NASTRAN Units

Units in NASTRAN are User Defined

Since no unit system is defined in a NASTRAN run it is the responsibility of the user to use consistent units. In general this means each matrix product below should produce a force unit:

Some commonly used consistent unit systems are:

Unit

English – 1

English – 2

SI

Time

Second

Second

Second

Geometry Location

Feet

Inch

Meter

Displacement

Feet

Inch

Meter

Acceleration

Feet/second²

Inch/second²

Meter/second²

Modulus of Elasticity

Lbf/feet²

Lbf/inch²

Newton/meter²

Applied Force

Lbf

Lbf

Newton

Applied Moment

Foot-lbf

Inch-lbf

Newton-meter

Mass

Lbf-sec²/feet or slug

Lbf-sec²/in or snail

Kilogram

Density

Slug/foot³

Snail/inch³

Kilogram/meter³

 

For example, consistent units on just the term to produce a force unit would be:

Unit System

[M]

F

SI

Kilogram

Meter/second²

Newton

English – 1

Slug or lbf-sec²/feet

Feet/second²

Lbf

English – 2

Snail (slinch) or lbf-sec²/inch

Inch/second²

Lbf

 

Global Changes to Mass Units

NASTRAN does allow a global change in Mass units with the parameter WTMASS. This is typically used when using English units so the user can enter concentrated masses, densities, etc with weight units since this is easier to think in terms of weight than actual English mass units. NASTRAN will then multiple the assembled structural mass matrix by the value on the PARAM WTMASS card hence converting the weight to mass. The relationship of mass to weight is: weight= mass*gravity. So to convert weight to mass is 1/gravity. Applicable WTMASS values would be:

Acceleration Units

feet/second²

Inch/second²

PARAM,WTMASS, value

1/32.2ft/s² = 0.031056

1/386.4in/s² = 0.002588