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| Structural analysis is mainly concerned with finding out the behaviour of a physical structure when subjected to force. This action can be in the form of load due to the weight of things such as people, furniture, wind, snow, etc. or some other kind of excitation such as an earthquake, shaking of the ground due to a blast nearby, etc. In essence all these loads are dynamic, including the self-weight of the structure because at some point in time these loads were not there. The distinction is made between the dynamic and the static analysis on the basis of whether the applied action has enough acceleration in comparison to the structure's natural frequency. If a load is applied sufficiently slowly, the inertia forces (Newton's second law of motion) can be ignored and the analysis can be simplified as static analysis.
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| '''Structural dynamics''', therefore, is a type of [[structural analysis]] which covers the behaviour of [[structure]]s subjected to [[Dynamics (physics)|dynamic]] (actions having high acceleration) loading. Dynamic loads include people, wind, waves, traffic, [[earthquake]]s, and blasts. Any structure can be subjected to dynamic loading. Dynamic analysis can be used to find dynamic [[Displacement (vector)|displacements]], time history, and [[modal analysis]].
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| A dynamic analysis is also related to the inertia forces developed by a structure when it is excited by means of dynamic loads applied suddenly (e.g., wind blasts, explosion, earthquake).
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| A [[statics|static]] load is one which varies very slowly. A dynamic load is one which changes with time fairly quickly in comparison to the structure's natural frequency. If it changes slowly, the structure's response may be determined with static analysis, but if it varies quickly (relative to the structure's ability to respond), the response must be determined with a dynamic analysis.
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| Dynamic analysis for simple structures can be carried out manually, but for complex structures [[finite element analysis]] can be used to calculate the mode shapes and frequencies.
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| ==Displacements==
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| A dynamic load can have a significantly larger effect than a static load of the same magnitude due to the structure's inability to respond quickly to the loading (by deflecting). The increase in the effect of a dynamic load is given by the [[dynamic amplification factor]] (DAF):
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| :<math> DAF = \frac{{u_{max}}}{{u_{static}}}</math>
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| where u is the deflection of the structure due to the applied load.
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| Graphs of dynamic amplification factors vs non-dimensional [[rise time]] (t<sub>r</sub>/T) exist for standard loading functions (for an explanation of rise time, see '''time history analysis''' below). Hence the DAF for a given loading can be read from the graph, the static deflection can be easily calculated for simple structures and the dynamic deflection found.
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| ==Time history analysis==
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| A {{Not a typo|full time}} history will give the response of a structure over time during and after the application of a load. To find the {{Not a typo|full time}} history of a structure's response, you must solve the structure's [[equation of motion]].
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| ===Example===
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| [[Image:Mass spring.svg|200px|right|Single degree of freedom system: simple mass spring model]]
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| A simple single [[Degrees of freedom (physics and chemistry)|degree of freedom]] [[system]] (a [[mass]], M, on a [[Spring (device)|spring]] of [[stiffness]], k for example) has the following equation of motion:
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| :<math>M{\ddot{x}} + kx = F(t)</math>
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| :
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| where <math>\ddot{x}</math> is the acceleration (the double [[derivative]] of the displacement) and x is the displacement.
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| If the loading F(t) is a [[Heaviside step function]] (the sudden application of a constant load), the solution to the equation of motion is:
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| :<math>x = \frac{{F_0}}{{k}}[1 - cos{(\omega t)}]</math>
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| :
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| where <math>\omega = \sqrt{\frac{{k}}{{M}}}</math> and the fundamental natural frequency, <math>f = \frac {\omega}{2\pi}</math>.
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| The static deflection of a single degree of freedom system is:
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| :<math>x_{static} = \frac{{F_0}}{{k}}</math>
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| :
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| so you can write, by combining the above formulae:
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| :<math>x = x_{static}[1 - cos(\omega t)]</math>
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| This gives the (theoretical) time history of the structure due to a load F(t), where the false assumption is made that there is no [[damping]].
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| Although this is too simplistic to apply to a real structure, the Heaviside Step Function is a reasonable model for the application of many real loads, such as the sudden addition of a piece of furniture, or the removal of a prop to a newly cast concrete floor. However, in reality loads are never applied instantaneously - they build up over a period of time (this may be very short indeed). This time is called the [[rise time]].
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| As the number of degrees of freedom of a structure increases it very quickly becomes too difficult to calculate the time history manually - real structures are analysed using [[non-linear]] [[finite element analysis]] software.
