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2024-07-15

# Influence of Stiffeners at Girder Flanges for Rigid Frame Joints

## How do the stiffeners on the outer flange edges of a rigid frame joint beam affect the maximum resistant moment as well as the rotational stiffness and the accompanying displacements on a frame structure?

The stiffener mounting in a rigid frame joint primarily affects the resistance of the joint.
To illustrate this, the graphic below shows a comparison of five joints, which are basically structurally the same and only differ in the stiffeners on the flanges of the girder.
The first considered joint is designed without stiffeners on the outer edges of the flange and the following ones with stiffeners of a thickness of 5, 8, 20, and 30 mm.
In this case, it is possible to determine an increase of the maximum absorbable moment of 12-33%.
As of the plate thickness of 20 mm, there is no further increase of the maximum resisting moment, because the maximum ultimate strains are always exceeded in the connected beam.

Furthermore, the stiffeners have an influence on the rotational stiffness of the joint and the associated effect on the deformations of the entire structure.
To illustrate the influence of the stiffeners on the deformation of the entire structure, three joint configurations were created, which are: plate and beam geometries as well as their materials are identical, but as a joint without stiffeners, with stiffeners of a thickness of 5 mm, and with stiffeners of a thickness of 30 mm.
To simplify, the determined rotational stiffnesses of a joint configuration are applied equally at all nodes of the frame structure (upon closer inspection, they would also be different for structurally identical joints, since the rotational stiffness depends on the ratio of the maximum moment acting on the joint to the maximum design moment of the joint).
The joint without stiffeners has a rotational stiffness of 138 [MNm/rad].
For the joint with 5mm-thick stiffeners, the rotational stiffness is 181 [MNm/rad], and 217 [MNm/rad] for the joint configuration with 30mm-thick stiffeners. As a further comparison model, the entire structure was modeled with completely rigidly connected beams.
As shown in the image below, the maximum horizontal displacements vary between 57 mm for the joint without stiffeners; 55 mm for a stiffener of 5 mm; 54 mm for a stiffener of 30 mm and 48 mm when neglecting rotational stiffness (completely rigid).

Author

Mr. Böttcher provides technical support for customers of Dlubal Software and takes care of their requests.

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