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Rigid Diaphragms

Requires: Advanced Level

Background

A Rigid Diaphragm in VisualAnalysis is an "old school" numerical way of constraining a group of nodes to displace as a unit.  They are often used to approximate the effects of floor systems on a 3D building frame without modeling the floor diaphragm itself.  This feature is particularly useful when the behavior the framing is the question and not the stresses or forces in the diaphragm itself.  Rather than having to model all the plates at various levels, simply specify a rigid diaphragm. 

Rigid Diaphragms offer no stability to your model and are not available in 2D models (plane grids, etc.).  If you are looking for model stability, or you need to investigate the stresses or forces in your diaphragm then you should actually model the diaphragm using plate elements.

Sloping or Flexible Diaphragms?

For years structural engineers have based their design on two “broad” categories of diaphragms:  rigid and flexible.  In the real world neither exists.  We have given you the option to include a rigid diaphragm in your structural models.  But in doing this you are forcing displacements at a given elevation to all be represented by a single translation and rotation motion.  This easy-to-use approximation allows you to simplify your design calculations, but may not be the best way to model your structure.  

Because your real structure lies somewhere between flexible and rigid, IES recommends that you think carefully before using a Rigid Diaphragm! Plate elements (or Meshed Areas) may be a more accurate way model the diaphragm for the floors and roof. 

Creating Rigid Diaphragms

Rigid diaphragms can be constructed from the Create tab of the Project Manager.  Using Rigid Diaphragms in VisualAnalysis is quite simple and can be specified in two different ways.  A "Simple" Rigid Diaphragm is specified by the plane in which the diaphragm is to act.  They can be modeled by either specifying a plane and an elevation (i.e. ZX at Y = 12 ft) or selecting members or nodes in the plane the rigid diaphragm is to act.  Any nodes falling within this plane will behave as part of the rigid diaphragm.  A simple rigid diaphragm is depicted graphically in the software by a tan rectangle running through the defined plane.

The other option is to define an "Advanced" Rigid Diaphragm which requires that you select all the nodes that are to be included within the diaphragm.  These nodes must lie in a plane and they define the extents of the diaphragm.  Advanced rigid diaphragms are depicted graphically with small tan rectangles surrounding each node included in the diaphragm. 

Note that rigid diaphragms can be selected in a Model View and edited using the Project Manager.  Rigid diaphragms can also be deleted in the same fashion as other elements in VisualAnalysis. 

Limitations of Rigid Diaphragms

As mentioned before, the rigid diaphragm feature of VisualAnalysis is meant to provide a way to quickly model the behavior of a slab or wall that is not explicitly part of your model.  A rigid diaphragm offers no external support or internal stability to the model.  In other words, the model must be able to stand by itself without the diaphragm.  If the analysis and design of the diaphragm is of interest, then the diaphragm would still have to be modeled in the typical way with plate elements.  Another limitation of using rigid diaphragms is that they must lie in a global plane (XY, YZ, ZX).

Rigid diaphragms may cause problems with nonlinear analysis, or dynamic analysis. If you start getting "really strange" behavior, you might try deleting the Rigid Diaphragms to see if they are the source of the problem!

Reporting Rigid Diaphragms

The only item to report for rigid diaphragms is a summary of your input data.  The summary includes properties of each of the rigid diaphragms that have been defined including the specified name, included nodes for an advanced diaphragm, or the plane and the elevation for a simple diaphragm.

Loads & Results

You may apply point loads to rigid diaphragms.  There are no specific results for a rigid diaphragm, though its effect should be visible in your nodal displacements. This is a "hack" way to quickly define a seismic force at a specific floor level, where the assumption is that the forces are distributed through the diaphragm which is not modelled.

References

Concepts and Applications of Finite Element Analysis 3rd Edition by Robert D. Cook, David S. Malkus, and Michael E. Plesha.  John Wiley & Sons, Inc.  Copyright 1974, 1981, 1989. ISBN 0-471-84788-7.

Linear Multipoint Constraints Applied Via Transformation as Part of a Direct Stiffness Assembly Process by Mark S. Shephard.  International Journal for Numerical Methods in Engineering. Vol 20, 2107-2122, 1984.

Efficient Seismic Analysis of High-Rise Building Structures with the Effects of Floor Slabs by Dong-Guen Lee, Hyun-Su Kim, Min Hah Chun.  Engineering Structures, 24, 613-623, 2002.