Design Supports and Deflection

The Design Supports & Deflection tab 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 boundary conditions for the "Compression Perpendicular to Grain" design check
2. Segmentation of the member or member set for the deflection analysis of the timber design

You can define the specifications for the serviceability limit state design of surfaces in the Deflection tab of the surface edit dialog box.

Members

Design Supports

As mentioned, design supports are required for the "Compression Perpendicular to Grain" design check at the support. You can define specific parameters for this design check in the Compression Perpendicular to Grain tab. Design supports also provide 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 intermediate 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 from the list or use the button to create a new type (see image New Design Support). You can use the button to edit the selected type, and 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.

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

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

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

Using 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 check is only performed in the case of compressive forces.

If a design support acts at an intermediate support, select the Inner Support check box. 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 force 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 compression stresses perpendicular to grain are too high, they can be absorbed by stiffening elements with bolts (only for EN 1995-1-1 and direct support). To do this, select the Reinforcement Elements check box. In the Reinforcement Elements tab, you can then define the properties of the fully threaded screws.

With the Active for Fire Design check box, you can control whether the support pressure design check is also to be performed for the fire resistance design.

If a design support is not to be considered for segmentation, deactivate the Active for Deflection Analysis 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 design supports for the respective directions of the deflection analysis are listed. For each design check point within 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 deviates from the segment length), select the 'User-Defined Lengths' check box. 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 Beam and Cantilever

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 each segment in the design check depending on the arrangement of the design supports: A segment with design supports on both sides or without a design support is assumed as a Beam segment type, a segment with a design support on one side as a Cantilever.

Design Direction

Use the 'Design Direction' to specify which deflection result values should be checked. The list provides the local axes y and z, the resulting deflection, and the local auxiliary axes y' and z' for selection. The segments below adjust accordingly.

Displacement Reference

With the options of 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 for the design check by the deformation values of the start and end nodes so that only the local deflections are checked.

Precamber

You can consider a precamber for each segment in the design check and thus reduce the value of the deflection. The precamber is applied as a single-wave form for beam segments and as a linear course for cantilever 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 check of the resulting direction, the portions of the precamber are converted into the resulting direction.

Example: In the following image, no design support was defined at the intermediate node No. 50. Consequently, only one segment is recognized by the program 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 checked for deformation, you can deactivate it via the check box:

Reinforcement Elements

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

Currently, only reinforcement 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 reinforcement elements that you have defined as Timber Bolts. Use the corresponding option in the list for this.

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

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

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

Surfaces

For the ultimate limit state design of surfaces, the stress components are examined. The design checks are based on the material properties and surface thicknesses. However, for the serviceability limit state design, surface-specific data is required. You can enter this in the Deflection tab of the 'Edit Surface' dialog box.

Surface Type

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

• Double-span beam
• Cantilever

The limit values are stored in the Serviceability Configurations dialog box for different 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 analysis. The list contains three options:

• Deformed user-defined reference plane: If the supports exhibit 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 by 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 check.

• Parallel surface at the location of the minimally deformed node: This option is recommended for a flexible support of the surface. The maximum deformation u_z is related to a reference plane shifted 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 check.

Reference Length and Definition Type

The limit value of the deflection depends on the reference length L_z. With the definition type options 'By Longest Boundary Line' and 'By Shortest Boundary Line' (default), RFEM determines the length of the longest or shortest edge from the surface geometry and sets the reference length automatically. If you want to define the reference length, select the 'Manual' definition type from 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 exists 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 realized 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 check is determined from the internal forces of the members connected to the node. In the structural model, all members meet at a node, as shown in the following example.

This simplification usually does not correspond to the actual conditions: Not every member transfers its axial or shear force directly into the support, but instead 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.

Using the check boxes, you can control which members cause compression perpendicular to grain forces and thus clearly define the support situation.