Murum, cura te ipsum

OpenSees has its fair share of element implementations that are computationally inefficient. Fortunately, most of those elements are never used. But among elements that are used, SFI-MVLEM is the undisputed champion. Whereas the standard MVLEM element uses a uniaxial material in each fiber, the SFI-MVLEM element accounts for the interaction of axial and shear stress ($latex \sigma_{11}$ … Continue reading Murum, cura te ipsum →

Secant Accelerated Newton Algorithm

I do not consider myself an expert with numerical methods. I know just enough to be dangerous, and root-finding algorithms is one of the subjects where I pose a threat. OpenSees uses root-finding algorithms like Newton-Raphson and Modified Newton to find the nodal response for which equilibrium is satisfied at every analysis time step. No … Continue reading Secant Accelerated Newton Algorithm →

Just Fillin’ Up the Tank

You're not going to conquer incremental dynamic analysis of 3D reinforced concrete frame models the first day you use OpenSees. Some try, but they all fail. Those who start with simple test cases and level up in complexity will succeed. The same goes for fluid-structure interaction. You will not conquer tsunami loading on structures the … Continue reading Just Fillin’ Up the Tank →

Static Analysis with Uniform Excitation

The UniformExcitation defines reference nodal loads in proportion to the mass (nodal plus element contributions), multiplied by negative acceleration, which is specified in a time series. There's nothing inherent in its implementation that ties the UniformExcitation to only dynamic analysis and earthquake excitations. So, if I had known sooner that the UniformExcitation load pattern works … Continue reading Static Analysis with Uniform Excitation →

Monte Carlo Simulation with OpenSeesMP

The parallel computing capabilities of OpenSeesSP and OpenSeesMP are easily confused. OpenSeesSP runs your script on a single processor with the other processors awaiting instructions on what to do. OpenSeesSP is ideal for assigning subdomains of a large model to each processor. The main processor, processor 0, directs traffic and solves the governing equations of … Continue reading Monte Carlo Simulation with OpenSeesMP →

Minimal MVLEM Example

Due to its simplicity and efficiency in modeling shear walls, the MVLEM (Multiple Vertical Line Element Model) is among the more frequently asked about elements in OpenSees. The MVLEM is also one of the better documented elements in OpenSees with documentation for both its 2D and 3D versions. Personally, I haven't used the MVLEM for … Continue reading Minimal MVLEM Example →

The OpenSees WCEE Bump

The 18th World Conference on Earthquake Engineering (18WCEE) takes place in Milan next week. With a couple thousand attendees, the conference will publish a lot of papers, some of which refer to OpenSees. Depending on how WCEE authors cite their use of OpenSees, there's usually a bump in OpenSees citations after the proceedings are posted … Continue reading The OpenSees WCEE Bump →

The Toy Reliability Problem

In Fundamentals of Structural Mechanics, Hjelmstad uses the "little BVP" to introduce boundary value problems. Chopra uses a two-story rigid shear frame throughout Dynamics of Structures. And in Structural and System Reliability, Der Kiureghian uses a "toy problem" for various types of reliability analyses. The toy reliability problem is defined by two random variables with … Continue reading The Toy Reliability Problem →

Earthquake Engineering Experts Hate Me for This One Weird Trick

I was a structural engineer who dabbled in earthquake engineering simulation. Then one day I stumbled upon a weird trick to get ground displacement history from a ground acceleration record. The trick was simple. It worked right away. I couldn't believe it. The trick did not require additional libraries or writing a loop. Just a … Continue reading Earthquake Engineering Experts Hate Me for This One Weird Trick →

Effective Earthquake Forces

There's nothing special about the UniformExcitation load pattern in OpenSees. The pattern is a convenience for defining effective earthquake forces due to uniform ground acceleration as plain 'ol mechanical loads on your model. Consider the equation of motion for uniform ground acceleration, with linear inertia and damping forces, and influence vector $latex {\boldsymbol\iota}$ for the … Continue reading Effective Earthquake Forces →