Design Supports and Deflection

The tab Design Supports & Deflection is available in the edit dialog box of a member or member set if the design properties of the object are activated (see image Activating design properties of a member). Design supports essentially have two functions:

1. Definition of the boundary conditions for the "Compression Perpendicular to Grain" design
2. Segmenting the member or member set for the deflection analysis of timber design

You can specify the default settings for the deflection analysis of surfaces in the Deflection tab of the "Edit Surface" dialog box.

Members

Design Supports

As mentioned, design supports are required for the "Compression Perpendicular to Grain" design at the support. You can specify specific parameters for this design in the Compression Perpendicular to Grain tab. Design supports also offer the option to segment members and member sets for the deflection analysis.

You can assign design supports not only to the member start and member end, but also to internal nodes. Therefore, nodes of the 'Node on Line/Member' type and standard nodes between members of a member set are automatically preset in the table.

Select a design support in the list or use the button to create a new type (see image New Design Support). Use the button to edit the selected type, or the button to select an already assigned design support in the model.

The dialog box is adapted to the Standard. If you have assigned a timber material to the member or member set, the Timber type is preset. Otherwise, select this option from the list.

Use the two Active options to control for which directions (z-axis and/or y-axis) a design support is present. If the member is rotated by 90°, for example, you can deactivate the 'Support in z/z'-axis' and activate it for the y-axis instead.

If no "Compression Perpendicular to Grain" design is to be performed, but the design support is to be used for segmenting the member or member set for the deflection analysis, deactivate the Direct Support option: This means that no entries for support geometry and position are necessary; the support is only used for segmentation for the deformation analysis. Alternatively, you could also select the 'General' design support type, for which the support pressure is not analyzed.

The Support Length is always related to the actual beam. Starting from the structural system or the node, it is displayed in half in the positive x-direction of the member and half in the negative x-direction in the dialog graphic.

With the Support from Edge option, the program automatically detects whether a design support receives compressive or tensile forces. Accordingly, the "Compression Perpendicular to Grain" design is only performed in the case of compressive forces.

If a design support acts at an intermediate support, check the Inner Support control field. Depending on the design standard, this specification is included in the determination of the effective support area.

With the Shear Force Reduction option, the shear design at the support is performed with the governing shear force. The shear force is considered in the design at a certain distance from the support edge. The distance depends on the design standard. This requires that the force acts on the opposite side of the support, i.e., usually at the top of the beam.

If the transverse compressive stresses are too high, they can be absorbed by stiffening elements using bolts (only for EN 1995-1-1 and direct support). To do this, check the Stiffening Elements control field. You can then define the properties of the fully threaded bolts in the Stiffening Elements tab.

Use the Active for Fire Design control field to control whether the support pressure design should also be performed for the fire case.

If a design support is not to be considered for segmentation, deactivate the Active for Deflection Design option.

Deflection Analysis

Segments and Reference Lengths

In the right dialog section of the Design Supports & Deflection tab, the segments resulting from the assignment of the design supports for the respective directions of the deflection analysis are listed. For each design location in a segment, the displayed length L_c is used as the reference length for determining the limit value.

If you want to change the automatically determined reference lengths (for example, because the reference length of a curved member differs from the segment length), check the 'User-Defined Lengths' control field. The values are then editable. However, these user-defined lengths are not automatically adjusted if you subsequently change the member length in the model.

Limit Values for Beams and Cantilever Beams

The limit values of the deflection for beams supported on both sides and cantilever beams are managed in the Serviceability Configurations. The corresponding limit value is applied for the design of each segment depending on the arrangement of the design supports: A segment with design supports on both sides or without design supports is assumed as the Beam segment type, a segment with a design support on one side as Cantilever Beam.

Design Direction

Use the 'Design Direction' to specify which result values of the deflection are to be checked. In the list, the local axes y and z, the resultant deflection, and the local auxiliary axes y' and z' are available for selection. The segments below adjust accordingly.

Displacement Reference

With the options in the 'Displacement Reference' list, you can influence the deflection values to be checked for the design:

• Undeformed System: The local deformation values u_y and u_z are taken directly from the results.
• Deformed Segment Ends: The deflection values are reduced by the deformation values of the start or end nodes for the design, so that only the local deflections are checked.

