When should I use the "Mesh 2-way Slabs in 3D analysis" option?

Introduction

In Tekla Structural Designer you have the option to mesh 2-way slabs in the 3D analysis.  As a default this option is not selected and typically we do not encourage it.
 

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So the discussion here is about:

1. When should slabs be meshed in 3D?
2. Since it is possible why would you not just do it all the time?

Traditional Structural Idealisation

Structural engineers have not traditionally created large FE models of entire buildings, the ability to do this is a relatively recent phenomenon. Traditionally engineers analysed one floor at a time, or even one continuous beam at a time. This traditional approach is catered for by the Grillage and FE Chasedowns.

When should slabs be meshed in 3D analysis?

Slabs should only be meshed if their stiffness is required in the 3D Analysis, example of where the slab stiffness is required is as follows:

1. Transfer slabs
2. When it is considered to be part of the lateral load-resisting system
3. Mat foundations (because they fall into both the above categories)
4. You have modeled 2-way panels on a slope and want to apply non-vertical loading to said panels (in this case the property can be found in the Slope properties instead of the level property)

Bear in mind that meshing can be applied selectively to floors/planes, e.g. if you need to mesh level 3 because it is a transfer level, then every other level does not need meshing.

Since it is possible, why would you not just do it all the time?

Because you can get results that are very different from traditional expectations.  This occurs because the slabs start sharing load with beams:

1. Slabs carry a proportion of the load directly to the columns – so shear forces and moments on beams can be much lower than you expect using traditional methods.
2. Slabs interact with columns forming a framework that helps resist lateral load – so the structure that is traditionally considered to provide lateral load resistance is designed for lower forces than you would expect.

Examples of meshing floors for the wrong reasons

• To apply wind load - If load is applied to the edge of a slab then you get a message indicating that the slab should be meshed.
  • It is actually better to consider applying the load to columns and walls or to use the Simple Wind Load option or Wind Wall > Decompose to rigid diaphragms option.
• To achieve diaphragm action - this is best achieved by using the slab panel diaphragm options which do not require the slab to be meshed in 3D analysis. Slab panel diaphragm options are; Rigid (default), Semi-rigid, and None.
  • For more on the Semi-rigid option see the Related Articles