iMechanica - Comments for "How to consider gravity in a dynamic analysis?"
https://imechanica.org/node/4744
Comments for "How to consider gravity in a dynamic analysis?"enconsider gravity in a dynamic analysis
https://imechanica.org/comment/11206#comment-11206
<a id="comment-11206"></a>
<p><em>In reply to <a href="https://imechanica.org/node/4744">How to consider gravity in a dynamic analysis?</a></em></p>
<div class="field field-name-comment-body field-type-text-long field-label-hidden"><div class="field-items"><div class="field-item even"><p>
hi ..this quesion always confused me .. but i think this better ans. for it
</p>
<p>
In my case,the analysis is a time-history analysis of a structure<br />
under earthquake.</p>
<p>First, gravity analysis, and then, dynamic analysis with constant<br />
gravity load (stress and deformation obtained by dynamic analysis<br />
will<br />
superimpose on those obtained by gravity analysis). However, if the<br />
structure is damage, the superimposition is not rational.</p>
<p>1. the equilibrium equation is Mx''+Cx'+kdx = -ma - mg -R or<br />
Mx''+Cx'+kdx = -ma -R ??</p>
<p>2. is [do one step dynamic analysis (t->t+dt) and do one gravity<br />
analysis at the same time, and then superimpose them] rational?</p>
<p>3. if the 2. is not ture, How to consider gravity in a dynamic<br />
analysis?
</p>
<p>
i hope it will you.
</p>
<p>
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</p>
<p>
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</p>
<p>
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</p>
<p>
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</p>
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</ul>Mon, 15 Jun 2009 09:31:40 +0000MichalClarkcomment 11206 at https://imechanica.orgDynamic structural analysis
https://imechanica.org/comment/10186#comment-10186
<a id="comment-10186"></a>
<p><em>In reply to <a href="https://imechanica.org/comment/9847#comment-9847">Dynamic structural analysis</a></em></p>
<div class="field field-name-comment-body field-type-text-long field-label-hidden"><div class="field-items"><div class="field-item even"><p>
Scott,
</p>
<p>
The question is not whether one should or should not separate the analysis into static and dynamic parts. It is a question of efficiency. When one applies the gravitational body force through an explicit algorithm, one needs to ramp up the gravity load slowly to minimize the inertial effects. Depending on the natural time scale of the problem of interest, this ramping of gravity load can be inordinately long for large 3-d finite element models. One can sometimes use a mass scaling technique to speed things up but in most cases, it is simply faster to solve the problem statically.
</p>
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</ul>Thu, 19 Mar 2009 21:15:28 +0000kwlimcomment 10186 at https://imechanica.orgRE: How to consider gravity in a dynamic analysis?
https://imechanica.org/comment/9856#comment-9856
<a id="comment-9856"></a>
<p><em>In reply to <a href="https://imechanica.org/node/4744">How to consider gravity in a dynamic analysis?</a></em></p>
<div class="field field-name-comment-body field-type-text-long field-label-hidden"><div class="field-items"><div class="field-item even"><p>
Hi,
</p>
<p>
Dynamics analysis should be departed from "equilibrium conditions" either in gravitational environment or not. Therefore, you should solve dynamics analysis out of gravity analysis (self weight ??). How to combine them? Save all results from gravity analysis and use them in dynamics analysis by considering zero gravity load.
</p>
<p>
</p>
<p>
</p>
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</ul>Thu, 19 Feb 2009 06:53:43 +0000Sugeng Waluyocomment 9856 at https://imechanica.orgDynamic structural analysis
https://imechanica.org/comment/9847#comment-9847
<a id="comment-9847"></a>
<p><em>In reply to <a href="https://imechanica.org/node/4744">How to consider gravity in a dynamic analysis?</a></em></p>
<div class="field field-name-comment-body field-type-text-long field-label-hidden"><div class="field-items"><div class="field-item even"><p>
It is interesting that these analyses should be separated. It has been some time since I have run eaerthquake damage simulations myself, but it is a simple matter to show (e.g, through free body diagrams) that the equilibrium equation is x" + 2zwx' + wwx = f/m, where the gravitational body force is folded into the RHS.
</p>
<p>
The functional form you show suggests you are doing a lumped mass type dynamic analysis (perhaps with damage softening and perhaps with associated damping changes). This system of equations can be well-treated with an explicit algorithm (e.g., Newmark) without the need to divorce a static calculation from a dynamic one. If this is a full structural finite element treatment, the situation does not appreciably change in terms of being able to do both analyses concurrently with an explicit integration algorithm.
</p>
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</ul>Wed, 18 Feb 2009 21:29:21 +0000Scott Johnsoncomment 9847 at https://imechanica.orgIMPORT analysis
https://imechanica.org/comment/9760#comment-9760
<a id="comment-9760"></a>
<p><em>In reply to <a href="https://imechanica.org/node/4744">How to consider gravity in a dynamic analysis?</a></em></p>
<div class="field field-name-comment-body field-type-text-long field-label-hidden"><div class="field-items"><div class="field-item even"><p>If you are using ABAQUS, you can perform a static analysis followed by a dynamic analysis. You can import your deformed geometry and stress state before you run your dynamic analysis. Not all elements are supported - check the documentation. Perhaps other software will be able to do the same.</p>
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</ul>Fri, 06 Feb 2009 04:17:52 +0000kwlimcomment 9760 at https://imechanica.org