#121: WTMASS Parameter
Global mass matrix scaling factor. The terms of the global mass matrix are multiplied by the value of WTMASS when they are generated. This parameter is used when material density is input in weight instead of mass units. It does not affect loads generated by GRAV or mass properties calculated by the Grid Point Weight Generator. The value of WTMASS is calculated using the relation:
rho_m = (1/g) rho_w, where rho_m = mass density, g = acceleration of gravity, rho_w = weight density.
WTMASS = 1/g
inch/sec^2: 1/386.4 in/s^2 = 0.002588
feet/sec^2: 1/32.2ft/s^2 = 0.031056
7G:
wtmass * mass * gravity = (1/386.4) * 6.724 * ( 7* 386.4) = 47.068
The default value for the factor is 1.0. Hence, the default value for WTMASS assumes that mass density is entered. However, in MMPDS tables densities of materials are given in ls/in^3, which is weight density. In this case, we should use factor 1/386.4 in/s^2 = 0.002588.
For example,
1) if the weight density of steel is entered as RHO=0.3 lb/in^3, using PARAM,WTMASS,0.002588 converts the weight density to mass density for the acceleration of gravity g=386.4 in/sec^2. The mass density becomes 7.76E-4 lbf-sec^2/in^4.
2) If the weight density of steel is entered as RHO = 80000 N/m^3 when using metric units, then using PARAM,WTMASS,0.102 converts the weight density to mass density for the acceleration of gravity g = 9.8 m/sec^2. The mass density, therefore, becomes 8160 kg/m^3
.
PARAM,WTMASS is used once per run, and it multiplies all weight/mass input (including CMASSi,CONMi, and nonstructural mass input).
Recent Comments