S13-1 NAS122, Section 13, August 2005 Copyright 2005 MSC.Software Corporation SECTION 13 ENFORCED MOTION

S13-2 NAS122, Section 13, August 2005 Copyright 2005 MSC.Software Corporation

S13-3 NAS122, Section 13, August 2005 Copyright 2005 MSC.Software Corporation ENFORCED MOTION n Used to analyze structures that have constrained motion (e.g. base motion). The motion could be displacement, velocity, or acceleration. n Common examples are earthquakes (for transient analysis) or swept sine shaker test simulation (for frequency response analysis). n Since Version 2001 of MSC.Nastran it is possible to specify enforced motion by using partitioned dynamic equations. n Prior to Version 2001 of MSC.Nastran a technique called the large mass method was commonly used to apply enforced motion. This latter approach is discussed in Section 16.

S13-4 NAS122, Section 13, August 2005 Copyright 2005 MSC.Software Corporation ENFORCED MOTION EQUATIONS n Use the n-set coordinates to define the matrix equation of motion in the time domain. n The n-set DOFs are partitioned into the f-set and s-set DOFs. n Separate the dynamic equation of motion into two equations, one for the f-set and the other for the s-set. From this the following equations are arrived at.

S13-5 NAS122, Section 13, August 2005 Copyright 2005 MSC.Software Corporation ENFORCED MOTION EQUATIONS (Cont.) n f-set equation. n s-set equation. n These equations are used when the enforced motion, {u s }, is given as a function of time.

S13-6 NAS122, Section 13, August 2005 Copyright 2005 MSC.Software Corporation n For frequency response there are corresponding equations of motion. n Use the n-set coordinates to define the matrix equation of motion in the frequency domain. n The n-set DOFs are partitioned into the f-set and s-set DOFs. ENFORCED MOTION EQUATIONS (Cont.)

S13-7 NAS122, Section 13, August 2005 Copyright 2005 MSC.Software Corporation n Separate the dynamic equation of motion into two equations, one for the f-set and the other for the s-set. From this the following equations are arrived at. n f-set equation. n s-set equation. n These equations are used when the enforced motion, {u s }, is given as a function of frequency. ENFORCED MOTION EQUATIONS (Cont.)

S13-8 NAS122, Section 13, August 2005 Copyright 2005 MSC.Software Corporation ENFORCED MOTION EQUATIONS (Cont.) n It is possible to calculate the relative motion (motion relative to the enforced motion, base motion). u where l {u f (t) } is the absolute motion of the f-set DOFs l {u f rel (t) } is the relative motion of the f-set DOFs l {u f s (t) } is the motion of f-set DOFs due to specified enforced motion of the s-set DOFs, computed from just static considerations. u Repeat the f-set equation obtained from the the n-set matrix equation of motion in the time domain. l This equation is solved for {u f (t)}. u Ignore the mass and damping, and set {P f (t)} to {0}. Determine the static response of the f-set DOFs to enforced motion of the s-set DOFs.

S13-9 NAS122, Section 13, August 2005 Copyright 2005 MSC.Software Corporation ENFORCED MOTION EQUATIONS (Cont.) u Solve for {u f s }. u From these equations the following equation for relative motion is found.

S13-10 NAS122, Section 13, August 2005 Copyright 2005 MSC.Software Corporation ENFORCED MOTION IN TRANSIENT ANALYSIS n Now review the MSC.Patran input for automatic enforced motion defined as u Acceleration u Displacement u Velocity n The analysis will be Transient

S13-11 NAS122, Section 13, August 2005 Copyright 2005 MSC.Software Corporation n First, a time dependent Load Case is created and is made current. This will ensure that all subsequent LBC forms relate to this time dependent loading. ENFORCED MOTION IN TRAN ANALY (Cont.)

S13-12 NAS122, Section 13, August 2005 Copyright 2005 MSC.Software Corporation n A time dependent field is set up, in this case a rectangular pulse input ENFORCED MOTION IN TRAN ANALY (Cont.)

S13-13 NAS122, Section 13, August 2005 Copyright 2005 MSC.Software Corporation n Now define the loading as Displacement, Velocity, or Acceleration. n Consider the first case where a Displacement was chosen. n The direction of the displacement is defined in Spatial Dependent. n The Time Dependent is defined in the field. n The grids where the enforced motion are applied are picked in Select Application Region. Grid 1 Z direction ENFORCED MOTION IN TRAN ANALY (Cont.)

S13-14 NAS122, Section 13, August 2005 Copyright 2005 MSC.Software Corporation n The Nastran Entry is identical to the form seen in the section on Transient analysis, except now the TYPE field, which is highlighted, points to a DISP type input n This defines an automatic enforced motion at grid 1 of a displacement time history n Two additional Nastran entries are required and are output by Patran u SPCD to define this as an enforced motion at GRID 1 u SPC1 to define GRID 1 as a constrained grid Grid 1 Z direction ENFORCED MOTION IN TRAN ANALY (Cont.)

S13-15 NAS122, Section 13, August 2005 Copyright 2005 MSC.Software Corporation n As an alternative, apply an acceleration in a similar manner. n Input 1 g n This equates to in/s^2 in this example n The TLOAD1 definition now changes to ACCE. n In this case the Magnitude of the motion is now set to in the SPCD definition. ENFORCED MOTION IN TRAN ANALY (Cont.)

S13-16 NAS122, Section 13, August 2005 Copyright 2005 MSC.Software Corporation WORKSHOP 8 DIRECT TRANSIENT RESPONSE WITH ENFORCED ACCELERATION n Please carry out Workshop 8. n This workshop uses the matrix partitioning method to determine the transient response due to a unit acceleration sine pulse of 250 Hz applied at the base, in the Z-direction. n Please do not hesitate to ask your tutors advice.

S13-17 NAS122, Section 13, August 2005 Copyright 2005 MSC.Software Corporation ENFORCED MOTION FOR FREQUENCY RESPONSE ANALYSIS n As in transient response, it is possible to specify enforced motion for frequency response in MSC.Nastran. The motion can be specified as displacement, velocity, or acceleration. n An alternative technique is to specify enforced motion using the large mass method.

S13-18 NAS122, Section 13, August 2005 Copyright 2005 MSC.Software Corporation PARAM,ENFMOTN n A new user parameter called ENFMOTN has been introduced in MSC.Nastran for use with the SPC/SPCD (matrix partitioning) method in SOL108,109,111,112, 146, and 200 n PARAM,ENFMOTN,ABSThe default value of ABS implies that the results of the analysis are absolute motion of the model. n PARAM,ENFMOTN,RELIf the value is specified as REL, then the results are motion relative to the enforced motion of the base. In the case of modal dynamic analysis (SOL 111 and SOL 112), this is equivalent to employing the large mass approach and excluding the rigid body modes from the analysis.