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Engineering Dynamics Analysis
and Design Services |
Analysis
Schemes
Specific
Dynamic Analysis Topics
Engineering
Dynamics Trouble Shooting
| Analysis
of Structures and Components Subjected to Dynamic Loading |
| Dynamic loading affects many structures
and components. Typical dynamic loads are from environmental phenomena, such as wind, wave
and earthquake, from vibrations induced by plant and machinery, or operational factors
such as vehicle dynamics. In analytical terms, all are similar in nature, but may have
particular properties which aid (or hinder) efficient solution. Dynamic analysis will
typically be carried out in the frequency and/or time domain depending on the structure,
materials and loading. A typical dynamic analysis will be carried out in increasing
levels of sophistication, from a hand assessment, through frequency domain studies to the
time domain. Obviously, each phase will only be undertaken if necessary, and if a previous
stage provides a conclusive answer the process will be stopped. |
| Hand
Calculation |
| In dynamic analysis the initial hand
calculations provide a great deal of important information, not the least of which are the
"expected" frequency bounds of the problem. These bounds are used to provide a
means of sanity checking in any more complex solutions, especially where complex time
domain solutions are called for. In many cases, these hand calculations can provide
initial design loadings to allow structural or component design to proceed ahead of the
major analysis. The advantage here is that dynamic analysis can be time consuming and,
therefore, early design information can allow a project to proceed whilst the more complex
analysis is carried out. In addition, this early data can give the designer a chance to
test the structural response and identify design changes, negating the need for costly
re-runs. |
| Frequency
Domain Analysis |
| Of the dynamic analysis procedures,
frequency domain studies tend to be the most cost effective, as they require the least
computational effort. (This should be taken as a rule of thumb only.) Frequency domain
analysis includes mode extraction and modal response, and harmonic analysis. (Some time domain techniques are extensions of mode extraction analysis.) Except
in clearly defined circumstances a mode frequency analysis will be carried out prior to a
time domain solution, to allow more accurate problem bounding and to act as a test of the
proposed solution model.
Mode frequency analysis will identify the natural frequencies of structures and their
sub systems, to allow analysis of the expected response of a structure. Once natural
frequencies are known, areas of possible interaction and resonance can be isolated and
modified if necessary.
Applying a response spectrum to the identified frequencies and mode shapes, it is
possible to identify the "steady state" responses of the structure. It is then
possible to analyse load paths, deflections and stressing. Modal response analysis will
also involve the combination of modes, using any of the common methods, direct sum,
absolute sum, 10% absolute sum, square root sum of squares (SRSS) and Complete Quadratic
Combination (CQC). |
| Time
Domain Analysis |
| Time domain analysis is used where
frequency domain analysis is inappropriate, or where a more detailed answer is required.
For example, time domain solutions are more appropriate for the analysis of varying
damping, non-linear structural responses and where differential effects are important,
such as displacement. In addition, time domain analysis does not assume a steady state
dynamic response, therefore, stress levels can be lower than from modal response analysis.
(Depending upon the modal response and the mode combination techniques used, time domain
analysis can result in higher loads, i.e. mode response analysis can be
non-conservative. Time domain analysis can be by direct integration, using implicit
or explicit integration schemes, or by mode superposition. Mode superposition analysis
will usually use the results of a mode extraction as the underlying basis for the time
domain solution. Direct integration solutions are well suited to problems where damping
variation, (either throughout the structure or varying by response), occurs and where
non-linear effects are likely to predominate.
Time domain results can be used to generate input for sub-structure analysis, either
using further time domain models or by generating "in structure" response
spectra for modal response analysis. |
| Explosion/Blast
Response |
| The response of structures to explosion or
blast loading is important, especially in safety related situations, such as chemical
processing or production areas. Structures will often resist much higher blast over
pressure than basic pseudo dynamic analysis will suggest, due to such effects as the
pressure pulse duration and local structural deformations and motions. Local and in some
case global non-linearities, such as material plastification can also help to resist
blast. Blast analysis by dynamic and non-linear dynamic analysis is one of the special
analysis services offered by CREA Consultants.
Experience has been gained in offshore oil and gas processing, defence and industrial
accident projects.
Time domain solutions can take account of mass and structural damping, differential
arrival times, change in material state, i.e. elasto-plastic and shear wave velocities.
These all add up to enable the "apparent" strength of the structure under blast
loading to be assessed.
