Get the Accel Out

In OpenSees, a UniformExcitation pattern is functionally equivalent to a regular load pattern, fitting into the framework of a time-varying scalar load factor and constant reference load vector. The scalar load factor is the input ground acceleration, $latex \ddot{u}_g(t)$, while the reference load vector is $latex {\bf P}_{ref}=-{\bf m}{\boldsymbol \iota}$ where $latex {\bf m}$ is … Continue reading Get the Accel Out

Gimme All Your Modal Damping

The GimmeMCK integrator is one of my more useful contributions to OpenSees. This integrator overcomes the limitations of the printA command and allows you to extract the individual mass, damping, and stiffness matrices, or some linear combination therein, in order to see what's assembled in an OpenSees model or to bootstrap new functionality. While getting … Continue reading Gimme All Your Modal Damping

More Than You or Eigen Chew

Software does exactly what you tell it to do. But problems arise when you don't know--or assume you know--what instructions you're giving. For example, many people like to use the 'fullGenLapack' eigenvalue solver in OpenSees because it will compute all N eigenpairs (eigenvalue with corresponding eigenvector) for a model whereas the default eigenvalue solver will … Continue reading More Than You or Eigen Chew

Interpolation of Ground Acceleration

There was a question on GitHub a few months about whether or not OpenSees uses linear interpolation when the analysis time step is smaller than the time step (digitization) of an input ground acceleration. This is a good question as I've used other software that does not interpolate and instead uses the acceleration of the … Continue reading Interpolation of Ground Acceleration

The Little IDA

In Fundamentals of Structural Mechanics, Hjelmstad solves a "little" one-dimensional Boundary Value Problem (BVP) for linear-elasticity before moving on to the three-dimensional BVP, which is full of mathematical difficulties. Hjelmstad's reasoning is sound. "Some of these difficulties are of great importance, while others are simply a nuisance. Reducing the issue to its simplest case helps … Continue reading The Little IDA

Multiple-Support Excitation

Structural systems typically have different ground accelerations at supports separated by long distances. These systems can be a single structure such as a long span bridge or multiple structures in a region. While most OpenSees analyses use uniform excitation with effective earthquake forces applied to the dynamic DOFs of the model, the framework also accommodates … Continue reading Multiple-Support Excitation