Mass Check
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Checking Dynamic Analysis Parameters

Learning Objectives: After reviewing this information, the reader will be able to:

  1. read a dynamic analysis output for global parameters.
  2. evaluate the appropriateness of a dynamic analysis, in other words if the correct number of modes have been determined and if the mass per story is approximately correct.

 

This page last updated on August 18, 1999

Mass per Story

Mass Participation

One aspect we usually monitor when performing dynamic analysis is the mass participation of various modes.  Mass participation might be thought of as the percentage of the building mass that is active in a given mode.  Mass participation indicates which modes have coupled degrees of freedom.

 

Below is an abbreviated output from a SAP2000 linear analysis of Example 10.5 from Chopra's Textbook.  (Note that to run the analysis we had to use actual values of mass and stiffness.  We chose values so that the fundamental period is around a half second.)

M O D A L  P E R I O D S  A N D  F R E Q U E N C I E S

MODE     PERIOD     FREQUENCY      FREQUENCY     EIGENVALUE

         (TIME)      (CYC/TIME)     (RAD/TIME)      (RAD/TIME)**2

1         0.503567      1.985832     12.477350      155.684252

2         0.189251      5.283990     33.200289      1102.259

M O D A L  P A R T I C I P A T I O N  F A C T O R S

FOR UNIT ACCELERATION LOADS IN GLOBAL COORDINATES

MODE PERIOD         UX              UY          UZ

1     0.503567     -0.651770      .000000     .000000

2     0.189251     0.316853      .000000     .000000

 

M O D A L  P A R T I C I P A T I N G  M A S S  R A T I O S

MODE PERIOD     INDIVIDUAL MODE (PERCENT)          CUMULATIVE SUM (PERCENT)

                    UX          UY            UZ          UX          UY          UZ

1   0.503567     80.8843     0.0000      0.0000     80.8843      0.0000     0.0000

2   0.189251     19.1157     0.0000      0.0000     100.0000      0.0000     0.0000