Use of Member Types and Hinges on Truss Example

Technical Article

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When modeling frameworks, RSTAB and RFEM offer various options for controlling the transfer of internal forces at the connection points of the members. On the one hand, you can use the member types to define whether only forces or also moments act on the connected members. On the other hand, you can exclude certain internal forces from the transfer by using hinges. A special form are scissor hinges, which allow for a realistic modeling of roof structures, for example.

Member types and hinges

When modeling simple member structures, the following member types are usually used.

  • Beam member: Rigid member that transfers all internal forces
  • Truss: Beam member with a moment hinge at both ends
  • Tension member: Member with stiffness EI that fails in the case of a compressive force
  • Compression member: Member with stiffness EI that fails in the case of a tensile force
  • Buckling member: Member with stiffness EI that fails if the buckling load is exceeded

Further member types are described in the online manual for RFEM .

In addition, nonlinear properties are possible for both members and hinges. This allows you to specify special failure criteria or nonlinear relationships between forces and strains. This technical article presents some options for modeling with member types and hinges using a simple example.

Roof structure

The roof system of a barn whose truss -like supporting structure was built structurally incorrectly is analyzed: In the case of the truss, a diagonal was "forgotten" so that visible deformations occurred even under the self -weight load. This member, which is indispensable for the load -bearing capacity, was subsequently added. In this way, damage could be avoided.

The RFEM model represents a section of the roof structure. The base purlins are assumed to be fixed supports, the ridge and central purlins as lateral supports with a small torsional spring. The load application is simplified by means of the rafters with corresponding catchment areas. As an example, only the load cases "Dead Weight and Structure" and "Snow" are analyzed.

Truss model with scissor hinges

The structure is modeled with beam members, which receive corresponding hinges at locations with moment releases. A pure truss model would not be correct because some members pass through the intersection points and thus transfer moments. The member type "Truss (N only)" is only used for the two headers. The missing diagonals can be displayed in the model with the "Null member" type. They are not taken into account in the calculation.

Hinges are usually related to the local xyz member axes. In the case of spatial member structures, this allows you to control the transfer of forces and moments to the members connected to the common node. In the model, local moment hinges are used for the ends of those members that do not transfer moments due to the simple timber connections - for example, the posts at the top and bottom. For the continuous members such as rafters and posts mentioned above, however, so -called "scissor hinges" are used. They ensure the continuous action for the moments on the respective crossing member pairs. Scissor hinges are always related to the global XZY axis system.

The use of scissor hinges is also described in FAQ 000177 and FAQ 001438 .

Certain member types such as truss or compression member are provided with hinges by definition so that they do not need to be defined additionally; the corresponding input fields are locked.

After calculating the model, relatively large deformations can already be seen for LC 1 (permanent loads).

The calculation of the design combination CO2 ends with an instability message.

Truss model with nonlinear acting hinge

In addition, the following scenario is to be analyzed for the example. If the connection of the center post to the bottom flange were designed as a pure tenon connection, the connection would loosen due to the tensile force in the post. This effect can be determined in the model by a nonlinearly acting normal force hinge, which only allows the connection to become effective in the case of compressive forces.

LC 1 is calculated in several iterations. Due to the tensile forces in the center post, the connection at the bottom flange is released so that the system is decoupled there.

An alternative modeling could be done in the form of a so-called nodal release (see online manual ).

Model of the renovated truss

The two null members are replaced by buckling members in the renovated structure. They act like truss members, but fail in the case of compressive forces beyond the buckling load. The calculation of the design combination CO 2 according to the second -order analysis now runs without interruption.

The deformation figure of the truss girder shows the effect of the infill. The deflections of the rafters are caused by the modeling of the load introduction that was selected for this structural section. They are not the subject of the investigation.

Summary

Using the example of a simple truss girder, it was shown how member types and hinges can be used to represent the distribution of internal forces in a model. So -called scissor hinges play an important role here, making it easier to model beam crossings in timber structures. You can also assign nonlinear properties to members and hinges. With null members, it is possible to analyze variants of the model in a time -saving manner.

Keywords

Truss Member type Hinge Scissor hinge Buckling member Member of type 'Null' Nonlinearity Rafters Posts Beam crossing

Reference

[1]   Eurocode 5: Design of timber structures - Part 1-1: General - Common rules and rules for buildings; EN 1995-1-1:2010-12
[2]   Manual RFEM. (2018). Tiefenbach: Dlubal Software.
[3]   Manual RSTAB. (2013). Tiefenbach: Dlubal Software.

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