Stiff Deck or Diaphragm Action is where you take account of the floor slab to distribute lateral loading back to the bracing frames or shear cores in pinned frames. This can be applied to horizontal and sloped surfaces but not vertical surfaces.
This approach actually models the stiffness of your floor realistically based on its Young’s Modulus and depth. This has the benefit of avoiding potential problems with ill-conditioned stiffness matrices due to large stiffness terms.
Under the bonnet the semi-rigid stiff-deck option generates a coarse finite element mesh, using triangular plane stress elements between existing nodes, meaning no extra degrees of freedom are created, so your model analyses just as quickly but with added accuracy.
Add stiff deck by clicking on members or panels or windowing several panels and add openings by deselecting members or panels (or not selecting them in the first place).
If you’re using stiff deck on your models there are a few things you should know about how it works. The stiff deck adds plane stress elements to the existing stiffness matrix in order to accurately model the diaphragm action of the deck. No out of plane bending stiffness is added. Here’s a diagram to illustrate this:
With the above model viewed on plan, here are the deflections under a horizontal load:
Important things to note at this point are…
1.There is no actual deck added to the structure, all the stiff-deck feature does is change values in the existing stiffness matrix in order to add extra stiffness to the in-plane (X and Z direction if your stiff-deck is flat) translational degrees of freedom.
2.The moment of inertia and neutral axis of the members and their vertical stiffness is unaffected. Therefore, vertical deflection of members is unaffected by stiff-deck.
3.The mass of the deck is not taken account of and has to be manually added to the model using area loading or member loading.
How to apply it to your model
1.Go to: Properties > Stiff deck Semi-Rigid
2.Enter a Stiff-Deck thickness and a value for E. These are used to alter the stiffness matrix (not to add loads to your model). Give the surface a relevant name.
3.Select Auto Select Main Border Members and also Add/Remove Member/s as per the image below:
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4.With these options selected you can now define the stiff deck in one of two ways.
a)The first is to select the border members one by one, adding them to the stiff deck definition manually.
b)The second approach is to window the members which will make up the stiff deck. Be careful with this approach to only window the members that will be part of the stiff deck, as windowing other members will confuse the automatic border identification.
The stiff deck areas will be shown as coloured surfaces on the model. If a surface has a boundary with members shown in red, but is not shaded then the stiff deck isn’t applied properly and the boundary should be checked for any members that may have been omitted. On adding these members, the surface should fill in as coloured.
5.To add further stiff deck areas for other surfaces, you can select the large + symbol to add a new group. The group members can also be cleared or the whole group or groups deleted using the other symbols in the same line.
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Notes for Analysis
1.Do not apply the stiff deck to surfaces that FE surfaces have been defined on. This can lead to erroneous results and problems with the FE mesh generator.
2.One of the great things about this semi-rigid stiff-deck approach is that it is fully compatible with the dynamic analysis engine.
3.Another thing to note is that, because the stiff deck feature doesn’t add stiffness to the vertical rotational degrees of freedom, the dynamic analysis can return mode shapes where the beams have lateral displacement caused by free rotation at their ends. These mode shapes are irrelevant and can simply be ignored.