
January 12, 2017
Use Mezzanine Levels for Wind Loading and Drift Calculations
Rigid diaphragms in RISA-3D are a powerful way to model how floor systems distribute lateral loads. By forcing all connected nodes to move together in-plane, they effectively capture the stiffness of a concrete slab or diaphragm deck—often simplifying analysis without sacrificing accuracy. However, when rigid diaphragms are combined with sloping members, they can introduce unexpected behavior that changes how the structure resists loads—sometimes creating a hidden “tension tie” that doesn’t exist in the real system. When Rigid Diaphragms Alter the Model’s Behavior Consider a simple moment frame with sloped beams under gravity loads—common in pre-engineered metal buildings. Model 1: No rigid diaphragm applied Model 2: Identical frame, but with a rigid diaphragm located at the eaves When reviewing the strong-axis bending moments, column base reactions, and thrust forces: The first frame behaves as expected. The second frame (with the rigid diaphragm) shows reduced bending moments and smaller thrust reactions at the column bases. At first glance, this might seem like an improvement—but it’s actually unrealistic behavior caused by the diaphragm. Why It Happens: The “Hidden Tension Tie” In the model with the rigid diaphragm, the diaphragm prevents the eaves from moving apart under load. This effectively turns the diaphragm into…
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These include:
In RISA-3D, RISAFloor, and RISAFoundation the cursor now provides additional information regarding the cursor coordinates. As you move your cursor around on your screen, a box adjacent to your cursor will appear and populate with the coordinates of the cursor.
RISAFloor v11 now includes the option to add parapets and parapet loading to a building.
RISAFloor does not record the applied area loads in a spreadsheet. To simplify modeling, it instead assumes a default area load over the entire diaphragm area. Additionally, you are free to apply area loads beyond the default loads. Whatever is drawn last will be considered the applied loading...
In a model that contains both RISAFloor and RISA-3D data it is possible to define your diaphragms as either flexible or rigid for lateral design. You can do this from the Diaphragms spreadsheet in either program:
When using RISA Integration between RISASection and RISA-3D, RISA-2D and/or RISAFloor, there are a few common mistakes that people make when attempting to access the RISASection files from the Shape Database.
RISAFloor has the ability to assign camber design rules which allow the user more control over which members are cambered. A camber is the slight upward curvature of a steel beam which is used to compensate for deflection. A user can assign a camber directly to a member or set up design rules to...
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