Bentley Communities
Bentley Communities
  • Site
  • User
  • Site
  • Search
  • User
  • Welcome
  • Products
  • Support
  • About
  • More
  • Cancel
RAM | STAAD | OpenTower
  • Product Communities
  • More
RAM | STAAD | OpenTower
RAM | STAAD | OpenTower Wiki RAM Frame - Center of Rigidity
    • Sign in
    • Working Remotely with RAM or STAAD
    • -RAM | STAAD | OpenTower Wiki
      • +Integrated Structural Modeling
      • +Limcon
      • +Microstran
      • +MStower
      • +RAM Concept
      • +RAM Connection
      • +RAM Elements
      • +RAM SBeam
      • -RAM Structural System
        • +Known Issues in Ram Structural System
        • +RAM Structural System - Feature Articles
        • +RAM Structural System Release Notes
        • -RAM Structural System Support Solutions
          • +RAM Structural System - General Topics
          • +RAM Structural System - RAM Concrete
          • +RAM Structural System - RAM Foundation
          • -RAM Structural System - RAM Frame
            • ASCE 7, AISC 360, and the Direct Analysis Method in the RAM Structural System
            • CoreBrace Buckling Restrained Braces
            • Earth Pressure in RAM Structural System
            • Missing Load Cases from Generated RAM Frame Load Combinations
            • Program Crash after Changing Beam Size in View/Update Dialog in RAM Frame - Steel Mode
            • RAM Frame - Additional Lateral Restraint
            • +RAM Frame - Building and Frame Story Shear
            • RAM Frame - Center of Rigidity
            • RAM Frame - Criteria - B1 & B2
            • +RAM Frame - Criteria - Diaphragms
            • RAM Frame - Diaphragm Constraints and Gravity Forces in Shear Walls
            • RAM Frame - DXF
            • RAM Frame - Dynamic Analysis FAQ
            • RAM Frame - Eigenvalue Error [TN]
            • +RAM Frame - Load Cases
            • RAM Frame - Masses
            • RAM Frame - Notional Loads
            • RAM Frame - Overlapping Node Numbers in Plan View
            • RAM Frame - Rigid Diaphragm Constraints and Frame Shear [TN]
            • RAM Frame - Rigid End Zone assumptions
            • RAM Frame - Second Order Effects with BS 5950-1:2000
            • RAM Frame - Seismic Loads [FAQ]
            • +RAM Frame - Steel Standard and Seismic Provisions
            • RAM Frame - Tension Only [FAQ]
            • RAM Frame - Wind Loads [FAQ]
            • RAM Frame Analysis Stalls
            • RAM Frame Numbers
            • RAM Frame P-Delta [TN]
            • +RAM Frame Troubleshooting [TN]
            • RAM Frame Wall Groups FAQ
            • RAM Instability In Finite Element Analysis [TN]
            • RAM Meshing and Segmentation [TN]
            • RAMSS Truss Modeling And Design [TN]
            • +Red Status Lights for Lateral Load Cases in RAM Frame
          • +RAM Structural System - RAM Modeler
          • +RAM Structural System - RAM Steel
          • +RAM Structural System Tutorial
          • +What is RAM DataAccess? [TN]
        • RAM Structural System V17.0 License Consolidation
        • RAM Structural System Videos
      • +Scenario Services
      • +STAAD Advanced Concrete Design (RCDC)
      • +STAAD RCDC FE
      • +STAAD Foundation Advanced
      • +STAAD.building
      • +OpenTower Designer
      • +STAAD.offshore
      • +STAAD.Pro
      • +Structural Analysis Support Solutions
      • +Structural Dashboard
      • +Structural WorkSuite
      • Structural Synchronizer Links
    • +Seminars and Events
    • +Webinars
    • STAAD.Pro V8i SS5, Build 20.07.10.65 (29 October 2014)
    • (05.21.2013) Bentley - Offshore eSeminar: Loads on Offshore Structures Attendee Questions
    • (06.29.2012) Bentley-RAM Powerful New Integrated Design and Analysis Tools for Your Complex Structures eSeminar Attendee Questions
    • (07.30.2013) Bentley Structural Software User Spotlight: John A. Martin & Associates' Structural Steel Project at LAX Q&A
    • (09.25.2012) Bentley–Integrated Structural Workflow from Desktop to Tablet eSeminar Attendee Questions
    • 1) Seminar Introduction
    • 3) Plant Session
    • 4) Attendee Quick Start Guide
    • Access Violation Error Temperature Loads on Shells
    • Argument Out of Range Error
    • Boundary Not Valid Error
    • Cannot Select Desired Column or Brace Section
    • Disable ProjectWise Integration with ISM
    • EC2:2004
    • I installed version (7.01.00) in Windows 7 and 8 systems, I get Microsoft compatibility error
    • Invalid Pointer Error When Assigning Spread Foundation
    • I’m getting error message while trying to analyze using the NS 3472 code.
    • Key not valid for use in specified state
    • Learn Event at Kolkata
    • Learn Event at Mumbai
    • Learn Event at New Delhi
    • libifcoremd.dll is missing from your computer
    • License Management Tool closes abruptly
    • License Management Tool fails to open
    • MFC application error after changing values in Optimize dialog
    • Not a Valid ISM DGN Repository Exception
    • RAM Concept 4.1 Release Notes
    • RAM Concept 4.1.1 Release Notes
    • RAM Concept 4.1.2 Release Notes
    • RAM Concept 4.1.3 Release Notes
    • RAM Concept 5.0.1 Release Notes
    • RAM Concept 5.0.2 Release Notes
    • RAM Concept 5.1.1 Release Notes
    • RAM Elements V12.5.1 Release Notes
    • Unable to Satisfy All Prerequisites for RAM Connection Release 9.0
    • Unable to Satisfy All Prerequisites for RAM Elements
    • V8i (SELECTseries 2) (Release 08.02.02) Release Notes
    • V8i (SELECTseries 3) (Release 08.02.03) Release Notes
    • V8i (SELECTseries 4) (Release 08.02.04) Release Notes
    • Xmllite.dll Error

     
     Questions about this article, topic, or product? Click here. 