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| ==Damping==
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| Any real structure will dissipate energy (mainly through friction). This can be modelled by modifying the DAF
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| :<math>DAF = 1 + e^{-c\pi}</math>
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| :
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| where <math>c=\frac{{\text {Damping Coefficient}}}{{\text{Critical Damping Coefficient}}}</math> and is typically 2%-10% depending on the type of construction:
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| * Bolted steel ~6%
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| * Reinforced concrete ~ 5%
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| * Welded steel ~ 2%
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| * Brick masonry ~ 10%
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| Generally damping would be ignored for non-transient events (such as wind loading or crowd loading), but would be important for transient events (for example, an [[impulse (physics)|impulse]] load such as an earthquake loading or bomb blast).
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| ==Modal analysis==
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| A [[modal analysis]] calculates the frequency [[Normal mode|modes]] or natural frequencies of a given system, but not necessarily its full-time history response to a given input. The natural frequency of a system is dependent only on the [[stiffness]] of the structure and the [[mass]] which participates with the structure (including self-weight). It is not dependent on the load function.
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| It is useful to know the modal frequencies of a structure as it allows you to ensure that the frequency of any applied periodic loading will not coincide with a modal frequency and hence cause [[resonance]], which leads to large [[oscillations]].
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| The method is:
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| # Find the natural modes (the shape adopted by a structure) and natural frequencies
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| # Calculate the response of each mode
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| # Optionally superpose the response of each mode to find the full modal response to a given loading
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| ===Energy method===
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| It is possible to calculate the frequency of different mode shape of system manually by the [[Energy principles in structural mechanics|energy method]]. For a given mode shape of a multiple degree of freedom system you can find an "equivalent" mass, stiffness and applied force for a single degree of freedom system. For simple structures the basic mode shapes can be found by inspection, but it is not a conservative method. Rayleigh's principle states:
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| "The frequency ω of an arbitrary mode of vibration, calculated by the energy method, is always greater than - or equal to - the fundamental frequency ω<sub>n</sub>."
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| For an assumed mode shape <math>\bar{u}(x)</math>, of a structural system with mass M; stiffness, EI ([[Young's modulus]], E, multiplied by the [[second moment of area]], I); and applied force, F(x):
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| :<math>\text{Equivalent mass, } M_{eq} = \int{M\bar{u}^2} du </math>
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| :
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| :<math>\text{Equivalent stiffness, }k_{eq} = \int{EI \bigg(\frac{{d^2\bar{u}}}{{dx^2}} \bigg)^2}dx</math>
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| :
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| :<math>\text{Equivalent force, }F_{eq} = \int{F\bar{u}}dx</math>
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| :
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| then, as above:
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| :<math>\omega = \sqrt{\frac{{k_{eq}}}{{M_{eq}}}}</math>
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| ===Modal response===
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| The complete modal response to a given load F(x,t) is <math>v(x,t)=\sum{u_n(x,t)}</math>. The summation can be carried out by one of three common methods:
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| * Superpose complete time histories of each mode (time consuming, but exact)
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| * Superpose the maximum amplitudes of each mode (quick but conservative)
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| * Superpose the square root of the sum of squares (good estimate for well-separated frequencies, but unsafe for closely spaced frequencies)
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| To superpose the individual modal responses manually, having calculated them by the energy method:
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| Assuming that the rise time t<sub>r</sub> is known (T = 2π/ω), it is possible to read the DAF from a standard graph. The static displacement can be calculated with <math>u_{static}=\frac{F_{1,eq}}{k_{1,eq}}</math>. The dynamic displacement for the chosen mode and applied force can then be found from:
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| :<math>u_{max} = u_{static}DAF</math>
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| ==Modal participation factor==
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| For real systems there is often mass participating in the [[forcing function (differential equations)|forcing function]] (such as the mass of ground in an [[earthquake]]) and mass participating in [[inertia]] effects (the mass of the structure itself, M<sub>eq</sub>). The [[modal participation factor]] Γ is a comparison of these two masses. For a single degree of freedom system Γ = 1.
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| :Γ <math> = \frac{\sum{M_n\bar{u}_n}}{\sum{M_n\bar{u}_n^2}}</math>
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| ==External links==
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| * [http://sites.google.com/site/dyssolve/ DYSSOLVE: Dynamic System Solver] - An encrypted-source, lightweight, free-of-charge software that can be used to solve basic structural dynamics problems.
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| * [http://structdynviblab.mcgill.ca/ Structural Dynamics and Vibration Laboratory of McGill University]
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| * [http://frame3dd.sourceforge.net Frame3DD open source 3D structural dynamics analysis program]
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| * [http://www.noisestructure.com/products/FRF.php Frequency response function from modal parameters]
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| * [http://vibrationdata.wordpress.com/category/structural-dynamics/ Structural Dynamics Tutorials & Matlab scripts]
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| [[Category:Structural analysis]]
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| [[Category:Dynamics]]
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