Global response parameters

Depending on the type of analysis and/or the input parameters defined in the Pre-Processor, up to seven different kinds of global response parameters results can be output in this module; (i) structural displacements, (ii) forces and moments at the supports, (iii) nodal velocities/accelerations, (iv) total inertia & damping forces, (v) hysteretic curves, (vi) performance criteria checks and (vii) code-based checks. Apart from the last two modules, where performance checks are displayed, in all the other results are defined in the global system of coordinates, as illustrated in the figure below, where it is noted that rotation/moment variables defined with regards to a particular axis, refer always to the rotation/moment around, not along, that same axis.

All of these parameters are briefly described hereafter:

Structural displacements
The user can obtain the displacement results of any given number of nodes, relative to one of the six available global degrees-of-freedom. Note that in dynamic analysis it is advisable for relative (with respect to a support), rather than absolute nodal displacements to be plotted. Indeed, due to the unrealistically large rigid body deformation of the foundation nodes (resulting from the uncorrected/unfiltered double-integration of the acceleration time-history), absolute displacements provide little information on the actual structural response characteristics, for which reason they are usually not considered when post-processing dynamic analysis.

Forces and Moments at Supports
Similarly to the structural deformations, the support forces and moments in every direction can be obtained for all restrained nodes. The possibility for outputting the total support force/moment in the specified direction, instead of individual support values, enables also the computation and plotting of total base shear values, for instance.  

Note: Evidently, the total moment support reaction does not include overturning effects, consisting simply of the sum of moments at the structure's supports.

Nodal Accelerations and Velocities
In dynamic time-history analyses, the response nodal accelerations and velocities can be obtained in exactly the same manner as nodal displacements are. The possibility of obtaining relative, as opposed to absolute, quantities is also available. The latter modality is usually adopted when accelerations are selected, whilst the former is usually considered when looking at velocity results.

Total Inertia & Damping Forces
Here, it is possible for the user to obtain the total values of inertia and viscous damping forces mobilised at every given time-step of a dynamic time-history analysis. It is noted that total viscous damping forces (which are the product, at every analysis step, of the damping matrix with the velocity vector) can be computed as the difference between the total internal forces (which are the product, at every analysis step, of the stiffness matrix with the displacement vector) and the total inertia forces (which are the product, at every analysis step, of the mass matrix with the acceleration vector). Evidently, the total internal forces are equal to the Forces and Moments at Supports, given above, and when no viscous damping is defined then the total inertia forces are simply equal to the forces at the supports.

Hysteretic Curves
The user is able to specify a translational/rotational global degree-of-freedom to be plotted against the corresponding total base-shear/base-moment or load factor (pushover analysis). In static analysis, such a plot represents the structure's capacity curve, whilst in time-history analysis this usually reflects the hysteretic response of the model. The possibility for relative displacement output is also available, as this is useful for the case of dynamic analysis post-processing.

Note: The supports reactions should in principle be equal to the internal forces of the base elements that are connected to the foundation nodes. In other words, one would normally expect the values obtained in Forces and Moments at Supports to be identical to those given in the Element Action Effects for the elements connected to the foundations. However, some factors may actually lead to differences in these two response parameters:
i) member action effects are given in the local reference system of each element, whilst reactions at supports are provided in the global coordinates system. Hence, in those cases where large displacements/rotations are incurred by the structure, differences in element shears and support horizontal reactions may be observed;
ii) in dynamic analyses featuring tangent stiffness proportional equivalent viscous damping, and in some cases only (typically, cantilevers with low/zero axial load), it may happen that differences between elements internal actions and support reactions are observed, due to spurious numerical responses (associated to the fact that the tangent stiffness proportional damping behaves hysteretically and thus may develop damping even for velocities equal to zero);
iii) the presence of offsets.