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

    - 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:

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.