Precamber

You can consider a precamber for the design for each segment, thereby reducing the value of the deflection. The precamber is applied as a single-wave shape for beam segments and as a linear distribution for cantilever beam segments. Enter the precamber w_c,z or w_c,y as a positive value if it is opposite to the local member axis z or y, respectively. For the design of the resultant direction, the components of the precamber are converted into the resultant direction.

Example: In the following image, no design support was defined at intermediate node no. 50. Consequently, the program only recognizes one segment, and the reference length corresponds to the member length.

If a design support is defined at the intermediate node, two segments are recognized. The reference length adjusts accordingly.

If a segment is not to be analyzed for deformation, you can deactivate it using the control field:

Stiffening Elements

This tab is available for a design according to EN 1995-1-1 if you have checked the Stiffening Elements option in the 'Base' tab. Here, you can define fully threaded bolts that are considered as transverse compression stiffening elements in the "Compression Perpendicular to Grain" design.

Currently, only stiffening elements of the 'Bolts' type are possible. Define the strengths and bolt lengths according to the manufacturer's specification. You can also use the properties of stiffening elements that you have defined as Timber Bolts. Use the corresponding option in the list for this purpose.

Select an already defined timber bolt or create a new type using the button.

In the 'Geometry in z-/y-Axis' section, define the number of bolts and their arrangement.

The bolts are designed for pushing in and buckling. Additionally, the design of the bearing capacity under compressive force perpendicular to grain is performed in the plane of the bolt tip. The load distribution angle of the 'Load distribution' can be considered linearly at 45° or – as described in [1] – nonlinearly (see also dialog graphic).

Surfaces

In the design of surfaces at the ultimate limit state, the stress components are analyzed. The designs are based on the material properties and surface thicknesses. For the serviceability limit state design, however, surface-specific data is required. You can specify this in the Deflection tab of the 'Edit Surface' dialog box.

Surface Type

Use the surface type to specify which limit values of the deflection are applied for the design. Two options are available for selection in the list:

• Doubly supported
• Cantilever beam

The limit values are stored in the Serviceability Configurations dialog box for various design situations of surfaces with one-sided or two-sided support.

Displacement Reference

The displacement reference controls which reference model is used for the deformation design. The list contains three selection options:

• Deformed User-Defined Reference Plane: If the supports have very different displacements, you should specify an inclined reference plane for the displacement u_z to be checked. Define the plane in the 'User-Defined Reference Plane' section using three points of the undeformed system. RFEM determines the deformation of the three definition points, places the reference plane through these displaced points, and uses the related maximum deformation u_z for the design.

• Parallel Surface at the Location of Minimum Deformed Node: This option is recommended for a flexible support of the surface. The maximum deformation u_z is related to a reference plane displaced parallel to the undeformed system, which RFEM places through the node with the smallest displacement value u_z,min.

• Undeformed System: The local deformations u_z are taken directly from the results and used for the design.

Reference Length and Definition Type

The limit value of the deflection depends on the reference length L_z. For the 'According to Maximum Boundary Line' and 'According to Minimum Boundary Line' definition type options (default), RFEM determines the length of the longest or shortest edge from the surface geometry and automatically sets the reference length. If you want to define the reference length, select the 'Manual' definition type in the list and then enter the value.

Compression Perpendicular to Grain

The Settings for 'Compression Perpendicular to Grain' tab is available in the edit dialog box of a member or member set if a Direct Support is present for a design support. Here, you can describe the support situation of a spatial system for nodes of the member or member set that are implemented in the model without nodal supports – for example, members with an (indirect) support on another member.

The compressive force required for the "Compression Perpendicular to Grain" design is determined from the internal forces of the members connected to the node. In the structural model, all members meet at a node, as the following example shows.

This simplification usually does not correspond to the actual conditions: Not every member transfers its normal or shear force directly into the support, but presses on another member, which in turn transfers this force with its internal forces into the support. This results in a multitude of support situations.

Use the control fields to control which members cause transverse compressive forces and thus clearly define the support situation. This is described in a technical article using an example:

• Design Supports for Timber Design – Serviceability Limit State, Compression Perpendicular to Grain and Shear Force Reduction