    RAM Frame - Center of Rigidity

      Product(s): RAM Structural System
      Version(s): Any
      Area:  Analysis; Reports

    Center of Rigidity

    In order to review the center of rigidity for a structure, a special Centers of Rigidity load case must be created under Loads – Load cases in RAM Frame. Once the analysis of that load case has been performed, a “Centers of Rigidity” report becomes available under the reports menu.

    For technical information on the method Ram Frame uses to calculate it see the RamFrameAnalysis.pdf manual section "7.22 Center of Rigidity Calculation".

    The Centers of Rigidity report lists the centers of rigidity of each rigid diaphragm at each of the levels that contain lateral members. 

    Center of Rigidity (CR) values reported are only used for load cases that require explicit calculation of CRs for use in calculation of load eccentricities (for example, ASCE 7-05 Wind Load Case). This information is never used directly for analysis, though it is used to determine the amount of eccentricity for some loads. It should be noted that analysis results always includes any torsional effects due to having center of rigidity and applied force resultant at different locations. In other words, the analysis always accounts for locations and stiffnesses of frame members and diaphragms. Hence, any torsional effects of the force being offset from the center are implicitly and correctly accounted in the analysis. The center of rigidity can be thought of as the location through which a lateral load would cause lateral deformation of the diaphragm (possibly at an angle) without causing rotation. The concept is analogous to that of a cross section’s “shear center”.

    The report also lists the "Centers of Mass", and for rigid diaphragms specifically, it includes the "Story Lateral Stiffness" of each diaphragm. The reported story stiffness is the inverse of the interstory drift that is calculated according to a unit load applied at the story.

    Below are the results for a 30’x10’ structure, 10 stories tall, with walls on 3 sides. The walls here intersect to create a "C" shape, but individual wall out-of-plane stiffness is ignored and the diaphragms are rigid. 

     

    Isometric view of “C” shaped wall model, center of mass at (15,5)

    For buildings with torsional irregularity, the lateral load may always be on the same side of the center of rigidity even after considering a 5% accidental eccentricity of the load. In this case, the torsion will always be “adding” to the shear on one side of the building and “subtracting” shear from the other side. To ignore these effects, an additional lateral load case with the same magnitude as the generated load, but with a location exactly at the center of rigidity can be added. This pure shear load case, can then be included in the load combos to yield a conservative design for the frame elements that otherwise would benefit from the eccentricity. Some codes may require such a conservative approach, but most building codes recognize the reality of the eccentricity and the effect it has on the structure. In those cases, the code may simply want an increase in the accidental torsion to be considered. This can be changed under Loads – Masses, within the Analysis mode of RAM Frame.

    Returning to the example above, an interesting phenomenon occurs in regard to the “Yr” location. Even though the levels are identical, the center of rigidity shifts from level to level because the flexibility of the supporting structure keeps increasing. The higher the level, the more eccentric the center of rigidity. In other words, at the top a lateral load must be applied farther from the middle in order to bend the channel without twisting it.

    A load applied to the diaphragm in line with the long wall (y=10’), actually produces a large diaphragm rotation. For a load applied to the center of the diaphragm (y=5’) the effect is even worse

     

    Channel Loaded Through Center of Rigidity

    Channel Loaded Through Long Wall

    It is worth noting that a structure like this with a high degree of eccentricity may also experience a significant shift in the center of rigidity location when using P-Delta analysis. For that reason, when closely evaluating the center of rigidity we recommend a first order analysis.

    Why is more than one center of rigidity shown on a level?

    The program calculates and reports a center of rigidity for each diaphragm on every level. The plotted center of rigidity values using Process - Results - Center of Rigidity are only for rigid diaphragms.

    Why can't I plot the center of rigidity?

    First a Center of Rigidity load case must be created and analyzed in order to plot the location using Process - Results - Center of Rigidity.

    Furthermore, this graphical result is only provided for rigid diaphragms.

    See Also

    RAM SS Walls FAQ

    RAM Frame P-Delta [TN]

    RAM Frame - Seismic Loads [FAQ]

    • analysis
    • RAM Frame
    • RAM Structural System
    • Center of Rigidity
    • Support Solutions
    • mass
    • Share
    • History
    • More
    • Cancel
    • Seth Guthrie Created by Bentley Colleague Seth Guthrie
    • When: Wed, Aug 28 2013 5:51 PM
    • Seth Guthrie Last revision by Bentley Colleague Seth Guthrie
    • When: Wed, Jun 27 2018 4:50 PM
    • Revisions: 17
    • Comments: 0
    Recommended
    Related
    Communities
    • Home
    • Getting Started
    • Community Central
    • Products
    • Support
    • Secure File Upload
    • Feedback
    Support and Services
    • Home
    • Product Support
    • Downloads
    • Subscription Services Portal
    Training and Learning
    • Home
    • About Bentley Institute
    • My Learning History
    • Reference Books
    Social Media
    •    LinkedIn
    •    Facebook
    •    Twitter
    •    YouTube
    •    RSS Feed
    •    Email

    © 2021 Bentley Systems, Incorporated  |  Contact Us  |  Privacy |  Terms of Use  |  Cookies