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:

image

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] image F
SI Kilogram Meter/second2 Newton
English-1 Slug or lbf-sec2/feet Feet/second2 Lbf
English-1 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

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