Answer:
Stiffeners used in rigid frame joints have a significant effect on the resistance of the joint. To illustrate this, the graphic shown below compares five joints that are basically structurally identical. They only differ in the design of the stiffeners on the girder flanges.
The first joint is designed without stiffeners on the outer edges of the flange; the others with stiffeners each with 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% to 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 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 this, three joint configurations are used that are identical in terms of bolt, end plate, and beam geometry, as well as materials, but are designed without stiffeners and with stiffeners of a thickness of 5 mm and 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.
A rotational stiffness of 138 [MNm/rad] is determined for the joint without stiffeners, a rotational stiffness of 181 [MNm/rad] for the joint with 5 mm thick stiffeners, and a rotational stiffness of 217 [MNm/rad] for the configuration with 30mm-thick stiffeners. For comparison, the entire structure is displayed as a model with completely rigidly connected beams.
As shown in the image above, 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).
