S3-1 NAS122, Section 3, August 2005 Copyright 2005 MSC.Software Corporation SECTION 3 MASS MODELING

S3-2 NAS122, Section 3, August 2005 Copyright 2005 MSC.Software Corporation

S3-3 NAS122, Section 3, August 2005 Copyright 2005 MSC.Software Corporation MASS MODELING n This section considers some of the implications of Mass Modeling n This section describes the Coupled versus Lumped mass representations previously mentioned. n Review the units of Mass n Also show how to set up different types of mass in Patran and the implications in Nastran u Density defined on a Material Property entry u Non-Structural mass defined on element Physical Property entry u Mass elements defined as CONM1, CONM2 or CMASS1

S3-4 NAS122, Section 3, August 2005 Copyright 2005 MSC.Software Corporation COUPLED VERSUS LUMPED MASS n Coupled mass is generally more accurate than lumped mass (however it really needs both methods to be assessed in each case) n Lumped mass is preferred for computational speed in dynamic analysis. n User-selectable coupled mass matrix for elements PARAM, COUPMASS, 1 to select coupled mass The default is lumped mass. n Elements which have either lumped or coupled mass: BAR, BEAM, CONROD, HEXA, PENTA, QUAD4, QUAD8, ROD, TETRA, TRIA3, TRIA6, TRIAX6, TUBE n Elements which have lumped mass only: CONEAX, SHEAR n Elements which have coupled mass only: BEND, HEX20, TRAPRG, TRIARG

S3-5 NAS122, Section 3, August 2005 Copyright 2005 MSC.Software Corporation COUPLED VERSUS LUMPED MASS (Cont.) n Lumped mass contains only diagonal, translational components (no rotational ones). n Coupled mass contains off-diagonal translational components as well as rotations for BAR (though no torsion), BEAM, and BEND elements.

S3-6 NAS122, Section 3, August 2005 Copyright 2005 MSC.Software Corporation ROD FINITE ELEMENT EXAMPLE n Stiffness matrix: n Classical consistent mass: Length = L, Area = A, Torsional Constant = J, Youngs Modulus = E, Shear modulus = G L

S3-7 NAS122, Section 3, August 2005 Copyright 2005 MSC.Software Corporation ROD FINITE ELEMENT EXAMPLE(Cont.) n Classical and MSC.NASTRAN lumped mass: n MSC.NASTRAN coupled mass: The translational terms represent the average of lumped mass and classical consistent mass. This average is found to be best for ROD and BAR elements.

S3-8 NAS122, Section 3, August 2005 Copyright 2005 MSC.Software Corporation JUSTIFICATION FOR MSC.NASTRAN COUPLED MASS CONVENTION n Consider a fixed-free rod n Exact quarter-wave natural frequency

S3-9 NAS122, Section 3, August 2005 Copyright 2005 MSC.Software Corporation JUSTIFICATION FOR MSC.NASTRAN COUPLED MASSS CONVENTION (Cont.) n Different approximations Lumped mass Classical consistent mass n MSC.NASTRAN Coupled mass

S3-10 NAS122, Section 3, August 2005 Copyright 2005 MSC.Software Corporation MASS UNITS n MSC.NASTRAN assumes consistent units. YOU MUST BE CAREFUL. n Weight units may be input instead of mass units if this is more convenient. You must then convert them to mass units using PARAM,WTMASS. n Weight-to-mass conversion: Mass = (1/G) Weight (G = Gravity Acceleration) Mass Density = (1/G) Weight Density n PARAM,WTMASS, factor performs conversion with factor = 1/G. The default value for factor is 1.0. n Example: Input RHO = 0.3 lb/in 3 for steel weight density. Use PARAM,WTMASS, for G = in/sec 2. n PARAM,WTMASS is used once per run and multiplies all weight/mass input (including MASSi, CONMi, and nonstructural mass input). Do not mix input types. Use all mass or all weight inputs.

S3-11 NAS122, Section 3, August 2005 Copyright 2005 MSC.Software Corporation MASS INPUT n The most common way to define Mass in a structure is via the material density of each of the materials in the structure. This is done using the Material Properties form. Every element that references the material property will build an element mass matrix. MATi entries MAT1MIDEGNURHOATREFGE MAT1230.0E E-4

S3-12 NAS122, Section 3, August 2005 Copyright 2005 MSC.Software Corporation MASS INPUT (Cont.) n Nonstructural mass Mass input on element property entry which is not associated with geometric properties of element. Input as mass/length for line elements and mass/area for elements with 2-D geometry. Examples are: payload distributed over a floor, insulation on beams, mass of electronic component modeled on a PCB.

S3-13 NAS122, Section 3, August 2005 Copyright 2005 MSC.Software Corporation MASS INPUT (Cont.) Scalar mass The Type of Grid Point Mass available are CONM1 and CONM2. CONM2 is most common. The selection between them is made using the Option: COUPLED – gives a CONM1 (A full 6x6 mass matrix) - The user defines half of the terms, symmetry is assumed. This is only used for advanced mass definitions and is not very commonly seen

S3-14 NAS122, Section 3, August 2005 Copyright 2005 MSC.Software Corporation MASS INPUT (Cont.) Lumped gives a CONM2 (concentrated mass), where the translational and rotational terms are defined

S3-15 NAS122, Section 3, August 2005 Copyright 2005 MSC.Software Corporation MASS INPUT (Cont.) u Grounded gives a CMASS1 (scalar mass) l This entry permits a mass with a value in a single direction, this can be a useful modeling technique when mass is only considered effective in that direction

S3-16 NAS122, Section 3, August 2005 Copyright 2005 MSC.Software Corporation MASS INPUT (Cont.) The most common form of Scalar element Mass input is via a CONM2 If only a mass term M is required then the other terms are left blank and the mass will have translational mass terms, equal in the 3 directions If inertia properties are required, then these are entered as appropriate If the mass cg is offset from the grid position, then this is also defined (this writes an MPC equation into the mass matrix – effectively a rigid offset) CONM2s are frequently defined with an offset and linked to more than one grid point with an RBE2 or RBE3, depending on the nature of the structure being attached to