February 13, 2012

How to Apply a Rigid Diaphragm in RISA-3D

In RISA-3D, rigid diaphragms can be added using the Diaphragm spreadsheet. Just enter the node label and then choose the horizontal plane.

Internally, all rigid diaphragms are membrane diaphragms that tie all of the nodes within the diaphragm plane together for both translation and rotation, but only within the plane of the diaphragm. This is done internally using rigid links.

Tags: RISA-3D, Diaphragms, New

June 20, 2019 | RISA-3D, Diaphragms, New, Shear Walls

Understanding Wall Panel Forces with Rigid Diaphragms

When modeling building structures using steel or composite steel floors, engineers typically utilize rigid diaphragms to distribute lateral loads (wind and seismic) to lateral load resisting elements such as shear walls, moment frames and braced frames. Rigid diaphragms represent a plane of very high rigidity and distribute load to elements which connect to them solely based on the stiffness of the elements. As a result, elements located within the extents of the diaphragm (such as wall panels) will have their forces “re-distributed” based on the relative stiffness of all the elements of that given level. Internally, this is accomplished by the fact that the rigid diaphragm is represented by rigid links that connect every node within the diaphragm (including those in a wall panel) at the level of the diaphragm. The best way to investigate how a rigid diaphragm is impacting the results on a given wall panel stack is to utilize both the Wall Panel Forces spreadsheet and the Internal Force Summation Tool for Walls. To easily review the results for a given wall stack, first select only the wall that you wish to evaluate the results for. Next, select the Filter Results button from the toolbar to the…

March 10, 2015 | RISAFloor, Concrete, Diaphragms, New, RISAFloorES

What is a Semi-Rigid Diaphragm?

RISAFloor Concrete Floor Slabs can design be transferred into RISA-3D for lateral design using either a Rigid Diaphragm or Semi-Rigid diaphragm. In RISA-3D, the wind and seismic loads are calculated and then applied into the diaphragm level to distribute the forces to the Lateral Resisting System. For example, looking at the plan view of a simple building the wind and seismic load is represented as a distributed load into the diaphragm. This building has three shear walls to resist the lateral forces. Rigid Diaphragm The Rigid Diaphragm is considered fully rigid along the entire plane and internally the program connects all nodes at that level together rigidly. As the lateral loads are applied to the Rigid diaphragm, they are distributed to the lateral walls based on the stiffness of the walls. Semi-Rigid Diaphragm The Semi-Rigid diaphragm is sub-meshed into a Finite Element Analysis (FEA) with plates that represent the stiffness of the diaphragm. As the loads are applied into a Semi-Rigid diaphragm the flow of the diaphragm deflects as shown below. This will often produce a more realistic distribution of the forces to the lateral resisting elements…

April 12, 2017 | RISAFloor, Diaphragms, New, News

Semi-Rigid Diaphragms now Available

RISAFloor now has the ability to model a Semi-Rigid, Rigid or Flexible diaphragms to distribute lateral loads. For more information about the different diaphragm types, click on the link below. Link: RISA Help: Diaphragms Link: What is a Semi-Rigid Diaphragm? On a beam supported floor, you create a diaphragm edge by clicking this icon: and then choose the Semi-Rigid option. The semi-rigid diaphragm is modeled using plates from the General- Material spreadsheet. The thickness in combination with the material stiffness should be based on the lateral stiffness you expect from your diaphragm. Existing models can be modified by changing the diaphragm type in the Diaphragms spreadsheet. The Semi-Rigid diaphragm will be automatically submeshed in RISA-3D. After a solution is present, you can view the submesh for reference (Model Display Options- Diaphragms tab)…