Design Supports and Deflection

The Bearing and Deflection Design tab is available in the member or member set editing dialog when the design properties of the object are enabled (see figure Enable design properties of a member). Design bearings essentially have two functions:

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

You can define the specifications for the deflection verification of surfaces in the Deflection tab of the surface editing dialog.

Members

Design Bearings

As mentioned, design bearings are required at the support for the "Compression Perpendicular to Grain" verification. Specific parameters for this verification can be defined in the Compression Perpendicular to Grain tab. Design bearings also provide the option to segment members and member sets for the deflection verification.

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

Select a design bearing from the list or create a new type using button (see figure New Design Bearing). Using button , you can modify the selected type; using button , you can select an already assigned design bearing in the model.

The dialog is tailored 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.

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

If no "Compression Perpendicular to Grain" verification is to be performed, but the design bearing is to serve for segmenting the member or member set for the deflection verification, deactivate the Direct Support option: This means no entries for support geometry and location are required; the support is used solely for segmentation for the deformation verification. Alternatively, you could select the design bearing type 'General', for which the bearing pressure is not checked.

The Bearing Length is always related to the actual beam. It is displayed in the dialog graphic – starting from the structural system or node – halfway in the positive x-direction of the member and halfway in the negative x-direction.

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

If a design bearing acts at an intermediate support, check the Internal Support box. Depending on the design code, this specification is used in the determination of the effective bearing area.

With the Shear Force Reduction option, the shear force verification at the support is performed with the governing shear force. In this case, the shear force is considered in the design at a certain distance from the edge of the support. The distance depends on the design code. This requires the force to act on the opposite side of the support, i.e. usually on the top side of the beam.

If the transverse compression stresses are too high, they can be resisted with reinforcement using screws (only for EN 1995-1-1 and direct support). To do this, check the Reinforcement Elements box. In the Reinforcement Elements tab, you can then define the properties of the fully threaded screws.

With the Active for Fire Design box, you can control whether the bearing pressure verification should also be performed for the fire case.

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

Deflection Verification

Segments and Reference Lengths

In the right dialog section of the Design Bearings and Deflection tab, the segments resulting from the assignment of the design bearings for the respective directions of the deflection verification are listed. For each verification 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 'Custom Lengths' box. The values are then editable. However, these custom lengths are not automatically adjusted if you subsequently change the member length in the model.

Limit Values for Beams and Cantilevers

The deflection limit values for beams and cantilevers supported at both ends are managed in the Serviceability Configurations. The corresponding limit value is applied for the verification of each segment depending on the arrangement of the design bearings: A segment with design bearings on both sides or without a design bearing is assumed as segment type Beam; a segment with a design bearing on one side as Cantilever.

Verification Direction

Use the 'Verification Direction' to define which result values of the deflection are to be checked. The list offers the local axes y and z, the resultant deflection, as well as the local auxiliary axes y' and z' for selection. The segments below adapt accordingly.

Displacement Reference

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

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

Camber

For the verification, you can take into account a camber for each segment and thereby reduce the deflection value. The camber is assumed as a single-curvature shape for beam segments and as a linear profile for cantilever segments. Enter the camber w_c,z or w_c,y as a positive value if it acts opposite to the local member axis z or y. For the verification of the resultant direction, the components of the camber are converted into the resultant direction.

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

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

If a segment is not to be checked for deformation, you can deactivate it using the box:

Reinforcement Elements

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

Currently, only reinforcement elements of the 'Screws' type are possible. Define the strengths and screw lengths according to the manufacturer's specifications. You can also use the properties of reinforcement elements that you have defined as Timber Screws. To do this, use the corresponding option in the list.

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

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

The screws are verified for compression and buckling. In addition, the transverse compression capacity is verified in the plane of the screw tip. The load distribution angle of the 'Load Distribution' can be taken into account linearly at 45° or nonlinearly as described in [1] (see also dialog graphic).

Surfaces

When designing surfaces in the ultimate limit state, the stress components are examined. The verifications are based on the material properties and surface thicknesses. For the serviceability limit state verification, however, surface-specific specifications are required. You can make these in the Deflection tab of the 'Edit Surface' dialog.

Surface Type

Use the surface type to define which deflection limit values are applied for the verification. The list offers two options:

• Double-supported
• Cantilever

The limit values are stored in the 'Serviceability Configurations' dialog 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 verification. The list contains three options:

• Deformed Custom Reference Plane: If the supports have very different displacements, you should specify an inclined reference plane for the displacement u_z to be verified. Define the plane in the 'Custom Reference Plane' section using three points of the undeformed system. RFEM determines the deformation of the three definition points, defines the reference plane through these displaced points and uses the referenced maximum deformation u_z for the verification.

• Parallel Plane at the Location of the Least Deformed Node: This option is recommended for a compliant support of the surface. The maximum deformation u_z is referenced to a reference plane shifted parallel to the undeformed system, which RFEM defines 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 verification.

Reference Length and Definition Type

The deflection limit value depends on the reference length L_z. With the definition type options 'According to Maximum Boundary Line' and 'According to Minimum 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 definition type 'Manual' in the list and then enter the value.

Compression Perpendicular to Grain

The Settings for 'Compression Perpendicular to Grain' tab is available in the member or member set editing dialog if a Direct Support exists for a design bearing. Here you can describe the support situation of a spatial system for nodes of the member or member set that are realized without node supports – for example, members supported on another member.

The compressive force required for the "Compression Perpendicular to Grain" verification is determined from the internal forces of the members connected to the nodes.

Description in progress.