created in a single plane, but the resulting mesh is can be affected by the slab properties and features as described below.

Slab thickness, vertical offsets, column drops and openings

The mesh of shell 2D solver elements is always created in the same plane, irrespective of whether slabs have different thicknesses, slabs items have been raised/lowered via vertical offsets, or column drops have been applied. Shells are not created inside slab openings and any loads placed within openings are not applied to the model.

Consider the example shown below. This features curved slab boundaries, circular and rectangular openings, thickened slab panels, lowered slab panels and a slab overhang. A column drop panel has also been inserted at one of the locations where the slab is supported by a column.

slab_physical_model_1.png

In the resulting FE solver model, since vertical offsets are not structurally significant the analysis mesh is formed at the same level relative to the top of the slab. The mesh properties do however reflect the change to the slab thicknesses in the different slab areas.

slab_solver_model_1.png
Note: Beam solver elements and slab meshes can only be offset vertically from one another by being defined in different construction levels.

Other slab properties

Rotation Angle

Different slab items in the same slab can have different rotation angles.

This property is used for the following where appropriate:

  • Span direction for 1-way load decomposition
  • To determine the 2D solver element local axes in the solver model
  • Bar direction for Slab on Beam and Flat Slabs.

Include in Diaphragm

This property is only available when the Diaphragm option is Semi-Rigid or Rigid. Individual slab items in the same slab can be included or excluded as required.

The effect of excluding a slab item depends on the Diaphragm option as follows:

  • Semi-Rigid - excluded slab items are not meshed with semi-rigid 2D solver elements
  • Rigid - internal nodes not considered in the nodal constraints
Note: Where 2 items share a boundary and one item is included and one excluded, then the nodes along the shared boundary are included in the diaphragm.
Note: When a slab item is excluded from the diaphragm this has no effect on the mesh of shell 2D solver elements used in some of the solver models for 2-way spanning slabs.

Mesh groups

To facilitate meshing, Tekla Structural Designer automatically gathers slab items and features together into mesh groups, and meshes them as a single entity. A mesh group contains one or more slab items with identical analysis attributes. Since slab depth is a key analysis attribute, by definition a slab step, or a column drop, will produce an additional mesh group.

Example: Mesh groups at a slab step

In the following image, you can see three separate slab items:

  1. d = 300 mm

  2. d = 100 mm

  3. d = 100 mm

Although there are three slab items in the previous image, there are only two different slab depths. That is why Tekla Structural Designer only creates two mesh groups, as shown in the following image:

  1. mesh group 1

  2. mesh group 2

Discontinuity of force contours at slab steps and column drops

When slab items on either side of a slab step are placed into different mesh groups, the solver nodes along the boundary are shared by both groups. Each node on the boundary reports a single value of deflection, but two values of force, one for each group. That is why there will be a discontinuity of force contours along the boundary.

See the following examples:

Example: Deflection contours (no discontinuity)

Example: Moment contours, discontinuity along boundary

The force discontinuity is a genuine result. The slab items share the same curvature at the step and have the same elastic modulus, so the moment must be directly proportional to the inertia of each slab item.

Note: Other programs may average the value across the boundary when generating the contours. However, we prefer the approach of Tekla Structural Designer because averaging would result in an unrealistically high design of the thinner slab.

Mesh group boundary warnings

Meshing may fail or produce undesirable results when there is challenging mesh group boundary geometry. In this case, Tekla Structural Designer warns you about the source points source of meshing issues.

Examples of possible warning triggers are:

  • short edges

  • distance between a hole and an edge

  • small area enclosed by a mesh group

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