Weight of dinamic mass

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Weight of dinamic mass
 
#1
Oct 29, 2014 18:33:34
Dear forum users, help to become better acquainted with the LIRA. Explain me please; what is the weight of dynamic mass in loads? Why do we need it? How to apply it?
 
 
 
#2
Oct 30, 2014 12:37:44
Dynamic masses are necessary for calculation of inertia forces. For instance, during calculation of wind with pulsation. The pulsation (inertia) supplement depends on the mass of the object and its distribution. Masses are set as a load in dynamic load cases. It is possible to determine them and apply to scheme nodes manually or handle existing load cases with gravity loads using software (the ones that are directed against the Z-axis HSC). Use tab “convert static load into masses”. Mass relates to weight and correlation weight=mass* acceleration of gravity. There are set not masses but their weights in the LIRA. Transfer of the weight into mass is carried out at the program level.
 
 
 
#3
Oct 30, 2014 17:12:03
Thank you. Tell me please once again, what means additional mass in the node during set of harmonic load?
 
 
 
#4
Nov 4, 2014 12:30:27
Additional mass at load is just some convenience when setting the mass. It lets you decrease the volume of information you set. For example, the problem has two dynamic loads: seismic and harmonic. The user decided to set the same distribution of masses for both dynamic loads. But, in the case of harmonic load, one more mass (or several masses) is acting on the node or nodes. This mass is called "additional" relative to mass distribution that is already set.
 
 
 
#5
Nov 5, 2014 13:50:47
I’ve got it. Thank you, Alexey. I have one more question off topic. How best to do? I am calculating pile foundation of disk filter. Its weight is set in static load case and harmonic load in dynamic one. But the fact is that this aggregate has rotating parts, which leads to formation of moment that is transferred to foundation. What is the best way to set it? In static load case or in dynamic one? Is it possible to represent it as couple of vertical forces or set it as a moment? Or am I mistaken somewhere?
 
 
 
#6
Nov 5, 2014 18:02:37
When aggregate is off, the moment does not occur. So, let the weight remains in static load case. Harmonic analysis for the work of the aggregate will show behaviour of the structure.  

Best regards,
E.Streletskii
 
 
 
#7
Nov 11, 2014 14:44:02
How the moment is taken into account in dynamic load case? One more question. Does it necessary to multiply dynamic load on safety factor (according SNiP 2.02.05-87 it is equal to 4)?
 
 
 
#8
Nov 17, 2014 17:00:12
1. It is necessary to judge structural reactions to the action of harmonics according to received mode shapes.  If the result of the calculation will be the torsional shape, that means there was a pair of inertial forces, which corresponds to it. It is possible to get the value of torsional moment expressly multiplying the couple of received inertial forces on their shoulder. But usually the user is interested in displacements and efforts.
2. There are specified design loads in the LIRA.

Best regards,
E.Streletskii
 
 
 
#9
Nov 17, 2014 17:50:58
It means that, it is not necessary to apply any moments in the beginning during calculation of foundation except for harmonic loads. Is it possible to determine its value according to the results?
 
 
 
#10
Nov 18, 2014 16:38:35
Exactly.
 
 
 
#11
Nov 18, 2014 16:39:54
Thank you Eugene

I have one more question according to my task. There are acting horizontal and vertical dynamic loads on the aggregate foundation. I set this loads in different load cases as harmonic load. Dead weight of aggregate is set in separate load case. Is it necessary to set aggregate dead weight as additional mass in the node of horizontal harmonics?
 
 
 
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