August 30, 2019
Vibration Analysis for Steel Joists
RISAFloor v14 includes Steel Joist Vibration Analysis per SJI Technical Digest #5. For more information, check out the video below.
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 v14 includes Steel Joist Vibration Analysis per SJI Technical Digest #5. For more information, check out the video below.
Strength Design (LRFD) according to the AWC-NDS 2015/2018 code has been implemented into RISAFloor v14.0. To select this code for your design, simply choose this code from the Codes tab within Global Model Settings.
RISAFloor v14 now includes floor vibration calculations per the 2nd Edition of AISC Design Guide #11. To select this code for your design, simply choose AISC DG 11 2nd Edition in the Beam Vibrations drop-down from the Codes tab within Global Model Settings.
RISAFloor is the RISA Building System’s floor design software which allows users to design with composite decks, elevated concrete slabs and more. When paired with RISA-3D, the two create a complete solution for the analysis and design of multi-story structures, including two buildings on the same...
The 2015 NBC compliant codes have been implemented into RISA-3D v17.0, RISAFloor v13.0 and RISAFoundation V.11.0!
The 2016 edition of ASCE Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE 7-16) is now available in RISA-3D v17.0, RISAFloor v13.0, and RISAFoundation v11.0!
Eurocode lateral torsional buckling capacity is calculated per equations in Annex F in the ENV 1993-1-1:1992. This calculation uses variables C1, C2 and C3. Since there is no generic formula in the Eurocode to calculate the moment gradient factor, C1, RISA will use the widely accepted López, Yong...
RISAFloor v13 includes a variety of modeling features which allows users to quickly create and manipulate geometry. These features include construction lines, saved selections and copy elements from floor to floor. Overall, these features can be used to help engineers streamline their modeling...
The new AWC NDS 2018 code has been implemented into RISA-3D v17.0 and RISAFloor v13.0. To select this code for your design, simply choose this code from the Codes tab within Model Settings.
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