# Discovering OpenSees -- Modeling Diaphragms in 2D Models with Linear and Nonlinear Elements

### From OpenSeesWiki

Floor diaphragm need to be axially rigid to assure proper distribution of seismic forces to all lateral force resisting elements (columns and walls). Common modeling approach for frame structures is to represent the structural components (beams and columns) by line elements located at the original member centerlines and having cross-sectional properties equal to those of components. The effect of a rigid diaphragm at a floor level is usually modeled by imposing rigid constraints on all nodes of that floor level and thus enforcing the same lateral displacements of all nodes at the floor level. These constrains enforce condition of zero axial strain on elements that are part of that floor level. For sections where the neutral axis does not shift as a consequence of bending in the beam, axial strains at elementâ€™s centerlines are zero, and thus rigid constraints can be applied to model the rigid diaphragm (e.g. steel sections, elastic materials). However, for nonlinear beam-column elements (force-based or displacement-based) with reinforced concrete fiber section where the neutral axis shifts due to bending in the beam axial strains at elementâ€™s centerlines are no longer zero. Thus, rigid constraints that enforce condition of zero axial strain on elements will change the response of the frame. In this web-learning session, the effect of a rigid constraints is demonstrated on a 2D frame with force-based beam column elements considering three types of sections: elastic, nonlinear steel, and nonlinear reinforced concrete.

This web-learning series covers:

- Introduction to problem
- Consequences of applying a rigid constraint on a force-based beam-column element with different types of sections (demonstrated on examples)
- Conclusions and summary

**PPT presentation of the seminar can be found here:**

**Video of the seminar can be found here:**

**OpenSees files used to demonstrate the effect of rigid constraints can be found here:**

- The main file that is to be sourced from the OpenSees interpreter:
- Supporting files to be stored in the same folder with the main file:
- RectangularRCsection2D.tcl (procedure for discretizing RC rectangular section into fibers)
- WSection.tcl (procedure for discretizing W steel section into fibers)
- A10000.tcl (ground motion)