Gravity and Mass

As indicated in the Materials module, users have the possibility of defining the materials specific weights, with which the distributed self-mass of the structure can then be calculated. Furthermore, in the Element Classes module, additional distributed mass may also be defined, which will serve to define any mass not associated to the self-weight of the structure (e.g. slab, finishings, infills, variable loading, etc). Lumped and distributed mass-only elements can also be defined and then added to the structure in the Element Connectivity module, so that users may model mass distributions that cannot be obtained using the aforementioned Materials/Sections facilities; e.g. water tank with concentrated mass on top. Finally, in the applied loads module, permanent distributed loads can be applied on the elements in every direction.

Here, it is possible for users to define if and how such mass is to be transformed into loads (see Gravity Settings), and which degrees of freedom are to be considered in a dynamic analysis (see Global Mass Directions), as well as, if and how mass is to be defined from loads.

Gravity Settings
There are three available options for defining Loads: i) Loads are not derived from masses, and are only explicitly defined in the Applied loading module. ii) Loads are derived from masses, based on the g value, but ONLY in the gravity direction, which is the default option, and iii) Loads are derived from Masses in any translational direction, according to user-defined coefficients. It is noted that, currently, the mass-derived loads are internally transformed into equivalent point forces/moments at  nodes of the element, as discussed in here, or distributed loads applied at the elements. In addition, the user may also define the value of acceleration of gravity g (which is to be multiplied by the masses in order to obtain the permanent loads) and also the direction in which the latter is to be considered. Clearly, for the vast majority of standard applications, the default values (g=9.81 m/s2, considered in the -z direction) need not to be modified. Finally, it is noted that stress-recovery may be employed to retrieve correct internal forces when distributed loads are defined (through the definition of material specific weight or of sectional/element additional mass, but not through the introduction of dmass elements).

Mass Settings
Three options are offered for defining mass in dynamic analysis, IDA and eigenvalue analysis: i) From the Frame Elements, based on the specific weight of their materials and their section's additional mass, as well as the Mass Elements (lmass and dmass), ii) From Loads, point and distributed, the mass is applied in the gravity direction ONLY, and its value is based on the g value, and iii) From both options i) and ii) above, i.e. from both Frame/Mass Elements and Loads. The first option is set by default.

Further, when running dynamic analyses, it may some times come handy to have the possibility of constraining the dynamic degrees-of-freedom to only a few directions of interest, in order to speed up the analyses or avoid the development of spurious response modes in those directions where the structural mesh was intentionally not adequately devised or refined. This can be done here, by unchecking those dofs that are not of interest (by default, all dofs are activated, i.e. checked). It is also noted that these settings take precedence over the 'mass directions' defined in the lumped/distributed mass elements, that is, if a given distributed mass element should define mass only in the x direction, for instance, but all dofs were to be selected in the Global Mass Directions settings, then even if such element mass contribution to the global Mass matrix of the structure would indeed be considered only in the x direction, the dynamic analysis will nonetheless consider all dofs as active.

Notes

  1. Loads defined in the Applied Loads module are always applied to the structural model, irrespective of the employed option for the masses-to-loads transformations.
  2. The mass-derived loads are internally transformed into equivalent nodal forces/moments, with the exception of elastic and inelastic frame elements, in which mass-derived loads are distributed along the element.
  3. Stress-recovery (Project Settings > Elements > Carry out Stress Recovery) may be employed to retrieve correct internal forces when distributed loads are defined (through the definition of material specific weight or of sectional/element additional mass, but not through the introduction of dmass elements).
  4. Analyses of large models featuring distributed mass/loading are inevitably longer than those where lumped masses, and corresponding point loads, are employed to model, in a more simplified fashion, the mass/weight of the structure. If users are not interested in obtaining information on the local stress state of structural elements (e.g. beam moment distribution), but are rather focused only on estimating the overall response of the structure (e.g. roof displacement and base shear), then the employment of a faster lumped mass/force modelling approach may prove to be a better option, with respect to its distributed counterpart.
  5. If in Gravity Settings the z-direction is set as that of gravity, then the program will consider only Mz (mass defined in z direction) values in the computation of gravity loading, independently of the values defined in other directions (e.g. Mx, My).