The Mechanically Separated Wall

The MVLEM element (and its SFI-MVLEM and E-SFI-MVLEM cousins) has a curious 3D implementation in OpenSees. In plane, the element is a two node link with a fiber section and some rigid beams while out of plane the element is a linear-elastic plate. The mechanics of the in-plane response are separate from the mechanics of … Continue reading The Mechanically Separated Wall →

Section Warping Analysis

Loading a ZeroLengthSection element is the easiest approach to compute the moment-curvature and shear force-shear deformation response of fiber sections in OpenSees. However, we cannot do a section warping analysis with a zero length element like we can with non-warping sections--at least not without modifying the ZeroLengthSection element to handle seven DOFs per node. That source code modification is … Continue reading Section Warping Analysis →

One Fiber at a Time

A few years after G3 became OpenSees, UCFyber became XTRACT. In those intervening years, to accommodate section data exported from UCFyber, we added the fiber command to OpenSees, allowing you to add a single fiber to a section based on the fiber's (y,z) coordinates, area, and material tag. Several section analysis software packages have sprung … Continue reading One Fiber at a Time →

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 →

Rectangular Hollow Structural Sections

A hollow structural section (HSS) is manufactured from steel and is either rectangular, square, or circular. This post will address rectangular HSS, inclusive of square HSS. Round HSS is a little easier to define and will be the subject of another post. There are a couple options in OpenSees for defining material nonlinear rectangular HSS … Continue reading Rectangular Hollow Structural Sections →

Section Integration

If we can use procedures in Tcl or Python to lay out fiber patches and layers for standard section types, e.g., doubly-reinforced RC sections and wide flange steel sections, then why do we need the SectionIntegration class? You cannot say that it is not useful to have commands that define fibers in terms of basic … Continue reading Section Integration →

How to Find the Neutral Axis

There is no recorder option in OpenSees to get the location of the neutral axis in a fiber section. Instead, you have to post-process the section deformations. The strain at any point in a 2D fiber section is $latex \varepsilon=\varepsilon_a - y\kappa_z$ where $latex \varepsilon_a$ is the section axial deformation, $latex \kappa_z$ is the section … Continue reading How to Find the Neutral Axis →

Nonlinear Sections, Elastic Elements

I often make seemingly minor tweaks to OpenSees--tweaks that don't usually make it into the documentation, but that in some cases could be quite useful. For example, did you know that you can create an elasticBeamColumn element by passing a section tag instead of directly specifying material and section properties--E, A, and Iz for 2D, … Continue reading Nonlinear Sections, Elastic Elements →

Full Fiber Circle

Circular layers of fibers are required for simulating longitudinal reinforcing steel in circular RC columns. Although the layer circ command accommodates fibers along an arc, I have never seen anyone use this command for anything other than a full circle. Some years before the extent of OpenSees GitHub history, I added a default constructor to … Continue reading Full Fiber Circle →

Rectangular Patches

Defining rectangular patches is one of the more tedious aspects of building a fiber section in OpenSees. Using the 'quad' patch command, you have to define the four corner points (I, J, K, and L) in counter-clockwise order from I to L around the patch. If you go clockwise, the fiber areas will be negative, … Continue reading Rectangular Patches →