Modeling a slab beam as a rib

Product Support
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Hello everyone,

I am currently modeling an edge beam as a concentrically loaded bending beam (member type "rib") in combination with an FE plate and have a few basic questions regarding the correct implementation in RFEM:

  1. One-sided effective width for the edge beam: Since it is an edge beam, the effective width is only present on the inside plate side. How can I set the effective width exactly to zero on the outer rib half or to half the web width?

  2. Variation of beff over multiple spans (member segmentation): The edge beam spans continuously over several supports. According to the standard, the values for beff change in the end span, over the intermediate supports, and in the interior spans. Do I necessarily have to divide the beam into individual members to assign the span-specific beff values to each rib? Or is it better to model the edge beam as one continuous member across all supports and define the effective width by segments?

  3. Avoidance of stiffness double counting (overlap of plate and web): Since the member is located concentrically in the plate axis, the geometries of the plate and beam web overlap in the model. Does RFEM automatically subtract the area moment of inertia or stiffness portion of the plate in the overlapping area for the member type "rib" to prevent double counting of bending stiffness? Or do I have to manually reduce the cross-section height of the beam by the plate thickness or adjust the stiffness myself?

I would greatly appreciate your support!

Best regards

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Hi neje1011,

Thank you for your message. Below are the answers to your questions:

  1. One-sided effective width:
    The deactivation of the outer width is explained step by step in this Dlubal training video (003086).

  2. Modeling as a continuous beam:
    Modeling as a continuous member using nodes of the type "On line" is recommended. You can find a comparison of the two variants in the Dlubal Knowledge Base article 001821.

  3. Double counting of stiffness:
    Manual reduction is not necessary. For the member cross-section, you only define the additional flange portion that is not already included in the area. More details can be found in the RFEM 6 Online Manual.

Best regards
Florian Heinrich

1 Like
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Hello Mr. Heinrich,

Thank you for your response. However, RFEM does not take into account the Steiner component from the eccentricity of the beam for its bending stiffness in the case of the concentric bending beam, or am I mistaken? Then one would have to calculate a separate equivalent bending stiffness for the beam from the moment of inertia I of the beam plus the Steiner component. And possibly calculate the equivalent height from that?

Best regards, neje1011

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Hi neje1011,

sorry for the late reply. This technical article explains very well how the rib is calculated in RFEM:

https://www.dlubal.com/de/support-und-schulungen/support/knowledge-base/001411

I hope this helps you further.

Best regards Florian Heinrich