November 16, 2016
Custom Column Strip Widths
RISAFloor v11 now includes the ability to create custom column strip widths in order to optimize the design of slab reinforcement.
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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RISAFloor v11 now includes the ability to create custom column strip widths in order to optimize the design of slab reinforcement.
RISAFloor ES allows you to model concrete slabs of any thickness. However, there might be sections of the building that require a thicker slab. The icon is shown below and it will allow you to draw a Polygon or Rectangular shape.
In RISAFloor ES, you can quickly thicken the slab in the column strip area with the Drop Panel tool. The column strip is typically the area of highest demand so increasing the thickness may help optimize the concrete usage.
The new ACI 318-14 code has been implemented into RISA-3D v14, RISAFloor v10, and RISAFoundation v8.
The batter dimensions can be set from Wall Footing Definitions:
In RISAFoundation, Design Strips are used to create regions in a slab for reinforcement design. Design strip size and shape is determined by the user, see the Help- Slab- Design Strip Width topic.
Per the ACI, you can add a Shear Cap to your column to increase the shear strength of the slab. In RISAFloor ES two-way slabs, adding a shear cap will thicken the plates around the column automatically. The Shear Cap will affect the Punching Shear and the One-Way shear calculations. As you draw the...
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.
Concrete buildings often have large cantilevers. In RISAFloor ES, the Support lines can be drawn to support points but also they can be drawn as cantilevers (shown below).
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