(See also Fire and Explosion
Response) |
| Seismic
Response Analysis |
| Dynamic analysis to study the response of
structures to seismic loading can be carried out in the time domain, frequency domain or a
combination of both. Seismic response analysis will measure forces induced in structures
by earthquake, study the effects on the supporting soil, and look at soil-structure and
structure-soil-structure interactions. Parametric analysis can be used to optimise
design to resist earthquake, and to design isolation schemes. Seismic
loading is displacement driven, therefore, it is often necessary to introduce additional
flexibility into structures to provide resistance. Other structures may be such that great
stiffnesses can be achieved, thus shifting the response out of the influence of the
earthquake.
CREA Consultants also produce the program DynaTool a time series analysis
program which will generate time histories and response spectra. Visit
the DynaTool Home Page. |
| Wind
and Wave Loading |
| Both wind and wave loading can be treated
as pseudo static, spectral or as time domain loading. Analysis by CREA Consultants
can look at all of these schemes, and as combinations. Wind loads are more usually
considered as steady state loads, this is due to the relative stiffness of the structure
being loaded leading to high natural frequencies relative to the loading frequency.
However, slender structures and exposed slender components of larger structures, are
increasingly analysed using dynamic analysis technology due to the dynamic effects of wind
gust and vortex shedding. These effects have frequencies which are of the same order as
the natural frequencies of the structures, and there is the possibility of dynamic
amplification of the loading.
Wave loads are of importance to structures which are permanently installed in the sea
or significantly sized lakes. The repeated loading, particularly those with a high return
frequency, lead to fatigue loading of structures. This is particularly important with the
steel jackets of offshore oil and gas production structures, where the wave loading
combined with currents can lead to fatigue damage to structural nodes.
In addition to repeated wave loading, there is the requirement to analyse offshore
structures against extreme waves to demonstrate survivability to say 1 in 10,000 year
events. The non-linear analysis program USFOS is used for
this type of analysis. |
| Impact
Analysis |
| Analysis of impacting bodies can be quite
complex, due to the nature of the objects involved. Following hand calculation of the
system to assess the type of impact, the decision can be made as to whether or not more
complex analysis will be of benefit. As with blast response analysis
the structural stiffness due to high speed loading can be apparently much higher than that
due to "normal" static loading. In addition to the effects of the initial
impact, rebound and subsequent impacts are of importance. If the impacting body rebounds,
less damage is likely than if it cannot rebound, or if it rebounds upwards and, therefore,
due to gravity, subsequent impacts occur. It may be that the target is such that the
incident object does not rebound and plastic deformation and penetration occur. Again in
common with blast loading, the supporting structure can be of importance, since it may
deflect and absorb impact energy in addition to the target area. For offshore structures
the non-linear analysis program USFOS is used for this type
of analysis. |
| Vibration
Isolation |
| Vibration isolation is an important aspect
of modern design and can be achieved in one of two ways, either prevent vibration from the
source transmitting into the surrounding structure, or isolate vibration sensitive
equipment or structures from the surrounding structure and hence the vibration source. Isolation
from the source is often used to provide seismic resistance, by building on rubber
bearings, and to isolate sensitive equipment, by building on massive foundations.
Isolation by the combined use of rubber isolators and massive foundations can also be
employed. When using rubber compounds for isolation the sensitivity to deflection has to
be taken into account, or fusible links should be considered.
Prevention of damage due to the transmission of vibration can also be achieved using
structural techniques such as pins and sliders.
Dynamic analysis techniques can be used to identify and design in or out dynamic
responses. Mode frequency analysis will identify natural frequencies of systems and sub
systems, time domain analysis will assess vibration transmission. |
| Nuisance
Vibration Transmission |
| Nuisance from transmitted vibration is not
confined to transmission within structures. Vibration from factories and industrial sites
can be transmitted through the ground to neighbouring residential sites. Vibration is
transmitted from road and rail, through service conduits, and many more routes. Studies
can be undertaken to attempt to identify the source of nuisance vibration, and hence to
suggest solutions. |
CREA Consultants:-
"Your" Engineering Analysis Department |
Engineering
Analysis & Design | Offshore Oil and Gas Structural
Analysis | Dynamic Analysis (Seismic, Vibration)
Fire and Explosion Response Analysis | Thermal
Analysis | Safety Engineering
ANSYS Consultancy & Analysis | USFOS
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© CREA Consultants Ltd,
High Peak, UK Last Updated 05 Jan 2002
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