Long Term Column Loading

Practically all analyses of reinforced concrete columns in OpenSees assume the loading is short term--concrete as strong as it was at 28 days out of the hopper. Depending on what you're doing, not accounting for long term load effects, i.e., concrete creep and shrinkage, may not be a big deal. But the effects of creep … Continue reading Long Term Column Loading →

Corotational Rigid Offsets

Unlike Linear and PDelta, rigid joint offsets are not an option for the Corotational geometric transformation in OpenSees. And the lack of corotational rigid offsets is not due to theoretical limitations, simply no one has taken the time to implement the equations. The two-dimensional case would not be terrible. But three-dimensional offsets? No thanks, not … Continue reading Corotational Rigid Offsets →

The Hard Dynamics

I am confident that you can use OpenSees to solve all reasonable problems from textbooks on statics, structural analysis, finite elements, structural dynamics, and (most of) strength of materials. But what about engineering dynamics? The rigid body dynamics that's way more difficult than deformable body dynamics. You know, kinematics and kinetics of particles and rigid … Continue reading The Hard Dynamics →

One and Only One

Two element formulations in OpenSees--forceBeamColumnCBDI and mixedBeamColumn--are capable of handling geometric nonlinearity within the basic system, i.e., P-little-delta effects. The CBDI formulation, described in Neuenhofer and Filippou (1998), approximates the transverse deflection using Lagrange polynomials fit through the curvature at each integration point. Due to the added computational expense and coding details, the forceBeamColumnCBDI element … Continue reading One and Only One →

Geometric Transformation

OpenSees offers three types of transformations between the basic system and global system for frame (beam-column) elements: Linear - small displacement assumptions for compatibility and equilibrium PDelta - small displacement assumption for compatibility with the $latex P-\Delta$ term included in equilibrium Corotational - large displacement assumption for compatibility and equilibrium Use the geomTransf command to … Continue reading Geometric Transformation →

Cable Analysis

Analyzing cables subject to transverse loads is straightforward in OpenSees. Use a mesh of corotational truss elements with elastic uniaxial material. Of course, you can use any uniaxial material you like. The only trick is you have to scramble the nodes up a little bit--if you try to analyze a perfectly straight cable, you'll get … Continue reading Cable Analysis →

Stability Challenge Results

I posted a modeling challenge for the famous, perhaps now infamous, three member truss example of OpenSees. The members are very slender, so I wanted to see how well we can account for geometric nonlinearity. First, the results. There were five entries--three reported a load factor of about 0.47 and two gave a load factor … Continue reading Stability Challenge Results →

Meshing for Column Loads

For material nonlinear analysis of frame models, you can improve the computed response by using more displacement-based elements or more integration points in a force-based element. The material nonlinearity occurs inside the basic system, also known as the natural system or the kernel. To capture geometric nonlinearity due to large displacements, you have to go … Continue reading Meshing for Column Loads →