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

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…

November 29, 2010 | RISA-3D, Results, Diaphragms, New

How to Check Frame Story Shears in RISA-3D

When using rigid diaphragm analysis in RISA-3D it can be useful to evaluate the story shear in each frame. This is a perfect application for the Internal Force Summation Tool. The first step is to solve the desired load combination. Next, select one of the braced frames in the model and using the selection tools on the far left side of the 3D window, turn off the unselected parts of the model to show only the frame which you are interested in. Next, snap the model into an elevation view of the frame in order to make the next step easier. Click on the "sigma" icon (Internal Force Summation Tool) below the selection toolbar and then click on two joints of the frame that are at the same elevation to define a horizontal cutting plane. Once the cutting plane is defined (by clicking the second joint) you can slide the plane up and down your frame to select the vertical location of your cutting plane. Click a third time for a dialog box which explains what each force is. Here, F1 represents the story shear…

April 23, 2020 | RISA-3D, Loads, RISAFloor, Diaphragms, New

How Do Flexible Diaphragms Work on Sloped Roofs?

RISAFloor supports the use of flexible diaphragms on both flat and sloped roofs. Before reviewing the workflow for utilizing a flexible diaphragm on a sloped roof, it makes sense to review and understand the basic behavior of flexible diaphragms within RISAFloor. To do this, please first review the article listed below: Link: How to Design Flexible Diaphragms in RISAFloor Once you have an understanding of flexible diaphragm behavior in RISAFloor, we will utilize a simple gable roof structure with shear walls and a single moment frame to demonstrate sloped roof flexible diaphragm behavior. After the model is created, and before the model is transferred to RISA-3D, the user has the option to consider a Ceiling Diaphragm for the load attribution. This option exists in the Global Model Settings. When a Ceiling Diaphragm is Specified When a Ceiling Diaphragm is specified, lateral load will be applied in the horizontal plane at the base elevation of the sloped roof. Since two diaphragm regions were defined within RISAFloor, the program knows that load is supposed to be attributed to the steel moment frame and the exterior walls. When transferring the model to RISA-3D, a warning message will appear stating that the load was…