Steel frame structures can have a very complex topology and may be unsuited for representation by linear or low order polynomial elements. Isogeometric analysis and a representation with block structured trivariate NURBS patches may be a better approach than traditional FEA to perform structural analasys for these frames.

 Above is an example of a steel frame structure, below we zoom into one of the joints

 The detail above is in the inital CAD system probably modelled using trimmed cylindrical surfaces and trimmed NURBS surfaces. We want to maintain the shape of the CAD model, but translate it to a representation fit for isogeometric analysis, i.e. a block structured representation where each block is a non-trimmed NURBS volume. Alternatively, an isogeometric modelling operation may be performed to remodl the frame structure. In both cases an approximation at the joint is inevitably, but a very good accuracy is reachable. The tubes can be represented exactly.

Creating a model fit for isogeometric analysis from a CAD model is a non-trivial task that is the subject of current research. It is the main topic in the project isogeomtric representation.