Add Stiff Deck, Semi Rigid

The semi-rigid stiff deck tool allows the diaphragm action of a floor plate or slab be be added to a model. This allows the model of the structure to include the ability of the floor structure to transfer lateral loads back to the bracing, cores of moment resisting frames that form the lateral stability system of the structure.

The semi-rigid stiff deck is based on an underlying Finite Element surface which models the slab based on the material thickness and Young Modulus (E-value) along with the area of the diaphragm defined. These properties are controlled by the user inputs. The semi-rigid stiff deck FE surface is not part of the MasterFrame FE module and does not need the MasterFrame FE module, being part of MasterFrame. The semi-rigid stiff deck is used to model the lateral stiffness of the floor plate only and so does not require the modification of the members making up the frame. The mesh of the semi-rigid stiff deck uses only the main nodes associated with the frame linear elements and the mesh is not displayed. The semi-rigid stiff deck FE surface cannot be loaded vertically. In effect, the semi-rigid stiff deck is dealt with by the software as a separate entity from any MasterFrame FE surfaces.

Earlier versions of MasterFrame used a rigid member based stiff deck system. This was applied on a member by member basis, and the rigid stiff deck system modified the member minor axis properties, in effect making the member infinitely stiff about it's minor axis. This system was satisfactory in most cases, but on floor with large floor voids, this system could lead to higher than expected axial forces in the members. The semi-rigid stiff deck method offers a more accurate modelling of the floor diaphragm, particularly on floors with floor voids.

Stiff-Deck Regions

To add a stiff deck region to a model, go to Create>Add Stiff Deck, Semi Rigid. This will open the Stiff-Deck Region pane on the right hand side of the screen. The screen layout is shown below.

 The Stiff-Deck Region pane contains controls to add a new stiff deck region, control the members selected for a region and inputs to define the properties of the stiff deck. The stiff-deck panel is shown below.

The stiff deck regions can be named by typing the desired name in the drop down area. If stiff deck regions have already be created, they can also be selected from the drop down, either by expanding the drop down, or navigating using the up and down arrows to the left of the drop down.

In addition to the drop downs, the row of icons allow for control of the input of members and regions defining the stiff-deck, and for clearing or deleting selections and stiff-decks. The icons are displayed as

. The icons have the following function:

    - Add/Remove item. Selects or de-selects individual members. With the auto select main boundary members, this option allows for the selection of panels without having to pick the boundary members individually

    - Find Item. Highlights any stiff regions associated with the member or area selected in the graphics window.

    - Add or Remove only. These options are available in Add/Remove mode. They controls the selection by only allow members to be added or removed when selecting members.

    - Add new groups. This tool adds a new stiff deck region

    - Clear current group. This option clears all selected members for the currently selected stiff deck region. The region is selected by the drop down.

   - Clear all groups. This option automatically clears all members selected in all created stiff deck regions.

    - Delete current group. Deletes the selected stiff deck region. The region is selected using the drop down.

    - Delete all groups. Deletes all created stiff deck regions.

In addition to the top icons, there are also the following two options:

  - for a selected stiff deck region, clicking on this icon automatically removes from the selection any members which are internal to the region, this, non-boundary members.

 - the software automatically selects the boundary members, allowing the stiff deck to be created by selecting panels rather than individual members.

Create a stiff deck region

A stiff deck region is created by specifying the stiff deck thickness and Young's modulus E-value, and then selecting the members which form the outer and inner boundaries of the region. The stiff-deck thickness and E-value are input by typing the required values into the input boxes.

The deck thickness is input in millimetres (mm). The Young's modulus input units are kN/mm². If incorrect units are used this can lead to an error in the analysis, due to the resulting ill-conditioning of the stiffness matrix.

When considering a floor slab or diaphragm where the cross-section is not constant, it is important to recognise that the diaphragm is bending in the plane of the diaphragm, not out of pane as in the case of a floor plate under gravity loading. Therefore, it is generally advisable to consider that part of the diaphragm which is constant over the floor area. For example, in the case of composite floors, this would be the thickness of the concrete above the ribs.

The boundary of the stiff deck region is selected graphically by clicking on the members or panels making up the stiff deck region, using the mouse pointer. Alternatively, area of the structure can be windowed around using the mouse. The selected members highlight in red and the member numbers appear in the right hand pane, in the lower rectangle. When a valid region is selected, the region will appear in colour, as shown below.

It is possible to create multiple stiff deck regions under one stiff deck title by picking members across several level in the structure, as shown in the image below. This can speed up the creation of stiff decks in a model, but has the disadvantage that if a modification is required to individual floors, they have to be removed from the original stiff deck region and then added as a new stiff deck region.

Voids can be created in the stiff deck by selecting both the members forming the external main boundary and then selecting the members to form the boundary of the opening.

When creating stiff deck regions, the viewing navigation, frame views and view filters can be used to assist in selecting the boundaries of a stiff deck region.

Limit on stiff deck application

The semi-rigid stiff deck is intended to model the diaphragm action of both horizontal and inclined surfaces, such as floor plate and roof structures. The stiff deck region was not intended to model the stiffness of walls and, therefore, it is not possible to create vertical stiff deck regions. The software has a tolerance on inclined surfaces, and will not permit the stiff deck to be created on a surface with a slope of greater than 80 degrees from horizontal.

The stiff deck will act as a diaphragm in the plane of the stiff deck region. Therefore, for inclined surfaces, the stiff deck will contribute to the stiffness of the structural elements in both the vertical and horizontal plane.

For members whose beta angle is such that the member beta angle orientates the member perpendicular to the plane of the stiff deck, the member minor axes moments be assumed to be taken wholly be the stiff deck.

Viewing Stiff Decks on a model

When in the Stiff-deck regions, stiff deck can be viewed individually by either scrolling through the created regions using the arrows to the left of the drop, by selecting individual stiff deck regions from the drop down, or by using the Find Item icon. This will select specific stiff deck regions which will be shown shaded if they are a valid region. This will only show the regions which have been created, which may not assist in identifying areas where a stiff deck region is not yet applied.

In the stiff-deck regions area, clicking on the  icon will display all valid stiff deck regions currently applied to a model. The stiff deck regions will be colour coded to identify the regions.

Within Masterframe, the stiff deck regions can be displayed using the  tool, found in the Supports and Restraints area of the toolbar.

Member Forces in the Plane of the Stiff-Deck

The Stiff-deck also has the effect of restraining the local bending and shear of members in the axis of stiff-deck. Member loads applied to the members in the axis of the deck are distributed to member ends for global load application, but no local bending moment or shear force will be present in the axis of the members in the plane of stiff-deck. 

Taking the above example steel frame, if for example we had vertically spanning wind panels distributing UDL load onto the exterior steel beam in the minor axis, then these loads would be distributed to the member ends, but no  local minor axis moment or shear force would be present in the member.

Equilibrium of Nodal Forces at the Stiff-Deck

The semi-rigid finite element stiff-deck will obviously contribute to the forces at a node. All static forces will be in equilibrium. At present it is not possible to directly view the force component in the stiff-deck at the node point, There is also no facility to view the distribution of membrane forces in the stiff-deck.

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:

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

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.