Diaphragm Modeling Tips

Modeling a Rigid Diaphragm in RISA-3D

In the Diaphragms spreadsheet:

1. Enter the node in the Node Label column that the diaphragm will be defined by.

2. Designate the Plane the diaphragm will act.

   

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   Internally, the Rigid Diaphragm ties all nodes at that elevation together with Rigid links.

   

To view the diaphragm:

1. Click the Toggle Display of Diaphragms button .

   This allows you to visually see the location of the diaphragms. Inactive diaphragms do not show up graphically.

   

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2. (Optional) From here you can run the Wind and Seismic load generators.

Axial Load Limitation

Members in the plane of the diaphragm will have no axial load attributed to them. This is because the internal rigid links that are created to achieve rigid behavior take the entire load. In these cases, the axial loads in these members will need to be considered outside of the program.

Openings in a RISA-3D Only Model

If there are openings within the defined diaphragm, where portions of the structure are not rigidly connected, it is possible to disconnect those locations from the diaphragm in the Node Coordinates spreadsheet.

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Use of Rigid Diaphragms with the Top of Member Offset

Never use Top of Member Offsets with the diaphragm feature. This combination will almost surely make the forces in the member incorrect. This is because a Vierendeel-type truss is created, where the internal rigid links created by the diaphragm feature acts as a top flange and the member acts as the bottom flange. What now is drawn as a single member has multiple internal members influencing these forces.

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Partial Diaphragms in a RISA-3D Only Model

There may be times when you want to model a partial diaphragm, i.e., a diaphragm that extends over only a portion of a floor or plane. For example, let’s say you are trying to model a floor that is composed of a relatively rigid section (thick concrete slab) and a relatively flexible section (corrugated steel decking). You would like a way to model a rigid diaphragm for only the rigid portion of the floor.

To accomplish this, specify that a node or group of nodes be detached from the diaphragm.

To detach a node or group of nodes from the diaphragm:

1. Select the Detach from Diaphragm option in the Node Coordinates spreadsheet or click a node.

2. Specify the node in the Information Coordinates window.

Another way this can be done is to offset the elevations of the nodes that comprise the rigid floor section so that they are all a little higher or a little lower than the surrounding floor. The offset only needs to be slightly larger than the Merge Tolerance, since this is the tolerance for other nodes to be on the same plane as the primary node. This works because the rigid diaphragm feature will only rigidly connect nodes that are at the same elevation as the primary node.  The other nodes, which are on the flexible portion of the floor and are now at a different elevation than the primary node, will not be incorporated into the diaphragm. This can also be used for a "twin tower" situation where you want each tower to act independently of the other.

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Partial Diaphragms in a Combined RISAFloor/RISA-3D Model

For buildings where a flexible diaphragm and rigid diaphragm occur on the same floor you can model the diaphragms using separate slab edges. This will require a gap between the framing of the two diaphragms however, such that load will not automatically be shared between the diaphragms.

The following image is an example of the gap:

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Sloped Roof Flexible Diaphragms (RISAFloor/RISA-3D Integration)

The flexible diaphragms at sloped roofs require members that are in the horizontal plane to attribute load to. These members must exist at the base roof elevation. For that reason, in the example below, the program reports "Loads are not attributed for Diaphragm". In the direction perpendicular to the ridge, there are no members for the program to attribute the wall wind loads to, so no loads are attributed to the diaphragm at all in that direction.

To correct this issue, simply draw horizontal bracing in the structure which can pick up the load and transmit it to the main lateral force resisting system.

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For additional advice on this topic, please see the RISA Tips & Tricks webpage at risa.com/post/support. Type in Search keywords: Sloped Roofs.