Strand7:  Web notes:  Gusset plates

Modelling gusset plates in Strand7

GussetPlate.zip
Frame structures usually adopt gusset plates to increase the joint stiffness. This web note proposes two approaches for modeling gusset plates in Strand7. The first is a simple approach that is suitable when only the forces in the beams are required. The second is a more thorough approach suitable for detailed investigation of the gusset plate itself.


Infinite rigidity of the joints

The first approach is based the following hypothesis: the membrane stiffness of the gusset plate is much greater than the beam bending stiffness. In this case the connection is modeled using rigid links. Once the structure has been solved the displacements obtained can be used to build a sub-model in which plate elements can be used to conduct a detailed (local) buckling load analysis. Note that the use of sub models is only appropriate when linear elastic behaviour is considered.

The connections are modelled using rigid links.

Direct modeling using both plate and beam elements

This approach is recommended when dealing with a limited number of gusset plates in the model. The procedure can be summarized in the following steps:

  1. Build the frame using beam elements.
  2. Build the gusset plates with the correct dimensions considering properly the beam cross section and therefore the offset of the gusset relative to the beam’s 3-axis.
  3. Refine the beam mesh close to the ends and link the nodes just created with the gusset plate using rigid links (see the following figure). The links will simulate the correct offset as well as the infinite stiffness of the beam cross section in its own plane.

Beam and plate model simulating the correct joint stiffness

Solid visualization of the plate and beam model

Comparison between the two approaches.

A simple 2D steel frame structure is analyzed and the displacements are compared.

Influence of the joint flexibility.

In this particular case the difference between the maximum displacement in the two approaches is about 20% using a gusset plate thickness of 10 mm and 1% using a 50 mm thickness. Even if the difference is not negligible, the rigid joint model involves a smaller number of total degrees of freedom and may be preferred when dealing with structures having many of such joints. Using rigid joints also produces an overestimate of the bending moments in the frame members, thus results in a more conservative design.