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# problem(s) with preloaded modal analysis in NASTRAN

## problem(s) with preloaded modal analysis in NASTRAN

(OP)
Hello everyone, this is my very first message in this forum, so I hope this is the right section to post my question.
I'm using Nastran for a study on bolted joints, both from a static and a modal point of view. I realised that Nastran allows to simulate the effect of pretensioning on a linear modal analysis, so I'm trying to perform such an analysis, but I'm experiencing some difficulties.

The problem is very simple, even if for a Nastran beginner as I am, it is a pretty mess: two plates linked with a preloaded bolt. To make the problem easier, I am modelling only the plates with 3D solid elements; bolt presesnce is considered as a pressure load acting over washer area. Using load/BC interface I defined the two plates as deformable bodies in contact, since this is expected to happen on mating surface. The first step was to run a linear static analysis, who went well. An image with the displacements coming from SOL101 is attached. Moreover, a detail of BC is reported: the point along x axis are not allowed to move in the y direction, while points along y axis are not allowed to move in the x direction. This is due to simmetry.
Here is the case control

SOL 101
CEND
\$ Direct Text Input for Global Case Control Data
TITLE = MSC.Nastran job created on 12-Nov-15 at 16:29:51
ECHO = NONE
SUBCASE 1
SUBTITLE=Default
NLSTEP = 1
BCONTACT = ALLBODY
SPC = 2
DISPLACEMENT(SORT1,REAL)=ALL
SPCFORCES(SORT1,REAL)=ALL
STRESS(SORT1,REAL,VONMISES,BILIN)=ALL
BOUTPUT(SORT1,REAL)=ALL

Then, as reported in some Nastran documents, I modified this input file by sostituting SOL103 in place of SOLSOL101, and entering the following rows just above the begin bulk command

SUBCASE 2
STATSUB = 1
SPC = 2
METHOD = 1
VECTOR = ALL

Finally, EIGRL, 1, , , 11 was added just below the begin bulk command. The fatal error in .f06 file is
*** USER FATAL MESSAGE 9238 (SUBDMAP PHASE0)
GENERAL CONTACT IS REQUESTED IN PRELOAD OR BUCKLING ANALYSIS.
USER ACTION: REMOVE THE BCONTACT REQUEST

Also, the following warning message appears

*** USER WARNING MESSAGE 601 (IFP1RULE)
NLSTEP command is only supported in SOL 101 linear contact analysis and SOL 400. All other cases will be ignored.

The first obvious question is: what steps in the case command am I missing? What did I do wrong?
The second is: if I want to study the free modal analysis of the assembly after pre-load, is it sufficient to delete the command SPC = 2 in the case or do I have to follow another procedure?

Thanks a lot in advice to everyone.
Greetings
Valerio

### RE: problem(s) with preloaded modal analysis in NASTRAN

(OP)
I forgot to mention that to constrain motion in z-direction, 0D grounded spring elements have been used.

Greetings
Valerio

### RE: problem(s) with preloaded modal analysis in NASTRAN

Better off with using the NASTRAN forum

Use translation assistance for Engineers forum

Note the rules include No Student posting

### RE: problem(s) with preloaded modal analysis in NASTRAN

First, I believe that a basic normal modes analysis ignores nonlinear aspects such as contact, and also ignores any load cases.

Once you understand that, then it is just a matter of making sure there is mass, stiffness, and there are constraints. The goal is to find the eigenvalues of the system.

Aerospace Stress Analysis and FEA Courses
http://www.stressebook.com
Stressing Stresslessly!

### RE: problem(s) with preloaded modal analysis in NASTRAN

(OP)
Yes, I can see the point, but considering a simple beam, this will have different natural frequencies in dependancy of the load acting upon it. If it is compressed with critical load, eigenvalue will be zero... So, according to this, in my bolted and preloaded structure I want to see how preload effect the modal problem, in terms of eigenvalues and eigenvectors. But I am still interwined with Nastran input file building!

Thanks for the attentions
greetings
Valerio

### RE: problem(s) with preloaded modal analysis in NASTRAN

You might want to research more on the direct and modal frequency response analysis types.

Aerospace Stress Analysis and FEA Courses
http://www.stressebook.com
Stressing Stresslessly!

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