Civil Engineering Reference
In-Depth Information
(4) Response spectrum analysis, which is a linear response spectrum analysis
that is often used to obtain an approximate upper bound of the peak sig-
nificant response of a system to a user-supplied input spectrum (such as
earthquake data) as a function of frequency. The method has a very low
computational cost and provides useful information about the spectral
behavior of a system. Response spectrum analysis can be performed
using the SIM architecture.
(5) Random response analysis, in which the linearized response of a model
to random excitation can be calculated based on the natural modes of the
system. This procedure is used when the structure is excited continu-
ously and the loading can be expressed statistically in terms of a “power
spectral density” function. The response is calculated in terms of statis-
tical quantities such as the mean value and the standard deviation of
nodal and element variables. Random response analysis can be per-
formed using the SIM architecture. SIM is a high-performance
software architecture available in ABAQUS [1.29] that can be used
to perform modal superposition dynamic analyses. The SIM architec-
ture is much more efficient than the traditional architecture for large-
scale linear dynamic analyses (both model size and number of modes)
with minimal output requests. SIM-based analyses can be used to
efficiently handle nondiagonal damping generated from element or
material contributions. Therefore, SIM-based procedures are an
efficient alternative to subspace-based linear dynamic procedures for
models with element damping or frequency-independent materials.
ABAQUS [1.29] relies on user-supplied model data and assumes that
the material's physical properties reflect experimental results. Examples of
meaningful material properties are a positive mass density per volume, a pos-
itive Young's modulus, and a positive value for any available damping coef-
ficients. However, in special cases, modelers may want to “adjust” a value of
density, mass, stiffness, or damping in a region or a part of the model to bring
the overall mass, stiffness, or damping to the expected required levels. Cer-
tain material options in ABAQUS allow modelers to introduce nonphysical
material properties to achieve this adjustment. Every nonconservative sys-
tem exhibits some energy loss that is attributed to material nonlinearity,
internal material friction, or external (mostly joint) frictional behavior. Con-
ventional engineering materials like steel and high-strength aluminum alloys
provide small amounts of internal material damping, not enough to prevent
large amplification at near-resonant frequencies. Damping properties
increase in modern composite fiber-reinforced materials, where the energy
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