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Posts: 8
Registered: ‎01-16-2013
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Definition of Blast accidental loads in Robot

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01-16-2013 01:04 AM

Hello,

I need to calculate Collision of truck with the bearing column of frame. The weight of the truck is 18t and his speed is 8,34m/s. What is the right way to do it ? 

 

Best regards

Igor

 

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Artur.Kosakowski
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Registered: ‎12-17-2010

Re: Definition of Blast accidental loads in Robot

01-16-2013 02:43 AM in reply to: IgorSitek

Most likely there are some rules that describe the equivalent static force that you could  try to search for. For the dynamic approach (time history analysis) check the attached example.

 

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Artur Kosakowski
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Posts: 8
Registered: ‎01-16-2013

Re: Definition of Blast accidental loads in Robot

01-16-2013 06:15 AM in reply to: Artur.Kosakowski

Hello,

I read carefully your answer, but still some things are unclear for me.

Could you,please, see my .doc file and comment it.

Is this possible to sent you robot model for review ?

 

Best regards

Igor

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Active Member
Posts: 8
Registered: ‎01-16-2013

Re: Definition of Blast accidental loads in Robot

01-16-2013 06:18 AM in reply to: IgorSitek

Please find my model here...

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Active Member
Posts: 8
Registered: ‎01-16-2013

Re: Definition of Blast accidental loads in Robot

01-16-2013 06:21 AM in reply to: IgorSitek

Sorry,

this is again me =) If you can correct necessary things in my model and sent me back - it will be greate.

 

Best regards

Igor

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Posts: 461
Registered: ‎06-23-2008

Re: Definition of Blast accidental loads in Robot

01-16-2013 09:21 AM in reply to: IgorSitek

Attached the modified file. Some of modifications made:

1/ switched off DSC to avoid unjustified message related to dampers in modal decomposition method

2/ load in load case 3 changed to 1 kN

3/ time history forcing functions defined using the conservation of momentum principle as described in more detail in my example supplied previosly by Artur:

m*v = F*t

Assumed constant force F during the impact lasting t  (precisely it should be integral of F in time domain) and checked 2 lengths t of impact: 0.02sec (case 5), and 0.01sec (case 9)

So for instance for t=0.02 sec: F=18000kg*8.33m/s/0.02sec = 7500000 N = 7500 kN

As it can be seen below no big difference between them - it is difficult to notice that these are pairs of overlapping diagrams:impact.png

 

Assumed modal damping 0.05 for all modes.

Other assumptions

Only the selfweight of the structure was considered in the mass of the model:

a/ no conversion of other loads to masses - converting load case 2 to mass would result in significantly increased inertia of frame thus significantly reducing displacements and completely changing frequency and shape of oscillations. After similar changes it may be necesary to divide bars of the frame to more precisely consider vibrations corresponding to higher modes

b/ truck treated only as the "source" of force and not mass - as if separating from the frame just after collision

c/ ...and of course everything linear and elastic

 

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Pawel Pulak
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Re: Definition of Blast accidental loads in Robot

01-17-2013 12:15 AM in reply to: pp2008

Hello,

1)      So, the force calculated by me 894,3 kN is too small, because it calculated for impact lasting t=0.168 sec ? Why did you take for the Time History analysis two lengths of impact t1=0.02 sec and t2=0,01 sec, does it comes from some Norms ?

2)      My perpose in Blast analysis to discover the response of structure from impact of truck. I need to ensure that if even one column isn’t resist the impact of truck, the structure itself will be strong enough and not collapse. What is the practical usage of  Time History analysis ? How I can get forces from it in proper form(table form or on the screen) for static analysis of the collided column ? If I know force F in advance, for instance, for t=0,01 sec it is F=15000Kn, why I can’t put it STREIGHT in Static analysis and calculate the capacity of the column and general deformation of structure , that benefit I can get from Time History analysis ?

3)      Now I found only how to get the forces in graphic view. Please, give me clear explanation how to use the results from Time History analysis ?

 

Best regards

Igor 

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Posts: 461
Registered: ‎06-23-2008

Re: Definition of Blast accidental loads in Robot

01-17-2013 07:03 AM in reply to: IgorSitek

1)      So, the force calculated by me 894,3 kN is too small, because it calculated for impact lasting t=0.168 sec ? Why did you take for the Time History analysis two lengths of impact t1=0.02 sec and t2=0,01 sec, does it comes from some Norms ?

----------------------------

In your original model you have defined the impact of 894.8*63=56340.9kN lasting 0.05 sec and then linearly decreasing to zero during next 0.05 sec.

When I have defined (attached model - see load case 15) the impact 894,3 kN lasting  t=0.168 sec (forcing function constant during 0.168 sec) the results comparable with shorter impacts but with bigger force (but the same F*t product) were received - see the screen capture below. In load case 15 smaller and delayed response can be observed because the impact time is already comparable with 1/4 of the periods of basic vibration modes of the frame.

impact1.png

I have taken for  the Time History analysis two lengths of impact to illustrate the effect that for short lengths of impact (short in relation to the periods of basic vibration modes of the structure) the response depends on the product F*t and not only the force F. I have not taken it from any norms - pure machanics.

 

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2)      My perpose in Blast analysis to discover the response of structure from impact of truck. I need to ensure that if even one column isn’t resist the impact of truck, the structure itself will be strong enough and not collapse. What is the practical usage of  Time History analysis ? How I can get forces from it in proper form(table form or on the screen) for static analysis of the collided column ? If I know force F in advance, for instance, for t=0,01 sec it is F=15000Kn, why I can’t put it STREIGHT in Static analysis and calculate the capacity of the column and general deformation of structure , that benefit I can get from Time History analysis ?

3)      Now I found only how to get the forces in graphic view. Please, give me clear explanation how to use the results from Time History analysis ?

-----------------------------

Time history analysis (THA) give results (forces, displacements, moments,etc) in series of time steps.

For instance in my examples it was defined with such paramaters that the results were output in 501 steps from t=0 sec to t=5 sec.

The results of all these steps can be seen in the tables when tables are filtered only to THA case - see below:

tha1.png

In the graphical window the results (deformed shapes, diagrammes of moments,...) can be also displayed for selected time (as if a single picture from the video) selecting THA load case and appropriate component of it:

tha2.png

Direct application of the force of 15000kN as static load case would give completely different (bigger) results than THA because it would correspond to static application of this load without vibrations (constant displacements, forces, moments etc). In case of THA the load is acting very short (0.01 sec in my example) and is resulting in variable in time displacements, forces, moments etc.

When using THA results in design it is the best to find time steps with the extreme forces or moments and to convert them to simple cases - see the green rectangle in the screen capture above. Such cases can be easily combined with other load cases and used in design.

 

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If this post answers your question please click "Accept as Solution". It will help everyone to find answer more quickly!

 

Regards,



Pawel Pulak
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