As always, we look forward to comments or suggestions you may have to make this newsletter a valuable source of information. We are here to help and support.

Anca Timmermans provides technical and sales support for NEiNastran, tests new software versions and updates, and writes documentation. Anca has previously held a similar position at MSC.Software in USA and Compumod in Australia. She has over 15 years experience in the field of civil and structural engineering, with almost 7 years of FEA and customer support. Ms. Timmermans earned a MS in Civil Engineering from Romania and a MS in Structural Engineering from Australia.

Customer Driven Solutions

Introduction

The MODALDATABASE feature in NEiNastran is designed so that the user can export a database file that includes the results of a modal analysis. This is a useful tool if the user is performing a modal transient or frequency response analysis and does not want NEiNastran to calculate the modes each time the dynamic analysis is run. The MODESET feature allows a reduced modal set to be generated that selects modes based on a criteria or selection. Users can define this modal set based on top modal effective mass contribution, a percent of modal effective mass, or by simply calling out the specific modes using a SET command.

Using these two features together can greatly save analysis time and expand flexibility.

Using the MODALDATABASE Feature

To have NEiNastran produce a modal database, open your .nas file (setup for a normal modes analysis) in the NEiNastran Editor. Then open the Eigenvalue Processor Parameters and double click MODALDATABASE. Select Store and hit OK. This is shown in Figure 1. After you run the analysis, a file will be created with the same name as the .NAS file but with an .MDB extension (this is the modal database). Now when you run your modal dynamic analysis you can use this database by setting MODALDATABASE=FETCH. Note that NEiNastran will look for an .MDB file with the same name as the .NAS file you are running, so you will need to rename the .MDB so it matches.

The MODESET feature is a Case Control command that must by typed in manually. To see the specific options of the MODESET entry, open the NEiNastran Reference Manual and go to page 3-53. If, for example, we type in MODESET, PERCENT, 2.5 in the Case Control this will direct NEiNastran to only consider the modes that exceed 2.5% of modal effective mass when performing the dynamic analysis solution.

Combining the Features

For large dynamic analyses you can run a normal modes analysis with a full range of frequencies you might expect and store this modal database. Then, you can use the MODESET command to run your dynamic model and look at the response of your structure based only on certain modes, or a criterion of modes (i.e., highest modal effective mass). In this way, you can gain valuable insight as to how your structure responds with minimal analysis time.

Established in 1991, Noran Engineering Inc. (NEi) was created with the idea of bringing a true Nastran analysis tool from the mainframe to the desktop. The company has experienced steady and consistent growth from the beginning, based primarily upon customer loyalty, referrals, cost factors and a true commitment to customer enhancements.
These concepts shape the way everyone at NEi does their job. In a small flexible organization it is possible to implement methods and processes that are infeasible in larger, stratified and compartmentalized companies. Working in this way, NEi can establish a diverse customer base in its short history.
Over the years, NEi is becoming one of the most respected mechanical engineering service providers within the CAE market. By sticking to their core principles and growing their skill and technology base, the company has positioned itself to continue providing the best solutions, at the best price in the best time.  

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