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The OpenSees interpreter does not process units. Units, however, can be used when entering values if these units are defined previously. The unit definition consists of two parts: the basic units are defined first, all other units are subsequently defined. The basic units are assigned a value of one and all OpenSees output is in these units. It is very important that all basic units are independent of each other. The unit-definition file can contain both metric and Imperial units, as can the basic units. Hence, the input files may contain mixed units.
Constants, such as p and g can also be defined apriori.
An example of unit and constant definition is given in the following:
# ----Units&Constants.tcl---------------------------------------------------------
set in 1.; # define basic units
set sec 1.;
set kip 1.;
set ksi [expr $kip/pow($in,2)]; # define dependent units
set psi [expr $ksi/1000.];
set ft [expr 12.*$in];
set lb [expr $kip/1000];
set pcf [expr $lb/pow($ft,3)];
set ksi [expr $kip/pow($in,2)];
set psi [expr $ksi/1000.];
set cm [expr $in/2.54]; # define metric units
set meter [expr 100.*$cm];
set MPa [expr 145*$psi];
set PI [expr 2*asin(1.0)]; # define constants
set g [expr 32.2*$ft/pow($sec,2)];
set U 1.e10; # a really large number
set u [expr 1/$U]; # a really small number